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diff --git a/39033-8.txt b/39033-8.txt new file mode 100644 index 0000000..ad4d544 --- /dev/null +++ b/39033-8.txt @@ -0,0 +1,4043 @@ +The Project Gutenberg EBook of An Introduction to Machine Drawing and +Design, by David Allan Low + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: An Introduction to Machine Drawing and Design + +Author: David Allan Low + +Release Date: March 4, 2012 [EBook #39033] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK AN INTRODUCTION TO MACHINE *** + + + + +Produced by Juliet Sutherland and the Online Distributed +Proofreading Team at http://www.pgdp.net + + + + + + + + + AN INTRODUCTION + TO + MACHINE DRAWING + AND + DESIGN + + BY + + DAVID ALLAN LOW + + (WHITWORTH SCHOLAR), M. INST. M.E. + + HEAD MASTER OF THE PEOPLE'S PALACE TECHNICAL SCHOOLS, LONDON + AUTHOR OF 'A TEXT-BOOK ON PRACTICAL SOLID OR DESCRIPTIVE GEOMETRY' + 'AN ELEMENTARY TEXT-BOOK OF APPLIED MECHANICS' ETC. + + [Illustration] + + _FOURTH EDITION_ + + LONDON + LONGMANS, GREEN, AND CO. + AND NEW YORK: 15 EAST 16th STREET + 1890 + + + + PRINTED BY + SPOTTISWOODE AND CO., NEW-STREET SQUARE + LONDON + + + + +PREFACE. + + +It is now generally recognised that the old-fashioned method of teaching +machine drawing is very unsatisfactory. In teaching by this method an +undimensioned scale drawing, often of a very elaborate description, is +placed before the student, who is required to _copy_ it. Very often the +student succeeds in making a good copy of the drawing placed before him +without learning very much about the object represented by it, and this +state of matters is sometimes not much improved by the presence of the +teacher, who is often simply an art master, knowing nothing about +machine design. It is related of one school that a pupil, after making a +copy of a particular drawing, had a discussion with his teacher as to +whether the object represented was a sewing machine or an electrical +machine. Evidently the publisher of the drawing example in this case did +not adopt the precaution which a backward student used at an examination +in machine design: he put on a full title above his drawing, for the +information of his examiner. + +Now, if machine drawing is to be of practical use to any one, he must be +able to understand the form and arrangement of the parts of a machine +from an inspection of suitable drawings of them without seeing the parts +themselves. Also he ought to be able to make suitable drawings of a +machine or parts of a machine from the machine or the parts themselves. + +In producing this work the author has aimed at placing before young +engineers and others, who wish to acquire the skill and knowledge +necessary for making the simpler _working drawings_ such as are produced +in engineers' drawing offices, a number of good exercises in drawing, +sufficient for one session's work, and at the same time a corresponding +amount of information on the design of machine details generally. + +The exercises set are of various kinds. In the first and simplest +certain views of some machine detail are given, generally drawn to a +small scale, which the student is asked to reproduce _to dimensions +marked on these views_, and he is expected to keep to these dimensions, +and not to measure anything from the given illustrations. In the second +kind of exercise the student is asked to reproduce certain views shown +_to dimensions given in words or in tabular form_. In the third kind of +exercise the student is required to make, in addition to certain views +shown to given dimensions, others which he can only draw correctly if he +thoroughly understands the design before him. In the fourth kind of +exercise the student is asked to make the necessary working drawings for +some part of a machine which has been previously described and +illustrated, _the dimensions to be calculated by rules given in the +text_. + +The illustrations for this work are all new, and have been specially +prepared by the author from _working drawings_, and he believes that +they will be found to represent the best modern practice. + +As exercises in drawing, those given in this book are not numbered +exactly in their order of difficulty, but unless on the recommendation +of a teacher, the student should take them up in the order given, +omitting the following:--26, 27, 28, 35, 40, 42, 43, 45, 49, 50, 54, 60, +61, as he comes to them, until he has been right through the book; +afterwards he should work out those which he omitted on first going over +the book. + +In addition to the exercises given in this work the student should +practise making freehand sketches of machine details from actual +machines or good models of them. Upon these sketches he should put the +proper dimensions, got by direct measurement from the machine or model +by himself. These sketches should be made in a note-book kept for the +purpose, and no opportunity should be lost of inserting a sketch of any +design which may be new to the student, always putting on the dimensions +if possible. These sketches form excellent examples from which to make +working drawings. The student should also note any rules which he may +meet with for proportioning machines, taking care, however, in each case +to state the source of such information for his future guidance and +reference. + +As machine drawing is simply the application of the principles of +descriptive geometry to the representation of machines, the student of +the former subject, if he is not already acquainted with the latter, +should commence to study it at once. + + D. A. L. + GLASGOW: _March_ 1887. + + +_PREFACE TO THE THIRD EDITION._ + +To this edition another chapter has been added, containing a number of +miscellaneous exercises, which it is hoped will add to the usefulness of +the work as a text-book in science classes. The latest examination paper +in machine drawing by the Science and Art Department has also been added +to the Appendix. + + D. A. L. + LONDON: _August_ 1888. + + + + +CONTENTS. + + + PAGE + I. INTRODUCTION 1 + II. RIVETED JOINTS 6 + III. SCREWS, BOLTS, AND NUTS 14 + IV. KEYS 22 + V. SHAFTING 24 + VI. SHAFT COUPLINGS 25 + VII. BEARINGS FOR SHAFTS 30 + VIII. PULLEYS 36 + IX. TOOTHED WHEELS 39 + X. CRANKS AND CRANKED SHAFTS 43 + XI. ECCENTRICS 47 + XII. CONNECTING RODS 49 + XIII. CROSS-HEADS 56 + XIV. PISTONS 57 + XV. STUFFING-BOXES 63 + XVI. VALVES 68 + XVII. MATERIALS USED IN MACHINE CONSTRUCTION 76 + XVIII. MISCELLANEOUS EXERCISES 81 + + APPENDIX A 99 + APPENDIX B 102 + INDEX 113 + + + + +AN INTRODUCTION + +TO + +MACHINE DRAWING AND DESIGN. + + + + +I. INTRODUCTION. + + +_Drawing Instruments._--For working the exercises in this book the +student should be provided with the following:--A well-seasoned yellow +pine _drawing-board_, 24 inches long, 17 inches wide, and 3/8 inch or +1/2 inch thick, provided with cross-bars on the back to give it strength +and to prevent warping. A =T= _square_, with a blade 24 inches long +attached permanently to the stock, _but not sunk into it_. One 45° and +one 60° _set square_. The short edges of the former may be about 6 +inches and the short edge of the latter about 5 inches long. A _pair of +compasses_ with pen and pencil attachments, and having legs from 5 +inches to 6 inches long. A _pair of dividers_, with screw adjustment if +possible. A _pair of small steel spring pencil bows_ for drawing small +circles, and a _pair of small steel spring pen bows_ for inking in the +same. A _drawing pen_ for inking in straight lines. All compasses should +have _round points_, and if possible _needle_ points. A piece of +india-rubber will also be required, besides two pencils, one marked H or +HH and one marked HB or F; the latter to be used for lining in a drawing +which is not to be inked in, or for freehand work. + +Pencils for mechanical drawing should be sharpened with a _chisel +point_, and those for freehand work with a _round point_. _Do not wet +the pencil_, as the lines afterwards made with it are very difficult to +rub out. + +Drawing-paper for working drawings may be secured to the board by +_drawing-pins_, but the paper for finished drawings or drawings upon +which there is to be a large amount of colouring should be _stretched_ +upon the board. + +The student should get the best instruments he can afford to buy, and he +should rather have a few good instruments than a large box of inferior +ones. + +_Drawing-paper._--The names and sizes of the sheets of drawing paper are +given in the following table:-- + + Inches + Demy 20 × 15 + Medium 22 × 17 + Royal 24 × 19 + Imperial 30 × 22 + Atlas 34 × 26 + Double Elephant 40 × 27 + Antiquarian 52 × 31 + +The above sizes must not be taken as exact. In practice they will be +found to vary in some cases as much as an inch. + +Cartridge-paper is made in sheets of various sizes, and also in rolls. + +Hand-made paper is the best, but it is expensive. Good cartridge-paper +is quite suitable for ordinary drawings. + +_Centre Lines._--Drawings of most parts of machines will be found to +be symmetrical about certain lines called _centre lines_. These lines +should be drawn first with great care. On a pencil drawing centre +lines should be thin continuous lines; in this book they are shown +thus -- - -- - --. + +After drawing the centre line of any part the dimensions of that part +must be marked off from the centre line, so as to insure that it really +is the centre line of that part: thus in making a drawing of a rivet, +such as is shown at (_a_) fig. 1, after drawing the centre line, half +the diameter of the rivet would be marked off on each side of that line, +in order to determine the lines for the sides of the rivet. + +_Inking._--For inking in drawings the best Indian ink should be used, +and not common writing ink. Common ink does not dry quick enough, and +rapidly corrodes the drawing pens. The pen should be filled by means of +a brush or a narrow strip of paper, and not by dipping the pen into the +ink. + +In cases where there are straight lines and arcs of circles touching one +another _ink in the arcs first_, then the straight lines; in this way it +is easier to hide the joints. + +_Colouring._--Camel's-hair or sable brushes should be used; the latter +are the best, but are much more expensive than the former. The colour +should be rubbed down in a dish, and the tint should be light. The +mistake which a beginner invariably makes is in having the colour of too +dark a tint. + +First go over the part to be coloured with the brush and _clean_ water +for the purpose of damping it. Next dry with clean blotting-paper to +take off any superfluous water. Then take another brush with the colour, +and beginning at the top, work from left to right and downwards. If it +is necessary to recolour any part let the first coating dry before +beginning. + +Engineers have adopted certain colours to represent particular +materials; these are given in the following table:-- + +_Table showing Colours used to represent Different Materials._ + + MATERIAL COLOUR + + Cast iron Payne's grey or neutral tint. + Wrought iron Prussian blue. + Steel Purple (mixture of Prussian blue and crimson lake). + Brass Gamboge with a little sienna or a very little red + added. + Copper A mixture of crimson lake and gamboge, the former + colour predominating. + Lead Light Indian ink with a very little indigo added. + Brickwork Crimson lake and burnt sienna. + Firebrick Yellow and Vandyke brown. + Greystones Light sepia or pale Indian ink, with a little + Prussian blue added. + Brown freestone Mixture of pale Indian ink, burnt sienna, and + carmine. + Soft woods For ground work, pale tint of sienna. + Hard woods For ground work, pale tint of sienna with a little + red added. + For graining woods use darker tint with a greater + proportion of red. + +_Printing._--A good drawing should have its title printed, a plain style +of letter being used for this purpose, such as the following:-- + + [Illustration: ABCDEFGHIJKLMNOPQRST + UVWXYZ + 1234567890] + + [Illustration: ABCDEFGHIJKLMNOPQR + STUVWXYZ + 1234567890] + +The following letters look well _if they are well made_, but they are +much more difficult to draw. + + [Illustration: ABCDEFGHIJKLMNOP + QRSTUVWXYZ + 1234567890] + +For remarks on a drawing the following style is most suitable:-- + + [Illustration: abcdefghijklmnopqrstuvwxyz] + +All printing should be done by freehand. + +_Border lines_ are seldom put on engineering drawings. + +_Working Drawings._--A good working drawing should be prepared in the +following manner. It must first be carefully outlined in pencil and then +inked in. After this all parts cut by planes of section should be +coloured, the colours used indicating the materials of which the parts +are made. Parts which are round may also be lightly shaded with the +brush and colours to suit the materials. The centre lines are now inked +in with _red_ or _blue ink_. The red ink may be prepared by rubbing down +the cake of crimson lake, and the blue ink in like manner from the cake +of Prussian blue. Next come the _distance_ or _dimension_ lines, which +should be put in with _blue_ or _red ink_, depending on which colour was +used for the centre lines. Dimension lines and centre lines are best put +in of different colour. The arrow-heads at the ends of the dimension +lines are now put in with _black ink_, and so are the figures for the +dimensions. The arrow-heads and the figures should be made with a common +writing pen. The dimensions should be put on neatly. Many a good drawing +has its appearance spoiled through being slovenly dimensioned. + +We may here point out the importance of putting the dimensions on a +working drawing. If the drawing is not dimensioned, the workman must get +his sizes from the drawing by applying his rule or a suitable scale. Now +this operation takes time, and is very liable to result in error. Time +is therefore saved, and the chance of error reduced, by marking the +sizes in figures. + +In practice it is not usual to send original drawings from the drawing +office to the workshop, but copies only. The copies may be produced by +various 'processes,' or they may be tracings drawn by hand. Many +engineers do not ink in their original drawings, but leave them in +pencil; especially is this the case if the drawings are not likely to be +much used. + +_Scales._--The best scales are made of ivory, and are twelve inches +long. Boxwood scales are much cheaper, although not so durable as those +made of ivory. If the student does not care to go to the expense of +ivory or boxwood scales, he can get paper ones very cheap, which will be +quite sufficient for his purpose. The divisions of the scale should be +marked down to its edge, so that measurements may be made by applying +the scale directly to the drawing. For working such exercises as are in +this book the student should be provided with the following scales:-- + + A scale of 1, or 12 inches to a foot. + " 1/2 " 6 " + " 1/3 " 4 " + " 1/4 " 3 " + " 1/6 " 2 " + +A scale of 1 is spoken of as 'full size,' and a scale of 1/2 as 'half +size.' + +Engineers in this country state dimensions of machines in feet, inches, +and fractions of an inch, the latter being the 1/2, 1/4, 1/8, 1/16, &c. +In making calculations it is generally more convenient to use decimal +fractions, and then substitute for the results the equivalent fractions +in eighths, sixteenths, &c. The following table will be found useful for +this purpose:-- + +_Decimal Equivalents of Fractions of an Inch._ + + +----------+--------------------+ + | Fraction | Decimal Equivalent | + +----------+--------------------+ + | 1/32 | .03125 | + | 1/16 | .0625 | + | 3/32 | .09375 | + | 1/8 | .125 | + | 5/32 | .15625 | + | 3/16 | .1875 | + | 7/32 | .21875 | + | 1/4 | .25 | + | 9/32 | .28125 | + | 5/16 | .3125 | + | 11/32 | .34375 | + | 3/8 | .375 | + | 13/32 | .40625 | + | 7/16 | .4375 | + | 15/32 | .46875 | + | 1/2 | .5 | + | 17/32 | .53125 | + | 9/16 | .5625 | + | 19/32 | .59375 | + | 5/8 | .625 | + | 21/32 | .65625 | + | 11/16 | .6875 | + | 23/32 | .71875 | + | 3/4 | .75 | + | 25/32 | .78125 | + | 13/16 | .8125 | + | 27/32 | .84375 | + | 7/8 | .875 | + | 29/32 | .90625 | + | 15/16 | .9375 | + | 31/32 | .96875 | + | 1 | 1.0 | + +----------+--------------------+ + +Engineers use a single accent (') to denote _feet_, and a double +accent (") to denote _inches_. Thus 2' 9" reads two feet nine inches. + + + + +II. RIVETED JOINTS. + + +Two plates or pieces to be riveted together have holes punched or +drilled in them in such a manner that one may be made to overlap the +other so that the holes in the one may be opposite the holes in the +other. The rivets, which are round bars of iron, or steel, or other +metal, are heated to redness and inserted in the holes; the head already +formed on the rivet, and called the tail, is then held up, and the point +is hammered or pressed so as to form another head. This process of +forming the second head on the rivet is known as riveting, and may be +done by hand-hammering or by a machine. + +_Forms of Rivet Heads._--In fig. 1 are shown four different forms of +rivet heads: (_a_) is a _snap head_, (_b_) a _conical head_ (_c_) a +_pan head_, and (_d_) _a countersunk head_. + +_Proportions of Rivet Heads._--The diameter of the snap head is about +1.7 times the diameter of the rivet, and its height about .6 of the +diameter of the rivet. The conical head has a diameter twice and a +height three quarters of the rivet diameter. The greatest diameter of +the pan head is about 1.6, and its height .7 of the rivet diameter. The +greatest diameter of the countersunk head may be one and a half, and its +depth a half of the diameter of the rivet. + +[Illustration: FIG. 1.] + +In fig. 1 at (_a_) and (_b_) are shown geometrical constructions devised +by the author for drawing the snap and conical head for any size of +rivet, the proportions being nearly the same as those given above. + +_Geometrical Construction for Proportioning Snap Heads._--With centre A, +and radius equal to half diameter of rivet, describe a circle cutting +the centre line of the rivet at B and C. With centre B and radius BC +describe the arc CD. Make BE equal to AD. With centre E and radius ED +describe the arc DFH. + +_Construction for Conical Head._--With centre K, and radius equal to +diameter of rivet, describe the semicircle LMN, cutting the side of the +rivet at M. With centre M and radius MN describe the arc NP to cut the +centre line of rivet at P. Join PL and PN. + +When a number of rivets of the same diameter have to be shown on the +same drawing the above constructions need only be performed on one +rivet. After the point E has been discovered the distance AE may be +measured off on all the other rivets, and the arcs corresponding to +DFH drawn with radii equal to ED. In like manner the height KP of the +conical head may be marked off on all rivets of the same diameter with +conical heads. + +_Caulking._--In order to make riveted joints steam- or water-tight the +edges of the plates and the edges of the heads of the rivets are burred +down by a blunt chisel or caulking tool as shown at Q and R. + +[Illustration: FIG. 2.] + +[Illustration: FIG. 3.] + + EXERCISE 1: _Forms of Rivets._--Draw, full size, the rivets and + rivet heads shown in fig. 1. The diameter of the rivet in each + case to be 1-1/8 inches, and the thickness of the plates 7/8 inch. + + EXERCISE 2: _Single Riveted Lap Joint._--Draw, full size, the + plan and sectional elevation of the _single riveted lap joint_ + shown in fig. 2. + +_Table showing the Proportions of Single Riveted Lap Joints for various +Thicknesses of Plates._ (_Plates and Rivets Wrought Iron._) + + +--------------+-------------+----------+--------------+ + | Thickness of | Diameter of | Pitch of | Width of lap | + | plates | rivets | rivets | | + +--------------+-------------+----------+--------------+ + | 1/4 | 9/16 | 1-5/8 | 1-3/4 | + | 5/16 | 5/8 | 1-3/4 | 2 | + | 3/8 | 11/16 | 1-7/8 | 2-1/4 | + | 7/16 | 3/4 | 2 | 2-1/2 | + | 1/2 | 13/16 | 2-1/8 | 2-3/4 | + | 9/16 | 7/8 | 2-1/4 | 2-7/8 | + | 5/8 | 15/16 | 2-5/16 | 3 | + | 11/16 | 1 | 2-3/8 | 3-1/8 | + | 3/4 | 1-1/16 | 2-1/2 | 3-1/4 | + +------------------------------------------------------+ + + All the dimensions are in inches. + +[Illustration: FIG. 4.] + + EXERCISE 3.--Draw, half size, a plan and section of a single + riveted lap joint for plates 3/4" thick to the dimensions given in + the above table. + + EXERCISE 4: _Double Riveted Lap Joint._--Draw, full size, the two + views of the _double riveted lap joint_ shown in fig. 3. + +_Table showing the Proportions of Double Riveted Lap Joints for various +Thicknesses of Plates._ (_Plates and Rivets Wrought Iron._) + + +-----------+-------------+----------+------------------+----------+ + | Thickness | Diameter of | Pitch of | Distance between | Width of | + | of plates | rivets | rivets | rows of rivets | lap | + +-----------+-------------+----------+------------------+----------+ + | 3/8 | 11/16 | 2-1/2 | 1-1/8 | 3-1/2 | + | 7/16 | 3/4 | 2-5/8 | 1-1/4 | 3-3/4 | + | 1/2 | 13/16 | 2-3/4 | 1-3/8 | 4 | + | 9/16 | 7/8 | 2-7/8 | 1-7/16 | 4-1/4 | + | 5/8 | 15/16 | 3 | 1-9/16 | 4-1/2 | + | 11/16 | 1 | 3-1/8 | 1-3/4 | 4-3/4 | + | 3/4 | 1-1/16 | 3-1/4 | 1-7/8 | 5 | + | 13/16 | 1-1/16 | 3-3/8 | 1-7/8 | 5 | + | 7/8 | 1-1/8 | 3-1/2 | 1-15/16 | 5-1/4 | + | 15/16 | 1-1/8 | 3-5/8 | 1-15/16 | 5-1/4 | + | 1 | 1-3/16 | 3-3/4 | 2 | 5-1/2 | + +-----------+-------------+----------+------------------+----------+ + +[Illustration: FIG. 5.] + + EXERCISE 5.--Draw, half size, a plan and section of a double + riveted lap joint for plates 7/8 inch thick to the dimensions + given in the above table. + + EXERCISE 6: _Single Riveted Butt Joints._--In fig. 4 are shown + _single riveted butt joints_. One of the sectional views shows a + butt joint with one _cover plate_ or _butt strap_; the other + sectional view shows the same joint with two cover plates; the + third view is a plan of both arrangements. Draw all these views + full size. + + EXERCISE 7.--Fig. 5 shows a plan and sectional elevation of the + connection of three plates together, which are in the same plane, + by means of single riveted butt joints and single cover plates. + The butt straps where they overlap are forged so as to fit one + another as shown, and thus form a close joint. Draw these views to + the scale of 6 inches to a foot. + + The plates are 1/2 inch thick and the butt straps 9/16 inch thick. + All other dimensions must be deduced from the table for single + riveted lap joints. + + EXERCISE 8.--The connection of three plates by single riveted lap + joints is shown in fig. 6. To make the joint close one plate has a + portion of its edge thinned out, and the plate above it is set up + at this part so as to lie close to the former. + + Draw the three views shown in fig. 6 to the same scale as the last + exercise. + + The plates are 7/16 inch thick. All other dimensions to be + obtained from table for single riveted lap joints. + + EXERCISE 9: _Corner of Wrought-iron Tank._--This exercise is to + illustrate the connection of plates which are at right angles to + one another by means of _angle irons_. Fig. 7 is a plan and + elevation of the corner of a wrought-iron tank. The sides of the + tank are riveted to a vertical angle iron, the cross section of + which is clearly shown in the plan. Another angle iron of the same + dimensions is used in the same way to connect the sides with the + bottom. The sides do not come quite up to the corner of the + vertical angle iron, excepting at the bottom where the horizontal + angle iron comes in. At this point the vertical plates meet one + another, and the edge formed is rounded over to fit the interior + of the bend of the horizontal angle iron so as to make the joint + tight. Draw half size. + + The dimensions are as follows: angle irons 2-1/2 inches × 2-1/2 + inches × 3/8 inch; plates 3/8 inch thick; rivets 11/16 inch + diameter and 2 inches pitch. + + EXERCISE 10: _Gusset Stay._--In order that the flat ends of a + steam boiler may not be bulged out by the pressure of the steam + they are strengthened by means of stays. One form of boiler stay, + called a 'gusset stay,' is shown in fig. 8. This stay consists of + a strip of wrought-iron plate which passes in a diagonal direction + from the flat end of the boiler to the cylindrical shell. One end + of this plate is placed between and riveted to two angle irons + which are riveted to the shell of the boiler. A similar + arrangement connects the other end of the stay plate to the flat + end of the boiler. In this example the stay or gusset plate is 3/4 + of an inch thick; the angle irons are 4 inches broad and 1/2 inch + thick. The rivets are 1 inch in diameter. The same figure also + illustrates the most common method of connecting the ends of a + boiler to the shell. The end plates are _flanged_ or bent over at + right angles and riveted to the shell as shown. The radius of the + inside curve at the angle of the flange is 1-1/4 inches. Draw this + example to a scale of 3 inches to 1 foot. + +[Illustration: FIG. 6.] + +[Illustration: FIG. 7.] + +[Illustration: FIG. 8.] + + + + +III. SCREWS, BOLTS, AND NUTS. + + +_Screw Threads._--The various forms of screw threads used in machine +construction are shown in fig. 9. The _Whitworth_ =V= thread is shown at +(_a_). This is the standard form of triangular thread used in this +country. The angle between the sides of the =V= is 55°, and one-sixth of +the total depth is rounded off both at the top and bottom. At (_b_) is +shown the _Sellers_ =V= thread, which is the standard triangular thread +used by engineers in America. In this form of thread the angle between +the sides of the =V= is 60°, and one-eighth of the total depth is cut +square off at the top and bottom. The _Square_ thread is shown at (_c_). +This form is principally used for transmitting motion. + +[Illustration: FIG. 9.] + +Comparing the triangular and square threads, the former is the stronger +of the two; but owing to the normal pressure on the =V= thread being +inclined to the axis of the screw, that pressure must be greater than +the pressure which is being transmitted by the screw; and therefore, +seeing that the normal pressure on the square thread is parallel, and +therefore equal to the pressure transmitted in the direction of the axis +of the screw, the friction of the =V= thread must be greater than the +friction of the square thread. In the case of the triangular thread +there is also a tendency of the pressure to burst the nut. The +_Buttress_ thread shown at (_e_) is designed to combine the advantages +of the =V= and square threads, but it only has these advantages when the +pressure is transmitted in one direction; if the direction of the +pressure be reversed, the friction and bursting action on the nut are +even greater than with the =V= thread, because of the greater +inclination of the slant side of the buttress thread. The angles of the +square thread are frequently rounded to a greater or less extent to +render them less easily damaged. If this rounding is carried to excess +we get the _Knuckle_ thread shown at (_d_). The rounding of the angles +increases both the strength and the friction. + + EXERCISE 11: _Forms of Screw Threads._--Draw to a scale of three + times full size the sections of screw threads as shown in fig. 9. + The pitch for the Whitworth, Sellers, and buttress threads to be + 3/8 inch, and the pitch of the square and knuckle threads to be 1/2 + inch. + +_Dimensions of Whitworth Screws._ + + +-----------------------------------+ + | Diameter | Number | Diameter | + | of screw | of threads | at bottom | + | | per inch | of thread | + +----------+------------+-----------+ + | 1/8 | 40 | .093 | + | 3/16 | 24 | .134 | + | 1/4 | 20 | .186 | + | 5/16 | 18 | .241 | + | 3/8 | 16 | .295 | + | 7/16 | 14 | .346 | + | 1/2 | 12 | .393 | + | 5/8 | 11 | .508 | + | 3/4 | 10 | .622 | + | 7/8 | 9 | .733 | + | 1 | 8 | .840 | + | 1-1/8 | 7 | .942 | + | 1-1/4 | 7 | 1.067 | + | 1-3/8 | 6 | 1.162 | + | 1-1/2 | 6 | 1.286 | + | 1-5/8 | 5 | 1.369 | + | 1-3/4 | 5 | 1.494 | + | 1-7/8 | 4-1/2 | 1.590 | + | 2 | 4-1/2 | 1.715 | + | 2-1/4 | 4 | 1.930 | + | 2-1/2 | 4 | 2.180 | + | 2-3/4 | 3-1/2 | 2.384 | + | 3 | 3-1/2 | 2.634 | + | 3-1/4 | 3-1/4 | 2.856 | + | 3-1/2 | 3-1/4 | 3.106 | + | 3-3/4 | 3 | 3.323 | + | 4 | 3 | 3.573 | + | 4-1/4 | 2-7/8 | 3.805 | + | 4-1/2 | 2-7/8 | 4.055 | + | 4-3/4 | 2-3/4 | 4.284 | + | 5 | 2-3/4 | 4.534 | + | 5-1/4 | 2-5/8 | 4.762 | + | 5-1/2 | 2-5/8 | 5.012 | + | 5-3/4 | 2-1/2 | 5.238 | + | 6 | 2-1/2 | 5.488 | + +-----------------------------------+ + +_Gas Threads_[1] (_Whitworth Standard_). + +[1] Used for wrought-iron and brass tubes. + + +-------------------------------------------------------------+ + | Diameter of Screw | 1/8 | 1/4 | 3/8 | 1/2 | 5/8 | 3/4 | 1 | + +-------------------+-----+-----+-----+-----+-----+-----+------ + | Number of threads | | | | | | | | + | per inch | 28 | 19 | 19 | 14 | 14 | 14 | 11 | + +-------------------------------------------------------------+ + + +-------------------------------------------------+ + | Diameter of Screw | 1-1/4 | 1-1/2 | 1-3/4 | 2 | + +-------------------+-------+-------+-------+-----+ + | Number of threads | | | | | + | per inch | 11 | 11 | 11 | 11 | + +-------------------------------------------------+ + +_Representation of Screws._--The correct method of representing screw +threads involves considerable trouble, and is seldom adopted by +engineers for working drawings. For an explanation of the method see the +author's Text-book on Practical Solid Geometry, Part II., problem 134. A +method very often adopted on working drawings is shown in fig. 15; here +the thin lines represent the points, and the thick lines the roots of +the threads. At fig. 16 is shown a more complete method. The simplest +method is illustrated by figs. 10, 11, 13, and 14. + +Here dotted lines are drawn parallel to the axis of the screw as far as +it extends, and at a distance from one another equal to the diameter of +the screw at the bottom of the thread. + +[Illustration: FIG. 10.] + +[Illustration: FIG. 11.] + +_Forms of Nuts._--The most common form of nut is the hexagonal shown in +figs. 10, 13, 14, 15, and 16; next to this comes the square nut shown in +fig. 11. The method of drawing these nuts will be understood by +reference to the figures; the small circles indicate the centres, and +the inclined lines passing through them the radii of the curves which +represent the chamfered or bevelled edge of the nut. In all the figures +but the first the chamfer is just sufficient to touch the middle points +of the sides, and in these cases the drawing of the nut is simpler. + +[Illustration: FIG. 12.] + +[Illustration: FIG. 13.] + +[Illustration: FIG. 14.] + +_Forms of Bolts._--At (_a_), fig. 12, is shown a bolt with a square head +and a square neck. If this form of bolt is passed through a square hole +the square neck prevents the bolt from turning when the nut is being +screwed up. Instead of a square neck a snug may be used for the same +purpose, as shown on the cup-headed bolt at (_b_). The snug fits into a +short groove cut in the side of the hole through which the bolt passes. +At (_a_) the diagonal lines are used to distinguish the flat side of the +neck from the round part of the bolt above it. At (_c_) is shown a +tee-headed bolt, and at (_d_) an eye-bolt. Fig. 13 represents a hook +bolt. A bolt with a countersunk head is shown in fig. 11. If the +countersunk head be lengthened so as to take up the whole of the +unscrewed part of the bolt, we get the taper bolt shown in fig. 14, +which is often used in the couplings of the screw shafts of steamships. +The taper bolt has the advantage of having no projecting head, and it +may also be made a tight fit in the hole with less trouble than a +parallel bolt. Bolts may also have hexagonal heads. + +[Illustration: FIG. 15] + +[Illustration: FIG. 16] + +_Studs_, or _stud bolts_, are shown in figs. 15 and 16; that in fig. 15 +is a _plain stud_, while that in fig. 16 has an intermediate collar +forged upon it, and is therefore called a _collared stud_. + +_Proportions of Nuts and Bolt-heads._--In the hexagonal nut the diameter +D across the flats is 1-1/2_d_ + 1/8, where _d_ is the diameter of the +bolt. The same rule gives the width of a square nut across the flats. A +rule very commonly used in making drawings of hexagonal nuts is to make +the diameter D, across the angles equal to 2_d_. H, the height of the +nut, is equal to the diameter of the bolt. In square and hexagonal +headed bolts the height of the head varies from _d_ to 2/3_d_; the other +dimensions are the same as for the corresponding nuts. + +_Washers_ are flat, circular, wrought-iron plates, having holes in their +centres of the same diameter as the bolts on which they are used. The +object of the washer is to give a smooth bearing surface for the nut to +turn upon, and it is used when the surfaces of the pieces to be +connected are rough, or when the bolt passes through a hole larger than +itself, as shown in fig. 10. The diameter of the washer is a little more +than the diameter of the nut across the angles, and its thickness about +1/8 of the diameter of the bolt. + + EXERCISE 12.--Draw, full size, the views shown in fig. 10 of an + hexagonal nut and washer for a bolt 1-1/4 inches in diameter. The + bolt passes through a hole 1-3/4 × 1-1/4. All the dimensions are + to be calculated from the rules which have just been given. + + EXERCISE 13.--Draw, full size, the plan and elevation of the + square nut and bolt with countersunk head shown in fig. 11, to the + dimensions given. + + EXERCISE 14.--Draw, full size, the elevation of the hook bolt with + hexagonal nut shown in fig. 13 to the dimensions given, and show + also a plan. + + EXERCISE 15.--Draw, to a scale of 4 inches to a foot, the conical + bolt for a marine shaft coupling shown in fig. 14. All the parts + are of wrought iron. + + EXERCISE 16.--Fig. 15 is a section of the mouth of a small + steam-engine cylinder, showing how the cover is attached; draw + this full size. + + EXERCISE 17.--Fig. 16 shows the central portion of the + india-rubber disc valve which is described on page 68. A is the + central boss of the grating, into which is screwed the stud B, + upon which is forged the collar C. The upper part of the stud is + screwed, and carries the guard D and an hexagonal nut E. F is the + india-rubber. The grating and guard are of brass. The stud and nut + are of wrought iron. Draw full size the view shown. + +_Lock Nuts._--In order that a nut may turn freely upon a bolt, there is +always a very small clearance space between the threads of the nut and +those of the bolt. This clearance is shown exaggerated at (_a_), fig. +17, where A is a portion of a bolt within a nut B. Suppose that the bolt +is stretched by a force W. When the nut B is screwed up, the upper +surfaces of the projecting threads of the nut will press on the under +surfaces of the threads of the bolt with a force P equal and opposite +to W, as shown at (_b_), fig. 17. When in this condition the nut has no +tendency to slacken back, because of the friction due to the pressure on +the nut. Now suppose that the tension W on the bolt is momentarily +diminished, then the friction which opposes the turning of the nut may +be so much diminished that a vibration may cause it to slacken back +through a small angle. If this is repeated a great many times the nut +may slacken back so far as to become useless. + +[Illustration: FIG. 17.] + +[Illustration: FIG. 18.] + +A very common arrangement for locking a nut is shown at (_a_), fig. 18. +C is an ordinary nut, and B one having half the thickness of C. B is +first screwed up tight so as to act on the bolt, as shown at (_b_), fig. +17. C is then screwed on top of B. When C is almost as tight as it can +be made, it is held by one spanner, while B is turned back through a +small angle with another. The action of the nuts upon the bolt and upon +one another is now as shown at (_b_), fig. 18. It will be seen that the +nuts are wedged tight on to the bolt, and that this action is +independent of the tension W in the bolt. The nuts will, therefore, +remain tight after the tension in the bolt is removed. + +It is evident that if the nuts are screwed up in the manner explained, +the outer nut C will carry the whole load on the bolt; hence C should be +the thicker of the two nuts. In practice, the thin nut, called the lock +nut, is often placed on the outside, for the reason that ordinary +spanners are too thick to act on the thin nut when placed under the +other. + +Another very common arrangement for locking a nut is shown in fig. 19. A +is the bolt and B the nut, the lower part of which is turned circular. A +groove C is also turned on the nut at this part. The circular part of +the nut fits into a circular recess in one of the parts connected by the +bolt. Through this part passes a set screw D, the point of which can be +made to press on the nut at the bottom of the groove C. D is turned back +when the nut B is being moved, and when B is tightened up, the set screw +is screwed up so as to press hard on the bottom of the groove C. The nut +B is thus prevented from slackening back. The screw thread is turned off +the set screw at the point where it enters the groove on the nut. + +[Illustration: FIG. 19] + +The use of the groove for receiving the point of the set screw is this: +The point of the set screw indents the nut and raises a bur which would +interfere with the free turning of the nut in the recess if the bur was +not at the bottom of a groove. Additional security is obtained by +drilling a hole through the point of the bolt, and fitting it with a +split pin E. + +Locking arrangements for nuts are exceedingly numerous, and many of them +are very ingenious, but want of space prevents us describing them. We +may point out, however, that many very good locking arrangements have +the defect of only locking the nut at certain points of a revolution, +say at every 30°. It will be noticed that the two arrangements which we +have described are not open to this objection. + + EXERCISE 18.--Draw, full size, a plan, front elevation, and side + elevation of the arrangement of nuts shown in fig. 18, for a bolt + 7/8 inch diameter. + + EXERCISE 19.--Draw the plan and elevation of the nut and locking + arrangement shown in fig. 19. Make also an elevation looking in + the direction of the arrow. Scale 6 inches to a foot. + + + + +IV. KEYS. + + +_Keys_ are wedges, generally rectangular in section, but sometimes +circular; they are made of wrought iron or steel, and are used for +securing wheels, pulleys, cranks, &c., to shafts. + +[Illustration: Fig. 20.] + +Various sections of keys are shown in fig. 20. At (_a_) is the _hollow_ +or _saddle key_. With this form of key it is not necessary to cut the +shaft in any way, but its holding power is small, and it is therefore +only used for light work. At (_b_) is the _key on a flat_, sometimes +called a _flat key_. The holding power of this key is much greater than +that of the saddle key. At (_c_) is the _sunk key_, a very secure and +very common form. + +The part of the shaft upon which a key rests is called the _key bed_ or +_key way_, and the recess in the boss of the wheel or pulley into which +the key fits is called the _key way_; both are also called _key seats_. +With saddle, flat, and sunk keys the key bed is parallel to the axis of +the shaft; but the key way is deeper at one end than the other to +accommodate the taper of the key. The sides of the key are parallel. + +The _round key_ or taper pin shown at (_d_) is in general only used for +wheels or cranks which have been previously shrunk on to their shafts or +forced on by great pressure. After the wheel or crank has been shrunk +on, a hole is drilled, half into the shaft and half into the wheel or +crank, to receive the pin. + +When the point of a key is inaccessible the other end is provided with a +_gib head_ as shown at (_e_), to enable the key to be withdrawn. + +A _sliding_ or _feather key_ secures a piece to a shaft so far as to +prevent the one from rotating without the other, but allows of relative +motion in the direction of the axis of the shaft. This form of key has +no taper, and it is secured to the piece carried by the shaft, but is +made a _sliding fit_ in the key way of the shaft. In one form of feather +key the part within the piece carried by the shaft is dovetailed as +shown at (_f_). In another form the key has a round projecting pin +forged upon it, which enters a corresponding hole as shown at (_g_). The +feather key may also be secured to the piece carried by the shaft by +means of one or more screws as shown at (_h_). The key way in the shaft +is made long enough to permit of the necessary sliding motion. + +_Cone Keys._--These are sometimes fitted to pulleys, and are shown in +fig. 32, page 38. In this case the eye of the pulley is tapered and is +larger than the shaft. The space between the shaft and the boss of the +pulley is filled with three _saddle_ or _cone keys_. These keys are made +of cast iron and are all cast together, and before being divided the +casting is bored to fit the shaft and turned to fit the eye of the +pulley. By this arrangement of keys the same pulley may be fixed on +shafts of different diameters by using keys of different thicknesses; +also the pulley may be bored out large enough to pass over any boss +which may be forged on the shaft. + +_Proportions of Keys._--The following rules are taken from Unwin's +'Machine Design,' pp. 142-43. + + Diameter of eye of wheel, or boss of shaft = _d_. + Width of key = 3/4_d_ + 1/8. + Mean thickness of sunk key = 1/8_d_ + 1/8. + " key on flat = 1/16_d_ + 1/16. + +The following table gives dimensions agreeing with average practice. + +_Dimensions of Keys._ + + D = diameter of shaft. + B = breadth of key. + T = thickness of sunk key. + T_{1} = thickness of flat key, also = thickness of saddle key. Taper + of key 1/8 inch per foot of length, _i.e._ 1 in 96. + + +---------------------------------------------------------------+ + | D | 3/4 | 1 | 1-1/4 | 1-1/2 | 1-3/4 | 2 | 2-1/4 | 2-1/2 | + +-----+-----+-----+-------+-------+-------+-----+-------+-------+ + | B | 5/16| 3/8 | 7/16 | 1/2 | 9/16 | 5/8 | 11/16 | 11/16 | + | T | 1/4 | 1/4 | 1/4 | 5/16 | 5/16 | 5/16| 3/8 | 3/8 | + |T_{1}| 3/16| 3/16| 3/16 | 3/16 | 1/4 | 1/4 | 1/4 | 5/16 | + +---------------------------------------------------------------+ + + +-------------------------------------------------------------------+ + | D | 2-3/4 | 3 | 3-1/2 | 4 | 4-1/2 | 5 | 5-1/2 | 6 | + +-----+-------+-----+-------+-------+-------+-------+-------+-------+ + | B | 3/4 | 7/8 | 1 | 1-1/8 | 1-1/4 | 1-3/8 | 1-1/2 | 1-5/8 | + | T | 3/8 | 7/16| 1/2 | 1/2 | 9/16 | 5/8 | 11/16 | 3/4 | + |T_{1}| 5/16 | 5/16| 3/8 | 7/16 | 1/2 | 1/2 | 9/16 | 5/8 | + +-------------------------------------------------------------------+ + + +-------------------------------------------------------+ + | D | 7 | 8 | 9 | 10 | 11 | 12 | + +-----+-------+-------+-------+--------+--------+-------+ + | B | 1-7/8 | 2-1/8 | 2-3/8 | 2-5/8 | 2-7/8 | 3-1/8 | + | T | 13/16 | 15/16 | 1 | 1-1/16 | 1-3/16 | 1-1/4 | + |T_{1}| 11/16 | 3/4 | 7/8 | 15/16 | 1-1/16 | 1-1/8 | + +-------------------------------------------------------+ + + + + +V. SHAFTING. + + +Shafting is nearly always cylindrical and made of wrought iron or steel. +Cast iron is rarely used for shafting. + +_Axles_ are shafts which are subjected to bending without twisting. + +The parts of a shaft or axle which rest upon the bearings or supports +are called _journals_, _pivots_, or _collars_. + +In journals the supporting pressure is at right angles to the axis of +the shaft, while in pivots and collars the pressure is parallel to that +axis. + +Shafts may be solid or hollow. Hollow shafts are stronger than solid +shafts for the same weight of material. Thus a hollow shaft having an +external diameter of 10-1/4 inches and an internal diameter of 7 inches +would have about the same weight as a solid shaft of the same material +7-1/2 inches in diameter, but the former would have about double the +strength of the latter. Hollow shafts are also stiffer and yield less to +bending action than solid shafts, which in some cases, as in propeller +shafts, is an objection. + + + + +VI. SHAFT COUPLINGS. + + +For convenience of making and handling, shafts used for transmitting +power are generally made in lengths not exceeding 30 feet. These lengths +are connected by couplings, of which we give several examples. + +[Illustration: FIGS. 21 and 22.] + +_Solid_, _Box_, or _Muff Couplings._--One form of box coupling is shown +in fig. 21. Here the ends of the shafts to be connected butt against one +another, meeting at the centre of the box, which is made of cast iron. +The shafts are made to rotate as one by being secured to the box by two +wrought-iron or steel keys, both driven from the same end of the box. A +clearance space is left between the head of the forward key and the +point of the hind one, to facilitate the driving of them out, as then +only one key needs to be started at a time. Sometimes a single key the +whole length of the box is used, in which case it is necessary that the +key ways in the shafts be of exactly the same depth. + +The half-lap coupling, introduced by Sir William Fairbairn, is shown in +fig. 22. In this form of box coupling the ends of the shafts overlap +within the box. It is evident that one shaft cannot rotate without the +other as long as the box remains over the lap. To keep the box in its +place it is fitted with a saddle key. + +It will be noticed that the lap joint is sloped in such a way as to +prevent the two lengths of shaft from being pulled asunder by forces +acting in the direction of their length. + +Half-lap couplings are not used for shafts above 5 inches in diameter. + +It may here be pointed out that the half-lap coupling is expensive to +make, and is now not much used. + +As shafts are weakened by cutting key ways in them, very often the ends +which carry couplings are enlarged in diameter, as shown in fig. 21, by +an amount equal to the thickness of the key. An objection to this +enlargement is that wheels and pulleys require either that their bosses +be bored out large enough to pass over it, or that they be split into +halves, which are bolted together after being placed on the shaft. + +_Dimensions of Box Couplings._ + + D = diameter of shaft. + T = thickness of metal in box. + L = length of box for butt coupling. + L_{1} = length of box for lap coupling. + _l_ = length of lap. + D_{1} = diameter of shaft at lap. + + +---------------------------------------------------------------+ + | D | 1-1/2 | 2 | 2-1/2 | 3 | 3-1/2 | 4 | + +-------+--------+--------+---------+-------+----------+--------+ + | T | 1-1/8 | 1-5/16 | 1-1/2 | 1-3/4 | 1-15/16 | 2-1/8 | + | L | 5-3/4 | 7 | 8-1/4 | 9-1/2 | 10-3/4 | 12 | + | L_{1} | 4-1/8 | 5-1/4 | 6-3/8 | 7-1/2 | 8-5/8 | 9-3/4 | + | _l_ | 1-7/16 | 1-7/8 | 2-5/16 | 2-3/4 | 3-3/16 | 3-5/8 | + | D_{2} | 2-5/16 | 3 | 3-11/16 | 4-3/8 | 5-1/16 | 5-3/4 | + +---------------------------------------------------------------+ + + +----------------------------------------------+ + | D | 4-1/2 | 5 | 5-1/2 | 6 | + +-------+---------+--------+--------+----------+ + | T | 2-5/16 | 2-1/2 | 2-3/4 | 2-15/16 | + | L | 13-1/4 | 14-1/2 | 15-3/4 | 17 | + | L_{1} | 10-7/8 | 12 | -- | -- | + | _l_ | 4-1/16 | 4-1/2 | -- | -- | + | D_{2} | 6-7/16 | 7-1/8 | -- | -- | + +----------------------------------------------+ + + Slope of lap 1 in 12. + + EXERCISE 20: _Solid Butt Coupling._--From the above table of + dimensions make a longitudinal and a transverse section of a solid + butt coupling for a shaft 2-1/2 inches in diameter. Scale 6 inches + to a foot. + + EXERCISE 21: _Fairbairn's Half-Lap Coupling._--Make the same views + as in the last exercise of a half-lap coupling for a 3-inch shaft + to the dimensions in the above table. Scale 6 inches to a foot. + +_Flange Couplings._--The form of coupling used for the shafts of marine +engines is shown in fig. 23. The ends of the different lengths of shaft +have flanges forged on them, which are turned along with the shaft. +These flanges butt against one another, and are connected by bolts. +These bolts may be parallel or tapered; generally they are tapered. A +parallel bolt must have a head, but a tapered bolt will act without one. +In fig. 23 the bolts are tapered, and also provided with heads. In fig. +14, page 17, is shown a tapered bolt without a head. The variation of +diameter in tapered bolts is 3/8 of an inch per foot of length. + +[Illustration: FIG. 23.] + +Sometimes a projection is formed on the centre of one flange which fits +into a corresponding recess in the centre of the other, for the purpose +of ensuring the shafts being in line. + +Occasionally a cross-key is fitted in between the flanges, being sunk +half into each, for the purpose of diminishing the shearing action on +the bolts. + + EXERCISE 22: _Marine Coupling._--Draw the elevation and section of + the coupling shown in fig. 23; also an elevation looking in the + direction of the arrow. Scale 3 inches to a foot. + +The following table gives the dimensions of a few marine couplings taken +from actual practice. + +_Examples of Marine Couplings._ + + +--------------------------------------------------------------------+ + | Diameter of shaft |2-3/8 | 9-3/4 | 12-7/8 |16-1/2 | 22-1/2 | 23 | + +--------------------+------+-------+--------+-------+--------+------+ + |Diameter of flange | 6 | 19 | 24 | 32 | 35 | 38 | + |Thickness of flange | 1 | 2-3/4 | 3-1/8 | 4-1/4 | 6 | 5 | + |Diameter of bolts | 3/4 | 2-3/4 | 2-11/16| 3-1/2 | 4-1/4 | 4-1/4| + |Number of bolts | 3 | 6 | 6 | 8 | 9 | 8 | + |Diameter of bolt | | | | | | | + | circle |4-1/8 | 14-1/8|18-13/16| 25 | 28-3/4 |30-3/8| + +--------------------------------------------------------------------+ + + All the above dimensions are in inches. + + EXERCISE 23.--Select one of the couplings from the above table, and + make the necessary working drawings for it to a suitable scale. + +The cast-iron flange coupling is shown in fig. 24. In this kind of +coupling a cast-iron centre or boss provided with a flange is secured to +the end of each shaft by a sunk key driven from the face of the flange. +These flanges are then connected by bolts and nuts as in the marine +coupling. + +To ensure the shafts being in line the end of one projects into the +flange of the other. + +In order that the face of each flange may be exactly perpendicular to +the axis of the shaft they should be 'faced' in the lathe, after being +keyed on to the shaft. + +If the coupling is in an exposed position, where the nuts and bolt-heads +would be liable to catch the clothes of workmen or an idle driving band +which might come in the way, the flanges should be made thicker, and be +provided with recesses for the nuts and bolt-heads. + +[Illustration: FIG. 24.] + +_Dimensions of Cast-iron Flange Couplings._ + + +--------------------------------------------------------------------+ + | |Diameter| | |Depth | |Diameter|Diameter| + |Diameter| of |Thickness|Diameter| at |Number| of | of bolt| + |of shaft| flange |of flange| of boss| boss | of | bolts | circle | + | D | F | T | B | L | bolts| d | C | + +--------|--------|---------|--------|------|------|--------|--------+ + | 1-1/2 | 7-1/4 | 7/8 | 3-1/2 |2-5/8 | 3 |5/8 | 5-1/2 | + | 2 | 8-7/8 | 1-1/16 | 4-3/8 |3-3/16| 4 | 3/4| 6-3/4 | + | 2-1/2 | 10-5/8 | 1-1/4 | 5-5/16 |3-3/4 | 4 |7/8 | 8-1/8 | + | 3 | 12-3/8 | 1-7/16 | 6-1/4 |4-5/16| 4 | 1 | 9-1/2 | + | 3-1/2 | 13-1/8 | 1-5/8 | 7-1/8 |4-7/8 | 4 | 1 |10-5/16 | + | 4 | 14 | 1-3/4 | 8 |5-7/16| 6 | 1 |11-1/4 | + | 4-1/2 | 15-5/8 | 2 | 8-7/8 |6 | 6 |1-1/8 |12-1/2 | + | 5 | 17-3/8 | 2-1/8 | 9-13/16|6-5/8 | 6 | 1-1/4|13-13/16| + | 5-1/2 | 18-1/4 | 2-5/16 |10-3/4 |7-1/4 | 6 |1-1/4 |14-3/4 | + | 6 | 19-7/8 | 2-1/2 |11-5/8 |7-3/4 | 6 | 1-3/8| 16 | + +--------------------------------------------------------------------+ + +The projection of the shaft _p_ varies from 1/4 inch in the small shafts +to 1/2 inch in the large ones. + + EXERCISE 24: _Cast-iron Flange Coupling._--Draw the views shown in + fig. 24 of a cast-iron flange coupling, for a shaft 4-1/2 inches in + diameter, to the dimensions given in the above table. Scale 4 inches + to a foot. + + + + +VII. BEARINGS FOR SHAFTS. + + +An example of a very simple form of bearing is shown in fig. 25, which +represents a brake shaft carrier of a locomotive tender. The bearing in +this example is made of cast iron and in one piece. Through the +oval-shaped flange two bolts pass for attaching the bearing to the +wrought-iron framing of the tender. With this form of bearing there is +no adjustment for wear, so that when it becomes worn it must be renewed. + +[Illustration: FIG. 25.] + + EXERCISE 25: _Brake Shaft Carrier._--Draw the elevation and + sectional plan of the bearing shown in fig. 25. Draw also a + vertical section through the axis. The latter view to be projected + from the first elevation. Scale 6 inches to a foot. + +_Pillow Block_, _Plummer Block_, or _Pedestal_.--The ordinary form of +plummer block is represented in fig. 26. A is the block proper, B the +sole through which pass the holding-down bolts. C is the cap. Between +the block and the cap is the brass bush, which is in halves, called +_brasses_ or _steps_. The bed for the steps in this example is +cylindrical, and is prepared by the easy process of boring. The steps +are not supported throughout their whole length, but at their ends only +where fitting strips are provided as shown. As the wear on a step is +generally greatest at the bottom, it is made thicker there than at the +sides, except where the fitting strips come in. To prevent the steps +turning within the block they are generally furnished with lugs, which +enter corresponding recesses in the block and cover. + +[Illustration: FIG. 26] + +In the block illustrated the journal is lubricated by a _needle +lubricator_; this consists of an inverted glass bottle fitted with a +wood stopper, through a hole in which passes a piece of wire, which has +one end in the oil within the bottle, and the other resting on the +journal of the shaft. The wire or needle does not fill the hole in the +stopper, but if the needle is kept from vibrating the oil does not +escape owing to capillary attraction. When, however, the shaft rotates, +the needle begins to vibrate, and the oil runs down slowly on to the +journal; oil is therefore only used when the shaft is running. + + EXERCISE 26: _Pillow Block for a Four-inch Shaft._--Draw the views + shown of this block in fig. 26. Make also separate drawings, full + size, of one of the steps. Scale 6 inches to a foot. + +_Proportions of Pillow Blocks._--The following rules may be used for +proportioning pillow blocks for shafts up to 8 inches diameter. It +should be remembered that the proportions used by different makers vary +considerably, but the following rules represent average practice. + + Diameter of journal = _d_. + Length of journal = _l_. + Height to centre = 1.05_d_ + .5. + Length of base = 3.6_d_ + 5. + Width of base = .8_l_. + " block = .7_l_. + Thickness of base = .3_d_ + .3. + " cap = .3_d_ + .4. + Diameter of bolts = .25_d_ + .25. + Distance between centres of cap bolts = 1.6_d_ + 1.5. + " " base bolts = 2.7_d_ + 4.2. + Thickness of step at bottom = _t_ = .09_d_ + .15. + " " sides = 3/4 _t_. + +The length of the journal varies very much in different cases, and +depends upon the speed of the shaft, the load which it carries, the +workmanship of the journal and bearing, and the method of lubrication. +For ordinary shafting one rule is to make _l_ = _d_ + 1. Some makers use +the rule _l_ = 1.5_d_; others make _l_ = 2_d_. + + EXERCISE 27: _Design for Pillow Block._--Make the necessary + working drawings for a pillow block for a shaft 5 inches in + diameter, and having a journal 7 inches long. + +[Illustration: FIG. 27.] + +_Brackets._--When a pillow block has to be fixed to a wall or column a +bracket such as that shown in figs. 27 and 28 may be used. The pillow +block rests between the _joggles_ A A, and is bolted down to the bracket +and secured in addition with keys at the ends of the base of the block, +in the same manner as is shown, for the attachment of the bracket to +the column. + + EXERCISE 28: _Pillar Bracket._--Fig. 27 shows a side elevation and + part horizontal section, and fig. 28 shows an end elevation of a + pillar bracket for carrying a pillow block for a 3-inch shaft. + Draw these views _properly projected from one another_, showing + the pillow block, which is to be proportioned by the rules given + on page 32. Draw also a plan of the whole. Scale 4 inches to a + foot. + +[Illustration: FIG. 28.] + +_Hangers._--When a shaft is suspended from a ceiling it is carried by +hangers, one form of which is shown in fig. 29, and which will be +readily understood. The cap of the bearing, it will be noticed, is +secured by means of a bolt, and also by a square key. + + EXERCISE 29: _Shaft Hanger._--Draw the two elevations shown in + fig. 29, and also a sectional plan. The section to be taken at a + point 5 inches above the centre of the shaft. Scale 6 inches to a + foot. + +_Wall Boxes._--In passing from one part of a building to another a shaft +may have to pass through a wall. In that case a neat appearance is given +to the opening and a suitable support obtained for a pillow block by +building into the wall a _wall box_, one form of which is shown in fig. +30. + + EXERCISE 30: _Wall Box._--Draw the views of the wall box shown in + fig. 30, and also a sectional plan; the plane of section to pass + through the box a little above the joggles for the pillow block. + Scale 3 inches to a foot. + +[Illustration: FIG. 29.] + +[Illustration: FIG. 30.] + + + + +VIII. PULLEYS. + + +_Velocity Ratio in Belt Gearing._--Let two pulleys A and B be connected +by a belt, and let their diameters be D_{1} and D_{2}; and let their +speeds, in revolutions per minute, be N_{1} and N_{2} respectively. If +there is no slipping, the speeds of the rims of the pulleys will be the +same as that of the belt, and will therefore be equal. Now the speed of +the rim of A is evidently = D_{1} × 3.1416 × N_{1}; while the speed of +the rim of B is = D_{2} × 3.1416 × N_{2}. Hence D_{1} × 3.1416 × N_{1} = +D_{2} × 3.1416 × N_{2}, and therefore + + N_{1} D_{2} + ----- = -----. + N_{2} D_{1} + +_Pulleys for Flat Bands._--In cross section the rim of a pulley for +carrying a flat band is generally curved as shown in figs. 31 and 32, +but very often the cross section is straight. The curved cross section +of the rim tends to keep the band from coming off as long as the pulley +is rotating. Sometimes the rim of the pulley is provided with flanges +which keep the band from falling off. + +Pulleys are generally made entirely of cast iron, but a great many +pulleys are now made in which the centre or nave only is of cast iron, +the arms being of wrought iron cast into the nave, while the rim is of +wrought sheet iron. + +The arms of pulleys when made of wrought iron are invariably straight, +but when made of cast iron they are very often curved. In fig. 31, which +shows an arrangement of two cast-iron pulleys, the arms are straight; +while in fig. 32, which shows another cast-iron pulley, the arms are +curved. Through unequal cooling, and therefore unequal contraction of a +cast-iron, pulley in the mould, the arms are generally in a state of +tension or compression; and if the arms are straight they are very +unyielding, so that the result of this initial stress is often the +breaking of an arm, or of the rim where it joins an arm. With the curved +arm, however, its shape permits it to yield, and thus cause a diminution +of the stress due to unequal contraction. + +The cross section of the arms of cast-iron pulleys is generally +elliptical. + +[Illustration: Fig. 31.] + + EXERCISE 31: _Fast and Loose Pulleys_.--Fig. 31 shows an + arrangement of fast and loose pulleys. A is the fast pulley, + secured to the shaft C by a sunk key; B is the loose pulley, which + turns freely upon the shaft. The loose pulley is prevented from + coming off by a collar D, which is secured to the shaft by a + tapered pin as shown. The nave or boss of the loose pulley is here + fitted with a brass liner, which may be renewed when it becomes + too much worn. Draw the elevations shown, completing the left-hand + one. Scale 6 inches to a foot. + + By the above arrangement of pulleys a machine may be stopped or + set in motion at pleasure. When the driving band is on the loose + pulley the machine is at rest, and when it is on the fast pulley + the machine is in motion. The driving band is shifted from the one + pulley to the other by pressing on that side of the band which is + advancing towards the pulleys. + +[Illustration: FIG. 32.] + + EXERCISE 32: _Cast-iron Pulley with Curved Arms and Cone + Keys_.--Draw a complete side elevation and a complete cross + section of the pulley represented in fig. 32 to a scale of 3 + inches to a foot. In drawing the side elevation of the arms first + draw the centre lines as shown; next draw three circles for each + arm, one at each end and one in the middle; the centres of these + circles being on the centre line of the arm, and their diameters + equal to the widths of the arm at the ends and at the middle + respectively. Arcs of circles are then drawn to touch these three + circles. The centres and radii of these arcs may be found by + trial. The cone keys for securing the pulley to the shaft were + described on p. 23. + +_Pulleys for Ropes_.--Ropes made of hemp are now extensively used for +transmitting power. These ropes vary in diameter from 1 inch to 2 +inches, and are run at a speed of about 4,500 feet per minute. The +pulleys for these ropes are made of cast iron, and have their rims +grooved as shown in fig. 33, which is a cross section of the rim of a +pulley carrying three ropes. The angle of the V is usually 45°, and the +rope rests on the sides of the groove, and not on the bottom, so that +it is wedged in, and has therefore a good hold of the pulley. The +diameter of the pulley should not be less than 30 times the diameter of +the rope. Two pulleys connected by ropes should not be less than thirty +feet apart from centre to centre, but this distance may be as much as +100 feet. + +[Illustration: FIG. 33.] + + EXERCISE 33: _Section of Rim of Rope Pulley._--Draw, half size, + the section of the rim of a rope pulley shown in fig. 33. + + + + +IX. TOOTHED WHEELS. + + +_Pitch Surfaces of Spur Wheels._--Let two smooth rollers be placed in +contact with their axes parallel, and let one of them rotate about its +axis; then if there is no slipping the other roller will rotate in the +opposite direction with the same surface velocity; and if D_{1}, D_{2} +be the diameters of the rollers, and N_{1}, N_{2} their speeds in +revolutions per minute, it follows as in belt gearing that-- + + N_{1} D_{2} + ----- = -----. + N_{2} D_{1} + +If there be considerable resistance to the motion of the follower +slipping may take place, and it may stop. To prevent this the rollers +may be provided with teeth; then they become _spur wheels_; and if the +teeth be so shaped that the ratio of the speeds of the toothed rollers +at any instant is the same as that of the smooth rollers, the surfaces +of the latter are called the _pitch surfaces_ of the former. + +_Pitch Circle._--A section of the pitch surface of a toothed wheel by a +plane perpendicular to its axis is a circle, and is called a _pitch +circle_. We may also say that the pitch circle is the edge of the pitch +surface. The pitch circle is generally traced on the side of a toothed +wheel, and is rather nearer the points of the teeth than the roots. + +_Pitch of Teeth._--The distance from the centre of one tooth to the +centre of the next, or from the front of one to the front of the next, +_measured at the pitch circle_, is called the _pitch of the teeth_. If D +be the diameter of the pitch circle of a wheel, _n_ the number of teeth, +and _p_ the pitch of the teeth, then D × 3.1416 = _n_ × _p_. + +[Illustration: FIG. 34.] + +By the diameter of a wheel is meant the diameter of its pitch circle. + +_Form and Proportions of Teeth._--The ordinary form of wheel teeth is +shown in fig. 34. The curves of the teeth should be cycloidal curves, +although they are generally drawn in as arcs of circles. It does not +fall within the scope of this work to discuss the correct forms of wheel +teeth. The student will find the theory of the teeth of wheels clearly +and fully explained in Goodeve's 'Elements of Mechanism,' and in Unwin's +'Machine Design.' + +The following proportions for the teeth of ordinary toothed wheels may +be taken as representing average practice:-- + + Pitch of teeth = _p_ = arc _a b c_ (fig. 34). + Thickness of tooth = _b c_ = .48_p_. + Width of space = _a b_ = .52_p_. + Total height of tooth = _h_ = .7_p_. + Height of tooth above pitch line = _k_ = .3_p_. + Depth of tooth below pitch line = _l_ = .4_p_. + Width of tooth = 2_p_ to 3_p_. + + EXERCISE 34: _Spur Wheel._--Fig. 35 shows the elevation and + sectional plan of a portion of a cast-iron spur wheel. The diameter + of the pitch circle is 23-7/8 inches, and the pitch of the teeth is + 1-1/2 inches, so that there will be 50 teeth in the wheel. The + wheel has six arms. Draw a complete elevation of the wheel and a + half sectional plan, also a half-plan without any section. Draw + also a cross section of one arm. Scale 4 inches to a foot. + +[Illustration: FIG. 35.] + +_Mortise Wheels._--When two wheels gearing together run at a high speed +the teeth of one are made of wood. These teeth, or cogs, as they are +generally called, have tenons formed on them, which fit into mortises in +the rim of the wheel. This wheel with the wooden teeth is called a +_mortise wheel_. An example of a mortise wheel is shown in fig. 36. + +[Illustration: FIG. 36.] + +_Bevil Wheels._--In bevil wheels the pitch surfaces are parts of cones. +Bevil wheels are used to connect shafts which are inclined to one +another, whereas spur wheels are used to connect parallel shafts. In +fig. 36 is shown a pair of bevil wheels in gear, one of them being a +mortise wheel. At (_a_) is a separate drawing, to a smaller scale, of +the pitch cones. The pitch cones are shown on the drawing of the +complete wheels by dotted lines. + +The diameters of bevil wheels are the diameters of the bases of their +pitch cones. + + EXERCISE 35: _Pair of Bevil Wheels._--Draw the sectional elevation + of the bevil wheels shown in gear in fig. 36. Commence by drawing + the centre lines of the shafts, which in this example are at right + angles to one another; then draw the pitch cones shown by dotted + lines. Next put in the teeth which come into the plane of the + section, then complete the sections of the wheels. The pinion or + smaller wheel has 25 teeth, and the wheel has 50 teeth, which makes + the pitch a little over 3 inches. Each tooth of the mortise wheel + is secured as shown by an iron pin 5/16 inch diameter. Scale 3 + inches to a foot. + + + + +X. CRANKS AND CRANKED SHAFTS. + + +The most important application of the crank is in the steam-engine, +where the reciprocating rectilineal motion of the piston is converted +into the rotary motion of the crank-shaft by means of the crank and +connecting rod. + +At one time steam-engine cranks were largely made of cast iron, now they +are always made of wrought iron or steel. The crank is either forged in +one piece with the shaft, or it is made separately and then keyed to it. + +_Overhung Crank._--Fig. 37 shows a wrought-iron overhung crank. A is the +crank-shaft, B the crank arm, provided at one end with a boss C, which +is bored out to fit the shaft; at the other end of the crank arm is a +boss D, which is bored out to receive the crank-pin E, which works in +one end of the connecting rod. The crank is secured to the shaft by the +sunk key F. It is also good practice to _shrink_ the crank on to the +shaft. The process of shrinking consists of boring out the crank a +little smaller than the shaft, and then heating it, which causes it to +expand sufficiently to go on to the shaft. As the crank cools, it +shrinks and grips the shaft firmly. The crank may also be shrunk on to +the crank-pin, the latter being then riveted over as shown in fig. 37. + +[Illustration: FIG. 37.] + +A good plan to adopt in preference to the shrinking process is to force +the parts together by hydraulic pressure. This method is adopted for +placing locomotive wheels on their axles, and for putting in crank-pins. +As to the amount of pressure to be used, the practice is to allow a +force of 10 tons for every inch of diameter of the pin, axle, or shaft. + +Instead of being riveted in, the crank pin may be prolonged and screwed, +and fitted with a nut. Another plan is to put a cotter through the crank +and the crank-pin. + +The distance from the centre of the crank-shaft to the centre of the +crank-pin is called the radius of the crank. The _throw_ of the crank is +twice the radius. In a direct-acting engine the throw of the crank is +equal to the stroke of the piston. + + EXERCISE 36: _Wrought-iron Overhung Crank._--Draw the two + elevations shown in fig. 37, also a plan. Scale 1-1/2 inches + to a foot. + + _Proportions of Overhung Cranks._ + + D = diameter of shaft. + _d_ = " crank-pin. + Length of large boss = .9 D. + Diameter " = 1.8 D. + Length of small boss = 1.1 _d_. + Diameter " = 1.8 _d_. + Width of crank arm at centre of shaft = 1.3 D. + " " crank-pin = 1.5 _d_. + The thickness of the crank arm may be roughly taken as = .7 D. + + EXERCISE 37.--Design a wrought-iron crank for an engine having a + stroke of 4 feet. The crank-shaft is 9 inches in diameter, and + the crank-pin is 4-3/4 inches in diameter and 6-1/2 inches long. + +[Illustration: FIG. 38.] + +_Locomotive Cranked Axle._--As an example of a cranked shaft we take the +cranked axle for a locomotive with inside cylinders shown in fig. 38; +here the crank and shaft or axle are forged in one piece. A is the wheel +seat, B the journal, C the crank-pin, and D and E the crank arms. Only +one half of the axle is shown in fig. 38, but the other half is exactly +the same. The cranks on the two halves are, however, at right angles to +one another. The ends of the crank arms are turned in the lathe, the +crank-pin ends being turned at the same time as the axle, and the other +ends at the same time as the crank-pin. This consideration determines +the centres for the arcs shown in the end view. + + EXERCISE 38.--Draw to a scale of 2 inches to a foot the side and + end elevations of the locomotive cranked axle partly shown in + fig. 38. The distance between the centre lines of the cylinders + is 2 feet. + +[Illustration: FIG. 39.] + +_Built-up Cranks._--The form of cranked shaft shown in fig. 38 is +largely used for marine engines, but for the very powerful engines now +fitted in large ships this design of shaft is very unreliable, the +built-up crank shown in fig. 39 being preferred, although it is much +heavier than the other. It will be seen from the figure that the shaft, +crank arms, and crank-pin are made separately. The arms are shrunk on to +the pin and the shaft, and secured to the latter by sunk keys. These +heavy shafts and cranks are generally made of steel. + + EXERCISE 39.--Keeping to the dimensions marked in fig. 39, draw + the views there shown of a built-up crank-shaft for a marine + engine. Scale 3/4 inch to a foot. + + + + +XI. ECCENTRICS. + + +The _eccentric_ is a particular form of crank, being a crank in which +the crank-pin is large enough to embrace the crank-shaft. In the +eccentric what corresponds to the crank-pin is called the sheave or +pulley. The advantage which an eccentric possesses over a crank is that +the shaft does not require to be divided at the point where the +eccentric is put on. The crank, however, has this advantage over the +eccentric, namely, that it can be used for converting circular into +reciprocating motion, or _vice versâ_, while the eccentric can only be +used for converting circular into reciprocating motion. This is owing to +the great leverage at which the friction of the eccentric acts. + +The chief application of the eccentric is in the steam-engine, where it +is used for working the valve gear. + +To permit of the sheave being placed on the shaft without going over the +end (which could not be done at all in the case of a cranked axle, and +would be a troublesome operation in most cases) it is generally made in +two pieces, as shown in fig. 40, which represents one of the eccentrics +of a locomotive. The two parts of the sheave are connected by two cotter +bolts. The part which embraces the sheave is called the eccentric strap, +and corresponds to, and is, in fact, a connecting rod end: the rod +proceeding from this is called the eccentric rod. + +The distance from the centre of the sheave to the centre of the shaft is +called the _radius_ or _eccentricity_ of the eccentric. The _throw_ is +twice the eccentricity. + +The sheave is generally made of cast iron. The strap may be of brass, +cast iron, or wrought iron; when the strap is made of wrought iron it is +commonly lined with brass. + +[Illustration: FIG. 40.] + + EXERCISE 40: _Locomotive Eccentric._--In fig. 40 D E is the + sheave, F H the strap, and K the eccentric rod. The sheave and + strap are made of cast iron, and the eccentric rod is made of + wrought iron. (_a_) is a vertical cross section through the + oil-box of the strap; (_b_) is a plan of the end of the eccentric + rod and part of the strap. All the nuts are locked by means of + cotters. Draw first the elevation, partly in section as shown. + Next draw two end elevations, one looking each way. Afterwards + draw a horizontal section through the centre, and also a plan. + Scale 4 inches to a foot. + + + + +XII. CONNECTING RODS. + + +The most familiar example of the use of a connecting rod is in the +steam-engine, where it is used to connect the rotating crank with the +reciprocating piston. The rod itself is made of wrought iron or steel, +and is generally circular or rectangular in section. The ends of the rod +are fitted with steps, which are held together in a variety of ways. + +_Strap End._--A form of connecting rod end, which is not so common as it +used to be, is shown in fig. 41. At (_a_) is shown a longitudinal +section with all the parts put together, while at (_b_), (_c_), _(d)_ +and (_e_) the details are shown separately. A B is the end of the rod +which butts against the brass bush C D, which is in two pieces. A +_strap_ E passes round the bush and on to the end of the rod as shown. +The arms of the strap have rectangular holes in them, which are not +quite opposite a similar hole in the rod when the parts are put +together. If a wedge or _cotter_ F be driven into these three holes they +will tend to come into line, and the parts of the bush will be pressed +together. To prevent the cotter opening out the strap, and to increase +the sliding surface, a _gib_ H is introduced. The gib is provided with +horns at its ends to keep it in its place. Sometimes two gibs are used, +one on each side of the cotter; this makes the sliding surface on both +sides of the cotter the same. The cotter is secured by a set screw K. +The unsectioned portion of fig. (_a_) to the right of the gib, or to the +left of the cotter, is called the _clearance_ or _draught._ + +[Illustration: FIG. 41.] + + EXERCISE 41: _Connecting Rod End._--Make the following views of + the connecting rod end illustrated by fig. 41. First, a vertical + section, the same as shown at (_a_). Second, a horizontal section. + Third, side elevation. Fourth, a plan. Or the first and third + views may be combined in a half vertical section and half + elevation; and the second and fourth views may be combined in + a half horizontal section and half plan. + + All the dimensions are to be taken from the detail drawings (_b_), + (_c_), (_d_), and (_e_), _but the details need not be drawn + separately_. The brass bush is shown at (_d_) by half elevation, + half vertical section, half plan, and half horizontal section. + The draught or clearance is 7-16ths of an inch. + +_Box End._--At (_a_), fig. 42, is shown what is known as a box end for a +connecting rod. The part which corresponds to the loose strap in the +last example is here forged in one piece with the connecting rod. In +this form the brass bush is provided with a flange all round on one +side, but on the opposite side the flange is omitted except at one end; +this is to allow of the bush being placed within the end of the rod. The +construction of the bush will be understood by reference to the sketch +shown at (_b_). The bush is in two parts, which are pressed tightly +together by means of a cotter. This cotter is prevented from slackening +back by two set screws. Each set screw is cut off square at the point, +and presses on the flat bottom of a very shallow groove cut on the side +of the cotter. + +The top, bottom, and ends of this box end are turned in the lathe at the +same time as the rod itself; this accounts for the curved sections of +these parts. + +It is clear from the construction of a box end that it is only suitable +for an overhung crank. + + EXERCISE 42: _Locomotive Connecting Rod._--In fig. 42 is shown a + connecting rod for an outside cylinder locomotive. (_a_) is the + crank-pin end, and (_c_) the cross-head end. The end (_a_) has just + been described under the head 'box end.' We may just add that in + this particular example the brass bush is lined with white metal as + shown, and that the construction of the oil-box is the same as that + on the coupling rod end shown in fig. 44. The end (_c_) is forked, + and through the prongs of the fork passes the cross-head pin, of + which a separate dimensioned drawing is shown at (_d_). Observe + that the tapered parts A and B of this pin are parts of the same + cone. The rotation of the pin is prevented by a small key as shown. + The cross-head pin need not be drawn separately, and the isometric + projection of the bush at (_b_) may be omitted, but all the other + views shown are to be drawn to a scale of 6 inches to a foot. + +_Marine Connecting Rod._--The form of connecting rod shown in fig. 43 is +that used in marine engines, but it is also used extensively in land +engines. A B is the crank-pin end, and C the cross-head end. The end A B +is forged in one piece, and after it is turned, planed, and bored it is +slotted across, so as to cut off the cap A. The parts A and B are held +together by two bolts as shown. This end of the rod is fitted with brass +steps, which are lined with white metal. The cross-head end is forked, +and through the prongs of the fork passes a pin D, which also passes +through the cross-head, which is forged on to the piston rod or attached +to it in some other way. + +[Illustration: FIG. 42.] + +[Illustration: FIG. 43.] + + EXERCISE 43: _Marine Connecting Rod._--Draw all the views shown in + fig. 43 of one form of marine connecting rod. For detail drawings + of the locking arrangement for the nuts see fig. 19, page 21. Scale + 4 inches to a foot. + +_Coupling Rods._--A rod used to transmit the motion of one crank to +another is called a _coupling rod_. A familiar example of the use of +coupling rods will be found in the locomotive. Coupling rods are made of +wrought iron or steel, and are generally of rectangular section. The +ends are now generally made solid and lined with solid brass bushes, +_without any adjustment for wear_. This form of coupling rod end is +found to answer very well in locomotive practice where the workmanship +and arrangements for lubrication are excellent. When the brass bush +becomes worn it is replaced by a new one. + +Fig. 44 shows an example of a locomotive coupling rod end for an outside +cylinder engine. In this case it is desirable to have the crank-pin +bearings for the coupling rods as short as possible, for a connecting +rod and coupling rod in this kind of engine work side by side on the +same crank-pin, which, being overhung, should be as short as convenient +for the sake of strength. The requisite bearing surface is obtained by +having a pin of large diameter. The brass bush is prevented from +rotating by means of the square key shown. The oil-box is cut out of the +solid, and has a wrought-iron cover slightly dovetailed at the edges. +This cover fits into a check round the top inner edge of the box, which +is originally parallel, but is made to close on the dovetailed edges of +the cover by riveting. A hole in the centre of this cover, which gives +access to the oil-box, is fitted with a screwed brass plug. The brass +plug has a screwed hole in the centre, through which oil may be +introduced to the box. Dust is kept out of the oil-box by screwing into +the hole in the brass plug a common cork. The oil is carried slowly but +regularly from the oil-box over to the bearing by a piece of cotton +wick. + +[Illustration: FIG. 44.] + + EXERCISE 44: _Coupling Rod End._--Draw first the side elevation and + plan, each partly in section as shown in fig. 44. Then instead of + the view to the left, which is an end elevation partly in section, + draw a complete end elevation looking to the right, and also a + complete vertical cross section through the centre of the bearing. + Scale 6 inches to a foot. + + + + +XIII. CROSS-HEADS. + + +An example of a steam-engine cross-head is shown in fig. 45. A is the +end of the piston rod which has forged upon it the cross-head B. The +cross-head pin shown at (_d_), fig. 42, and to which the connecting rod +is attached, works in the bearing C. Projecting pieces D, forged on the +top and bottom of the cross-head, carry the slide blocks E which work on +the slide bars, and thus guide the motion of the piston rod. + +[Illustration: FIG. 45.] + + EXERCISE 45: _Locomotive Cross-head._--In fig. 45 are shown side + and end elevations, partly in section, of the cross-head and slide + blocks for an outside cylinder locomotive. Draw these views half + size, showing also on the end elevation the cross-head pin and a + vertical section of the connecting rod end from fig. 42. The bush + in the cross-head which forms the bearing for the cross-head pin is + of wrought iron, case-hardened, and is prevented from rotating by + the key shown. The cross-head is of wrought iron, and the slide + blocks are of cast iron, and are fitted with white metal strips as + shown. A short brass tube leads oil from the upper slide block into + a hole in the cross-head as shown, which carries it to a slot in + the bush which distributes it over the cross-head pin. + + + + +XIV. PISTONS. + + +A _piston_ is generally a cylindrical piece which slides backwards and +forwards inside a hollow cylinder. The piston may be moved by the action +of fluid pressure upon it as in a steam-engine, or it may be used to +give motion to a fluid as in a pump. + +A piston is usually attached to a rod, called a _piston rod_, which +passes through the end of the cylinder inside which the piston works, +and which serves to transmit the motion of the piston to some piece +outside the cylinder, or _vice versâ_. + +[Illustration: FIG. 46.] + +A _plunger_ is a piston made in one piece with its piston rod, the +piston and the rod being of the same diameter. + +A piston which is provided with one or more valves which allow the +fluid to pass through it from one side to the other is called a +_bucket_. + +_Simple Piston._--The simplest form of piston is a plain cylinder +fitting accurately another, inside which it moves. Such a piston works +with very little friction, but as there is no adjustment for wear, such +a piston is not suitable for a high fluid pressure if it has to work +constantly. This simple form of piston is used in the steam-engine +indicator, and also in pumps. + +Fig. 46 shows the piston of the circulation pump of a marine engine. +A is the cast-iron casing or barrel of the pump; B is a brass liner +fitting tightly into the former at its ends, and secured by eight +screwed Muntz metal pins C, four at each end; D is the piston, which is +made of brass, and is attached to a Muntz metal piston rod E. The liner +is bored out smooth and true from end to end, and the piston is turned +so as to be a sliding fit to the liner. The wear in this form of piston +is diminished by making the rubbing surface large. + + EXERCISE 46: _Piston for Circulating Pump._--Draw the vertical + sectional elevation of the piston, &c., shown in fig. 46, also a + half plan and half horizontal section through the centre. Scale 4 + inches to a foot. + +_Pump Bucket._--The next form of piston which we illustrate is shown in +fig. 47. This represents the air-pump bucket of a marine engine. The +bucket is made of brass, and is provided with six india-rubber disc +valves. The rod is in this case made of Muntz metal. Air-pump rods for +marine engines are very often made of wrought iron cased with brass. It +will be observed that there is a wide groove around the bucket, which is +filled with hempen rope or gasket. This gasket forms an elastic packing +which prevents leakage. This is an old-fashioned form of packing, and is +now only used for pump buckets. + +[Illustration: FIG. 47.] + + EXERCISE 47: _Air-pump Bucket._--Draw the sectional elevation of + the air-pump bucket shown in fig. 47. Also draw a half plan looking + downwards and a half plan looking upwards. Scale 4 inches to a + foot. + +_Ramsbottom's Packing._--The form of packing used in the air-pump +bucket, fig. 47, is not suitable for steam pistons. For the latter the +packing is now always metallic. The simplest form of metallic packing is +that known as Ramsbottom's. This form is very largely used for +locomotive pistons, and for small pistons in many kinds of engines +besides. A locomotive piston for an 18-inch cylinder with Ramsbottom's +packing is shown in fig. 48. The particular piston there illustrated is +made of brass, and is secured to a wrought-iron piston rod by a brass +nut. Two circumferential grooves of rectangular section are turned out +of the piston, and into these fit two corresponding rings, which may be +of brass, cast iron, or steel. In this example the rings are of cast +iron. These rings are first turned a little larger in diameter than the +bore of the cylinder (in this example 1/2 inch), and then sprung over +the piston into the groves prepared for them. Their own elasticity +causes the rings to press outwards on the cylinder. At the point where a +ring is split a leakage of steam will take place, but with quick-running +pistons this leakage is unimportant. The points where the rings are cut +should be placed diametrically opposite, so as to diminish the leakage +of steam. + +[Illustration: FIG. 48.] + + EXERCISE 48: _Locomotive Piston._--A part elevation and part + section of a locomotive piston, for a cylinder having a bore 18 + inches in diameter, is shown in fig. 48. Draw this, and also a view + looking on the nut in the direction of the axis of the piston rod. + Scale 6 inches to a foot. + + _Note._--The reason why the part of the piston rod within the + piston has such a quick taper is that the piston has to be taken + off the rod while it is in the cylinder. The cross-head being + forged on the end of the piston rod prevents the piston and piston + rod being withdrawn together. + +_Large Pistons._--Pistons of large diameter are generally provided with +two cast-iron packing rings placed within the same groove. These rings +are pressed outwards against the cylinder, and also against the sides of +the groove by one or more springs. One form of this packing +(Lancaster's) is shown in fig. 49. Here one spring only is used, and it +is first made a straight spiral spring, and then bent round and its ends +united. The action of the spring will be clearly understood from the +illustration. For the purpose of admitting the packing rings the piston +is divided into two parts, one the piston proper, and the other the +_junk ring_. In fig. 49, A is the junk ring, which is secured to the +piston by means of bolts as shown. + +[Illustration: FIG. 49.] + + EXERCISE 49: _Marine Engine Piston._--The piston illustrated by + fig. 49 is for the high-pressure cylinder of a marine engine. The + piston, junk ring, and packing rings are of cast iron. The piston + rod and nut are of wrought iron, so also are the junk ring bolts. + The nuts for the latter are of brass. The spiral spring is made + from steel wire 3/8 inch diameter. An enlarged section of one of + the packing rings is shown at (_a_). A front elevation of the + locking arrangement for the piston rod nut is shown at (_b_). A + sectional plan of one of the nuts for the junk ring bolts is shown + at (_c_). + + First draw the vertical section of this piston, next draw a plan, + one-third of which is to show the piston complete, one-third to + show the junk ring removed, and the remaining third to be a + horizontal section through between the packing rings. The details + (_a_) and (_c_) need not be drawn separately. Scale 3 inches to a + foot. + +_Proportions of Marine Engine Pistons._--Mr. Seaton, in his 'Manual of +Marine Engineering,' gives the following rules for designing marine +engine pistons:-- + + D = diameter of piston in inches. + _p_ = effective pressure in lbs. per square inch. + _x_ = D/50 × [sqrt (_p_)] + 1. + + Thickness of front of piston near boss 0.2 × _x_. + " " " rim 0.17 × _x_. + " back of piston 0.18 × _x_. + " boss around rod 0.3 × _x_. + " flange inside packing ring 0.23 × _x_. + " " at edge 0.25 × _x_. + " junk ring at edge 0.23 × _x_. + " " inside packing ring. 0.21 × _x_. + " " at bolt-holes 0.35 × _x_. + " metal around piston edge 0.25 × _x_. + Breadth of packing ring 0.63 × _x_. + Depth of piston at centre 1.4 × _x_. + Lap of junk ring on piston 0.45 × _x_. + Space between piston body and packing ring 0.3 × _x_. + Diameter of junk-ring bolts 0.1 × _x_ + .25 inch. + Pitch of junk-ring bolts 10 diameters. + Number of webs in piston (D + 20)/12. + Thickness " 0.18 × _x_. + + EXERCISE 50: _Design for Marine Engine Piston._--Calculate by + Seaton's rules the dimensions for a marine engine piston 40 inches + in diameter, and subjected to an effective pressure of 36 lbs. per + square inch. Then make the necessary working drawings for this + piston to a scale of, say, 3 inches to a foot. + + _Note._--Take the dimensions got by calculation to the nearest + 1-16th of an inch. + + + + +XV. STUFFING-BOXES. + + +[Illustration: FIG. 50.] + +In fig. 50 is shown a gland and stuffing-box for the piston rod of a +vertical engine. A B is the piston rod, C D a portion of the cylinder +cover, and E F the _stuffing-box_. Fitting into the bottom of the +stuffing-box is a brass bush H. The space K around the rod A B is filled +with _packing_, of which there is a variety of kinds, the simplest +being greased hempen rope. The packing is compressed by screwing down +the cast-iron gland L M, which is lined with a brass bush N. In this +case the gland is screwed down by means of three stud-bolts P, which are +screwed into a flange cast on the stuffing-box. Surrounding the rod on +the top of the gland there is a recess R for holding the lubricant. + +[Illustration: FIG. 51.] + +[Illustration: FIG. 52.] + +The object of the gland and stuffing-box is to allow the piston rod to +move backwards and forwards freely without any leakage of steam. + +Fig. 51 shows a gland and stuffing-box for a horizontal rod. The +essential difference between this example and the last is in the mode of +lubrication. The gland flange has cast within it an oil-box which is +covered by a lid; this lid is kept shut or open by the action of a small +spring as shown. A piece of cotton wick (not shown in the figure) has +one end trailing in the oil in the oil-box, while the other is carried +over and passed down the hole A B. The wick acts as a siphon, and drops +the oil gradually on to the rod. In this example only two bolts are used +for screwing in the gland; and the flanges of the gland and stuffing-box +are not circular, but oval-shaped. + +In the case of small rods the gland is made entirely of brass, and no +liner is then necessary. Fig. 52 shows a form of gland and stuffing-box +sometimes used for small rods. The stuffing-box is screwed externally, +and carries a nut A B which moves the gland. + + EXERCISE 51: _Gland and Stuffing-box for a Vertical Rod._--Draw the + views shown in fig. 50 to the dimensions given. Scale 6 inches to a + foot. + + EXERCISE 52: _Gland and Stuffing-box for a Horizontal Rod._--Fig. + 51 shows a plan, half in section, and an elevation half of which is + a section through the gland flange. Draw these to a scale of 6 + inches to a foot, using the dimensions marked in the figure. + + EXERCISE 53: _Screwed Gland and Stuffing-box._--Draw, full size, + the views shown in fig. 52 to the given dimensions. + +A more elaborate form of gland and stuffing-box is shown in fig. 53. +This is for a large marine engine with inverted cylinders, such as is +used on board large ocean steamers. The stuffing-box is cast separate +from the cylinder cover to which it is afterwards bolted. The lubricant +is first introduced to the oil-boxes marked A, from which it passes to +the recess B, where it comes in contact with the piston rod. To prevent +the lubricant from being wasted by running down the rod, the main gland +is provided with a shallow gland and stuffing-box which is filled with +soft cotton packing, which soaks up the lubricant. + +The main gland is screwed up by means of six bolts, and to prevent the +gland from locking itself in the stuffing-box, it is necessary that the +nuts should be turned together. This is done in a simple and ingenious +manner. One-half of each nut is provided with teeth, and these gear with +a toothed wheel which has a rim only; this rim is held up by a ring C. +When one nut is turned, all the rest follow in the same direction. + +[Illustration: FIG. 53.] + + EXERCISE 54: _Gland and Stuffing-box for Piston Rod of Large + Inverted Cylinder Engine._--The lower view in fig. 53 is a half + plan looking upwards, and a half section of the gland looking + downwards. The upper view is a vertical section. Complete all these + views and add an elevation. Scale 3 inches to a foot. + + _Note._--The large nuts, the wheel, the supporting ring, and small + gland are made of brass. + +_Dimensions of Stuffing-boxes and Glands._ + + _d_ = diameter of rod. _t__{1} = thickness of + _d__{1} = diameter of box (inside). stuffing-box flange. + _l_ = length of stuffing-box _t__{2} = thickness of gland + bush. flange. + _l__{1} = length of packing space. _t__{3} = thickness of bushes in + _l__{2} = length of gland. box and gland. + _t_ = thickness of metal in _d__{2} = diameter of gland bolts. + stuffing-box. _n_ = number of bolts. + + +----------------------------------------------------------+ + | _d_ | _d__{1} | _l_ | _l__{1} | _l__{2} | _t_ | _t__{1} | + +-----+---------+-----+---------+---------+------+---------+ + |1 | 1-3/4 | 3/4| 2 | 1-1/2 | 7/16| 1/2 | + |1-1/2| 2-1/2 |1-1/4| 2-5/8 | 2 | 9/16| 11/16 | + |2 | 3-1/2 |1-3/4| 3-1/4 | 2-1/2 | 11/16| 7/8 | + |2-1/2| 4-1/8 |2-1/4| 3-7/8 | 2-7/8 | 13/16| 1-1/16 | + |3 | 4-3/4 |2-3/4| 4-1/2 | 3-1/4 | 15/16| 1-1/4 | + |3-1/2| 5-1/4 | 3 | 5-1/8 | 3-5/8 |1 | 1-3/8 | + |4 | 5-7/8 |3-1/4| 5-3/4 | 4 |1 | 1-3/8 | + |4-1/2| 6-3/8 |3-1/2| 6-3/8 | 4-3/8 |1-1/16| 1-9/16 | + |5 | 7 |3-3/4| 7 | 4-5/8 |1-1/16| 1-9/16 | + |6 | 8 |4-1/4| 8-1/4 | 5 |1-1/8 | 1-11/16 | + +----------------------------------------------------------+ + + +-------------------------------------------------+ + | _d_ | _t__{2} | _t__{3} | _d__{2} | _n_ | + +-----+-----------------+---------+---------+-----+ + |1 | _t__{2}=_t_ | 3/16 | 7/16 | 2 | + |1-1/2| when gland | 1/4 | 5/8 | 2 | + |2 | flange is | 5/16 | 3/4 | 2 | + |2-1/2| made of cast | 5/16 | 7/8 | 2 | + |3 | iron and | 3/8 | 1 | 2 | + |3-1/2| _t__{2}=_t__{1} | 3/8 | 1 | 2 | + |4 | when gland | 7/16 | 1 | 2 | + |4-1/2| flange is | 7/16 | 7/8 | 4 | + |5 | made of | 7/16 | 1 | 4 | + |6 | brass. | 1/2 | 1-1/4 | 4 | + +-------------------------------------------------+ + +The proportions of glands and stuffing-boxes vary considerably but the +above table represents average practice. + + EXERCISE 55:--Make the necessary working drawings for a gland and + stuffing-box for a locomotive engine piston rod 2-1/2 inches in + diameter, to the dimensions given in the table. + + + + +XVI. VALVES. + + +Professor Unwin divides valves, according to their construction into +three classes as follows:--(1) flap valves, which bond or turn upon a +hinge; (2) lift valves, which rise perpendicularly to the seat; (3) +sliding valves, which move parallel to the seat. + +Examples of flap valves are shown in figs. 54 and 55; two forms of lift +valves are shown in figs. 56 and 57, and in figs. 58 and 59 are shown +two forms of slide valve. The slide valve shown in fig. 58 moves in a +straight line, while that shown in fig. 59 (called a cock) moves in +circle. + +_India-rubber Valves._--In india-rubber valves there is a grating +covered by a piece of india-rubber, which may be rectangular, but is +generally circular, and which is held down along one edge if +rectangular, or at the centre if circular. Water or other fluid can pass +freely upwards through the grating, but when it attempts to return the +elasticity of the india-rubber, and the pressure of the water upon it, +cause it to lie close on the grating, and thus prevent the return of the +water. The india-rubber is prevented from rising too high by a +perforated guard. In fig. 54 is shown an example of an india-rubber disc +valve. A is the grating, B the india-rubber, C the guard secured to the +grating or seat by the stud D and nut E. The grating is held in position +by bolts and nuts F. The grating and guard are generally of brass. + +India-rubber disc valves are also shown on the air-pump bucket, fig. 47. + + EXERCISE 56: _India-rubber Disc Valve._--Fig. 54 shows a vertical + section and a plan of an india-rubber disc valve. In the plan + one-half of the guard and india-rubber are supposed to be removed + so as to show the grating or seat. Draw these views, and also an + elevation. A detail drawing of the central stud is shown in fig. + 16, page 18. In fig. 54 the elevation of the guard is drawn as it + is usually drawn in practice, but if the student has a sufficient + knowledge of descriptive geometry he should draw the elevation + completely showing the perforations. Scale 6 inches to a foot. + +[Illustration: FIG. 54.] + +[Illustration: FIG. 55.] + +_Kinghorn's Metallic Valve._--The action of this valve is the same as +that of an india-rubber valve, but a thin sheet of metal (phosphor +bronze) takes the place of the india-rubber. + +This valve is now largely used in the pumps of marine engines, and is +shown in fig. 55 as applied to an air-pump bucket. Three valves like the +one shown are arranged round the bucket. + + EXERCISE 57: _Kinghorn's Metallic Valve._--Fig. 55 shows an + elevation and plan of one form of this valve. In the plan one-half + of the guard and metal sheet are supposed to be removed, so as to + show the grating, which in this case is part of an air-pump bucket. + Draw the views shown, and also a vertical section of the guard + through the centres of the bolts. All the parts are of brass except + the valve proper, which is of phosphor bronze. Scale 6 inches to a + foot. + +_Conical Disc Valves._--A very common form of valve is that shown in +figs. 56 and 57. This form of valve consists of a disc, the edge of +which (called the face) is conical. The conical edge of this disc fits +accurately on a corresponding seat. The angle which the valve face makes +with its axis is generally 45°. If the disc is raised, either by the +action of the fluid as in the india-rubber valve, or by other means, an +opening is formed around the disc through which the fluid can pass. The +valve is guided in rising and falling either by three feathers +underneath it, as in fig. 56, or by a central spindle which moves freely +through a hole in the centre of a bridge which stretches across the +seat, as in fig. 57. The lift of the valve is limited by a stop above +it, which forms part of the casing containing the valve. The lift should +in no case exceed one-fourth of the diameter of the valve, and it is +generally much less than this. The guiding feathers (fig. 56) are +notched immediately under the disc for the purpose of making available +the full circumferential opening of the valve for the passage of the +fluid. These notches also prevent the feathers from interfering with the +turning or scraping of the valve face. + +Conical disc valves and their seats are nearly always made of brass. + + EXERCISE 58: _Conical Disc Valves._--Draw, half size, the plans and + elevations shown in figs. 56 and 57. In fig. 57 the valve is shown + open in the elevation, and in the plan it is removed altogether in + order to show the seat with its guide bridge. + +[Illustration: Plan of Valve. FIG. 56.] + +[Illustration: Plan of Seat. FIG. 57.] + +_Simple Slide Valve._--The form of valve shown in fig. 58, often called +the _locomotive slide valve_, is very largely used in all classes of +steam-engines for distributing the steam in the steam cylinders. The +valve is shown separately at (_d_), (_e_), and (_f_), while at (_a_), +(_b_), and (_c_) is shown its connection with the steam cylinder. + +It will be observed that the valve itself is in the shape of a box with +one side open, the edges of the open side being flanged. When the valve +is in its middle position, as shown at (_a_), two of these flanged edges +completely cover two rectangular openings S_{1} and S_{2}, called _steam +ports_, while the hollow part of the valve is opposite to a third port +E, called the _exhaust port_. As shown at (_a_) the piston P would be +moving upwards and the valve downwards. By the time the piston has +reached the top of its stroke the valve will have moved so far down as +to partly uncover the steam port S_{1}, and admit steam from the valve +casing C through S_{1} and the passage P_{1} to the top of the piston. +The pressure of this steam on the top of the piston will force the +latter down. While the above action has been going on, the port S_{2} +will have become uncovered, and the hollow part of the valve will be +opposite both the steam port S_{2} and the exhaust port E, so that the +steam from the under side of the piston, and which forced the piston up, +can now escape by the passage P_{2}, the steam port S_{2}, and the +exhaust port E to the exhaust outlet O, and thence into the atmosphere, +if it is a non-condensing engine, or into the condenser if it is a +condensing engine, or into another cylinder if it is a compound engine. +After the piston has performed, a certain part of its downward stroke, +the valve, which has been moving downwards, will commence to move +upwards, and when it has reached a certain point it will cover the port +S_{1}, and shut off the supply of steam to the top of the piston. It is +generally arranged that the steam shall be cut off before the piston +reaches the end of the stroke. When the piston reaches the bottom of its +stroke the valve has moved far enough up to uncover the port S_{2} and +admit steam to the bottom of the piston, and to uncover the port S_{1} +and allow the steam to escape from the top of the piston through the +passage P_{1}, the port S_{1}, the port E, and outlet O. In this way the +piston is moved up and down in the cylinder. + +The valve is attached to a valve spindle S by nuts as shown, the hole in +the valve through which the spindle passes being oval-shaped to permit +of the valve adjusting itself so as to always press on its seat. + +When the valve is in its middle position it generally more than covers +the steam ports. The amount which the valve projects over the steam port +on the outside, the valve being in its middle position, is called the +_outside lap_ of the valve, and the amount which it projects on the +inside is called the _inside lap_. When the term lap is used without any +qualification, outside lap is to be understood. In fig. 58 it will be +seen that the valve has no inside lap, and that the outside lap is +three-eighths of an inch. The inside lap is generally small compared +with the outside lap. + +[Illustration: FIG. 58.] + +When the piston is at the beginning of its stroke the steam port is +generally open by a small amount called the _lead_ of the valve. + +The reciprocating motion of the slide valve is nearly always derived +from an eccentric fixed on the crank-shaft of the engine. Slide valves +are generally made of brass, bronze, or cast iron. + + EXERCISE 59: _Simple Slide Valve._--At (_d_), fig. 58, is shown a + sectional elevation of a simple slide valve for a steam-engine, the + section being taken through the centre line of the valve spindle, + while at (_e_) is shown a cross section and elevation, and at (_f_) + a plan of the same. Draw all these views full size, and also a + sectional elevation at A B. The valve is made of brass, and the + valve spindle and nuts of wrought iron. + + EXERCISE 60: _Slide Valve Casing, &c., for Steam-engine._--Draw, + half size, the views shown at (_a_), (_b_), and (_c_), fig. 58; + also a sectional plan at L M. (_b_) is an elevation of the valve + casing with the cover and the valve removed. (_a_) is a sectional + elevation, the section being taken through the axes of the steam + cylinder and valve spindle. (_c_) is a sectional plan, the section + being a horizontal one through the centre of the exhaust port. The + inlet and outlet for the steam are clearly shown in the sectional + plan: in the sectional elevation their positions are shown by + dotted circles. + + The stroke of the piston is in this case 12 inches, so that from + the dimensions given at (_a_) it must come within a quarter of an + inch of each end of the cylinder; this is called the _cylinder + clearance_. + + The piston has three Ramsbottom rings, a quarter of an inch wide + and a quarter of an inch apart. + + The steam cylinder and valve casing are made of cast iron. + +_Cocks._--A cock consists of a slightly conical plug which fits into a +corresponding casing cast on a pipe. Through the plug is a hole which +may be made by turning the plug to form a continuation of the hole in +the pipe, and thus allow the fluid to pass, or it may be turned round so +that the solid part of the plug lies across the hole in the pipe, and +thus prevent the fluid from passing. As the student will be quite +familiar with the common water cock or tap such as is used in +dwelling-houses we need not illustrate it here. + +[Illustration: FIG. 59.] + +Fig. 59 shows a cock of considerable size, which may be used for water +or steam under high pressure. The plug in this example is hollow, and is +prevented from coming out by a cover which is secured to the casing by +four stud bolts. An annular ridge of rectangular section projecting from +the under side of the cover, and fitting into a corresponding recess on +the top of the casing, serves to ensure that the cover and plug are +concentric, and prevents leakage. Leakage at the neck of the plug is +prevented by a gland and stuffing-box. The top end of the plug is made +square to receive a handle for turning it. The size of a cock is taken +from the bore of the pipe in which it is placed; thus fig. 59 shows a +2-1/4-inch cock. + + EXERCISE 61: 2-1/4-_inch Steam or Water Cock._--First draw the + views of this cock shown in fig. 59, then draw a half end elevation + and half cross section through the centre of the plug. Scale 6 + inches to a foot. + + Instead of drawing the parts of the pipe on the two sides of the + plug in the same straight line as in fig. 59, one may be shown + proceeding from the bottom of the casing, so that the fluid will + have to pass through the bottom of the plug and through one side. + This is a common arrangement. + + All the parts of the valve and casing in this example are made of + brass. + + + + +XVII. MATERIALS USED IN MACHINE CONSTRUCTION. + + +_Cast Iron._--The essential constituents of cast iron are iron and +carbon, the latter forming from 2 to 5 per cent. of the total weight. +Cast iron, however, usually contains varying small amounts of silicon, +sulphur, phosphorus, and manganese. + +In cast iron the carbon may exist partly in the free state and partly in +chemical combination with the iron. + +In _white cast iron_ the whole of the carbon is in chemical combination +with the iron, while in _grey cast iron_ the carbon is principally in +the free state, that is, simply mixed mechanically with the iron. It is +the free carbon which gives the grey iron its dark appearance. A mixture +of the white and grey varieties of cast iron when melted produces +_mottled cast iron_. The greater the amount of carbon chemically +combined with the iron, the whiter, harder, and more brittle does it +become. + +The white cast iron is stronger than the grey, but being more brittle it +is not so suitable for resisting suddenly applied loads. White iron +melts at a lower temperature than grey iron, but after melting it does +not flow so well, or is not so liquid as the grey iron. White iron +contracts while grey iron expands on solidifying. The grey iron, +therefore, makes finer castings than the white. Castings after +solidifying contract in cooling about 1/8 of an inch per foot. Castings +possessing various degrees of strength and hardness are produced by +melting mixtures of various proportions of white and grey cast irons. +White cast iron has a higher specific gravity than grey cast iron. + +Cast iron gives little or no warning before breaking. The thickness of +the metal throughout a casting in cast iron should be as uniform as +possible, so that it may cool and therefore contract uniformly +throughout; otherwise some parts may be in a state of initial strain +after the casting has cooled, and will therefore be easier to fracture. +Re-entrant angles should be avoided; such should be rounded out with +fillets. + +The presence of phosphorus in cast iron makes it more fusible, and also +more brittle. The presence of sulphur diminishes the strength +considerably. + +The grey varieties of cast iron are called _foundry irons_ or _foundry +pigs_, while the white varieties are called _forge irons_ or _forge +pigs_, from the fact that they are used for conversion into wrought +iron. + +Amongst iron manufacturers the different varieties of cast iron are +designated by the numbers 1, 2, 3, &c., the lowest number being applied +to the greyest variety. + +_Chilled Castings._--When grey cast iron is melted a portion of the free +carbon combines chemically with the iron; this, however, separates out +again if the iron is allowed to cool slowly; but if it is suddenly +cooled a greater amount of the carbon remains in chemical combination, +and a whiter and harder iron is produced. Advantage is taken of this in +making _chilled castings_. In this process the whole or a part of the +mould is lined with cast iron, which, being a comparatively good +conductor of heat, chills a portion of the melted metal next to it, +changing it into a hard white iron to a depth varying from 1/8 to 1/2 an +inch. To protect the cast-iron lining of the mould from the molten metal +it is painted with loam. + +_Malleable Cast Iron._--This is prepared by imbedding a casting in +powdered red hematite (an oxide of iron), and keeping it at a bright red +heat for a length of time varying from several hours to several days +according to the size of the casting. By this process a portion of the +carbon in the casting is removed, and the strength and toughness of the +latter become more like the strength and toughness of wrought or +malleable iron. + +_Wrought or Malleable Iron._--This is nearly pure iron, and is made from +cast iron by the puddling process, which consists chiefly of raising the +cast iron to a high temperature in a reverberatory furnace in the +presence of air, which unites with the carbon and passes off as gas. In +other words the carbon is burned out. The iron is removed from the +puddling furnace in soft spongy masses called _blooms_, which are +subjected to a process of squeezing or hammering called _shingling_. +These shingled blooms still contain enough heat to enable them to be +rolled into rough _puddled bars_. These puddled bars are of very +inferior quality, having less than half the strength of good wrought +iron. The puddled bars are cut into pieces which are piled together, +reheated, and again rolled into bars, which are called _merchant bars_. +This process of piling, reheating, and re-rolling may be repeated +several times, depending on the quality of iron required. Up to a +certain point the quality of the iron is improved by reheating and +rolling or hammering, but beyond that a repetition of the process +diminishes the strength of the iron. + +The process of piling and rolling gives wrought iron a fibrous +structure. When subjected to vibrations for a long time, the structure +becomes crystalline and the iron brittle. The crystalline structure +induced in this way may be removed by the process of _annealing_, which +consists in heating the iron in a furnace, and then allowing it to cool +slowly. + +_Forging and Welding._--The process of pressing or hammering wrought +iron when at a red or white heat into any desired shape is called +_forging_. If at a white heat two pieces of wrought iron be brought +together, their surfaces being clean, they may be pressed or hammered +together, so as to form one piece. This is called _welding_, and is a +very valuable property of wrought iron. + +_Steel._--This is a compound of iron with a small per-centage of carbon, +and is made either by adding carbon to wrought iron, or by removing some +of the carbon from cast iron. + +In the _cementation_ process, bars of wrought iron are imbedded in +powdered charcoal in a fireclay trough, and kept at a high temperature +in a furnace for several days. The iron combines with a portion of the +carbon to form _blister steel_, so named because of the blisters which +are found on the surface of the bars when they are removed from the +furnace. + +The bars of blister steel are broken into pieces about 18 inches long, +and tied together in bundles by strong steel wire. These bundles are +raised to a welding heat in a furnace, and then hammered or rolled into +bars of _shear steel_. + +To form _cast steel_ the bars of blister steel are broken into pieces +and melted into crucibles. + +In the _Siemens-Martin_ process for making steel, cast and wrought iron +are melted together on the hearth of a regenerative gas-furnace. + +_Bessemer steel_ is made by pouring melted cast iron into a vessel +called a converter, through which a blast of air is then urged. By this +means the carbon is burned out, and comparatively pure iron remains. To +this is added a certain quantity of 'spiegeleisen,' which is a compound +of iron, carbon, and manganese. + +_Hardening and Tempering of Steel._--Steel, if heated to redness and +cooled suddenly, as by immersion in water, is hardened. The degree of +hardness produced varies with the rate of cooling; the more rapidly the +heated steel is cooled, the harder does it become. Hardened steel is +softened by the process of _annealing_, which consists in heating the +hardened steel to redness, and then allowing it to cool slowly. Hardened +steel is _tempered_, or has its degree of hardness lowered, by being +heated to a temperature considerably below that of a red heat, and then +cooling suddenly. The higher the temperature the hardened steel is +raised to, the lower does its 'temper' become. + +_Case-hardening._--This is the name given to the process by which the +surfaces of articles made of wrought iron are converted into steel, and +consists in heating the articles in contact with substances rich in +carbon, such as bone-dust, horn shavings, or yellow prussiate of potash. +This process is generally applied to the articles after they are +completely finished by the machine tools or by hand. The coating of +steel produced on the article by this process is hardened by cooling the +article suddenly in water. + +_Copper._--This metal has a reddish brown colour, and when pure is very +malleable and ductile, either when cold or hot, so that it may be rolled +or hammered into thin plates, or drawn into wire. Slight traces of +impurities cause brittleness, although from 2 to 4 per cent. of +phosphorus increases its tenacity and fluidity. Copper is a good +conductor of heat and of electricity. Copper is largely used for making +alloys. + +_Alloys._--_Brass_ contains two parts by weight of copper to one of +zinc. _Muntz metal_ consists of three parts of copper to two of zinc. +Alloys consisting of copper and tin are called _bronze_ or _gun-metal_. +Bronze is harder the greater the proportion of tin which it contains; +five parts of copper to one of tin produce a very hard bronze, and ten +of copper to one of tin is the composition of a soft bronze. _Phosphor +bronze_ contains copper and tin with a little phosphorus; it has this +advantage over ordinary bronze, that it may be remelted without +deteriorating in quality. This alloy also has the advantage that it may +be made to possess great strength accompanied with hardness, or less +strength with a high degree of toughness. + +_Wood._--In the early days of machines wood was largely used in their +construction, but it is now used to a very limited extent in that +direction. _Beech_ and _hornbeam_ are used for the cogs of mortise +wheels. _Yellow pine_ is much used by pattern-makers. _Box_, a heavy, +hard, yellow-coloured wood, is used for the sheaves of pulley blocks, +and sometimes for bearings in machines. _Lignum-vitć_ is a very hard +dark-coloured wood, and remarkable for its high specific gravity, being +1-1/3 times the weight of the same volume of water. This wood is much +used for bearings of machines which are under water. + + + + +XVIII. MISCELLANEOUS EXERCISES. + + +The illustrations in this chapter are in most cases not drawn to scale; +they are also in some parts incomplete, and in others some of the lines +are purposely drawn wrong. The student must keep to the dimensions +marked on the drawings, and where no sizes are given he must use his own +judgment in proportioning the parts. All errors must be corrected, and +any details required, but not shown completely in the illustrations, +must be filled in. + + EXERCISE 62: _Single Riveted Butt Joint with Tee-iron Cover + Strap._--Two views, one a side elevation and the other a sectional + elevation, of a riveted joint are shown in fig. 60. Draw these + views, and also a plan projected from one of them. Show the rivets + completely in all the views. Scale 4 inches to a foot. + +[Illustration: FIG. 60.] + +[Illustration: FIG. 61.] + + EXERCISE 63: _Girder Stay for Steam Boiler._--The flat crown of the + fire-box of locomotive and marine boilers is generally supported or + stayed by means of girder stays, an example of which is shown in + fig. 61. A B is the side elevation of a portion of one of these + girders. Each girder is supported at its ends by the plates forming + the vertical sides of the fire-box. The flat crown is bolted to the + girders as shown. Observe that the girders are in contact with the + crown only in the neighbourhood of the bolts. Consider carefully + this part of the design, and then answer the following questions: + (1) What objections are there to supporting the girders at the ends + only without the contact pieces at the bolts? (2) What objections + are there to having the girders in contact with the crown plate of + the fire-box throughout their whole length? + + Draw the views shown in fig. 61, and from the right-hand one + project a plan. Scale 4 inches to a foot. + +[Illustration: FIG. 62.] + + EXERCISE 64: _End of Bar Stay for Steam Boiler._--On page 12 one + form of stay for supporting the flat end of a steam boiler is + described. Another form of stay for the same purpose is shown in + fig. 62. A B is a portion of the end of a steam boiler. C D is one + end of a bar which extends from one end of the boiler to the + other. The ends of this bar are screwed, and when the bar is of + wrought iron the screwed parts are generally larger in diameter + than the rest of the bar. When made of steel the bar is generally + of uniform diameter throughout. In the case of wrought-iron bar + stays the enlarged ends are welded on to the smaller parts. + Welding is not so reliable with steel as with wrought iron. Write + out answers to the following questions: (1) What is the advantage + of having the screwed part of the bar larger in diameter than the + rest? (2) Why are steel bar stays not generally enlarged at their + screwed ends? + + Draw the views shown in fig. 62, and project from one of them a + third view. Scale 4 inches to a foot. + + EXERCISE 65: _Knuckle Joint._--Draw the plan and elevation of this + joint shown in fig. 63, and also draw an end elevation looking in + the direction of the arrow. The parts at A and B are octagonal in + cross section. Scale 4 inches to a foot. + +[Illustration: FIG. 63.] + + EXERCISE 66: _Locomotive Coupling Rod Ends._--A form of knuckle + joint used on locomotive coupling rods is shown in fig. 64. + + In this case two rods meet and work on the same pin, as shown at + (a) fig. 64. Draw, in addition to the views shown in fig. 64, a + plan and a vertical section through the axis of the pin. Scale 6 + inches to a foot. + + Would it be practicable to replace the two rods A B and B C by a + single rod working on the crank pins at A, B, and C? Give reasons + for your answer. + +[Illustration: FIG. 64.] + + EXERCISE 67: _Bell Crank Lever._--Draw the plan and elevation of + the lever shown in fig. 65. Scale 6 inches to a foot. + +[Illustration: FIG. 65.] + + EXERCISE 68: _Back Stay for Lathe._--Draw a plan and two elevations + of the stay shown in fig. 66. Make all necessary corrections and + show all the details in each view. Scale full size. + +[Illustration: FIG. 66.] + +[Illustration: FIG. 67.] + + EXERCISE 69: _Conical Disc Valve and Casing._--Draw, half size, the + views shown in fig. 67 of the conical disc valve and casing, and + also add an elevation looking in the direction of the arrow. + + EXERCISE 70: _Connecting Rod End._--The student should carefully + compare this connecting rod end (fig. 68) with those illustrated on + pages 50 and 52. The lower part of fig. 68 is a half plan and half + horizontal section, and the upper part is a half side elevation and + half vertical section. Draw these views and also an end elevation. + Scale 6 inches to a foot. + +[Illustration: FIG. 68.] + +[Illustration: FIG. 69.] + +[Illustration: FIG. 70.] + +[Illustration: FIG. 71.] + +[Illustration: FIG. 72.] + +[Illustration: FIG. 73.] + +[Illustration: FIG. 74.] + + EXERCISE 71: _Engine Cross-head._--The cross-head shown in fig. 69 + is for an inverted cylinder marine engine. A is the piston rod, and + B B are pins, forged in one piece with C, to which the forked end + of the connecting rod is attached. Draw the upper view with the + central part in section as shown. Make the right-hand half of the + lower view a plan without any section, and make the left-hand half + a horizontal section through the axis of the pins B B. Scale 4 + inches to a foot. + + EXERCISE 72: _Ratchet Lever._--The lever shown in fig. 70 is used + for turning the horizontal screw of a traversing screw jack. Draw + the two views shown, and from one of them project a plan. Scale + full size. + + EXERCISE 73: _Steam Whistle._--Draw, full size, the elevation and + section of the steam whistle shown in fig. 71. Draw also horizontal + sections at A B, C D, and E F. + +[Illustration: FIG. 75.] + + EXERCISE 74: _Screw Coupling for Railway Carriages._--Draw the + three views of the screw coupling shown in fig. 72. Scale 6 inches + to a foot. + + If the link A is fixed, through what distance will the link B move + for two turns of the lever? + +[Illustration: FIG. 76.] + + EXERCISE 75: _Loose Headstock for a 6-inch Lathe._--Two views of + this headstock are shown in fig. 73. On one of these views a few of + the chief dimensions are marked. The details, fully dimensioned, + are shown separately in figs. 74, 75, and 76. + + Explain clearly how the centre is moved backwards and forwards, and + also how the spindle containing it is locked when it is not + required to move. + + Draw, half-size, the views shown in fig. 73, and from the + left-hand view project a plan. Draw also the detail of the locking + arrangement shown in fig. 74. + + + + +APPENDIX A. + +_SCIENCE AND ART DEPARTMENT, SOUTH KENSINGTON._ + + +SYLLABUS. + +SUBJECT II.--MACHINE CONSTRUCTION AND DRAWING. + +It is assumed that the student has already learnt to draw to scale, and +that he can draw two or more views of the same object in simple or +orthographic projection. To pass in machine construction and drawing, he +must be able to apply this knowledge to the representation of machinery. +He must be acquainted with the form and purpose of the simpler parts of +which machines are built up and must have had some practice in drawing +them. To test his knowledge, rough dimensioned sketches, more or less +incomplete, of simple machine details will be given him, and he will be +required to produce a complete drawing in pencil to a given scale. Two +or more views of at least one subject will be required, and these must +be so drawn as to be properly projected one from the other, _in order to +show that the student appreciates that he is producing a representation +of a solid piece of machinery, and not merely copying a sketch. No +credit will be given unless some knowledge of projection is shown._ The +centre lines of the drawings should be shown, and parts cut by planes of +section should be indicated by diagonal shading. Bolts and other +fastenings should be carefully shown where required. Any indication that +a candidate has merely copied the sketches given, without understanding +the part represented, will invalidate his examination. + + +FIRST STAGE OR ELEMENTARY COURSE. + +In the elementary stage, a knowledge is required of the simple parts +only of _machines in common use_. _Some_ of these are enumerated in the +following list. The student should be practised in drawing them till he +recognises their forms, and the object of the arrangement should be +explained to him. He should also know the simple technical terms used in +describing them. + +A few very simple questions relating to the arrangement, proportions, +and strength of the simplest machine details will be set in the +examination paper. + +In drawing the examples set to test a student's knowledge and skill in +machine drawing, it must be remembered that only a limited time is +available. It is only possible to set an example to be drawn in pencil, +and the points which will receive attention are (1) accuracy of scale +and projection; (2) power of reading a drawing, shown by the ability to +transfer portions of the mechanism and dimensions from one view to +another; (3) knowledge of machines, as shown by the ability to fill in +small details, such as nuts, keys, etc., omitted in the sketches given. +Bearing in mind the limited time available, the student should try to +make his outline clear and decisive and complete. But the diagonal lines +necessary for sectional parts may be done rapidly, though neatly, by +freehand if necessary. + +_Riveted Joints._--Forms of rivets and arrangement of rivets in lap and +butt joints with single and double riveting. Junction of plates by angle +and T-irons. + +_Bolts, Studs, and Set Screws._--Forms of these fastenings. Forms and +proportions of nuts and bolt-heads. Arrangement of flanges for bolting. + +_Pins, Keys, and Cotters._--Form of ordinary knuckle joint. Use of split +pins. Connection of parts by a key. Connection of parts by a cotter. Gib +and cotter. + +_Pipes and Cylinders._--Forms of ordinary pipes and cylinders and their +flanges and covers. + +_Shafting._--Forms of shafts and axles and of journals and pivots. Use +of collars and bosses. Half-lap coupling. Box coupling. Flange coupling. + +_Pedestals and Plummer Blocks._--Simplest forms of pedestals and hangers +for shafts. Form and arrangement of brass steps. Arrangements for +fixing pedestals and for neutralising the effects of wear. + +_Toothed Gearing._--Forms of ordinary spur and bevil wheels. Meaning of +the terms pitch, breadth of face, thickness of tooth, pitch line, rim, +nave, arm. Mode of drawing bevil wheels in section. + +_Belt Pulleys._--Forms of belt pulleys for flat and round belts. Stepped +speed cones. Drawing of pulleys with curved arms. + +_Cranks and Levers._--Forms of ordinary cast-iron and wrought-iron +cranks and levers. Modes of fixing crank pin. Modes of fixing crank +shaft. Double cranks. Form of eccentrics. + +_Links._--Most simple forms of connecting rod ends, open or closed. Use +of steps in connecting rods. Use of cotters to tighten the steps. + +_Pistons._--Simple forms of piston. Use of piston packing. Modes of +attaching piston rod. + +_Stuffing-Boxes._--Simple form of stuffing-box and gland. Use of +packing. Mode of tightening gland. + +_Valves._--Simple conical of puppet valve. Simple slide valve. Cock or +conical sliding valve. + + + + +APPENDIX B. + +_EXAMINATION PAPERS SET BY THE SCIENCE AND ART DEPARTMENT._ + + +SUBJECT II.--MACHINE CONSTRUCTION AND DRAWING. + +_Examiners_, PROF. T. A. HEARSON, M.Inst.C.E., and J. HARRISON, ESQ., +M.Inst.M.E. + +GENERAL INSTRUCTIONS. + +_If the rules are not attended to, the paper will be cancelled._ + +You may take the Elementary, or the Advanced, or the Honours paper, but +you must confine yourself to one of them. + +Put the number of the question before your answer. + +You are expected to prove your knowledge of machinery as well as your +power of drawing neatly to scale. You are therefore to supply details +omitted in the sketches, to fill in parts left incomplete, and to +indicate, by diagonal lines, parts cut by planes of section. + +No credit will be given unless some knowledge of projection is shown, so +that at least two views of one of the examples will be required properly +projected one from the other. The centre lines should be clearly drawn. +The figured dimensions need not be inserted. + +Your answers should be clearly and cleanly drawn in pencil. No extra +marks will be allowed for inking in. + +All figures must be drawn on the single sheet of paper supplied, for no +second sheet will be allowed. + +The value attached to each question is shown in brackets after the +question. But a full and correct answer to an easy question will in all +cases secure a larger number of marks than an incomplete or inexact +answer to a more difficult one. + +Your name is not given to the Examiner, and you are forbidden to write +to him about your answers. + +You are to confine your answers _strictly_ to the questions proposed. + +A single accent (') signifies _feet_; a double accent (") _inches_. + +_The examination in this subject lasts for four hours._ + + * * * * * + +First Stage or Elementary Examination. 1885. + +INSTRUCTIONS. + +Read the General Instructions above. + +Answer briefly any three, but not more than three, of the following +questions, and draw two, but not more than two, of the examples. + +_Questions._ + + (_a._) Show two methods by which a cotter may be prevented from + slacking back. (6.) + + (_b._) Sketch the brasses for a bearing, and show how they are + prevented from turning in the pedestal. (6.) + + (_c._) Explain the object of the construction of the connecting rod + end shown in fig. 78. Describe how the adjustment must be made and + how it is locked. (10.) + + (_d._) Show the form of the Whitworth screw thread by drawing to + scale a part section of two or three threads taking a pitch of + 1-1/2 inches. Figure the dimensions on the sketch. How many threads + to the inch are used on an inch bolt? (10.) + + (_e._) Make a sketch showing how the adjustment is made in the + sliding parts of machine tools: as, for example, in the slide rest + of a lathe. (10.) + + (_f._) Describe with sketches two methods by which the joints are + made in connecting lengths of cast-iron pipes. (6.) + +_Examples to be drawn._ + + 1. Jaw for four-screw dog chuck for 5" lathe. Draw the two views as + shown (fig. 77). Scale full size. + + (Note.--The other three jaws of the chuck are not to be drawn.) + (35.) + + 2. Connecting rod end. Draw the two views as shown, partly in + section (fig. 78). Draw full size. (35.) + + 3. Hooke's coupling. Draw the three views shown (fig. 79), adding + any omitted lines where the views are incomplete. Draw to scale of + 1/4 full size. (35.) + +[Illustration: FIGS. 77 AND 78.] + +[Illustration: FIG. 79.] + + * * * * * + +First Stage or Elementary Examination. 1886. + +INSTRUCTIONS. + +Read the General Instructions (page 102). + +Answer briefly any three, but not more than three, of the following +questions, and draw two, but not more than two, of the examples. + +_Questions._ + + (_a._) Give sketches showing how the cutting tool of a lathe or + other machine is secured in place. (6.) + + (_b._) Make a sketch of a stud, describe how it is screwed into + place, and state some circumstances under which it is used in + preference to a bolt. (6.) + + (_c._) Give sketches showing one method of attaching the valve rod + to an ordinary slide valve. (6.) + + (_d._) Sketch a connecting rod end, with strap, gib, and cotter. + Explain the use of the gib. (10.) + + (_e._) Explain the use of the quadrant for change wheels for a + screw-cutting lathe shown in Example 1, fig. 80, by making a + sketch showing it in place on a lathe with wheels in gear. (10.) + + (_f._) Sketch one form of hanger suitable for supporting mill-shafting. + (10.) + +_Examples to be drawn._ + + 1. Quadrant for change wheels for screw-cutting lathe. Draw the two + views shown (fig. 80). Scale half-size. (35.) + + 2. Crank-shaft. Draw the two views as shown, partly in section (fig + 81). Scale 1/8 full size. (35.) + + 3. Ball bearing for tricycle. Draw the two views as shown, partly + in section (fig. 82). Draw full size. (35.) + +[Illustration: FIGS. 80 AND 81.] + +[Illustration: FIG. 82.] + + * * * * * + +First Stage or Elementary Examination. 1887. + +INSTRUCTIONS. + +Read the General Instructions (page 102). + +Answer briefly any three, but not more than three, of the following +questions, and draw two, but not more than two, of the examples. + +_Questions._ + + (_a._) Explain how the piston rings in Example 1, fig. 84, are made + so that the piston may work steam-tight in the cylinder. How are + these rings got into place? (8.) + + (_b._) Give two views of a double riveted lap joint for boiler-plates. + (8.) + + (_c._) Show by sketches how a wheel is fixed on a shaft by means of + a sunk key. Explain how the key may be withdrawn when it cannot be + driven from the point end. (8.) + + (_d._) Give sketches showing the construction of a conical metal + lift or puppet valve and seating. (10.) + + (_e._) With the aid of sketches explain how a piston rod is made to + work steam-tight through the end of the cylinder. (10.) + + (_f._) Explain how the slotting machine ram of Example 8, fig. 85, + may be made to move up and down when at work. How is the length of + the stroke altered, and what is the object of the slotway in the + upper part of the ram? (10.) + +_Examples to be drawn._ + + 1. Piston for steam-engine. Draw and complete the two views shown + (fig. 84), the top half of the left-hand view to be in section. + Scale 1/2 size. (30.) + + 2. Plan and sectional elevation of a footstep bearing for an + upright shaft (fig. 83). Draw and complete these views. Scale + 1/4 size. (35.) + + 3. Ram of slotting machine. Draw and complete the two elevations + shown (fig. 85). The tool-holders must be drawn in their proper + positions in the ram, and not separate as in the diagram. Scale + 1/4 size. (35.) + +[Illustration: FIGS. 83 AND 84.] + +[Illustration: FIG. 85.] + + * * * * * + +First Stage or Elementary Examination. 1888. + +INSTRUCTIONS. + +Read the General Instructions on p. 102. + +Answer briefly any three, but not more than three, of the following +questions, and draw two, but not more than two, of the examples. + +_Questions._ + + (_a._) Give sketches showing how the separate lengths of a line of + shafting may be connected together. (8.) + + (_b._) What is the object of using chipping or facing strips in + fitting up machine parts? Give one or two examples. (8.) + + (_c._) Give sketches showing how you would grip and drive a round + iron bar for the purpose of turning it between the centres of a + lathe. (10.) + + (_d._) Explain the action of the governor shown in Example 1 + (fig. 86). (10.) + + (_e._) Describe in detail how the mud-hole door in Example 2 + (fig. 88) is removed for the purpose of cleaning the boiler and + how it is replaced and the joint made steam-tight. (10.) + + (_f._) Describe how the parts of the spur wheel in Example 3 + (fig. 87) are put together, and explain why the wheel is made + in segments. (10.) + +_Examples to be drawn._ + + 1. Loaded governor for small gas engine. Draw and complete the two + views, partly in section as shown (fig. 86). Scale full size. (35.) + + 2. Mud-hole mouth-piece for Lancashire boiler. Draw and complete + the two views shown (fig. 88). Scale 3/8ths. (35.) + + 3. Point for segments of large spur wheel. Draw and complete the + views shown (fig. 87). Scale 3/16ths. + + _Note._--As the radius of the wheel is too large for your + instruments, the circumference at the joint may be set out + straight, as in a rack. (35.) + +[Illustration: FIGS. 86 AND 87.] + +[Illustration: FIG. 88.] + + + + +INDEX + + + Air-pump bucket, 58 + Alloys, 80 + Angle irons, 12 + Annealing, 79, 80 + Axles, 24 + + + Back stay for lathe, 86 + Bar stay, 83 + Bearings for shafts, 30 + Beech-wood, 81 + Bell crank lever, 86 + Bessemer steel, 79 + Bevil wheels, 43 + Blister steel, 79 + Blooms, 78 + Bolt-heads, proportions of, 18 + Bolts, forms of, 17 + Border lines, 4 + Box couplings, 25 + -- end, connecting rod, 51 + Box-wood, 81 + Brackets, 33 + Brake shaft carrier, 30 + Brass, 80 + Brasses, 30 + Bucket, 58 + Built-up cranks, 46 + Bush, 30, 49, 51, 54, 56, 63 + Butt joints, 10, 11 + -- strap, 10 + Buttress screw thread, 15 + + + Case-hardening, 80 + Cast iron, 76 + Cast iron flange coupling, 28, 29 + -- steel, 79 + Caulking, 8 + Cementation process, 79 + Centre lines, 2, 4 + Chilled castings, 78 + Circulating pump piston, 58 + Clearance, cylinder, 74 + -- of cotter, 49 + Cocks, 74 + Cogs, 41 + -- wood for, 81 + Collared stud, 18 + Collars, 24 + Colouring, 3 + Colours for different materials, 3 + Compasses, 1 + Cone keys, 23, 38 + Conical disc valve, 70, 71, 89 + -- head, 7 + Connecting rod, locomotive, 51 + -- -- marine, 51 + -- rods, 49, 89 + Construction for rivet heads, 7 + Contraction of castings, 77 + Copper, 80 + Cotters, 48, 49 + Countersunk head, 7, 18 + Coupling rod ends, 55, 84 + -- rods, 54 + -- screw, 96 + Couplings, shaft, 25 + Cover plate, 10 + Cranked axle, 45 + Cranks, 43 + -- built-up, 46 + Cross-head pin, 51 + Cross-heads, 56, 89 + Cross-key, 28 + Cup-headed bolt, 17 + + + Decimal equivalents, 6 + Dimension lines, 5 + Dimensions, 5 + -- of box couplings, 26 + -- cast-iron flange couplings, 29 + -- keys, 24 + -- stuffing-boxes and glands, 67 + -- Whitworth screws, 15 + Distance lines, 5 + Dividers, 1 + Draught of cotter, 49 + Drawing board, 1 + -- instruments, 1 + -- paper, 2 + -- pen, 1 + -- pins, 2 + + + Eccentrics, 47 + Exhaust port, 71 + Eye-bolt, 18 + + + Fairbairn's coupling, 26 + Fast and loose pulleys, 37 + Feather key, 23 + Flange couplings, 27 + Flap valves, 68 + Flat key, 22 + Forge irons, 77 + Forging, 79 + Form of wheel teeth, 40 + Forms of nuts, 16 + -- rivet heads, 7 + -- screw threads, 15 + Foundry irons, 77 + + + Gasket, 58 + Gas threads, 15 + Gib, 49 + -- head, 23 + Girder stay, 81 + Gland, 64 + Grey cast iron, 77 + Gun-metal, 80 + Gusset stay, 12 + + + Half-lap coupling, 26 + Hangers, 34 + Hardening of steel, 80 + Headstock lathe, 96 + Hexagonal nut, 16 + Hollow key, 22 + Hook bolt, 18 + Hornbeam, 81 + + + India-rubber disc valves, 58, 68 + Inking drawings, 2 + Inside lap of valve, 72 + + + Joggles, 33 + Joint, knuckle, 84 + Journals, 24 + -- length of, 32 + Junk ring, 61 + + + Keys, 22 + -- proportions of, 23 + Kinghorn's metallic valve, 70 + Knuckle joint, 84 + -- screw thread, 15 + + + Lancaster's piston packing, 61 + Lap joints, 8, 9, 10, 12 + -- of slide valve, 72 + Lathe headstock, 96 + Lead of valve, 74 + Lever, bell crank, 86 + -- ratchet, 96 + Lignum-vitć, 81 + Locking arrangements for nuts, 21, 62 + Lock nuts, 19 + Locomotive connecting rod, 51 + -- cranked axle, 45 + -- cross-head, 56 + Locomotive eccentric, 47 + -- piston, 60 + Lubricator, needle, 32 + + + Malleable cast iron, 78 + -- iron, 78 + Marine connecting rod, 51 + -- coupling, 28 + -- crank-shaft, 46 + -- piston, 61 + Merchant bars, 78 + Mortise wheels, 41 + Mottled cast iron, 77 + Muff couplings, 25 + Muntz metal, 80 + + + Needle lubricator, 32 + Nuts, forms of, 16 + -- lock, 19 + -- proportions of, 18 + + + Oil-box, 54, 65 + Outside lap of slide valve, 72 + Overhung crank, 43 + -- cranks, proportions of, 45 + + + Packing, 63 + Pan head, 7 + Pedestal, shaft, 30 + Pencils, drawing, 1 + Phosphor bronze, 80 + Pillar bracket, 34 + Pillow block, 30, 32 + Pin, cross-head, 51, 54 + -- split, 21 + Piston rod, 57 + Pistons, 57 + Pitch circle, 40 + -- of wheel teeth, 40 + -- surfaces of wheels, 39, 43 + Pivots, 24 + Plummer block, 30 + Plunger, 57 + Printing, 4 + Proportions of bolt-heads, 18 + -- keys, 23 + Proportions of lap joints, 9, 10 + -- marine engine pistons, 62 + -- nuts, 18 + -- overhung cranks, 45 + -- pillow blocks, 32 + -- rivet heads, 7 + -- wheel teeth, 40 + Puddled bars, 78 + Puddling process, 78 + Pulley, eccentric, 47 + Pulleys, 36 + Pump bucket, 58 + + + Ramsbottom's packing, 60 + Ratchet lever, 96 + Riveted joints, 8 + Rivet heads, forms of, 7, 8 + -- -- proportions of, 7 + Riveting, 7 + Rivets, 6 + Rope pulley, 39 + Round key, 23 + + + Saddle key, 22 + Scales, 5 + Screw coupling, 96 + Screwed gland and stuffing-box, 65 + Screw threads, 14, 15 + Screws, representation of, 16 + Sellers =V= screw thread, 14 + Set screw, 21, 49 + -- squares, 1 + Shaft couplings, 25 + -- hanger, 34 + Shafting, 24 + Shear steel, 79 + Sheave, eccentric, 47 + Shingling, 78 + Shrinking, process of, 44 + Siemens-Martin steel, 79 + Slide blocks, 56 + -- valves, 68, 71 + Sliding key, 23 + Snap head, 7 + Snug, 17 + Spiegeleisen, 80 + Spring bows, 1 + Spur wheel, 41 + Square nut, 16 + -- screw thread, 14 + Stay, back, for lathe, 86 + -- bar, 83 + -- girder, 81 + -- gusset, 12 + Steam ports, 71 + -- whistle, 96 + Steel, 79 + Steps, 30 + Strap, 49 + -- eccentric, 47 + -- end of connecting rod, 49 + Stud bolts, 18 + Studs, 18 + Stuffing-boxes, 63 + Sunk key, 22 + + + Taper bolt, 18, 27 + -- pin, 23 + Tee-headed bolt, 18 + Tee-iron cover strap, 81 + Tee square, 1 + Teeth of wheels, form and proportions of, 40 + Teeth, pitch of, 40 + Tempering of steel, 80 + Throw of crank, 44 + -- eccentric, 47 + Toothed wheels, 39 + + + Valve Kinghorn's metallic, 70 + -- slide, 68, 71 + Valves, 68 + -- conical disc, 70 + -- india-rubber, 58, 68 + Velocity ratio in belt gearing, 36 + + + Wall boxes, 34 + Washers, 19 + Welding, 79 + Whistle, steam, 96 + White cast iron, 77 + Whitworth screws, dimensions of, 15 + -- =V= screw thread, 14 + Wood, 81 + Working drawings, 4 + Wrought iron, 78 + + + Yellow pine, 81 + + +PRINTED BY + +SPOTTISWOODE AND CO., 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