In a recent Memorandum on the Teaching of Engineering in Evening Technical Schools, the Board of Education, in the sections dealing with Engineering Drawing, directs the attention of teachers and students to the great importance of "Machine Design."

During the student's First Year's Course, the memorandum points out, the weights of objects of simple design should be estimated; also, it is useful, wherever possible, to introduce simple numerical exercises. Simple designs, for example, such as a mild steel eye bolt to carry a snatch block might be set; or the student might determine the diameter of two long bolts to be used with a 20-ton hydraulic jack to pull a flywheel off a shaft; or determine the diameter of the four bolts supporting the top crosshead of a hydraulic press.

In the second year, students might design a cotter connection for a steel tie bar; estimate the strength of bolts in flanges and cylinder covers; find the stresses in cotters connecting crossheads to piston rods, and so on.

If the student is also studying Practical Mathematics, the sizes and proportions of the machine details he is drawing afford excellent scope for exercises. Thus, with given data he can find the probable stresses in the various parts; later, he can proceed to carry out the design of some important machine detail or some machine, such as a winch, or a crane. He can also make working drawings to suitable scales when all the dimensions are obtained.

In the following pages an attempt is made to provide exercises of this kind. As a rule only those details are referred to which admit of numerical calculation. The design of details which depend on empirical data alone are probably most easily obtained from one of the numerous "Pocket books" dealing with the subject. Fully dimensioned drawings may be found in the author's Machine Construction and Drawing (Macmillan). The scheme of work outlined in the following pages is that used for several years by the writer, but teachers and others may take the subject in any order they deem best.

The exercises are numerous and have been arranged to correspond with the usual draughtsman's accuracy; the dimensions are to the nearest sixteenth of an inch, or to the accuracy obtain- able by four-figure logarithms. The exercises throughout have been checked carefully, and it is hoped that no serious errors will be found by those using the book.

Without making the book unduly large, it is not possible to deal fully with the important subject of "Compound Stress"; but students who desire to pursue the subject further should consult Applied Mechanics for Engineers, by J. Duncan (Macmillan). Proofs of some of the more important portions, which have a direct bearing on Machine Design, are included. In other cases, the results alone are given, together with applications of them; necessary proofs of them may be obtained from any of the numerous books on Strength of Materials.

Elementary details - for example, the forms and proportions of rivet heads, bolts, screws, and such data - are not included; they may be obtained from the author's Machine Construction and Drawing.

Shaft couplings. For convenience in manufacture and in handling, the shafts used in factories and for similar purposes are usually made in lengths of from 20 to 30 ft. long. These separate lengths are fastened together by means of cast-iron couplings, secured to the shafts by means of keys. Exceptions occur in marine shafts, in which the couplings are in one piece with the shafts.

Box or muff coupling. A box or muff coupling, shown in Fig. 92, may be used to connect two lengths of shafting. The coupling may be fastened by means of a key, which extends the full length of the coupling. It is much better to use two keys, as in Fig. 92. With this arrangement it is not necessary that the depth of the keyway in each shaft shall be the same. In addition, two keys may be secured more tightly than a single long key, and when a space is left between them as shown, one key can be released and used §,8 a driver to release the other. The ends of the shafts may be enlarged to avoid weakening the shafts by cutting the keyways.

Flange couplings. A flange or face plate coupling (except in the case of marine shafts) is usually made of cast iron. The halves of the couplings are bored to fit the shafts, the keyways out and the holes made for the bolts. The two parts are then keyed to the shafts, the head of the key being at the end of the shaft. Rotation of the bolts during the process of screwing home the nuts is prevented by means of a small snug, or pin, inserted close to the head of the bolt. One coupling is made to enter a short distance 1/4 in. or 3/4 in. into the other. This plan ensures that the couplings are in line with each other (Pig. 93). The proportional unit is D, the diameter of the shaft.

When the torque, or horse-power, transmitted by a shaft is known, the diameter of the shaft can be obtained by calculation (p. 131). The diameter of the coupling bolts can also be found (pp. 143, 144) ; when these are known the dimensions of the remaining parts of the coupling are obtainable from the proportional dimensions (Fig. 93). If the ends of the shafts are enlarged, the proportional values (Fig. 92) may be used.

To avoid any mischance due to projecting bolts, heads and nuts becoming entangled with clothing, the flanges are in some cases recessed, as in Fig. 94. The flanges are slightly thicker than those in Fig. 93; the proportions are given in Fig. 94, the proportional unit being d, the diameter of the bolt.

Couplings for marine shafts. In the shafts for marine engines, the flanges are usually forged in one piece with, and form part of, the shaft. The separate couplings are secured by bolts and are usually filleted into one another, as in Fig. 93.

The following table gives the dimensions in inches of some couplings taken from actual practice:

In marine couplings the number of bolts usually are 6, 8, 9 or 1 2. In the case of large shafts these bolts are taper, the taper being 3/8 in. per foot of length. Also the bolts are sometimes made with, but often without, heads.

In marine engine practice, there is no fixed rule for the design of a flange coupling; an examination of the couplings in use also fails to give any satisfactory rule. It is necessary to obtain the diameter of the coupling bolts before the dimensions of the coupling can be found, and for this purpose the radius of the bolt circle may be taken equal to 0-8D, where D is the diameter of the shaft.

As the bolts fit tightly into the bolt holes and the flanges are forged solid with the shafts, a projecting piece on one coupling fitted into a corresponding recess on the other is not necessary.

Hooke's joint or universal coupling. When the axes of two shafts are inclined to one another, a form of coupling known as Hooke's Joint or coupling may be used. One advantage in the use of this coupling is that the angle between the axes of the shafts may be altered whilst the shafts are in motion. Thus, in the mechanism of a motor-car, universal joints or couplings must be introduced between the gear box and the back axle. In this manner the driving shaft can be out of line in any direction without interfering in any way with the transmission of the rotation from the gear box to the back axle. The shafts A and B are forked at the ends as shown in Fig. 97. The joint is made by two pins P and P of equal size; these allow the forked ends to turn freely about their axes.