This little volume is intended primarily as a class-room text book on the subject of elementary machine design. It is the result of the author's experience, extending over more than fifteen years, both in practical design and in teaching of this subject. The pre-requisites on the part of the student are thorough courses in machine drawing and elementary mechanics.

Based on the brief outline of the strength of materials given in Chapter I the author has attempted to develop the equations for the design of the more common machine elements. This has generally been done very concisely and frequently only an outline of the deduction has been given. Empirical formulae and rule of thumb methods, so much used in elementary texts on this subject, have been avoided as far as possible. There are many factors which in practice affect the design of machine parts that cannot be discussed profitably in the class-room; for instance, cost of construction, capacity of shop machinery, etc. For this reason the teacher must be content if the student acquires the power to analyze the forces and the resultant stresses in machine parts and to apply the proper equations for their design.

The illustrations have been carefully chosen to show typical constructions rather than a great variety which tend to confuse the inexperienced student. The standard text books as well as the American and European technical press have been freely consulted in the preparation of the manuscript.


CHAPTER VII - COUPLINGS AND CLUTCHES

Couplings. When a long line of shafting is to be installed it is necessary to connect the ends of the lengths which make up this line, as the usual length of a piece of shafting is 20 ft. to 25 ft. The machine part used is called a shaft coupling. There are several different kinds of couplings used for this purpose.

The Sellers coupling (Fig. 7-4), drives by means of friction between the conical bushings and the shaft. These bushings are split lengthwise and the bolts tighten them around the shaft, due to an equal taper on the inside of sleeve. The length L may be 3d+2 in. and the outside diameter D is 2d+2 in.

The universal coupling shown in Fig. 7-5 is used to connect two shafts which meet at an angle or where some flexibility is necessary as in motor cars. It should be remembered that with one coupling connecting two shafts which meet at an angle the transmission of motion will not be uniform and to obtain such motion of the driven shaft two couplings must be used.

A flexible coupling is advantageous where two shafts are to be coupled and accurate alignment cannot be maintained, or if a prime mover, such as an electric motor, is coupled directly to a shaft. Figs. 7-6 and 7-7 show two types of this coupling. In the first the connection is a band of leather or cotton, while the latter has a number of leather links connecting the two halves of coupling. The outside diameter D may be made 5d, the length of hubs L is 1 3/4d, and width w of band or links is d.

Clutches. Couplings which may be readily disengaged are usually termed clutches. These are used not only to connect two shafts but also to couple pulleys, gears, sprocket wheels, hoisting drums and similar machine parts to their shafts. The variations in construction are very numerous. They may, however, be divided into two classes, positive clutches and friction clutches, accordingly as the motion is transmitted by positive contact pressure or by friction between surfaces..

Positive Clutches. The jaw clutch shown in Figs. 7-8 and 7-8a is the commonest type of positive clutch. One part of this clutch is keyed tightly to its shaft while the other slides on a feather key and therefore can be moved axially until its teeth engage with those of the other half. Fig. 7-8 shows a clutch capable of transmitting rotation in both directions, while that shown in Fig. 7-8a can do so in one direction only. Such clutches can be thrown into or out of action only at very low speeds. They are frequently used in conveying machinery. The sliding member for large clutches should have two feather keys placed 180 apart, otherwise it is difficult to move same along the shaft.

Pin clutches are of the positive type in which the motion is transmitted by means of a steel pin. They are largely used in punching and shearing machinery.

Friction Clutches. These may be divided into cone, disc, cylindrical and coil clutches, according to the surfaces in contact. Figs. 7-9 to 7-12 show examples of each of these types. In all the motion is transmitted from the driving part to the driven by means of friction between two or more surfaces. These surfaces are pressed together with sufficient force so that the friction between them will overcome the resistance to motion offered by the driven member. The contact surfaces may be steel, cast iron, bronze, brass, wood, cork, leather or fiber.