Chapter XI: Cams and Eccentrics

More or less confusion arises from the terms _cams_ and _eccentrics_. A cam is a wheel which may be either regular in shape, like a _heart-wheel_, or irregular, like a _wiper-wheel_.

The object in all forms of cams is to change motion from a regular into an irregular, or reversely, and the motion may be accelerated or retarded at certain points, or inverted into an intermittent or reciprocating movement, dependent on the shape of the cam.

A cam may be in the shape of a slotted or grooved plate, like the needle bar of a sewing machine, where a crank pin works in the slot, and this transmits an irregular vertical movement to the needle.

A cam may have its edge provided with teeth, which engage with the teeth of the engaging wheel, and thus impart, not only an irregular motion but also a turning movement, such forms being largely used to give a quickly rising or falling motion.

What are called _wiper-wheels_ are designed to give an abrupt motion and such types are used in trip hammers, and to operate stamp mills. In harvesters, printing presses, sewing machines, and mechanism of that type, the cam is used in a variety of forms, some of them very ingenious and complicated.

Cams are also used for cutting machines, or in tracing apparatus where it would be impossible to use ordinary mechanism. All such forms are special, requiring care and study to make their movements co-relate with the other parts of the mechanism that they are connected up with.

Simple Cams.--Fig. 88 shows a form of the most simple character, used, with some modifications, to a larger extent than any other. It is called the _heart-shaped_ cam, and is the regular type.

Fig. 89 is an elliptical cam, which is also regular. What is meant by _regular_ is a form that is the same in each half portion of its rotation.

Fig. 90 is a double elliptic, which gives a regular movement double the number of times of that produced by the preceding figure, and the differences between the measurements across the major and minor axes may vary, relatively, to any extent.

Wiper Wheels.--Wiper wheels are cams which give a quick motion to mechanism, the most common form being the single wiper, as shown in Fig. 91.

The double wiper cam, Fig. 92, has, in some mechanism, a pronounced difference between the lengths of the two fingers which form the wipers.

The form of cam shown in Fig. 93 is one much used in iron works for setting in motion the tilt hammer. Only three fingers are shown, and by enlarging the cam at least a dozen of these projecting points may be employed.

Cam Sectors.--Fig. 94 shows a type of cam which is designed for rock shafts. The object of this form of cam is to impart a gradually increasing motion to a shaft. Assuming that A is the driving shaft, and B the driven shaft, the cam C, with its short end D, in contact with the long end E of the sector F, causes the shaft B to travel at a more accelerated speed as the other edges G, H, approach each other.

Cylinder Cam.--Fig. 95 shows one form of cylinder A with a groove B in it, which serves as a means for moving a bar C back and forth. The bar has a projecting pin D, which travels in the groove.

This form of movement may be modified in many ways, as for instance in Fig. 96, where the drum E has a sinuous groove F to reciprocate a bar G to and fro, the groove being either regular, so as to give a continuous back and forth movement of the bar; or adapted to give an irregular motion to the bar.

Double Cam Motion.--Cams may also be so arranged that a single one will produce motions in different directions successively, as illustrated in Fig. 97. The horizontal bar A, hinged at B to the upper end of a link C, has its free end resting on the cam D.

The arm A has also a right-angled arm E extending downwardly, and is kept in contact with the cam by means of a spring F. Connecting rods G, H, may be hinged to the arm E and bar A, respectively, so as to give motion to them in opposite directions as the cam revolves.

Eccentrics.--An eccentric is one in which the cam or wheel itself is circular in form, but is mounted on a shaft out of its true center. An eccentric may be a cam, but a cam is not always eccentric in its shape. The term is one in direct contrast with the word _eccentric_.

Fig. 98 shows the wheel, or the cam, which is regular in outline, that is circular in form, but is mounted on the shaft out of its true center. In this case it is properly called an eccentric cam but in enginery parlance it is known as the eccentric, as represented in Fig. 99.

Triangularly-Formed Eccentric.--Fig. 100 illustrates a form of cam which has been used on engines. The yoke A being integral with the bar B, gives a reciprocating motion to the latter, and the triangular form of the cam C, which is mounted on the shaft D, makes a stop motion at each half-revolution, then produces a quick motion, and a slight stop only, at the half turn, and the return is then as sudden as the motion in the other direction.