A cam is an applied form of the ordinary wedge. The simple wedge is used to split apart the piece into which it enters, or to pry up heavy weights. It does not automatically repeat its work. The work of the wedge is finished when it is once driven home; and its function is not to produce motion, but to give mechanical advantage to the blows which drive it.
If we take a simple wedge and fasten it to some piece - say, a disk centered on a shaft, which is capable of continuous or periodic rotation, and allow the face of the wedge to rub against another guided piece, called the follower, we have a cam. In Fig. 76 is shown a double wedge in which either sloping side produces against the follower the action just referred to. When we rotate the cam, it "wedges" the follower along a fixed path. When the follower reaches the top of the wedge, it may drop back to its original position, drawn by gravity or by the force of a spring, or it may be eased back by another wedge in the reverse position of the other, as shown in Fig. 76. This cycle will be repeated as long as we choose to rotate the cam. The cam, therefore, is essentially a repeating wedge, and its function is primarily one of motion rather than of great force. The wedge principle, however, enables very powerful cams to be made in cases where but little motion is desired. The motion of the cam is usually a rotation, but it may be an oscillation, or a straight-line reciprocating motion.
Fig. 74. Conventional Drawing for U. S. Standard Thread.
Fig. 75. Conventional Drawing for Square Thread.
In designing a cam, it is not only essential that the proper layout be made to produce, theoretically, the required motion of the follower, but that the wedge action be such that the cam will drive easily. Referring again to the wedge as a machine, a thin wedge, for example, may be forced under a heavy weight with a sharp blow, whereas a thick broad wedge cannot be made to lift the weight. For precisely the same reasons, cams designed with thin wedges will drive their followers with ease, while cams may be designed so steep in their wedge action that they drive with difficulty, and may even lock the follower in its path, on account of excessive side pressure. This is a very important element in a design, and the analysis will be brought out more clearly in the discussions of the line of pressure.
The actual laying out of a cam is simple in principle, although somewhat tedious, especially in complicated cams. Several positions of the follower in its path are chosen, the follower drawn in those positions, and then the face of the cam is drawn tangent thereto. In order to do this, the several positions of the follower in its path may be laid down on the drawing paper as shown in Fig. 77. This fixes the limit through which the cam must move the follower. Now develop the cam itself on a separate piece of transparent paper or tracing cloth, Fig. 78, and place this over the follower layout with the centers of the two drawings coinciding, and a pin through this center of rotation of the cam. As the cam is rotated about this axis so as to correspond to the various positions of the follower shown on the drawing underneath, the follower can be traced in on the upper drawing in each position.
Fig. 76. Simple Diagram of a Cam.
When the movement of the cam is complete, we have on the cloth a series of drawings of the follower; and, if we draw a tangent line to these, the line of the cam will be produced. This method, however, from the drawing-board standpoint, is clumsy and inaccurate, because of the wearing of the pinhole and the error of transferring the shape of the follower to the tracing cloth. It is readily seen by reference to Figs. 77 and 78, that the same result will be attained in a much easier and more accurate manner if we artificially rotate the follower about the cam on the drawing-board by means of a pair of compasses, laying off equal angles to complete one revolution of the cam and showing the follower in position at each step. This is shown in Fig. 81, and is the method usually followed in cam design. This does not mean that the tracing-cloth method should be discarded, for it is useful in studying complicated cam movements; and also in testing the cam development when it is completed, to make sure that no error has been made.
Fig. 77. Layout for Positions of Cam Follower.
Fig. 78. Layout for Cam.