Fig. 229.
Fig. 230.
Fig. 231.
If we were to cut out a cam to the outline thus obtained, and revolve it at a uniform velocity, it would move a point held against its perimeter at a uniform velocity throughout the whole of the cam revolution. But such a point would rapidly become worn away and dulled, which would, as the point broadened, vary the motion imparted to it, as will be seen presently. To avoid this wear a roller is used in place of a point, and the diameter of the roller affects the action of the cam, causing it to accelerate the cam action at one and retard it at another part of the cam revolution, hence the pitch line obtained by the process in [Fig. 225] represents the path of the centre of the roller, and from this pitch line we may mark out the actual cam by the construction shown in [Fig. 226]. A pair of compasses are set to the radius of the roller r, and from points (such as at a, b, e, f), as the pitch line, arcs of circles are struck, and a line drawn to just meet the crowns of these arcs will give the outline of the actual cam. The motion of the roller, however, in approaching and receding from the cam centre c, must be in a straight line g g that passes through the centre c of the cam. Suppose, for example, that instead of the roller lifting and falling in the line g g its arm is horizontal, as in [Fig. 227], and that this arm being pivoted the roller moves in an arc of a circle as d d, and the motion imparted to the arm will no longer be uniform. Furthermore, different diameters of roller require different forms of cam to accomplish the same motion, or, in other words, with a given cam the action will vary with different diameters of roller. Suppose, for example, that in [Fig. 228] we have a cam that is to operate a roller along the line a a, and that b represents a large and c a small roller, and with the cam in the position shown in the figure, c will have contact with the cam edge at point d, while b will have contact at the point e, and it follows that on account of the enlarged diameter of roller b over roller c, its action is at this point quicker under a given amount of cam motion, which has occurred because the point of contact has advanced upon the roller surface—rolling along it, as it were. In [Fig. 229] we find that as the cam moves forward this action continues on both the large and the small roller, its effect being greater upon the large than upon the small one, and as this rolling motion of the point of contact evidently occurs easily, a quick roller motion is obtained without shock or vibration. Continuing the cam motion, we find in [Fig. 230] that the point of contact is receding toward the line of motion on the large roller and advancing upon the small one, while in [Fig. 231] the two have contact at about the same point, the forward motion being about completed.
