Cams in general, are divided into two classes: uniform motion cams and uniformly accelerated motion cams. The uniform motion cam moves the follower at the same rate of speed from the beginning to the end of the stroke; but as the movement is started from zero to the full speed of the uniform motion and stops in the same abrupt way, there is a distinct shock at the beginning and end of the stroke, if the movement is at all rapid.
In machinery working at a high rate of speed, therefore, it is important that cams are constructed that sudden shocks are avoided when starting the motion or when reversing the direction of motion of the follower. The cam best suited for high speeds is one where the speed at first is slow and then accelerated at a uniform rate until the maximum speed is reached. The speed is then again uniformly slowed until the rate of motion of the follower is zero or nearly zero when the reversal takes place.
A cam constructed along these lines is called a uniformly accelerated motion cam. The distances which the follower passes through during equal periods of time increase in the same ratio as the distances passed through in consecutive seconds by a freely falling body acted upon by gravity alone. A cam constructed on these lines, therefore, is often called a gravity-curve cam.