CAM MECHANISMS

8.1 Cams, Cam Types and Classification of Cams

Simple cam mechanisms are 3-link kinematic chains with one cam pair. The other two kinematic pairs can either be revolute or prismatic pair. Hence, we can have 3 kinematic chains with joints R-C-R, R-C-P, P-C-P and 7 different simple cam mechanisms. Both of the kinematic elements that form the cam pair can be of a complex shape, but usually one of the kinematic elements is of a simple shape like a straight line or a circle. In order to reduce the friction and to convert sliding friction to rolling, a roller is attached. The link (which is usually the driven link) containing the simple kinematic element is the follower and the complex curved link (cam) is the driving input link. The resulting mechanisms are schematically shown below

Classification of cams in terms of their kinematic structure as shown was not found sufficient in practice. Several different classifications are used.

According the shape cams are radial, face, wedge, cylindrical, spherical, helical, conical, 3 dimensional ,etc.

Cam pairs are classified as force closed or form closed. Force closed cam pairs are more common since it is expensive to manufacture form closed cams.
In form closed cams theoretically the cam and the follower is in contact at two points and there is no need for an external force to keep the surfaces in contact. However, for the system to operate there must be some clearence between the roller and the slot in which it operates. Hence the contact will be at one point and it will change when the direction of force changes.

Followers, in general, are classified in one of two ways:
1. The construction of the surface in contact (roller, flat faced, cylindrical, spherical)
2. The type of movement (translating or oscillating)
The translating follower is also classified as radial or off-set according to the location of movement with respect to the cam rotation axis.

In describing a cam mechanism one must use as many of these classifications as necessary for the clear understanding, such as: force closed radial cam with translating flat faced follower, (.a), form closed wedge cam with translating roller follower (.b) or form closed cylindrical cam with translating follower (c).