SEED Unit Design - Cam Mechanisms < >

Cam mechanisms are used to convert a simple input motion, such as a shaft rotating at nominally "constant" speed, into a complex output motion. The output motion from cam mechanisms is versatile as the displacement, velocity and acceleration are all controlled accurately; it can also include stationary periods. Each movement can be specified independently of the remainder of the operating sequence. Besides the familiar application of the cam-operated valves of petrol engines the mechanism is used in textile manufacture, automatic assembly, wrapping machinery and many special-purpose devices.

Examples of alternative configurations in common use are illustrated in SEED Design Guide "Planar Mechanisms" and textbooks on kinematics, dynamics and mechanisms, such as Grosjean. The most common forms have output members which trace circular arc (swinging-arm follower, see Figure 2a) or linear (translating follower, see Figure 2b) paths. The design process is more complex for the former type as it involves more independent dimensions.

Provided the number of independent dimensions needed to specify the mechanism is restricted to those defining a linear path the kinematic design synthesis can be completed readily on a computer by using a spreadsheet. This approach has the benefit of combining computation with graphical presentations to illustrate the significance of the various independent parameters, so helping you to optimize the dimensions. By re-arranging the terms in the kinematic equations to obtain non-dimensional parameters it is easy to re-run the same program repeatedly to investigate the significance of changing values, so encouraging the 'What if?" approach.

This Guide assumes that the procedure given in SEED Design Guide "Planar Mechanisms" has been followed, from the preparation of the layout drawing and a timing chart, to the decision to use a cam mechanism having one of the configurations shown in Figures 2b or 6. These drawings show cams which drive a translating follower through a freely pivoted roller. A spring provides the force closure between the roller and cam to maintain continuous contact between the roller and the cam profile.

Books and commercial software packages, which cover the design, dynamics and strength of most configurations of the cam mechanism are cited in "Sources of Information".

Fig 1 General Design Procedure

Fig 2a Cams Driving a Swinging-Arm Follower
Courtesy ESDU International plc

Fig 2b Cam driving an offset translating follower
Courtesy ESDU International plc