SEED Guides Tutor's Guide - Cam Mechanisms < >

4.1 The cam law is the equation defining the follower displacement as a function of cam rotation. There are a vast number of alternative laws; in the author's opinion too much fuss is made of these alternatives. The non-dimensional forms of the equations for a few laws are given in 'Cam Mechanisms - Unit Design', pages 9 to 11 and 12, Figures 11 to 13 and equations (10) to (13). See also ESDU "Item" 82006, Rothbart, Jensen (1987), Reeve, etc.

4.2 The fundamental criteria to select a cam law for good dynamic performance are defined in 'Cam Mechanisms - Unit Design' Guide by equations (14) & (15), page 13. These rules assume perfect manufacture of the profile which is one reason for suggesting that the students compare the follower displacements around the start and finish of DRD motions for alternative motions to decide whether the theoretical performance is achievable. [See Figure S1 and assignment proposal 12.2]

Figure S1

4.3 Warning! Some laws, e.g., modified sinusoidal acceleration motion [which you may find abbreviated to "mod. sine'] have complex, non-continuous equations and hence are relatively difficult to enter on the spreadsheet [the transition may not occur at an increment in the listing; as a check it may be desirable to select an increment which includes this step]. For this reason cycloidal motion or 3-4-5 polynomial motion [which have simple, continuous equations meeting the fundamental dynamic requirements for DRD and DFD motions, see 'Cam Mechanisms - Unit Design' Guide equations (14) & (1 5)] are recommended as they illustrate the principles well.

4.4 Assignment 12.2 is intended to emphasise a criterion of cam law selection which contradicts the choice based on the superficial comparison of maximum accelerations without considering their relative location and hence the consequences on maximum spring force.

The following sections expand interlinked aspects of cam design.