SEED Guides Tutor's Guide - Cam Mechanisms < >

8.1 Refer to the flow chart, Fig 1 of 'Cam Mechanisms - Unit Design' guide Also note that the meaning of optimum" depends upon the requirements of a particular design. The controlling requirement could, for example, be any of:-

8.1.1 least swept radius for a specified rise or fall action,

8.1.2 least angle of cam rotation for a specified rise or fall action,

8.1.3 maximum lift for given segment angle and prime circle radius,

8.1.4 specified maximum force on roller,

8.1.5 specified maximum rev/min of the roller,

8.1.6 specified maximum contact stress, or

8.1.7 specified maximum driving torque,

remembering that the remainder also apply to an extent depending on the application. The kinematic constraints imposed by the maximum acceptable pressure angle and prevention of profile undercutting apply in every case and provide the basis for the 'Cam Mechanisms - Unit Design' guide . It is usually effective - and easiest - to begin by determining the minimum size compatible with the maximum permitted pressure angle to obtain a starting dimension.

Data on driving torque is contained in ESDU hardcopy "Item" 82006; see also assignment 12.9.

8.2 NB - The maximum force normal to the cam profile is unlikely to occur at the location of the:-

8.2.1 peak follower acceleration,

or

8.2.2 peak pressure angle, or, in the case of maximum contact stress,

8.2.3 minimum radius of convex profile curvature.

8.3 The simplest approach is to begin with a radial follower [i.e., the follower path extended intersects the centre of cam rotation]. Ensure that the maximum pressure angle is less than the limiting value and the minimum radius of profile curvature exceeds the limiting value. The former test is more straightforward and often imposes the kinematic minimum; therefore it is the recommended starting place. The larger of the values of prime circle radius, R_a , obtained from these independent tests imposes the absolute kinematic minimum for that configuration. If it is too large try the procedures given below.

8.4 In practice the design must also satisfy the additional requirements for:

8.4.1 acceptable values of contact stress in the cam disc and in the roller,

8.4.2 the static and dynamic load capacities of the roller,
        the shear strength of the roller stud,
        the maximum rotational speed of the roller.
        [see Section 14.6].

8.4.3 acceptable peak driving torque; strength of drive shaft, fastening, etc.

8.4.4 feasible stiffness, deflection, natural frequency, etc of the spring.

Any of these may require a larger cam than that determined solely from kinematic considerations.

8.5.1 the minimum radius of profile curvature will be reduced, so increasing the probability of undercutting [see Section 7 above],

8.5.2 hence the contact stress will increase [but still be acceptable?], see 8.5.5

8.5.3 the roller is more expensive,

8.5.4 the increased mass moment of inertia of the roller assembly raises the probability of roller slip,

8.5.5 the roller has larger radius, acting to reduce the maximum contact stress!

8.8 Profile blending. Referring to the 'Cam Mechanisms - Unit Design' guide , page 13, equations (14), Figures 20 and 21 and Appendix A; a DRD motion can be modified by blending portions of two complete DRD motions so that the resultant is formed from the combination of one motion from inner dwell to the position of maximum follower velocity [and zero acceleration] and another motion for the remainder to outer dwell. Blending the profile for one motion has no effect on the remainder of the cycle. [Different cam laws can be used for the 2 motions provided the blending criteria are met.]

If the location of maximum follower velocity is displaced from mid-stroke towards outer dwell then the notional segment angle and the notional lift of the inner portion both become larger than those for the resultant motion and conversely for the outer portion. The maximum pressure angle always occurs between inner dwell and maximum follower velocity. The consequences are:

Alternatively,

8.8.6 biasing the maximum follower velocity from mid-stroke towards inner dwell reverses the situation so a weaker spring [having a lower natural frequency] is needed, the contact stress is reduced, but the cam has to be enlarged.

8.9 The same principles apply to the application of profile blending to DFD and to DRFD motions. They are particularly applicable to the latter in which case blending can occur at mid-stroke or [since SHM is particularly suitable for the intermediate portion but neither end] at the cam angle which equalises the [absolute values of] follower accelerations. The spreadsheet approach is readily applicable to the synthesis of motions blended at maximum velocity, but not to more complex motions such as those involving an intermediate precision point. [Blending computation must use increments of cam angle - not non-dimensional parameters - to obtain sensible charts; see section 10.8].