POA * ONLINE SUBSCRIPTION ONLY

INTRODUCTION

This Data Item describes a procedure for determining the minimum allowable size of a disc cam driving a radial translating roller follower (see Sketch 2.1). Minimisation of disc cam size is an important objective in the design of industrial machinery because it leads to:

(a) minimum space and weight,

(b) minimum camshaft out-of-balance force,

(c) minimum angular velocity and minimum fluctuation of angular acceleration of roller follower and hence increased life

and (d) minimum cam blank material cost and cutting cost.

Estimating the minimum allowable cam size at an early stage of the detail design may lead to a significant reduction in the number of subsequent iterations in the design process (see Derivations 1 and 2). An appropriate space envelope can be determined for the cam/follower subsystem.

Cam size is characterised by the largest distance from the cam centre to the cam profile. This is the sum of the reference circle radius and the maximum follower lift. Usually the lift is a known value, set during the design specification, and shown on a timing diagram. However the reference circle radius is often chosen arbitrarily. A realistic initial estimate of the minimum allowable value of reference circle radius for a given lift is obtained in this Item by consideration of the kinematic constraints (see Section 2.2). The procedure is most easily implemented using a computer program supplied with this Data Item. However, hand calculation is possible, in many cases, using the graphical design data (see Figures 1 to 12).

It is important to realise that the value of reference circle radius obtained using the procedure in this Item is the limiting minimum value for design purposes. To check that contact stress does not exceed an allowable value see ESDU 93001 (Reference 13) and to ensure that roller size is suitable see ESDU 91026 (Reference 11).

The method described in this Item is applicable only to either a DRD (dwell-rise-dwell or dwell-return-dwell) or a DRRD (dwell-rise-return-dwell) follower motion defined by a single-law. Follower displacement for a single-law motion segment is defined by the equation(s) for one cam law (see Reference 6). ESDU 92014 and ESDU 93002 (References 12 and 14) show the extension of this method to determine the minimum size of cams with more complex motion segments formed by blending parts of single-law profiles.

ESDU 82023 (Reference 7) presents a geometric construction procedure by which the minimum reference circle radius may be obtained. This geometric procedure is applicable to offset as well as radial followers but it is iterative and so does not lead directly to the optimum solution.