ESDU 94008:2010

INTRODUCTION

A cam/follower system will fail to perform its intended function
adequately if the surfaces of the cam and/or the follower are worn
excessively. This type of early service failure can often be
avoided if the system is designed such that an adequate lubricant
film is present in the contact region throughout the life of the
system. Therefore the prediction of the thickness of such a
lubricant film under a variety of operating conditions is a vital
step in the systematic design of a cam mechanism (see ESDU 94017,
Reference 28).

This Data Item presents a procedure for calculating values of
lubricant film thickness in the contact region between a disc cam
and its follower (either rolling or sliding contact). To determine
film thickness it is necessary to find the radius of curvature of
the cam profile, the lubricant entrainment velocity and the contact
force at the interface between the cam and the follower for a
series of angular positions of the cam. This analysis is time
consuming and is best carried out using the computer program (see
Section 7) supplied with this Data Item. The program determines
values of minimum film thickness at intervals of one degree of cam
rotation (assuming constant angular velocity of the cam) and finds
the smallest value of film thickness during the cycle. The program
can be used to determine suitable values of rate and preload for a
return spring designed to ensure that the cam and follower remain
in contact. Alternatively, if a spring of known rate and preload is
specified the program checks that contact is maintained. A check to
ensure that profile undercutting (see ESDU 92005, Reference 23)
does not occur is also included.

The calculation method presented in this Data Item is based on
the theory of elastohydrodynamic lubrication (see Taylor, Reference
38, or Zhu and Taylor, Reference 39) in which changes in the
viscosity of the lubricant and elastic deformation of the
components in contact are significant. The ESDU Tribology Series
contains procedures for calculating film thicknesses in lubricated
Hertzian contacts and these are given in ESDU 85027 and ESDU 89045
for two dimensional line contacts and three dimensional point
contacts, respectively (Derivations 2 and 3). Computer programs
that implement these procedures are also available (see ESDU 91037
and ESDU 91038, References 21 and 22, respectively). The algorithm
used to calculate film thickness in the program (see Section 7)
supplied with the current Data Item is exactly the same as that
used in ESDU 91037 (for the reasons discussed in Appendix A).

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