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).
ESDU 94008 C-2010 history
2010ESDU 94008 C-2010 Lubricant film thickness between disc cams and followers