Page 264 - Cam Design Handbook

THB9 9/19/03 7:26 PM Page 252

252 CAM DESIGN HANDBOOK

a = pressure viscosity coefficient of fluid, lb/in -1
D= relationship between m 1, m 2, and E 1, E 2 of materials
h 0 = lubricant absolute viscosity, lbf-s/in 2
L= film parameter
m 1, m 2 = Poisson’s ratio
m c, m f = Poisson’s ratio for cam and follower, respectively, in
r c, r f = radii of curvature of cam and follower, respectively, in
s max = maximum compressive stress lb/in 2
t 1 , t 2 = rms roughness of two surfaces in contact
w = load per unit cylinder length, lb/in

9.1 INTRODUCTION

This chapter considers the mating cam and follower surfaces to determine the optimum
performance, wear, and life of these moving parts. Note that no general methodology is
available for the design of the cam and follower surfaces even though they have been
around for a long time. The lack of reliable design data is primarily due to the special
complexity of cam-follower systems. In this chapter, selected methods and available data
are presented that should be used with discretion.
Some of the factors involved in cam-follower surface contact are: (a) for the system:
choice of mating materials, lubricant and additive used, contaminants in the lubricant,
lubrication system chosen, fluctuating external load, and reversal of the inertia and torques
(affecting the crossover shock in the backlash); (b) for the cam: materials selected, surface
roughness, surface finish, surface hardness, changing radius of curvature, and the prior
history of machining; and (c) for the follower: materials used, surface hardness and finish,
and the roller diameter and inertia (affecting rolling and sliding due to the changing roller
acceleration). Therefore, experience and performance in the laboratory or field is the
critical need in all designs.
Despite the numerous variables listed, the design information in this chapter presents
an acceptable approach to optimizing cam material life performance. It is suggested that
selective periodic replacement of the follower roller be done in the field. Adding new
rollers, rather than replacing the cam, is an easy and the least costly method of extending
the life of the contacting cam and roller follower.
It should be stated that cams and the following are of the same basic mechanical family,
the study of which is generically referred to as “contact mechanics”:

• cams
• gearing
• rolling-element bearings
• traction friction drives
• intermittent motion mechanisms
• chains
• belts
• wire rope.
All the foregoing have in common rolling and/or sliding action coupled with reason-
ably high concentrated contact (hertz) stresses. In the field of contact mechanics the failure
mechanism among these various machine elements is similar and is related to material