Page 286 - Cam Design Handbook

THB9 9/19/03 7:26 PM Page 274

274 CAM DESIGN HANDBOOK

tion of protective surface films to reduce wear and surface damage. The effectiveness
of the films is established by their physical properties, which include shear strength, thick-
ness, surface adhesion, film cohesion, melting point, and solubility. Operating variables
that affect the lubricant include load, speed, temperature, and environmental atmos-
phere. Additions in the lubricant include antiwear and antifoam agents, antioxidants, and
viscosity improvers.
9.5.2 Elastohydrodynamic Film (EHD)

This section applies to roller followers only. Sliding followers are assumed to function in
the boundary and mixed lubrication regimes. The elastohydrodynamic film is experienced
by cast iron (with its free graphite and porosity) which operates with limited lubrication
and gives good sliding and wear properties. Also, this section is written based on theories
and tests from both rolling-element bearings and gearing (Zaretsky, 1997). Specific design
data is not available for cam-follower elements. However, the information contained here
is of a fundamental nature (rolling fatigue) so it can be applied to cams and followers since
all these systems show similar surface fatigue wear. In application, some modifications
and refinements may be necessary to account for the skidding action of the follower roller
at high speeds and for larger cams. The information is presented as a potential design basis
for the cam designer in selecting the best material and lubricant available and to optimize
the performance of the machine. The important point is that the film that separates rolling-
contact surfaces is elastohydrodynamic (EHD) film.
Grubin (1949) solved the EHD problem by assuming that the elastic displacements of
contacting lubricating surfaces, heavily loaded, would be similar to the dry hertzian
contact, Fig. 9.11.
The dimensionless minimum film thickness is written as a function of the other three
parameters
f U W G)
H = ( , ¢,
which develops to
h -0091.
H = =195 G 073 ( W¢) (9.16)
.
.
R x
in which G is the material parameter, U is speed parameter, and W¢ is the load parameter.
The variables are related in Eq. (9.16) by four dimensionless groupings.
h
Film thickness H = , dimensionless
R
x
w
Load parameter W ¢ = , dimensionless
¢ ER
x
Speed parameter U = u E R ¢ , dimensionless
h 0 x
Materials parameter G = a¢E¢, dimensionless
where
h = EHD film thickness, in
R x = equivalent radius in the rolling direction, in
W¢= EHD load parameter, w/E¢R x
w = load per unit cylinder length, lb/in
E¢= effective Young’s modulus, lb/in 2