Page 279 - Cam Design Handbook

THB9 9/19/03 7:26 PM Page 267

CAM MATERIALS AND LUBRICATION 267

velocity of 600ft/min. Table 9.3 also includes static hertzian stress values, which are pre-
sented for information and comparison only and do not relate to the fatigue tests.
In plotting K versus life, a least-square curve fitting line is drawn through the normal
scatter of significant points obtained from tests. The equation for the K factor as a func-
tion of life is derived from the slope of these fitting lines
B - log N
log K = 10 (9.15)
10
A
where constants A and B are given in Table 9.3 and N is the number of stress cycles for
which a K value is required. For numbers other than 100 million cycles the load stress
factors can be interpreted from the load-life curves similar to Fig. 9.9. Figure 9.9 presents
typical load-life test data obtained from the roll test machine. Comparisons are given for
pure rolling and up to 300 percent sliding to indicate trends and slope comparison of the
materials. Figure 9.9a shows nodular iron and class 45 cast iron and Fig. 9.9b shows cast
bronze.
Let us consider the design approach for choosing the optimum combination of mate-
rials. In fatigue design criteria, this choice is a modification of Fail-Safe Design by Fuchs
and Stephens (1980).
Fail-safe design recognizes that fatigue cracks may occur and arranges the structure so
that cracks will not lead to a failure before they are detected. In cam-follower system
design we design and operate the machine so that the roller follower will wear out before
the cam is ever affected. In this manner the rollers will be periodically replaced. This is
the easiest and least expensive solution to the fatigue problem. Therefore the roller diam-
eter should be measured periodically and replaced when it has shown a reduction in size
but before destructive wear of either the roller or cam occurs. Experience over time will
be necessary to succeed with this approach to discover compatible materials for each par-
ticular machine. It is suggested that dependent on the size and reliability of the system a
reduction of the roller diameter from -0.0004 to -0.002 in requires a replacement. For
example, the author designed a newspaper folding mechanism run at 70,000 papers per
hour with commercial ball-bearing rollers SAE 52100 hardened RC60-62. They were
measured every six months and if the diameters were reduced by -0.001 in they were
replaced. The cam material is SAE 8620 air-hardened die steel RC 58-60 and still running
with accuracy and some surface polish over 35 years as well as operating properly and at
a minimum cost.
Next, let us discuss some of the test data in Table 9.3 in which it was stated that the
rollers were of equal size and the softer material of the combination failed first. However,
this is not the case with cam-follower machines which use a roller smaller than the cam
by a ratio of between 1 and 6 or 1 and 10 depending on the design. Accordingly the fol-
lower roller surface has more fatigue stress cycles than the cam surface and will tend to
fail first. This fact slants the test data of Table 9.3 positively to the periodic replacement
of the roller follower. Also, it may be noted that the roller will wear out first even though
it is a harder and more expensive surface material than that of the cam.
Occasionally, a soft metal ring may be added to the commercial roller follower as a
choice over the manufactured bearing surface. A shortcoming is the larger rotational
moment of inertia of the roller and more skidding produced in contact with the cam.

EXAMPLE A cam is loaded by a roller follower with about 9 percent sliding. The
cam is 4340 steel and 280 BHN hardness. The roller follower (tool steel 60-62 RC) is
1-in diameter and has a width of 3/4 in. At a point on the cam which has a radius of
curvature of 2 in find the allowable contact forces to prevent significant wear before 100
million cycles.