FAILURE OF CAM SYSTEMS—
STRESS, WEAR, CORROSION
are only three ways in which parts or systems can “fail”:
. Most systems are subject
to all three types of possible failure. Failure by obsolescence is somewhat
arbitrary. Failure by breakage is often sudden and may be permanent. We will
not address failure by breakage as it applies to general machine elements such
as follower arms, bearings, etc. That topic is well covered in many other
references such as . We will address only the failure mechanisms to which
the camfollower interface are typically subject. These fall under the general
rubric of wear.
by “wearing out” is generally a gradual process and is sometimes repairable.
Ultimately, any system that does not fall victim to one of the other two modes
of failure will inevitably wear out if kept in service long enough. Wear is the
final mode of failure, that nothing escapes. Thus, we should realize that we
cannot design to avoid all types of wear completely, only to postpone them.
is a broad
term that encompasses many types of failures, all of which involve changes to
of the part. Some of these so-called
are still not completely understood, and rival
theories exist in some cases. Most experts describe five general categories of
erosion, corrosion wear
. In general, the
(noncorrosive) wear rate is inversely proportional to hardness.
following sections discuss these topics in detail. In addition, there are other
types of surface failure that do not fit neatly into one of the five categories
or that can fit into more than one.
has aspects of the
last two categories as does
. For simplicity, we
will discuss these hybrids in concert with one of the five main categories
listed above. Table 12-0 shows the variables used in this chapter.
Portions of this chapter were adapted from R. L.
Design: An Integrated Approach
, 2ed, Prentice-Hall, 2000, with permission.
from wear usually involves the loss of some material from the surfaces of solid
parts in the system. The wear motions of interest are sliding, rolling, or some
combination of both. It only requires the loss of a very small volume of
material to render the entire system nonfunctional. Rabinowicz estimates
that a 4 000-lb automobile, when completely “worn out,” will have lost only a
few ounces of metal from its working surfaces. Moreover, these damaged surfaces
will not be visible without extensive disassembly, so it is often difficult to
monitor and anticipate the effects of wear before failure occurs.
Copyright 2004, Industrial
Press, Inc., New York, NY