Thomas Industrial Library

FAILURE OF CAM SYSTEMS—

STRESS, WEAR, CORROSION

12.0 INTRODUCTION

There are only three ways in which parts or systems can “fail”: obsolescence , breakage, or wearing out . 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 [1]. We will address only the failure mechanisms to which the camfollower interface are typically subject. These fall under the general rubric of wear.

Failure 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.

Wear is a broad term that encompasses many types of failures, all of which involve changes to the surface of the part. Some of these so-called wear mechanisms are still not completely understood, and rival theories exist in some cases. Most experts describe five general categories of wear: adhesive wear , abrasive wear , erosion, corrosion wear , and surface fatigue . In general, the (noncorrosive) wear rate is inversely proportional to hardness.

The 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. Corrosion fatigue has aspects of the last two categories as does fretting corrosion . 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. Norton, Machine Design: An Integrated Approach , 2ed, Prentice-Hall, 2000, with permission.

Failure 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[2] 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.