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The book takes the subject from an introductory level through advanced topics needed to properly design, model, analyze, specify, and manufacture cam-follower systems.
Cam Design and Manufacturing Handbook
(Cam Systems Failure - Surface Geometry)

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   by Robert L. Norton
Published By:
Industrial Press Inc.
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12.1 SURFACE GEOMETRY

 

Before discussing the types of wear mechanisms in detail, it will be useful to define the characteristics of an engineering surface that are relevant to these processes. (Material strength and hardness will also be factors in wear.) Most solid surfaces that are subject to wear in machinery will be either machined, ground, or EDM’d, though some will be as-cast or as-forged. In any case, the surface will have some degree of roughness that is concomitant with its finishing process. Its degree of roughness or smoothness will have an effect on both the type and degree of wear that it will experience.

 

Even an apparently smooth surface will have microscopic irregularities. These can be measured by any of several methods. A profilometer passes a lightly loaded, hard (e.g., diamond) stylus over the surface at controlled (low) velocity and records its undulations. The stylus has a very small (about 0.5 m) radius tip that acts, in effect, as a lowpass filter, since contours smaller than its radius are not sensed. Nevertheless, it gives a reasonably accurate profile of the surface with a resolution of 0.125 m or better. Figure 12-1 shows the profiles and SEM* photographs (100 x ) of both ( a ) ground and ( b ) machined surfaces of hardened steel cams. The profiles were measured with a Hommel T- 20 profilometer that digitizes 8 000 data points over the sample length (here 2.5 mm). The microscopic “mountain peaks” on the surfaces are called asperities .

 

From these profiles a number of statistical measures may be calculated. ISO defines at least 19 such parameters. Some of them are shown in Figure 12-2 along with their mathematical definitions. Perhaps the most commonly used parameters are Ra , which is the average of the absolute values of the measured points, and Rq , which is their rms average. These are very similar in both value and meaning. Unfortunately, many engineers specify only one of these two parameters, neither of which tells enough about the surface. For example, the two surfaces shown in Figures 12-3 a and b have the same Ra and Rq values, but are clearly different in nature. One has predominantly positive, and the other predominantly negative, features. These two surfaces will react quite differently to sliding or rolling against another surface.

 

In order to differentiate these surfaces that have identical Ra or Rq values, other parameters should be calculated. Skewness Sk is a measure of the shape or symmetry of the amplitude distribution of the surface rougness contour. A negative value of Sk indicates that the surface has a predominance of valleys (Figure 12-3 a ) and a positive Sk defines a predominance of peaks (Figure 12-3 b ). Many other parameters can be computed (see Figure 12-2). For example, Rt defines the largest peak-to-valley dimension in the sample length, Rp the largest peak height above the mean line, and Rpm the average of the five largest peak heights. All the roughness measurements are calculated from an electronically filtered measurement that zeros out any slow-changing waves in the surface. An average line is computed from which all peak/valley measurements are then made. In addition to these roughness measurements (denoted by R ), the waviness Wt of the surface can also be computed. The Wt computation filters out the high-frequency contours and preserves the long-period undulations of the raw surface measurement. If you want to completely characterize the surface-finish condition, note that using only Ra or Rq is not sufficient. A cam surface finish specification should, at a minimum, include limits for Ra, Rt, Rpm, Sk , and Wt .

 

* Scanning Electron Microscope

 

Copyright 2004, Industrial Press, Inc., New York, NY

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