From Cam Design and Manufacturing Handbook


When two surfaces are pressed together under load, their apparent area of contact A a is easily calculated from geometry, but their real area of contact A r is affected by the asperities present on their surfaces and is more difficult to accurately determine. Figure 12-4 shows two parts in contact. The tops of the asperities will initially contact the mating part and the initial area of contact will be extremely small. The resulting stresses in the asperities will be very high and can easily exceed the compressive yield strength of the material. As the mating force is increased, the asperity tips will yield and spread until their combined area is sufficient to reduce the average stress to a sustainable level, i.e., some compressive penetration strength of the weaker material.

Figure 12-4: The actual contact between two surfaces is only at the asperity tips

We can get a measure of a material's compressive penetration strength from conventional hardness tests (Brinell, Rockwell, etc.), that force a very smooth stylus into the material and deform (yield) the material to the stylus' shape. The penetration strength S p is easily calculated from these test data and tends to be of the order of 3 times the compressive yield strength S yc of most materials.[3]

The real area of contact can then be estimated from


where F is the force applied normal to the surface and the strengths are as defined in the above...

Copyright Industrial Press Inc., 2002 under license agreement with Books24x7

Products & Services
Styli and Probes
Styli and probes are slender, rod-shaped stems and contact tips or points used to probe surfaces in conjunction with profilometers, SPMs, CMMs, gages and dimensional scanners.
Metal Sheet
Metal sheet is metal or alloy stock supplied or available in the form of sheet or foil. It has a thickness between 0.006" and 0.250", and a width of 24" (609.6 mm) or more.
Metal Strip
Metal strip is narrow, thin stock that is usually 3/16 in. (4.76 mm) or less in thickness and under 24 in. (609.6 mm) in width. Metal strips are formed to precise thicknesses and/or width requirements.
Washers are disks of metal or non-metallic material placed beneath a nut, an axle bearing, or a joint, to relieve friction, prevent leakage, isolate, prevent loosening, or distribute pressure.
Metal Foils and Foil Stock
Metal foils and foil stock are very thin, metal-mill products with a thickness that is usually less than 0.006 in. Copper foil and aluminum foil are the most common types of metal foils and foil stock.

Topics of Interest

12.3 ADHESIVE WEAR When (clean) surfaces such as those shown in Figure 12-1 (p. 338) are pressed against one another under load, some of the asperities in contact will tend to adhere to one another...

Bearings sans liquid lubricants find use in everything from computer printers to rocket engines. Director, Center for Rotating Machinery Dry bearings, as the name implies, work with little or no...

13.1 Real and Apparent Contact It has been tacitly assumed so far in this book that the surfaces of contacting bodies are topographically smooth; that the actual surfaces follow precisely the gently...

Mass-produced switch contacts have an rms roughness ranging from 0.5 to 1 10 in. These "smooth" surfaces, in reality, comprise microscopic peaks and valleys. As a result, two mating contacts...

Bharat Bhushan and Michael Nosonovsky Summary A model, which explains scale effects in mechanical properties and tribology is presented. Mechanical properties are scale dependent based on the strain...