3.10 Tribology, Lubrication, and Bearing Design
Francis E. Kennedy, E. Richard Booser, and Donald F. Wilcock
Introduction
Tribology, the science and technology of contacting surfaces involving friction, wear, and lubrication,
is extremely important in nearly all mechanical components. A major focus of the ?eld is on friction,
its consequences, especially wear and its reduction through lubrication and material surface engineering.
The improper solution of tribological problems is responsible for huge economic losses in our society,
including shortened component lives, excessive equipment downtime, and large expenditures of energy.
It is particularly important that engineers use appropriate means to reduce friction and wear in mechanical
systems through the proper selection of bearings, lubricants, and materials for all contacting surfaces.
The aim of this section is to assist in that endeavor.
Sliding Friction and its Consequences
Coefficient of Friction
If two stationary contacting bodies are held together by a normal force W and a tangential force is
applied to one of them, the tangential force can be increased until it reaches a magnitude suf?cient to
initiate sliding. The ratio of the friction force at incipient sliding to the normal force is known as the
static coef?cient of friction, fs. After sliding begins, the friction force always acts in the direction opposing
motion and the ratio between that friction force and the applied normal force is the kinetic coef?cient
of friction, fk.
Generally, fk is slightly smaller than fs and both coef?cients are independent of the size or shape of
the contacting surfaces. Both coef?cients are very much dependent on the materials and cleanliness of
the two contacting surfaces. For ordinary metallic surfaces, the friction coef?cient is not very sensitive
to surface roughness. For ultrasmooth or very rough surfaces, however, the friction coef?cient can be
larger. Typical friction coef?cient values are given in Table 3.10.1. Generally, friction coef?cients are
greatest when the two surfaces are identical metals, slightly lower with dissimilar but mutually soluble
metals, still lower for metal against nonmetal, and lowest for dissimilar nonmetals
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