Page 307 - Tribology in Machine Design

292 Tribology in machine design

Cose II. The coefficient of friction considered as variable
As a matter of fact, the friction coefficient is not constant but varies with
different loads, speeds, lubricants and gear materials, as well as with
different types of types of surface finish and many other factors. Actual tests
carried out on gears have revealed that the form of the relationship between
the average friction coefficients and the pitch line velocities is very much the
same as in the case of journal bearings. At low speeds, the values of the
friction coefficient are high, decreasing rapidly to a certain minimum value
with increasing velocity, and then rising slowly with further increase in
velocity. There is, however, one important difference in the lubrication
mechanism operating in plain journal bearings and in gears. In the case of
the journal bearings, hydrodynamic lubrication is usually a dominant type
of lubrication while in gears, elastohydrodynamic lubrication is the main
mechanism. It is known that the nature of sliding between involute gear
teeth consists of sliding in one direction during approach, reducing to zero
at the pitch point where the direction of sliding changes, and increasing
again as the contact progresses through the recess action. This is shown, in a
schematic way, in Fig. 8.8.
Since the direction of sliding changes at the pitch point, we may conclude
that the coefficient of friction will assume the value characteristic for a thick

Figure 8.8