Page 308 - Tribology in Machine Design

Lubrication and efficiency of involute gears 293

film lubrication regime during the period of engagement of a pair of mating
teeth. It is possible to set up an efficiency equation in various ways. The
chances are, however, that the most that can be determined by experiment is
to establish some average values of friction coefficient for the approach
action and similarly for the recess action.
Experimental results suggest that at very low pitch line velocities (up to
1
1.5 m min~ ) the friction of approach period appears to be approximately
double that of the recess period on hobbed, milled and shaped gears made
of cast-iron, soft-steel, bronze and aluminium. On hardened and ground
steel gears, however, the difference between the friction of approach and the
friction of recess is almost non-existent. When the contact passes through
the pitch point, a significant increase in friction (about 150 per cent) takes
place. Thus, introducing different average values for the friction coefficients
of the approach and recess, gives

8.8.2. Summary of efficiency formulae
In order to collate the material presented in the previous section the
following summary is made: when N l5 N 2 is the number of teeth on driver
and driven gear, respectively, i is the gear ratio, /? a, /? r is the arc of approach
and recess on the driver, respectively,/is the average coefficient of friction,/ a
is the average coefficient of friction during the approach period and/ r is the