Page 259 - Tribology in Machine Design

244 Tribology in machine design

Equivalent Young modulus

Material parameter

Speed parameter

Load parameter

Therefore

6.8. Heating at the The assumption in the analysis presented in the previous section, was that
inlet to the contact the lubricant properties are those at the inlet zone temperature and the
system is isothermal. The inlet zone lubricant temperature can be, and
frequently is, higher than the bulk lubricant temperature in the system.
There are basically two mechanisms responsible for the increase in the
lubricant temperature at the inlet to the contact. The first is viscous heating
of the lubricant in the inlet zone and the second is the conduction of thermal
energy accumulated in the bulk of the contacting solids to the inlet zone.
This second mechanism is probably only important in pure sliding where
the conduction can occur through the stationary solid. The heating at the
inlet zone is significant only at high surface velocities and can be subjected
to certain simplified analysis. Under conditions of high surface velocity or
high lubricant viscosity the effect of inlet heating due to shear on effective
viscosity ought to be considered. The engineering approach to this problem
is to use a thermal reduction factor, T f, which can be multiplied by the
isothermal film thickness, /J 0, to give a better estimate of the actual film
thickness.
The thermal correction factor is a weak function of load and material
parameters. As a first approximation, the following expression may be used
to determine the thermal correction factor for line contacts

In eqn (6.28) the thermal loading factor, T h is defined as