Page 94 - Tribology in Machine Design

80 Tribology in machine design

3.7.3. Refinement for thermal bulging in the conjunction zone
Thermal bulging relates to the fact that friction heating can cause both
thermal stresses and thermoelastic strains in the conjunction region. The
thermoelastic strains may result in local surface bulging, which may shift
and concentrate the load onto a smaller region, thereby causing higher flash
temperatures. A dimensionless thermal bulging parameter, K, has the form

where all the variables are as defined above except, e is the coefficient of
linear thermal expansion (1/°C). Note: p H is the maximum Hertz pressure
that would occur under conditions of elastic contact in the absence of
thermal bulging. In other words, it can be calculated using Hertz theory. In
general, for most applications

and for this range there is a good approximation to the relation between the
maximum conjunction pressure resulting from thermal bulging, p k, and the
maximum pressure in the absence of thermal bulging, p H, namely

and the ratio of the contact widths w k and W H, respectively, is

which, when substituted into the flash temperature expressions, eqn (3.9a),
results simply in a correction factor multiplying the original flash
temperature relation

where the second subscript, k, refers to the flash temperature value
corrected for the thermal bulge phenomena.
The thermal bulging phenomena can lead to a thermoelastic instability in
which the bulge wears, relieving the local stress concentration, which then
shifts the load to another location where further wear occurs.

3.7.4. The effect of surface layers and lubricant films

The thermal effects of surface layers on surface temperature increase may be
important if they are thick and of low thermal conductivity relative to the
bulk solid. If the thermal conductivity of the layer is low, it will raise the
surface temperature, but to have a significant influence, it must be thick
compared to molecular dimensions. Another effect of excessive surface
temperature will be the desorption of the boundary lubricating film leading
to direct metal-metal contacts which in turn could lead to a further increase