Page 87 - Tribology in Machine Design

74 Tribology in machine design

2. provision for careful alignment or minimum slope by deflection of
parallel surfaces, or the provision of crowned surfaces as has been done
for gear teeth, bearing rollers and cam followers;
3. cleaner steels, with fewer entrapped oxides (as by vacuum melting);
4. material and treatments to give higher hardness and strength at and
near the surface, and if carburized, a sufficient case depth (at least
somewhat greater than the depth to maximum shear stress) and a strong
core;
5. smoother surfaces, free of fine cracks, by polishing, by careful running-in
or by avoidance of coarse machining and grinding and of nicks in
handling;
6. oil of higher viscosity and lower corrosiveness, free of moisture and in
sufficient supply at the contacting surfaces. No lubricant on some
surfaces with pure rolling and low velocity;
7. provision for increased film thickness of asperity-height ratio, the so-
called lambda ratio (/> 1.5).

3.7. Thermal effects in The surface temperature generated in contact areas has a major influence
surface contacts on wear, scuffing, material properties and material degradation. The
friction process converts mechanical energy primarily into thermal energy
which results in a temperature rise. In concentrated contacts, which may be
separated by a full elastohydrodynamic film, thin-film boundary lubricated
contacts, or essentially unlubricated contacts, the friction intensity may be
sufficiently large to cause a substantial temperature rise on the surface. The
methods to estimate surface temperature rise presented in this section are
all based on simplifying assumptions but nevertheless can be used in design
processes. Although the temperature predicted may not be precise, it will
give an indication of the level of temperature to expect and thereby give the
designer some confidence that it can be ignored, or it will alert the designer
to possible difficulties that may be encountered because of excessive
temperatures.
The most significant assumption, involved in calculating a surface
temperature, is the actual or anticipated coefficient of friction between the
two surfaces where the temperature rise is sought. The coefficient of friction
will depend on the nature of the surface and can vary widely depending on
whether the surfaces are dry/unlubricated or if they are lubricated by
boundary lubricants, solids, greases, hydrodynamic or elastohydrody-
namic films. The coefficient of friction enters to the first power and is, in
general, relatively unpredictable. If measurements of the coefficient of
friction are available for the system under consideration, they frequently
show substantial fluctuations. Another assumption is that all the energy is
conducted into the solids in contact, which are assumed to be at a bulk
temperature some distance away from the contact area. However, the
presence of a lubricant in the immediate vicinity of the contact results in
convection heat transfer, thereby cooling the surfaces close to the contact.
This would generally tend to lower the predicted temperature.
The calculations focus on the flash temperature. That is the temperature