Page 25 - Tribology in Machine Design

12 Tribology in machine design

additives are generally satisfactory under high-torque low-speed conditions
but are sometimes less so at high speeds. The prevailing modes of failure are
pitting and scuffing.

1.2.8. Worm gears

Worm gears are somewhat special because of the degree of conformity
which is greater than in any other type of gear. It can be classified as a screw
pair within the family of lower pairs. However, it represents a fairly critical
situation in view of the very high degree of relative sliding. From the wear
point of view, the only suitable combination of materials is phos-
phor-bronze with hardened steel. Also essential is a good surface finish and
accurate, rigid positioning. Lubricants used to lubricate a worm gear
usually contain surface active additives and the prevailing mode of
lubrication is mixed or boundary lubrication. Therefore, the wear is mild
and probably corrosive as a result of the action of boundary lubricants.
It clearly follows from the discussion presented above that the engineer
responsible for the tribological aspect of design, be it bearings or other
systems involving moving parts, must be expected to be able to analyse the
situation with which he is confronted and bring to bear the appropriate
knowledge for its solution. He must reasonably expect the information to
be presented to him in such a form that he is able to see it in relation to other
aspects of the subject and to assess its relevant to his own system.
Furthermore, it is obvious that a correct appreciation of a tribological
situation requires a high degree of scientific sophistication, but the same can
also be said of many other aspects of modern engineering.
The inclusion of the basic principles of tribology, as well as tribodesign,
within an engineering design course generally does not place too great an
additional burden on students, because it should call for the basic principles
of the material which is required in any engineering course. For example, a
study of the dynamics of fluids will allow an easy transition to the theory of
hydrodynamic lubrication. Knowledge of thermodynamics and heat
transfer can also be put to good use, and indeed a basic knowledge of
engineering materials must be drawn upon.