Page 24 - Tribology in Machine Design

Introduction to the concept of tribodesign 11

pinion and gear teeth. This type of damage is believed to be due to abrasion
by hard debris detached from the tip wedge. There are indications of
subsurface fatigue due to cyclic Hertzian stress. The growth of fatigue
cracks can be related to the effect of lubricant trapped in an incipient crack
during successive cycles. Because of conservative design factors, the great
majority of gear systems now in use is not seriously affected by lubrication
deficiency. However, in really compact designs, which require a high degree
of reliability at high operating stresses, speeds or temperatures, the
lubricant truly becomes an engineering material.
Over the years, a number of methods have been suggested to predict the
adequate lubrication of gears. In general, they have served a design purpose
but with strong limits to the gear size and operating conditions. The search
has continued and, gradually, as the range of speeds and loads continues to
expand, designers are moving away from the strictly empirical approach.
Two concepts of defining adequate lubrication have received some
popularity in recent years. One is the minimum film thickness concept; the
other is the critical temperature criteria. They both have a theoretical
background but their application to a mode of failure remains hypothetical.
Not long ago, the common opinion was that only a small proportion of
the load of counterformal surfaces was carried by hydrodynamic pressure.
It was felt that monomolecular or equivalent films, even with non-reactive
lubricants, were responsible for the amazing performance of gears.
Breakthroughs in the theory of elastohydrodynamic lubrication have
shown that this is not likely to be the case. Low-speed gears operating at
over 2000 MPa, with a film thickness of several micrometers, show no
distress or wear after thousands of hours of operation. High-speed gears
operating at computed film thicknesses over 150/im frequently fail by
scuffing in drives from gas turbines. This, however, casts a shadow over the
importance of elastohydrodynamics. The second concept - one gaining
acceptance as a design criterion for lubricant failure - is the critical
temperature hypothesis. The criterion is very simple. Scuffing will occur
when a critical temperature is reached, which is characteristic of the
particular combination of the lubricant and the materials of tooth faces.

1.2.7. Hypoid gears

Hypoid gears are normally used in right-angle drives associated with the
axles of automobiles. Tooth actions combine the rolling action charac-
teristic of spiral-bevel gears with a degree of sliding which makes this type of
gear critical from the point of view of surface loading. Successful operation
of a hypoid gear is dependent on the provision of the so-called extreme
pressure oils, that is, oils containing additives which form surface protective
layers at elevated temperatures. There are several types of additives for
compounding hypoid lubricants. Lead-soap, active sulphur additives may
prevent scuffing in drives which have not yet been run-in, particularly when
the gears have not been phosphated. They are usually not satisfactory
under high torque but are effective at high speed. Lead-sulphur chlorine