Page 297 - Tribology in Machine Design

282 Tribology in machine design

Knowing both the thickness of the oil film, eqn (8.10) and the roughness of
the gear tooth surfaces, the parameter A can be determined. It is standard
practice to assume a thick film lubrication regime and no danger of scuffing,
when A is greater than 1.2. In the case when A is less than 1.0, some steps
should be taken to secure the gear set against a high probability of scoring.
This scoring is a direct result of insufficient thickness of the oil film. The
usual remedy is to use an oil containing surface-active additives.

8.5. Gear pitting It is not absolutely necessary to have contact between interacting gear teeth
in order to produce wear, provided the running time is long enough. The
Hertzian stresses produced in the contact zone of interacting gear teeth can
lead to a fatigue which is regarded as a standard mode of failure. It takes the
form of pitting; a pit being a small crater left in the surface as a result of a
fragment of metal falling out. The presence of a lubricant does not prevent
this, for under elastohydrodynamic conditions the surface pressure distri-
bution is essentially that found by Hertz for unlubricated contacts. It could
be argued that pitting is caused by lubrication in the sense that without
lubrication the surface would fail long before pitting could appear.
However, there are some reasons to believe that the lubricant is forced into
the surface cracks by the passage of very high pressure and the lubricant
then acts as a wedge to help open up and extend the cracks.
It is known from experiment that smooth surfaces pit less readily. It was
found from tests run on a disc machine using a small slide/roll ratio, that
increasing the oil film thickness, also reduced the tendency to pit and that
the ratio of the surface roughness to the oil film thickness, was the dominant
parameter. The correlation between the number of revolutions before
pitting occurred, and the surface roughness to oil film thickness ratio, holds
over a 500-fold variation in the above-mentioned ratio.
It must be emphasized, however, that the surface roughness measured
was the initial value, and that the roughness when pitting occurred was very
much less. Fatigue failures can originate either at or beneath the surface and