Page 298 - Tribology in Machine Design

Lubrication and efficiency of involute gears 283

it is clear that there are, at least, two competing failure mechanisms; one
associated with inclusions in the material of the gear and the other with
surface roughness and lubrication. A material with many internal imperfec-
tions is likely to fail by subsurface fatigue, and the life of the gear made of it
will show little dependence on the ratio of the oil film thickness to the
surface roughness. An inclusion free material will have a much longer life
depending strongly on the oil film thickness to surface roughness ratio.
Improvements in the manufacture of materials, mean that in practice,
surface originated failures dominate, and therefore that surface roughness is
now a critical factor.
There are a number of different types of pitting described in the literature
on gears, however, all of them stem from two basic mechanisms. The basis
used here for the classification of fatigue-related gear teeth failure,
commonly called pitting, is the location at which the failure process
originates.

8.5.1. Surface originated pitting
It has been found that in the case of through-hardened gears (hardness in
the range of 180 to 400 HB) pitting usually originates at the surface of the
tooth. Due to the stresses developed in the contact zone, small cracks are
created on the surface. These cracks grow inwards and after reaching some
depth they eventually turn upwards. As a result of that small metal particles
are detached from the bulk and fall out. In most cases pitting is initiated in
the vicinity of the pitch line. At the pitch point there is only pure rolling
while above and below it there is an increasing amount of sliding along with
rolling. Experiments suggest that pitting usually starts at the pitch line; a
fact never fully explained, and progresses below the pitch line towards
dedendum. It sometimes happens, especially with gears having a small
number of teeth (less than twenty), that pitting begins at mid-dedendum or
even lower. Usually, the dedendum part of the tooth is the first to undergo
pitting, and only in considerably overloaded gears or in gears which have
suffered a significant dedendum wear, is pitting attacking the addendum
part of the tooth observed. The technique used to measure the wear extent
due to pitting consists in taking a replica of the worn tooth profile and then
cutting it normal to the tooth surface. Figure 8.5 shows, schematically, a
Figure 8.5 typical worn involute tooth.

8.5.2. Evaluation of surface pitting risk
Only a very approximate evaluation of pitting risk is possible and that is
done simply by comparing the contact stress with a certain value
characteristic for the material of the gear. If the working contact stress does
not exceed that value then the probability that there will be pitting in the
design life of the gear is rather small. The practical implementation of this
simple rule is somewhat difficult. This is because the evaluation of such
things as material quality, tooth accuracy and lubrication efficiency always