The oils used to
lubricate aircraft turbine
gears are almost
without exception ones which
conform to specifications of military agencies
of various nations or those
issued by engine manufacturers. Even though the oils
have been approved for use,
this does not necessarily eliminate
the chance for lubricating problems but
does reduce the possibility. If the lubricant is actually at
fault, in case of trouble with aircraft gears, the first correction is to use oil
from another lot. The oil in question may have oxidized, lost its load carrying
ability, or become contaminated. Contamination may arise from unexpected
sources. Thus, failure of aircraft engine pinion sets is cited in Lubrication^32
where: “At one engine overhaul shop, engine pinion gear set failure began to
occur during green runs”. Finally, the trouble was traced to abrasive material used
to clean parts by blasting. Such abrasives were being “built into” the engines
at overhaul. Later these abrasives were picked up by the circulating oil on test
stands.
The steps by which this problem was
solved illustrate the investigation sometimes necessary before the reasons for gear
trouble can be pinpointed. To determine
if the failures were due
to overhaul practice
or to test stand operation, pinion gear
set which had operated
satisfactorily in flight service for
1,400 hours was installed in
a freshly overhauled engine. On the test stand this gear set
promptly failed. Abrasive scoring was found on both the gear and the bushings.
Therefore, the deposit from the oil screen was examined under a microscope
followed by photomicrographs and emission spectrographic analysis. All of this
pointed to alpha aluminum oxide
as the primary
offender, this being the material
used to clean parts
by blasting.
