Aircraft Reciprocating-Engine Failure - Pilot und Flugzeug

Bearing clearance
Bearing clearance is an important variable that affects the nature of bearing
lubrication through its affect on oil-film thickness and oil-film temperature, figure
8.46. If the radial clearance is too small, the increase in oil film temperature and
decrease in oil film thickness will lead to boundary lubrication.
Figure 8.46:
The effect of bearing clearance on oil-film thickness (Shigley
and Mischke, 1989, p.508)
Crankshaft journals may wear during engine operation. The effects of journal wear
may be corrected during engine overhaul by polishing the journals to an ‘undersize’
diameter, and installing ‘oversize’ bearing inserts. For Lycoming engines, journals
may be reduced in diameter by 0.003in during overhaul. The oversize bearing
inserts include a marking ‘M03’, (journal, minus 0.003in) with the bearing part
number marking.
Two powertrain failure events investigated in this study were associated with
crankshafts that had been polished to an undersize condition during overhaul;
occurrence 2000/2276, VH-ODE and occurrence 2002/3474, VH-ACZ. The
physical evidence available for occurrence 2002/3474, VH-ACZ, indicated that
bearing damage, through boundary lubrication, differed from other occurrences
investigated, figures 8.47 and 8.48. It is apparent that boundary lubrication affected
the surface of both the rod and cap inserts, and was localised to several
circumferential tracks. A detailed metrological inspection of the journals revealed
that polishing to an undersize state had left the journals in an out-of-round
condition, figure 8.49.
– 167 –