7/31/2019 Mill - Drive Gearbox Input Pinion Shaft Failure
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Problem Statement: Poor life & frequent failure of Sag Mill
drive gearbox input pinion
shaft at Mines.
Observations: The above problem was studied in detail and
following observations
were made:
It was reported that in last three years, total 3 input pinion
shafts of the main mill
drive gearbox have failed and the failed pinion shafts gave ~12
months life (refer
plate -1 & 2).
The speed rating of the gearbox is 1470/232.59 and it was
designed for a service
life factor of 1.92.
Department confirmed that each failure was reported to OEM via
emails but from
OEM no satisfactory answer for the failures of input pinion
shaft was given as a
result every time the failed shaft was replaced with available
spare shaft and the
problem reappeared after sometime.
It was observed that all three failures of input pinion shaft
took place from the
same location i.e. the position where pinion is mounted on the
pinion shaft (refer
plate -2, rough sketch).
Clear beach marks (progressive lines) were observed on the
fractured surface of
both failed shafts. The beach marks covers around 90% of the
fractured surface
area and is an indication of unidirectional fatigue mode failure
(refer plate 3 & 4).
The instantaneous zone area (final fracture area) was found very
small ~10% of
the fractured surface area indicates that the designer has taken
sufficient service
factor during designing. It also indicates that the failure
slowly propagated across
the fatigue zone, and then very rapidly crossed the
instantaneous zone (refer
plate-3&4).
It was observed that in all the failed pinion shafts, the
surface of the pinion shaft
had peeled off along its circumference i.e. spalling, which is
an indication of failure
due to high surface hardness. Moreover, presence of beach marks
in the failed
surface (in spalling area) indicate that the failure took place
in fatigue mode and
originates from the keyway fillet radius due to severe stress
concentration (refer
plate - 3 & 4)
Recommendations:
Above observations suggest that the component failed due to
rotating bending
fatigue under low nominal load and severe stress concentration
created due to
high surface hardness (improper heat treatment) and sharp fillet
radius in the
keyway at the location of failure. This aspect may be checked at
OEM end.
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