Page 283 - Root Cause Failure Analysis

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GEARBOXES OR REDUCERS

This chapter identifies common gearbox (also called reducer) problems and their
causes. Table 26-1 lists the more common gearbox failure modes. A primary cause of
failure is that, with few exceptions, gear sets are designed for operation in one direc-
tion only. Failure often is caused by inappropriate bidirectional operation of the gear-
box or backward installation of the gear set. Unless specifically manufactured for
bidirectional operation, the “nonpower” side of the gear’s teeth is not finished. There-
fore, this side is rougher and does not provide the same tolerance as the finished
“power” side.

Note that it has become standard practice in some plants to reverse the pinion or
bullgear in an effort to extend the gear set’s useful life. While this practice permits
longer operation times, the torsional power generated by a reversed gear set is not as
uniform and consistent as when the gears are properly installed.

Gear overload is another leading cause of failure. In some instances, the overload is
constant, which is an indication that the gearbox is not suitable for the application. In
other cases, the overload is intermittent and occurs only when the speed changes or
specific production demands cause a momentary spike in the torsional load require-
ment of the gearbox.

Misalignment, both real and induced, is another primary root cause of gear failure.
The only way to assure that gears are properly aligned is to hard blue the gears imme-
diately following installation. After the gears have run for a short time, their wear pat-
tern should be visually inspected. If the pattern does not conform to vendor’s
specifications, the alignment should be adjusted.

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