Page 198 - Root Cause Failure Analysis

186 Root Cause Failure Analysis

and the parallel relationship of the pinion and gear shafts should be verified for any
gearbox that is being investigated.

OPERATING METHODS
Two primary operating parameters govern effective operation of gear sets or gear-
boxes: maximum torsional power rating and transitional torsional requirements.

Each gear set has a specific maximum horsepower rating. This is the maximum tor-
sional power that the gear set can generate without excessive wear or gear damage.
Operating procedures should ensure that the maximum horsepower is not exceeded
throughout the entire operating envelope. If the gear set was properly designed for the
application, its maximum horsepower rating should be suitable for steady-state opera-
tion at any point within the design operating envelope. As a result, it should be able to
provide sufficient torsional power at any set point within the envelope.

Two factors may cause overload on a gear set: excessive load or speed transients.
Many processes are subjected to radical changes in the process or production loads.
These changes can have a serious effect on gear-set performance and reliability.

Operating procedures should establish boundaries that limit the maximum load varia-
tions that can be used in normal operation. These limits should be well within the
acceptable load rating of the gear set.

The second factor, speed transients, is a leading cause of gear-reliability problems.
The momentary change in torsional load created by rapid changes in speed can have a
dramatic, negative impact on gear sets. These transients often exceed the maximum
horsepower rating of the gears and may result in failure. Operating procedures should
ensure that torsional power requirements during startup, process-speed changes, and
shutdown do not exceed the recommended horsepower rating of the gear set.