The Effect of Start-Up Load Conditions on Gearbox Performance and Life - Failure Analysis and Case Study


When gearboxes are used in applications in which the connected load has high inertia, the starting torque transmitted by the gearbox can be much higher than the rated load of the prime mover. Power plants often require several evaporative cooling towers or large banks of air cooled condensers (ACC) to discharge waste heat. Because of the very large size of the fans used in these applications, they fall into this category of high inertia starting load devices. When started from zero speed, a very high torque is required to accelerate the fan to normal operating speed. If the fan is started infrequently and run continuously for long periods of time, this high starting torque is of minimal significance. However, when the fan is started and stopped frequently, the number of cycles at the high starting torque can accumulate to a point where they can cause extensive fatigue damage, even if the gear system is adequately rated.

Where the gear unit is marginally rated, very early, catastrophic gear failure is often the result. As part of the overall investigation of several failures in such gearboxes, we measured starting torque on a typical installation, examined many failed gears, and calculated the load capacity ratings for the gearboxes under actual operating conditions. This paper describes the failures observed, the testing conducted, the data analyses and the effect of the high measured starting torque on the life and performance of the gear systems. The test results revealed surprising results, especially during starts where the fan was already wind-milling due to natural air flow in the ACC bank.
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