Extracting the root of a puzzling gear tooth failure

How we resolved a problem in a power generation drivetrain.

By Eric Olson and Chris Hurrell

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Figure 1. The serious malfunction, which  consisted of broken pinion teeth in the gearbox, took the entire system  out of service. The root cause of the failures was a mystery to both the  unit’s owner and the gearbox OEM.
Figure 1. The serious malfunction, which consisted of broken pinion teeth in the gearbox, took the entire system out of service. The root cause of the failures was a mystery to both the unit’s owner and the gearbox OEM.

A 37-MW electrical power generation system driven by an aeroderivative gas turbine (Figure 1) suffered a puzzling failure in its parallel-shaft gearbox (Figure 2). After only several thousand hours of operation under typical service conditions, the teeth broke on the pinion of a double helical gear set (Figures 3 and 4).

The original equipment manufacturer (OEM) asked us to uncover the underlying cause of the failed gear teeth. Previous studies of this matter proposed gear mesh misalignment as the root cause of the failure. The basis for this conclusion was founded primarily on bluing patterns observed on the pinion’s turbine-end helix.

Because the gear tooth failures were attributed to misalignment, the OEM had begun to redesign the gear tooth profile on the replacement gear to better tolerate misalignment. Nonetheless, the OEM suspected that the gear mesh misalignment theory didn’t explain the gear tooth failures completely. At the time, however, a better explanation wasn’t apparent.

Figure 2. The drivetrain’s gearbox was  instrumented with accelerometers, velocity transducers, and proximity  probes to record the response during the routine operating conditions.
Figure 2. The drivetrain’s gearbox was instrumented with accelerometers, velocity transducers and proximity probes to record the response during the routine operating conditions.

Testing and analysis

We performed a comprehensive on-site evaluation of the drivetrain to capture data that potentially could suggest a better explanation for the failures. This mechanical testing included operating forced response and impact modal testing, vibration monitoring during partial and transient load events, and measuring critical dimensions in the gearbox and shaft position as load and temperature varied. Accelerometers, velocity transducers and a multi-channel data acquisition system captured structural and rotor vibration measurements. Radial proximity probes at each end of both gearbox shafts and an axial proximity probe measured movement of the input (pinion) shaft (Figure 2).

Figure 3. The gear tooth failures were severe enough that sizeable fragments of the pinion not only fractured, but had broken completely free from the gear during routine operating conditions.
Figure 3. The gear tooth failures were severe enough that sizeable fragments of the pinion not only fractured, but had broken completely free from the gear during routine operating conditions.

An Essinger Bar system measured the movement of each gearbox bearing housing. Temperature mapping across the gearbox at each operating condition identified possible anomalies in the temperature distributions, which could have been related to distortion and gear misalignment.

The recorded test data showed that structural vibration occurring in the gearbox during different test load conditions was inconclusive. The motion of the gearbox bearing housings relative to the baseplate was inadequate to cause the misalignment that others suggested, again based on an interpretation of the bluing pattern.

Figure 4. Additional scratches in the gear teeth occurred as broken gear tooth fragments dispersed through the gearbox.
Figure 4. Additional scratches in the gear teeth occurred as broken gear tooth fragments dispersed through the gearbox.

Moreover, gear analysis tools failed to predict magnitudes of mechanical stress that would have caused gear tooth failures at the location where they had occurred (apex edge of the double helix chevron, always on the same side of the apex), even when mesh misalignment was considered in the analysis.

On the other hand, we saw considerable pinion rotor radial and axial vibration, particularly when the gearbox was operated at low loads or at idle speed. Significant shifts in the pinion’s axial position and relatively high-amplitude axial vibration were apparent while the drivetrain operated at idle speed.

The axial vibration was intensified by the excitation of the pinion’s axial vibration frequency. The pinion’s radial vibration rose when the gearbox was operated at synchronous speed but under no load. The primary component of this vibration mode occurred at 43% of running speed, the result of bounded rotordynamic instability. These conditions could have contributed to gear tooth failures, but were insufficient to provide a complete explanation of the high stresses at the failure location.

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