Low-Speed Corrugation Press Gearbox
This next example is from a heavy-duty corrugation press in a paper plant.
The gearbox in question had an input shaft with a 23-tooth pinion driving a large bull gear with 132 teeth, which in turn drives another bull gear of the same size. The two gears are connected to large steel rollers about 24 inches in diameter. The two output gears turn at 52 RPM and the pinion turns at 302 RPM.
The lubricating oil in the gearbox was regularly subjected to analysis, and the last report stated that there were iron particles in the oil. The maintenance supervisor asked if we could determine the source of the oil contamination, and the first thing we did was to examine the vibration spectra measured near the bearings. The spectra looked normal, without evidence of bearing tones, so we suspected that the metal was coming from one or more of the gears. See the vibration spectrum in the figure below:
The problem then was to identify the faulty gear (if any), so as to allow the maintenance effort to proceed without delay.
We decided to perform synchronous averaging of the gearbox vibration signature.
When we did synchronous averaging on each of the two bull gears, there was no indication of any defect in the waveform. But, when we performed the same test on the pinion, the waveform told another story.
The waveform shows a little over two revolutions of the pinion. There is an obvious area on the gear where the meshing with the bull gear was very noisy and non-uniform. We called for an inspection of the gear, and an access plate was removed so we could look at the gears. We found that the keyway in the pinion shaft was badly worn such that the gear could be rotated back and forth on the shaft by about 1/2 tooth at the edge of the gear. There was also visible clearance between the shaft and the bore of the pinion. The bull gears showed no sign of damage. The averaged waveform is shown below:
The figure below is a photograph of the pinion showing the spalling in the pinion bore caused by the gear turning back and forth on the shaft:
We called one of the engineers at the gearbox factory and described the situation. He said the problem occurred during installation when the interference fit between the pinion and the shaft was too loose. He said the shaft and pinion would have to be replaced, and very soon, to avoid a catastrophic failure.
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