Tabletting Press Example
Following is a simplified diagram of the drive mechanism for a large tabletting press. The speed of the large ring gear is 56 RPM. This machine was moved from one building to another, and encountered an accident while en route. It was decided to perform a vibration analysis on it to find any hidden damage.
The following vibration spectra were measured eight days apart. The lower spectrum was the first to be collected. Note the gear mesh frequency at 3640 RPM, with the two prominent sidebands spaced at the pinion RPM from the GMF. This indicated modulation of the ring gear speed at the pinion rate, and is probably caused by a bent pinion shaft or uneven pinion tooth wear.
It was then decided to collect a synchronous averaged waveform with a time span of about 3 ring gear revolutions. The waveform from the earlier measurement is shown below:
This is not remarkable in that it does not show any significant repetitive pattern.
Now, note the upper spectrum in the previous figure, made 8 days later. Here we see sidebands around the GMF at the ring gear turning speed. This indicates the ring gear vibration level is fluctuating once per revolution. This could indicate a localized fault in the ring gear that causes momentary level changes. Again, a synchronous averaged waveform was collected, and is shown below. The time duration is about 3 seconds, so the plot encompasses 3 revolutions of the ring gear. Note the very definite repeating pattern of a high vibration level. The conclusion was that the ring gear was damaged by the accident, probably by impacting the pinion, and the damage increased rapidly as the machine ran.
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