Accelerometers are commonly used on high speed gear units that are built according to API 613. But the use of the signals from the accelerometers is very often subject to discussion. The questions discussed the most are:
- Should the accelerometer signal be used as acceleration or velocity?
- Should trip points be based on RMS (Root Mean Square) or 0-peak values?
- What frequency range should be analyzed?
- What values should be used for the alarm and trip set points?
There are no general answers to these questions. Especially for acceleration signals, the choices are complex and need to be analyzed case by case. Field data recorded on many different production machines indicate that the total acceleration value can vary strongly depending on speed. It is presumed that this is due to resonances excited by the gear mesh frequency. In some of these resonances, the frequency spectrum of the acceleration signal was dominated by sidebands of the gear mesh frequencies, which could not be explained at first.
To be able to use the acceleration signal for condition monitoring, it is necessary to:
- Record the vibration baseline data during a loaded test (if such test is performed) and during commissioning. To record the baseline completely, vary the speed and power over the whole specified range or at least over the expected operating range.
- Store the baseline data. This is the vibration signature of the gear unit in new, undamaged condition and is going to be used as a reference for any vibration data that will be recorded later, during operation.
- Compare any acceleration values measured during operation with the baseline value recorded at the same operating conditions (speed, power, lube oil pressure, lube oil temperature).
- If the total acceleration value recorded during operation shows a strong increase related to the baseline data recorded at them same operating conditions, then the vibration data should be investigated in detail.