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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?
This article contains excerpts from the paper, "High frequency vibrations on gears" by Dietmar Sterns of RENK Aktiengesellschaft and Michael Elbs of ISMB Dautermann GmbH at the 2017 Turbomachinery Symposium.
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. Ask the OEM and/or a specialist on machinery diagnosis for support.
In general, do not use the total value of the acceleration signal for machinery protection, i.e. for triggering a trip signal. However, the total value of the acceleration signal can be used for machinery diagnosis. It may be used to trigger an alarm, but the alarm value should depend on operating parameters such as speed and power. This variable alarm value should be set in accordance with the baseline values.
The value of the acceleration signal in terms of condition monitoring can be increased by applying frequency filters on the acceleration signal, so that only the frequency band of interest is monitored. For example, a frequency band around the gear mesh frequency may be monitored. For fixed speed applications, this can be achieved quite easily, as the frequency filters can be set to constant values. For speed variable drives, the limit values of the frequency filters need to be proportional to the speed. More complex evaluation methods of the acceleration signal could increase further its value for condition monitoring. Such methods exist and are already in application for roller bearings. For gears they need to be developed respectively to be integrated into monitoring systems.