There are not only meshing frequency and side frequency components in gear vibration signal, but also other vibration components. In order to effectively identify gear fault, these components need to be identified and distinguished.
The signals generated by the modulation of gear signals are generally symmetrical to the zero level. However, due to the influence of additional pulse, the measured signal is not necessarily symmetrical to the zero line. The additional pulse is directly superimposed on the normal vibration of the gear, rather than in the form of modulation, which is easy to distinguish in the time domain, as shown in Figure 8.
In frequency domain, the additional pulse and modulation effect are also easy to distinguish. The modulation produces a series of side frequency components on the spectrum. These side frequencies are centered on the meshing frequency and its harmonic frequency, and the additional pulse is the low harmonic of the gear rotation frequency.
Figure 8 decomposition ofvibration signal into additional pulses
The main causes of additional pulse are gear dynamic balance, alignment and mechanical looseness. The additional pulse is not necessarily directly related to the defect of the gear itself. The influence of the additional pulse will not exceed the low frequency range, that is, below the meshing frequency.
The serious local fault of gear, such as serious peeling off and broken teeth, will also produce additional pulse. At this time, in the low frequency range, it shows the increase of gear rotation frequency and its harmonic component.
The hidden spectrum is a frequency component of power spectrum, which is caused by periodic defects in the process of machining. The error of indexing worm and gear on the table of gear hobbing machine. The implied spectrum has the following characteristics.
(1) The hidden spectrum is generally corresponding to the number of teeth of a indexing, so it must be a harmonic of a specific rotation frequency.
(2) The hidden spectral line is produced by geometric error, and the working load of gear has little effect on it, and it will gradually reduce with the running in and wear of gear.
Because the position of measuring point is usually selected on the bearing seat when measuring the vibration of gear, the measured signal must contain the component of bearing vibration. The vibration level of the normal bearing is obviously lower than that of the gear, which is generally one order of magnitude smaller. Therefore, the frequency component of the bearing vibration is not obvious in the range of the gear vibration frequency. The characteristic frequency components of the vibration signals of journal bearings can be found in the low frequency range, that is, the rotation frequency and its low harmonic frequency range. The characteristic frequency range of rolling bearing is wider than that of gear, so the diagnosis of rolling bearing should not be carried out in the range of gear vibration, but in the high frequency range or other methods.
When the rolling bearing has a serious fault, there may be obvious characteristic frequency components in the frequency band of gear vibration. These components sometimes appear alone, sometimes cross modulation with gear vibration components, and sum frequency and difference frequency components appear, and sum frequency and difference frequency will change with the change of their basic components.