Theory

Safety and reliability are most important concerns in the field of engineering. With progress in the field of engineering the rotating machines are operating at higher speed and running for longer period of time. To increase the service life of a machine, early and accurate fault identification is a major concern for all. Most machines have gearboxes, bearings and power transmission elements. The fault problems in these units are more common.There are several signal analysis techniques for the condition monitoring of the gearbox.

Vibration response based fault detection method

1. Tooth Profile Deviations

i. Load Effects

Tooth deflection under load tends to give a signal waveform of a stepped nature due to periodically varying compliance as the load is shared between different number of teeths.

ii. Machining Errors

The machining process used in manufacturing the gear often introduces profile errors on the gear teeth.

iii. Ghost Components

The term 'Ghost Component' is used to describe periodic faults which are introduced into the gear tooth profiles during the machining process and corresponds to a different number of teeth to those actually being cut.

iv. Uniform Wear

Systematic wear is caused by the sliding action between the teeth which is present at either side of the pitch circle,but not at the pitch circle itself.Thus wear will not be uniform over the profile of the tooth and will cause a distortion of the tooth profile.

2. Amplitude Modulation Effects

Randall explained amplitude modulation by the sensitivity of the vibration amplitude to the tooth loading.If the load fluctuates, it is to be expected that the amplitude of the vibration will vary accordingly.

3. Frequency modulation Effects

Vibrations in the rotational speed of the gears and/or vibratios in the tooth spacing will give a frequency modulation of the tooth-meshing frequency..

4. Additive Impulses

Most local faults associated with tooth meshing will cause an additive impulse in addition to the amplitude and frequency modulation effects.Whereas the modulation effects cause changes in the signal which are symmetrical about zero line,additive impulses cause a shift in local mean position of the signal,that is,the portion affected by the additive impulse is no longer symmetrical about zero line