It is necessary to set the Fmax high enough to see 3x the highest gear mesh frequency and sidebands.Collect 6-10 rotations of each shaft with adequate resolution to see sidebands and detail.Time Synchronous Averaging: (Commonly used when attempting to diagnose gear faults)It is used to average away all of the vibration sources that are not synchronous with the tach pulse, which is taken from the shaft of the gear of interest.
There are two types of gearbox faults
Shaft Turns related faults;
Where Spectrum data represents the fault condition better.
The gearmesh frequency (and or 2xGM and 3xGM) will change in amplitude and the sidebands around the gearmesh peaks will change in amplitude.
Eaxample; Misalignment, Bent Shaft
Gear Meshing related faults;
Where Time Waveform data represents the fault condition better.
As the change in vibration will be momentary, the vibration will change once per revolution.
The spectrum will change, but it will change in less predictable ways
You may be able to diagnose how many teeth are damaged, and using techniques like time synchronous averaging, you will be able to perform this analysis even in complex gearboxes.
Example; Cracked, Broken or Worn Teath.