Gearbox noise is sometimes difficult to diagnose without sophisticated vibration measurement tools. However, some general guidelines are as follows:
Gear noise from a deteriorating gear set is usually a high pitch “whine”. This whine is generated by the tooth meshing frequency. For example; an input gear with 30 teeth running at 1800 RPM would have a meshing frequency of 900 HZ. Compare this to the shaft rotation frequency of only 30 HZ (1800/60).
Bearings rotate at input shaft speed or some increasing or decreasing multiple. The rotational frequency of the shaft is several orders of magnitude below the gear mesh frequency. Therefore, the noise produced by bearings as they fail is closer to a “growl” or at least a frequency much lower than gear mesh frequency. In the advanced stages of failure, the balls or rollers may slide producing a screech.
Pinpointing the noise is easier said than done especially on systems without torsionally tuned couplings. Noise transfers via shafts and other rigid members and often sounds as though it is coming from a gearbox when it may actually be transmitted there from another source. A “listening rod” may be used to help locate the noise. Troublesome bearings usually generate excessive heat. Look for hot spots on the gearbox.
When it comes to noise, be certain that the gearbox is the culprit. Many a gearbox has been torn down only to be found in good condition.