Page 307 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 287

Gear Noise

Unfortunately, gear units are a noise source. Manufacturers are work-
ing on solutions, but they have not found any economical answers. Many
factors, some of which are listed below, can contribute to gear noise.
However, the causes are not limited to the items on this list.

1. Tooth Spacing Errors-Spacing errors are usually caused by man-
ufacturing problems, damage to the teeth during assembly/disas-
sembly, or occasionally during operation when foreign objects
pass through the mesh. The spacing error may cover only one
tooth space or a number of tooth spaces. This type of error usually
will cause a bumping noise or a cyclic noise with a frequency
equal to the rotating speed of the gear or pinion with the spacing
error. In most cases, the gear will just run roughly and have re-
duced life.
2. Involute Error-Involute is the curve form used for the mating
tooth surfaces. It is not necessary that the involute be “textbook”
but the tooth flank curves must be conjugate, that is, matching.
This error can occur when these surfaces are either manufactured
incorrectly or destroyed by wear or scoring.
3. Surface Finish-Surface finish on gear teeth very seldom causes
noise except in extreme cases of scoring or abrasive wear.
4. Lead Error (Helix angle error)-Lead error is only important if
the leads (helix angles) of the gear and pinion are not matched.
When the leads do not match, the gear may be quiet when new,
and as wear occurs, the gear will become noisy.
5. Wear on Tooth Flanks-Wear only causes noise when it is severe
and when the gear teeth do not wear evenly and maintain conju-
gate action.
6. Pitting-Tooth flank pitting is not a great influence on noise un-
less it is very severe.
7. Resonance-Exciting the natural frequency of gears, shafting,
housing, or supports can produce high noise levels. The most
troublesome resonances tend to be in the housings and support
bases and can sometimes be corrected with additional stiffening.
8. Tooth Deflections-Under load, teeth deflect and tend to lose their
conjugate forms. Gear manufacturers modify the involute form so
that under load, interference does not occur, and conjugate action
is maintained. Excessive tooth deflections can cause noise.
9. Improper Tip or Root Relief-This is one method of making the
tooth deflection corrections as required in No. 8. The higher the
load on a gear tooth, the more tip or root relief is required to pre-
vent interference or to maintain lubricant film.