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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 7, Misalignment

Section 13, Higher Frequency Harmonics

The more frequent sources for higher frequency harmonics originate from the following:

Defective or worn gear teeth. These can result in harmonic frequencies of number of gear teeth x rpm and also harmonics of that frequency.

Gear shaft centerline orbit. Sometimes the gear teeth may be OK but the gear's centerline orbits around it's axis of rotation with an excessive displacement amplitude. This causes the gear's rotating pitch circle to present itself to its mating gear as if it were an eccentric pitch circle. (The pitch circle may be actually concentric as revealed in a slow roll, but when running at operating speed, unbalance or misalignment can cause the pitch circle to run "eccentrically.") This causes certain portions of the geartooth's curved surface to not properly roll against the correct portion of the mating tooth's curved surface. The error in mating surfaces is caused by only a few units of displacement, but it still results in impulses that are greater than they should be. This enlarged amplitude vibration at gearmesh frequency can then have enough vibratory energy to travel further throughout the machine, enabling it to reach a resonant section. If so, the amplitude at gearmesh frequency is magnified.

The solution is to always determine if the gear has excessive displacement at amplitudes that result from unbalance or misalignment. If so, correcting the unbalance or misalignment will reduce the vibration at the gearmesh frequency. As of this date, between unbalance and misalignment, the most frequent source for high amplitudes at gearmesh frequency has proven to be shaft misalignment. If there are no symptoms for excessive unbalance, misalignment or other sources, then analyze for defects that are part of the gear.

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