Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 10, Gear Vibration
Section 2, Gearmesh Frequency Harmonics and Sidebands
When the excessive vibration occurs at a frequency of number of gear teeth x rpm, there are two main possibilities for its source. One is that there may be a resonance to this frequency. The other includes actual defects such as machining errors, wear, cracked teeth, etc., or large centerline orbits due to unbalance or misalignment. The most likely and easiest parts to check for possible resonance are the external non-rotating parts, such as the gearbox cover, a section of the metal fabricated base, beams and even the concrete floor.
In one case, a papermachine could not reach its new higher speed since the number of gearmesh frequencies x rpm vibration was in resonance with the concrete floor. The engineers were afraid to go to a higher speed because they assumed the vibration would only get worse. Actually, all they had to do was to determine the actual resonant frequency of the floor by using a variable speed vibration shaker, and then determine if their new operating speed would result in a gearmesh vibration frequency that would be at least 20 to 25 percent above that. That was determined to be the case and then they were able to run at top operating speed.
Most published articles on gear vibration discuss the torsional resonances of the shaft supporting the gear. This, of course, is very possible but not common to the writer's experience. Perhaps that is due to the writer's working only on developed machinery and not on new designs. Therefore, the original equipment manufacturers have probably already de-bugged the possible torsional or bending resonant frequencies. However, torsional resonance may still occur, since it may have changed from the original design frequencies by the variance in weights of such attached parts as coupling and pulleys.
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