Troubleshooting: Gear Defects
Planetary gear systems are somewhat more complex than standard gear pairs due to the planet gear centers rotate around the sun gear at a rate called the train frequency. The sun gear RPM, a planet gear RPM or the train frequency can modulate the tooth mesh frequency. This can produce complex sets of sidebands in the spectrum, and can be difficult to interpret.
A Cracked or Broken Tooth generates a high amplitude 1x RPM of this gear, plus it excites the gear natural frequency side-banded at its running speed. It is best detected in Time Waveform which shows a pronounced spike every time the problem tooth tries to mesh with teeth on the mating gear. Time between impacts corresponds to 1/speed of gear with the problem. Amplitudes of impact spike in Time Waveform will often be higher than that of 1x Gear RPM in FFT.
Planetary Gear Defect
The ring gear in a planetary gearbox often has a region where the planetary teeth do not mesh smoothly, generating “impacting” each time a planetary gear passes over that region. In the spectra, this produces a periodic activity at the rate of the number of planetary gears times carrier turning speed.
An example signature presented is from a planetary gearbox where the ring gear has 89 teeth and the planetary gear has 36 teeth. A defect in the planetary teeth produces an impact at 89/36 or 2.47 orders of the carrier. When the defect in the planetary teeth aligns with the defect in the ring gear, the amplitude in the time waveform increases. This happens about every two turns of the carrier that corresponds to five turns of the planetary gear.
The spectrum of a cracked gear or cracked gear tooth has high amplitude peaks at the turning speed of the shaft with the bad gear and much harmonic activity. Usually gear mesh frequency and harmonics are also seen.
The waveform has a very high-impacting at turning speed due to the crack trying to open and close as the tooth is loaded and unloaded. If the waveform is displayed in the circular format with running speed correctly set, the plot shows this energy as the crack tries to open and close.
Information courtesy of Emerson Controls.