Gear Related Problems
Normal Spectrum shows 1x and 2x RPM, along with Gear Mesh Frequency (GMF). GMF commonly will have running speed sidebands around it relative to the shaft speed which the gear is attached to. All peaks are of low amplitude and no natural gear frequencies are excited.
A key indicator of gear tooth wear is excitation of the Gear Natural Frequency, along with sidebands around it spaced at the running speed of the bad gear. Gear Mesh Frequency (GMF) may or may not change in amplitude, although high amplitude sidebands surrounding GMF usually occur when wear is noticeable. Sidebands may be a better wear indicator than Gear Mesh Frequencies themselves. (see Vibration Case History number 8).
Gear Mesh frequencies are often very sensitive to load. High GMF amplitudes do not necessarily indicate a problem, particularly if sideband frequencies remain low and no gear natural frequencies are excited. Each analysis should be performed with the system at maximum operating load.
Fairly high amplitude sidebands around GMF often suggest gear eccentricity, backlash or non-parallel shafts which allow the rotation of one gear to "modulate" the running speed of the other. The gear with the problem is indicated by the spacing of the sideband frequencies. Improper backlash normally excites GMF and Gear Natural Frequencies, both of which will be sidebanded at 1x RPM. GMF amplitudes will often decrease with increasing load if backlash is the problem.
Gear Misalignment almost always excites second order or higher GMF harmonics which are sidebanded at running speed. Often will show only small amplitude 1x GMF, but much higher levels at 2x or 3x GMF. Important to set the Fmax high enough to capture at least 2 GMF harmonics if the transducer has the capability.
A Cracked or Broken Tooth
will generate a high amplitude 1x RPM of this gear, plus it will excite the gear natural
frequency (fn) sidebanded at its running speed. It is best detected in Time
Waveform which will show a pronounced spike every time the problem tooth tries to mesh
with teeth on the mating gear. Time between impacts (
Hunting Tooth Frequency (fHT) is particularly effective for detecting faults on both the gear and the pinion that might have occurred during the manufacturing process or due to mishandling. It can cause quite a high vibration, but since it occurs at low frequencies, predominantly less than 600 CPM, it is often missed. A gear set with this tooth repeat problem normally emits a "growling" sound from the drive. The maximum effect occurs when the faulty pinion and gear teeth both enter mesh at the same time (on some drives, this may occur once every 10 or 20 revolutions, depending on the fHT formula). Note the TGear and TPinion refer to the number of teeth on the gear and pinion respectively. Na = number of unique assembly phases for a given tooth combination which equals the product of prime factors common to the number of teeth on each gear.
-::- This site is designed and authored by David Stevens IEng MIET FIDiagE MICML -::- |
) will correspond to 1/speed of gear with the problem.
Amplitudes of impact spike in Time Waveform will often be much higher than that of 1x Gear
RPM in FFT.
