Gear Fault Detection (Sound and Vibration Measurement Suite)
December 2007
NI Part Number:
372416A-01
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You can use the envelope detection technique to perform machine fault detection on a gear. For a normal gear, the gear meshing frequency and its harmonics dominate the frequency or order power spectrum of the envelope signal. The gear meshing harmonics might have some narrow sidebands that spread across one to two orders. These narrow sidebands are the result of the geometric and assembly imperfections, such as tooth imperfection and alignment imperfection, of the gear pair. A local gear fault, such as a tooth crack, produces an additional modulating effect on the normal meshing vibration signal and generates sidebands in the power spectrum of the envelope signal that spread over a wide frequency range. These sidebands might indicate faults on the gear teeth.
Use the OAT Envelope Detection VI to calculate the envelope signal of the vibration signal of a gear. Use the Waveform Output instances of the OAT Envelope Detection VI to perform envelope detection when the rotational speed of the gear is constant. Use the Even-Angle Signal Output instances of the OAT Envelope Detection VI to perform envelope detection when the rotational speed of the gear is variable. You must manually select whether to use the 1 Channel or the N Channels instance of the OAT Envelope Detection VI.
For constant-speed tests, specify the center frequency [Hz] and the span [Hz] inputs of the OAT Envelope Detection VI. For variable-speed tests, set the unit of the center frequency/order input to order to specify a center order for performing envelope detection. You specify a center order instead of a center frequency because the order does not change with the rotational speed of the gear.
The output of the OAT Envelope Detection VI is an envelope signal. You can use the SVFA Power Spectrum VI to display the frequency power spectrum and the OAT Order Power Spectrum VI to display the order power spectrum of the envelope signal.
Performing Gear Fault Detection
The following example demonstrates how to use the OAT Envelope Detection VI to detect gear faults.
The gear pair under test is a 12-tooth pinion gear driving a 38-tooth bull gear. In this variable-speed test, the pinion gear speed increases from 700 RPM to 2,400 RPM and then coasts down to 700 RPM.
Because the gear speed is variable and the vibration signal is on only one channel, use the Even-Angle Signal Output 1 Channel instance of the OAT Envelope Detection VI to calculate the even-angle envelope signal of the vibration signal. In this example, one of the dominating gear meshing harmonics of the gear pair is the 72nd order. Therefore, set the value of the center frequency/order input to 72, and set the unit to order. Set the span [Order] input to 11. Then run the OAT Envelope Detection VI to acquire the envelope signal of the vibration signal.
Use the OAT Order Power Spectrum VI to compute the order power spectrum of the resulting even-angle envelope signal. If the gear pair has no fault, the sidebands around the gear meshing harmonics are negligible, as shown in the following front panel.
If a gear fault exists, the power spectrum of the envelope signal exhibits significant sidebands that spread over the span you specify, as shown in the following front panel.
You thus can use the envelope detection technique to distinguish normal gears from faulty gears.