Gearbox Degradation

Analyzing Gearbox Degradation Using Time-Frequency Signature Analysis

SpectraQuest Inc. 8205 Hermitage Road Richmond, VA 23228

(804)261-3300 www.spectraquest.com

March, 2006

Abstract: In this work, vibration signature caused by gear tooth seeded fault is studied in the time and frequency domains. The experiment was conducted on a Machinery Fault Simulator TM (MFS). The pinion gear in the gearbox was intentionally faulted with increasing severities, and a vibration signal was collected for each case. The signal was analyzed in a time and frequency domain. The results indicated that the vibration signature of a faulted bevel gear tooth is a pulse in time domain. Because of this impulse signal, strong sidebands arise in the spectrum around the mesh frequency. Test Setup The experiment is conducted on a Machinery Fault Simulator TM (MFS). The MFS and the position of the gearbox in the MFS are illustrated in Fig. 1.

Figure 1. MFS and Gearbox in MFS

The gearbox is shown in Fig. 2(a). The transmission ratio is 1.5:1. There are two straight bevel gears in the gearbox and they are shown in Fig. 2(b). The number of teeth on the pinion is 18. The gearbox input shaft is connected to a sheave and driven by a “V” belt drive. The gearbox output shaft is connected with a variable brake loader. The tachometer is installed on the rotor deck with a bracket and used to measure the speed of the belt sheave connected to the gearbox input shaft as illustrated in Fig. 3.

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(a) gearbox (b) pinion and gear

Figure 2. Gearbox

Figure 3. Installation of Tachometer

The vibration data was collected by a tri-axial accelerometer installed on the top of the gearbox. The accelerometer is also shown in Fig.3. The frequency limit was set to 5 KHz during data acquisition. During the test, the motor speed was kept at 1000 RPM and the brake loader was set to 2.5. First, the baseline of the gearbox vibration data is collected. Then the gearbox is disassembled and the pinion gear is taken out. The surface of one of the pinion’s teeth on the meshing side is milled using a drill mill. After the damage is done to the tooth, the pinion gear is put back into the gearbox. Vibration data is collected

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again under the same running conditions (speed and load) as before. This process is repeated four more times with additional damage applied to the same pinion tooth for each case scenario. The five damage severities are denoted as Severity Level 1 to Level 5, from the least to the most. The final pinion tooth damage is illustrated in Fig. 4. The main body of the faulted tooth has been milled out and the damage is referred to as the “missing of a tooth”.

Figure 4. Damage Severity at Level 5

Test Results Time Domain Signal The baseline waveforms illustrated in Fig.5. Fig. 5(a) and (b) are of the tachometer signal and gearbox acceleration in the axial direction, respectively. The time between two successive pulses in the tachometer signal is the rotation period of the pinion. The time scales for both Figures 5(a) and 5(b) show the identical time range.

(a) Tachometer signal

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(b) Acceleration signal

Figure 5. Baseline Signals in Time Domain

The waveforms for the damaged pinion with Damage Severity Level 1 are illustrated in Fig. 6. Comparing Fig. 6(b) with Fig. 5(b), notice that there are impulses appearing in Fig. 6(b). By a careful study of the position of the faulted pinion tooth, the positions of the pulses indicated by arrows in Fig. 6(b) correspond to the instances when the faulted pinion tooth enters meshing.



Figure 6. Time Signals (Damage Severity Level 1)

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The waveforms for the damaged pinion with Damage Severity Levels 2 through 5 are illustrated in Fig. 7 through Fig.10. Notice that the impulses caused by the faulted tooth in meshing are consistently clear in each of the four faulted case scenarios. Comparing the signal amplitudes in Fig. 10(b) and Fig. 7(b), it can be concluded that impulse amplitude increases with damage.





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Frequency Domain Signal The spectrum of the baseline vibration data is illustrated in Figure 11. Fig. 11(a) is the spectrum example as it appears in the baseband spectrum reading, and Fig. 11(b) is the spectrum around the mesh frequency with a frequency range from 250 Hz to 350 Hz (the mesh frequency is around 300 Hz, 1000 RPM/60*18 = 300 Hz). The mesh frequency and its harmonics are illustrated clearly in Fig. 11(a). Fig. 11(b) shows that there is a pinion rotation modulation sideband occurring around the mesh frequency component. However, the sideband is not dominant.

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(a) Baseband Spectrum

(b) Spectrum around Mesh Frequency Component

Figure 11. Baseline Vibration Signal Spectrum

The spectrum for the damaged pinion with Damage Severity Level 1 is illustrated in Fig. 12. Comparing Fig. 12(a) with Fig.11(a), notice that although the mesh frequency and its harmonics are still clearly visible, there are more spectrum lines appearing. Most of the spectrum lines can be attributed to the pinion speed sidebands around the mesh frequencies and its harmonics which are illustrated by Fig. 12(b). By comparing Fig. 12(b) with Fig. 11(b), it is easy to see that there are more pinion sidebands appearing.

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Figure 12. Vibration Signal Spectrum (Damage Severity Level 1)

The spectrums for the damaged pinion with Damage Severity Levels 2 through 5 are illustrated in Fig. 13 through Fig.16. A careful study of these figures indicates that the pinion sidebands become more and more dominant as the damage severity levels increase. In particular, Damage Severity Levels 4 and 5 reveal that the mesh frequency components are almost immersed by the pinion sidebands as illustrated in Fig. 15(a) and 16(a). And, Fig. 15(b) and 16(b) indicate the sidebands have higher amplitude than the mesh frequency component under these same two Damage Severity Level examples.

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Summary and Remarks In this test, the vibration signature caused by “bevel gear tooth seeded fault” is studied in the Time and Frequency domains. The pinion gear in the gearbox is intentionally faulted with increasing severity levels while a vibration signal is collected for each case. Then the Time and Frequency domain is used to analyze the resulting vibration signal. The two observations can be summarized as:

1. When a faulted bevel gear tooth enters meshing, it creates an impact pulse in the time domain vibration signal. The amplitude of impulse increases with damage.

2. The pinion sidebands increase with damage.

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Gearbox Degradation