Gearbox Bearing Failure on a 1500 rpm Drive Train
A 75 kW, 4-pole induction motor drives a belt conveyor through a two-stage helical gearbox. Motor speed is 1480 rpm; gearbox ratio is 25:1; output shaft speed is about 59 rpm. A vibration analyst reports a growing peak at 118 Hz on the gearbox input bearing, with sidebands at 1.5 Hz spacing. Bearing geometry: 9 balls, ball diameter 12 mm, pitch diameter 62 mm, contact angle 0 degrees. The gearbox has a 23-tooth input pinion and a 92-tooth intermediate gear, then a 19-tooth pinion and a 95-tooth output gear. Oil analysis shows iron particles at 45 ppm, up from 12 ppm three months ago. The maintenance team wants to run to the next planned shutdown in 6 weeks. Production cannot stop the conveyor for more than one 8-hour shift.
1. Identify the likely fault and compute the relevant bearing frequencies to confirm.
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Shaft speed on the input shaft: 1480/60 = 24.67 Hz. BPFO = (n/2) x f_r x (1 - (d/D) cos(contact angle)) = (9/2) x 24.67 x (1 - (12/62)) = 4.5 x 24.67 x 0.8065 = 89.5 Hz. BPFI = (n/2) x f_r x (1 + d/D) = 4.5 x 24.67 x 1.1935 = 132.5 Hz. BSF = (D/(2d)) x f_r x (1 - (d/D)^2) = (62/24) x 24.67 x (1 - 0.0375) = 2.583 x 24.67 x 0.9625 = 61.3 Hz. FTF = (f_r/2)(1 - d/D) = 12.33 x 0.8065 = 9.95 Hz. The observed 118 Hz does not match BPFO (89.5), BPFI (132.5), or BSF (61.3) directly. However, 118 Hz is close to 2 x 59 Hz (output shaft) plus something, or it could be a gear mesh frequency: input mesh = 23 x 24.67 = 567 Hz; intermediate shaft speed = 1480 x 23/92 = 370 rpm = 6.17 Hz; second mesh = 19 x 6.17 = 117.2 Hz. That matches 118 Hz almost exactly. So the dominant peak is the second-stage gear mesh frequency, not a bearing defect frequency. The 1.5 Hz sidebands suggest modulation at a shaft speed related to the output (59 rpm = 0.98 Hz) or intermediate (6.17 Hz) - 1.5 Hz does not match either exactly, so it may be a beat with line frequency or a hunting tooth frequency. The iron rise suggests wear, but the spectral evidence points to gear mesh, possibly a cracked or worn tooth on the second stage, not a bearing.
2. Given the ambiguity, what further measurements would you take before deciding to run to shutdown?
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(1) High-frequency envelope (demodulation) analysis around the bearing resonances (typically 500 Hz-10 kHz) to look for BPFO/BPFI/BSF harmonics and their sidebands at shaft speed - bearing defects show up in envelope spectra even when the raw spectrum is dominated by gear mesh. (2) Time-waveform analysis to look for periodic impacts. (3) Check the gear mesh sidebands more carefully: sidebands at input shaft speed around 567 Hz indicate input pinion problems; sidebands at intermediate shaft speed around 117 Hz indicate intermediate gear problems. (4) Oil analysis trend: sample every two weeks; a rapid rise in iron and the presence of large (>15 micron) particles suggests active wear. (5) Check gearbox oil temperature and filter differential pressure. (6) Inspect the gearbox mounting and coupling alignment, since misalignment produces 2x running speed and can mimic gear mesh. (7) Review load history - if the conveyor has been running at higher tonnage, mesh forces increase. Only after these should the run-to-shutdown decision be made.
3. The team decides to run for 6 weeks. What monitoring and contingency plan do you put in place?
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Set a vibration alarm at 118 Hz amplitude growth of 3 dB above current, and an envelope alarm at any bearing frequency exceeding a defined threshold. Increase oil sampling to weekly and set a particle-count and iron-concentration limit (e.g., iron above 60 ppm triggers action). Install continuous vibration monitoring with a trip interlock if overall velocity exceeds ISO 10816 zone D (about 7.1 mm/s RMS for this machine class). Prepare a spare gearbox or at least a spare bearing set and the special tools, and pre-book the 8-hour shift. Define a decision point at week 3: if the trend continues upward, bring the shutdown forward. Brief operators to report unusual noise or oil leaks. Document the risk acceptance with production management, since a catastrophic gearbox failure could cause secondary damage to the motor coupling and a much longer outage than one shift.