Diagnosing a Persistent Gearbox Noise: Lessons from a Refurbished Unit

A newsprint mill recently reached out after discovering a persistent noise in a single‑stage gearbox that had just returned from a local refurbishment. From day one, the unit was flagged for weekly vibration monitoring in an effort to pinpoint the source of the issue.

Vibration data pointed to a potential outer‑race fault on the input‑shaft support bearings, prompting the decision to remove the gearbox from service and bring it to us for a detailed investigation.

Our first step was a “blue‑in” of the gear set to assess mesh contact. The results were telling: contact varied alternately on opposite sides of the gear wheel at 180°, indicating that the wheel’s center of rotation was not perfectly aligned with the shaft—essentially a wobble. The pinion’s tooth profile also lacked the typical relief that helps maintain quiet operation.

After consulting with a vibration specialist, we reviewed the frequency data. While the fault frequency matched an outer‑race signature, the amplitude was below the level that would warrant immediate removal, and the frequency did not correlate with any gear cycle. The actual fault was uncovered during a physical inspection: the bearing on the drive end of the input shaft had been compromised by current leakage, pointing to a grounding problem between the motor and gearbox, rather than a remanufacturing defect.

Although this grounding issue explained the bearing fault, it could not account for the noise experienced from day one, indicating that the root cause lay elsewhere.

Key Takeaway: Relying solely on vibration analysis to detect all faults can lead to premature shutdowns and misdiagnosis.

1. If a gearbox is noisy, measure the noise against industry standards and consider all components—including gears—before concluding a bearing failure.
2. Vibration analysis is often equated with bearing faults because their signatures are distinct, but that focus can blind technicians to other sources of vibration.

Additional Insight: Variable‑speed drives are notorious for inducing current leakage that damages bearings. Proper insulation techniques must be applied to prevent such damage across all connected machinery.

Read the full case study at PEM Mag.

At J/E, we design, sell, and rebuild gearboxes. For more information on our products and services, visit our contact page or email info@je-bearing.com.