Vibration & Oil Analysis Case Study - Large Aggregate Mixing Gearbox Fault

Applied Diagnostics Ltd - Precision with Pride


This epicyclic gearbox is driven by a 250kW motor and is used to produce large batches of aggregate mix for the building industry. Due to its high criticality the unit is monitored as part of a Condition Monitoring programme. An unplanned failure could cause a severe reduction in the output of this plant.

Gearbox Trend Monitoring


Frequency Spectrum Analysis

The gearbox waveform peak-to-peak trend had remained relatively steady for a number of years; the sudden increase in amplitude indicated a potential problem. An inspection of the frequency spectrum was made to diagnose the cause of the increase:



An inspection of the frequency spectrum revealed harmonics matching one of the Epicyclic gear components. While it is not unusual to see these, a very large number of run speed sidebands were modulated either side of each harmonic, indicating severe gear wear. As well routine vibration testing on this unit, oil samples are also collected to track and monitor the condition of the oil and gearbox. The oil results should confirm the extent of any wear.

Oil Analysis Tri-Vector Results & Iron Trend

Tri-vector plots provide a quick and easy way of viewing the overall condition of the oil. The oil sample on the left was collected when the suspected gear problem was detected. This highlights a severe wear problem, prompting a closer inspection of the oil analysis results. The Iron count was found to be extremely high, confirming internal gear component wear. The associated trend showed a marked reduction following the gearbox repair and replacement of the oil.




In view of the vibration and oil analysis results an immediate internal inspection of the epicylic gearbox was recommended. This revealed that several tooth flanks had snapped off the main input planet gear, there was also pitting to the gear surface indicating general gear wear. The gearbox was scheduled for an immediate overhaul and refilled with new oil prior to being put back in service. Considerable cost savings were made by adopting a Pro-Active maintenance strategy rather than the previously used Re-Active run to failure strategy.

Gear Damage Photos