The Use of Log Scales to Analyze Bearing Failures

by David Gluzman


This short paper describes how the log scale can be used to identify bearing defect frequencies and other smaller peaks within the spectrum. A case history is provided with plots that illustrate the effectiveness of the log scale at identifying small bearing peaks in the presence of a resonant peak of high amplitude.


“A two-pole, 40 HP, vertical motor-generator unit exhibited high radial nonsynchronous vibration levels in the upper frequency range of the spectrum. At the bottom bearing of the motor ra-dial readings were 0.70 IPS at 1,533.0 Hz (Figure 1). At the top bearing radial readings were 0.19 IPS at 1,118.8 Hz (Figure 2). The bottom bearing is an FAF W310PP deep groove ball bearing to hold thrust load. The top bearing is an FAG NU208 cylindrical roller bearing. The best option was to take both measurements at the bearing housing through a short straight lubrication line with a pad mounted at the end.

The situation justified an immediate overhaul. The inner race of the bottom bearing had severe damage entirely around the race (Figure 3). The damage was assessed as a stage 3 rolling element bearing failure. No damage was found in the balls, cage, or outer race. Considering the similar vibration pattern in the radial and axial direction, it was surprising that no damage was observed in the top bearing.”

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