Identifying Mechanical Looseness

by Robert H. Jonas


In this short case history the author used trending, spectral analysis, time waveform analysis along with on-site inspections to identify internal looseness within a 400 HP motor. It provides a good example of the characteristics one looks for when looseness is present within machinery.


“During a routine monthly vibration survey of a paper pulping drive system, a large increase in overall energy was noted on the 400 HP drive motor (895 RPM); Figure 1. A spectrum showed multiple harmonics of the operating speed of the drive motor; Figure 2. A slight knocking could be heard from the inboard (drive end) bearing area of the drive motor.
The motor feet and support frame were inspected for any indications of mechanical looseness; no relative motion was detected. No problems – e.g., torn boot, loose flange, loose shaft – were detected when the coupling was visually inspected using a strobe light.
A time waveform analysis was conducted to help identify the source of the problem and its severity; Figure 3. The waveform shows a once-per-revolution impact with a strong modulation. The energy was greatest at the inboard bearing of the drive motor, indicating a possible loose bearing on the shaft or end bell.”

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