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A Multidisciplinary Approach Gets to the Root Cause of a Failed Shaft

An 8-inch diameter pinion shaft, which was driving a 12-foot diameter bull ring gear on a ball mill, failed after a period of rough operation.  The crack originated in the area of a keyway at a clutch hub, Figure 1.  This geometry acts as a stress concentrator and crack nucleation, in this region, is not uncommon.

 

Figure 1.  Location of failure.
 

During our examination of the broken shaft, we observed that the surface of the fracture was relatively smooth, with characteristic “beach marks” indicating the direction of crack propagation.  Using SEM (scanning electron microscopy), we found fatigue striations to confirm the suspected nature of crack propagation.

These findings, combined with the 45-degree slant of the fractured end of the shaft, implied that torsional fatigue stresses drove the failure.  However, our analysis of the shaft loads indicated that the operating stresses on the shaft were considerably below the fatigue endurance limit of the material, even after the stress concentration factors in the keyway were taken into consideration.

As the team members discussed the conflict between design and actual operating experience, we considered approaches to “on-line” evaluation of shaft stresses that combined two traditional measurement techniques:  strain gauging and vibrational analysis.

Because of the uncertainty in the source of the shaft stresses, we decided to attach strain sensing devices (Note: Strain gauges are typically spot welded or glued to the part.) to the pinion shaft in order to measure bending and torsion in the area of the failure.

The gauges stretch or compress in concert with the surface to which they are affixed. The stretching and/or compression changes the electrical resistance of the wire grid imbedded in the gauge.  The change in resistance is directly related to the strain (or movement) of the gauge and the underlying metal.  By measuring the change in resistance, the strain, Δε, is derived.  In the elastic strain range of a material’s mechanical properties, there is a direct correlation between the strain and the applied stress, Δσ.  This alternative stress provides a measurement of the fatigue stresses on the component.

Since the changing strain appeared to be periodic, we decided to analyze the strain gauge data in terms of a vibration phenomena.

Putting all of the data together, the source of the torsional stress on the shaft was discovered.  It was determined that the periodic torsional stress had a frequency which corresponded to the rpm of the ball mill.  It was concluded that a defect in the ball mill ring gear was exciting a system torsional resonance.  Although the information was detectable from the vibration spectrum, it had been overshadowed by the gear mesh problem.  Without the spectrum analysis of the strain gauge data, we would not have pinpointed the problem and would most likely have had similar shaft failures in the future. 

Additional lessons learned from this case:

The first step toward getting away from simply reacting to equipment failures is to treat the failed component as a valuable source of information.  An incident occurred at the time of the initial failure that, unfortunately, is a typical experience in the field.  The pinion gear was reversible, end for end.  The failed end of the shaft in the clutch hub was totally useless to the area and was set aside.

The maintenance crew very carefully placed the shaft so that the flat butt of the shaft was in the air and the end with the fatigue crack, with all of the failure information, was embedded in the mud and stones outside of the mill shed door, thus damaging valuable evidence.

 

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