The Case of the Shivering Shafts
Synopsis:
Coverage for machine repair often hinges on cause: accidental damage, breakdown, or wear-and-tear. Truth is in the Details.
Background:
A $100k segment of an industrial machine contained two slow turning driveshafts that carried hundreds of horsepower into the machine. The machine segment was claimed to have failed. It was claimed that an accident within the machine damaged the driveshafts, requiring replacement of the machine segment.
Analysis:
TI was requested to determine the cause and extent of damage. The shafts and associated bearings were sent to TI for laboratory analysis. The shafts were set up on precision roller V-blocks on TI’s Grade AA laboratory surface plate and checked for runout at the shaft ends. The shaft ends were found to wobble (run-out) only .0010” and .0006”, indicating the off-axis deviations of the shaft ends were only .0005” and .0003”.
Both ends of the shafts are splined and the splines were cut to be in rotational alignment. The alignment of the splines was tested and the splines were found to still be precisely in alignment showing that neither shaft had been twisted. However, the splines on one end of each shaft were badly degraded and after cleaning they were examined and photographed under stereomicroscope.
Analysis of the splines found that they were 70% missing due to advanced wear from fretting, a vibratory abrasion process common to high power splined joints. No spline was deformed due to impact.
The bearings were also inspected under stereomicroscope and found to be suffering from an advanced state of wear but not impact damage due to an accident.
TI also inspected the machine itself and found that a guide bushing was entirely missing from a machine input shaft which resulted in the weight of the input shaft and its 30-pound coupling hanging on the end of the splined shaft, thus creating the high amplitude, low frequency vibration that was transmitted back into the splines and caused the fretting. The damage was the result of long term wear and tear.
