Figure 1.
Figure 2.
Figure 3.
Background
A failure of an overhead crane electric hoist unit occurred at a steel stockholders warehouse. The crane had only been in service for a period of approximately six months. The hoist drum shown in figure 1, failed as the unit was transferring a load of steel reinforcing bars across the warehouse.
Findings
An examination of the failed unit revealed that the rope drum and motor casing were severely worn at the gear end. The bearing cage at the gear end was severely distorted and had split in two releasing some of the elements as shown in figure 2. The bearing at the motor end was found to be intact.
Closer inspection of the damaged bearing cage revealed that the resistance spot welds, which join the two sides of the cage together, had not fused properly and, that they were not distributed evenly around the cage. This feature is shown in figure 3. A subsequent examination of the motor end bearing revealed signs of separation between the two sides of the cage, although it was only at an early stage of development.
The results of the failure analysis indicated that the primary cause of failure was the detachment of the two sides of the bearing cage, resulting in the misalignment and eventual displacement of the bearing elements. This was a result of defective welding which indicates that there had been a problem with poor quality control during manufacture.
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