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INDUSTRIAL ACCIDENT

Stacker Crane Hoist Failure

This paper discusses the cause of failure of an intermediate shaft in a stacker crane hoist gear case.

Background

The shaft failed while the crane was in operation, causing the hoist to crash to the ground from a height of about 35’. The emergency brakes to the hoist cab in which the operator sits, detached from the hoist and immediately locked to the rails on the mast, suspending the operator 35’ in the air. He was not injured.

The assessment was required to determine the primary and contributory causes of the failure.

Investigation

The site of the accident was visited and photographs were taken. Various components, including the fractured intermediate pinion shaft, were removed from the hoist gear case for detailed examinations. Macroscopic and microscopic examinations and analysis were performed on the fracture surfaces.

The fracture surface of the pinion shaft was examined in detail visually and under low optical magnification. The shaft had fractured in fatigue as a result of a low overstress, high cycle, reversed bending load. A grind relief groove a the change in section of the pinion shaft, immediately adjacent to the pinion gear, provided a stress concentration which initiated the crack. The two origins of the cracks were diametrically opposite each other.

A detailed examination was conducted of the helical gear which fits to the spline of the intermediate pinion shaft that failed. Examination of the gear teeth revealed signs of surface fatigue. In particular, there was an initial pitting and wear pattern on the helical gears which indicated non-uniform loading. It was apparent from the pattern that the loading was limited to less than half of the available tooth contact surface on less than half of the circumference of the gear and was transposed on the other half of the gear circumference. The pitting and wear pattern suggested that the helical gear was subjected to a wobble or unbalance with a frequency of one cycle per revolution.

Testing

The fracture surface of the intermediate pinion shaft was examined under a Hitachi S-520 scanning electron microscope (SEM) to establish roughly the number of cycles to failure. An acetate replica of the fracture surface was made which was then mounted in preparation for placing in the SEM. Two areas of the acetate replica were magnified 10,000 times and 20,000 times respectively. The fatigue striations were identified and an average number per unit length was calculated. As a result, roughly 400,000 striations to failure was estimated.

Since it is known that 15 to 20% of the total cycles to failure are related to crack propagation, it was estimated that this shaft experienced in the range of 2,000,000 to 2,500,000 cycles until failure.

Analysis

Detailed examination and testing revealed that the intermediate pinion shaft failed in reversed bending fatigue under low nominal stress after roughly 2,000,000 to 2,500,000 cycles. Evidence suggested that a bending load caused shaft vibration or whip which exceeded the fatigue limit of the shaft at the grind relief groove adjacent to the pinion gear.

The surface fatigue pattern on the helical gear teeth the tooth breakage patterns suggested misalignment of the helical gear fitted to the spline in the intermediate pinion shaft. This resulted in bending of the pinion shaft due to inertial loading by this heavy mass (i.e. imbalance or wobble).

Conclusions

It was concluded that the primary cause of failure of the intermediate pinion shaft was likely a misalignment of the helical gear fitted to the spline of the shaft. This resulted in bending of the pinion shaft due to large inertial loading by this heavy mass (imbalance). This bending load caused shaft vibration or whip which exceeded the fatigue limit of the shaft.

Contributing to the failure was the grind relief groove design feature of the intermediate pinion shaft which reduced the fatigue strength of the shaft by providing local stress increases and multiple crack initiation sights in the eventual fracture plane.

 
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