Failure of a Stacking Hoist
Staff posted on October 24, 2006 |
Failure of a Stacking Hoist
Department of Philosophy and Department of Mechanical Engineering, Texas A&M University

The Lakewood Company, a fairly large manufacturing firm, has designed, manufactured and sold hundreds of self-contained storage/retrieval systems which are used for storing heavy parts such as dies, which must intermittently be taken from storage and used elsewhere in the plant. The system consists of two large sets of storage racks, facing each other, with an integral crane that runs between them on 2 bridge crane girders on the top back edges of the storage racks (see Figure 1).

Figure 1

The crane has fork-lift appendages that can slip under a pallet on any of the several levels of the storage racks, retrieve a pallet off the shelf, retract the pallet into the aisle, rotate it parallel to the aisle, and move it to the end of the storage racks. The pallet is subsequently placed at the end of the aisle on the ground for pickup by a wheeled fork lift and transportation to its final destination.

The Buchannan company, a large construction vehicle manufacturer, purchased one of these Lakewood "Hi-Stak" units and was using it extensively until one of their employees, West Michaels, had an accident and was seriously injured while operating the crane. (See the appended accident report written by Buchanan.)

According to eye witnesses, West had retrieved a 1,460 pound die from the top shelf, and was pulling it to the end of the aisle. The load was well within the crane's advertised limit of 2,000 pounds. The next thing West knew, as stated in the accident report, he was thrown about 20 feet. His next recollection was awakening in the hospital bed.

The accident caused the crane wheels on the back side of the pallet to be pulled free of the lower flange of the A588 steel W6X12 girder on the left side. (See Figures 2 and 3).

Figure 2  Figure 3
The sequence of events shown in the MPEG Simulation gives an idea of how the accident happened. (View Movie)
Same events, Top View. (View Movie)
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