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A Komatsu 685-E haul truck was operating at a surface coal mine when the bolts for the third stage piston bearing retainer from both Hoist Cylinders failed, allowing the dump bed to fall abruptly onto the frame of the truck. The truck operator was not injured; however the operator withstood a severe jolt.
The Komatsu truck utilizes a hoist limit switch intended to stop the cylinders from full extension. However, it was determined that this switch was not properly maintained and allowed the cylinders to extend beyond their normal limit. When the cylinders extended to the maximum travel, a “slide hammer” effect occurred between the piston and barrel head, causing repeated jolts. This condition most likely stressed the bolts and caused the failure over the dump cycles. Upon inspection of the two hoist cylinders, it was determined that one of the cylinders had six bolts that held the retainer and the other cylinder had 12 bolts.