I recently traveled to a client's dynamometer facility to perform dc motor thermal tests. I installed thermocouples at critical test points, but was delayed in spooling up the motor until electricians on a scissors-lift platform truck finished replacing a faulty transformer for the overhead light in the dyno test bay. Seeing workers correctly using lifting equipment reminded me of a recently closed case in which misused lifting equipment had fatal consequences.
The Scene of the Crime
The case began a few years ago when a plant ordered a new forklift. Due to a delivery delay, the dealer furnished a loaner forklift. Similar in function and capacity to the unit on order, this forklift was a short-wheelbase, three-wheeled electric-powered machine.
Shortly after the loaner hit the docks, the plant maintenance man hijacked the forklift and its driver to fix some overhead wiring. The driver maneuvered between the plant's presses and production machines and stopped as directed. Rushing to complete a simple wire routing change, the maintenance man skipped the use of a guarded work platform, or even a pallet to stand on. With one foot on each fork, he ordered the driver to raise him to the wiring runway over the machines, about 8 ft high. Job done, the order was called to lower away. While lowering, the forks suddenly dropped from about 6 ft to about 3 ft. The sudden fall and stop spilled the worker from his precarious perch. He struck his head on a nearby machine and was knocked unconscious. He died at the hospital soon after.
The man's family sued the forklift manufacturer and the dealer, citing faulty design and poor maintenance. They sued the employer citing lack of training and negligent forklift operation. The attorney representing the employer retained me to find the cause of this accident.
At an independent forklift repair/storage shop, I inspected the forklift. The lifting and lowering functions tested OK with the mast vertical and tilted. I repeated the tests with a 250-lb load. Lifting appeared normal, but sometimes during lowering, the forks stalled while the telescoping lift cylinder continued to retract. Holding the control handle forward, the forks dropped to 3 or 4 ft and then continued to lower at a normal speed. This malfunction repeated about every four or five lifts.
Suspecting a part critical to mast/cylinder sequencing was broken or missing, I pored over the mast structure. I peered at a spring-loaded latch that sequenced the mast channels and cylinder to keep the lifting chains in tension. During some lifts, a crosshead atop the cylinder ram rotated slightly, losing contact with the latch. The intermediate and inner mast channels then lowered out of sequence with the lift cylinder. The forks stalled while hydraulic fluid drained from the lift cylinder, putting slack in the cylinder chains, leaving the load supported by the mast channel chains.
When load forces overcame friction in the latch, the mast channels dropped rapidly until the slack in the cylinder chains ran out. The forks and load slammed to a near stop, with lowering again dependent on the position of the lowering control handle. Mystery solved? No, a question remained: why did the cylinder crosshead rotate during some lifts? Looking further, I saw the two cylinder chain anchors were not tensioned equally.
The Smoking Gun
I concluded the worn latch, plus unequally adjusted chains, caused the mast to mis-cycle and drop the forks.
Further discovery shows the forklift was a recent trade. Because of tardy delivery of the new truck, the dealer failed to inspect the forklift before lending it.
All parties took home important lessons that good design must consider performance of older and worn components, and safety procedures and training cannot be bypassed.