An attorney representing an insurance company that provided liability coverage for a Chicago-area forklift dealer called me to report an accident with one of the dealer's rental units. At an industrial equipment show being set up at McCormick Place on Chicago's lakefront, workers were attempting to place a process heat exchanger unit on heavy tubular legs already erected on the show floor. The heat exchanger was on a large, pallet-like shipping base with openings that would allow the heat exchanger to be lowered and bolted onto the support legs.
The Scene of the Crime
Two workers stood on the pallet as the forklift operator raised the load. As the driver slowly advanced, he found the heat exchanger had to be raised higher to let the unit's body clear the legs. He would then lower the heat exchanger so its hard points set on the pads on top of the legs.
As though urging the forklift to reach maximum lift, the driver pulled the lift control handle fully back and revved the engine. A brief moment later there was a loud popping noise, accompanied by a gush of hydraulic fluid and then a head-turning blast of noise as the heat exchanger crashed to the floor. The impact threw the two workers like bronco riders. They were spared injury when they landed on rolls of carpeting waiting to be installed after the exhibits were completed. The crashed heat exchanger did not fare as well, ending up with smashed piping, fittings, servo valves and instrumentation.
I had to find the cause of the crash. There was no question the forklift driver and the two riders were all guilty of unsafe work practices, but the accident seemed to be due to other causes. At the dealer, I inspected the forklift and saw that the central, single-lift cylinder had separated from the base casting. The elevating mechanism was a simplex-type mast, with a single 4-inch O.D. lift cylinder. At the top, its piston rod pushed a crosshead with two sheaves. One end of each lift chain was anchored to the bottom of the mast. Each chain went over each sheave with the other end of each chain attached to a plate-like carriage upon which the forks were hung.
The mast was removed and laid on the shop floor for inspection. I inspected the threads on the mast cylinder that mated with the threads in the cylinder base. The peaks of the threads on the tube were not sharply formed, but the threads appeared to be full depth. With good lighting, I examined the cast cap for cracks or other flaws, but found none. As I turned the cap to examine its threads, I found a thin spiral of metal filling the threads in the cap. Turning my attention to the cylinder, I found a few intact threads away from the open end of the tube. I used micrometers to measure the major and minor diameters of the good threads and then several points along the tube threads. I could see there was a slight taper of a few thousandths of an inch with the smaller diameter at the open end of the cylinder. I suspected the taper was due to flawed machining procedures.
At my request, the shop mechanic fitted a pressure gauge to the lift hose. We started the engine, revved it up and gently pulled back on the lift lever and observed 3,300 psi at relief pressure. The shop manual for this truck specified 2,200 psi. When the mechanic went to set the proper relief setting, he found the lock nut loose. He screwed the relief valve out to lower the relief pressure and cinched up the lock nut.
The Smoking Gun
The sequence of failure now seemed clear. Records showed that prior to the rental, the lift cylinder had been disassembled to repack the piston seals. The leaky seals may have prompted a wrong-headed attempt to correct lift performance by raising relief pressure, but I wasn't able to confirm why or how the pressure setting got to 3,300 psi. Upon assembly, the technician had failed to check or reset the relief pressure. The cylinder's threads were tapered so there was diminished shear area in the threads. As long as the relief pressure was correctly set, there was no problem. But, high relief pressure and the repacked cylinder resulted in a high force causing the cylinder to tear the tapered threads and blow the cylinder apart.