Design News for Mechanical and Design Engineers

This compact, 1,000 lb-capacity forklift was operating on new pavement, unloading building supplies at a new residential subdivision. At day’s end, the operator set down the last load and sped toward the construction crew and his wife, who came to pick him up. Known as a “hot dog” driver, he apparently tried to finish his task with a flourish as he sped up and then attempted a sharp right turn close to those present. The forklift pivoted on its two left wheels and tipped onto its left side. The driver attempted to bail out on that side and was fatally crushed by the overhead guard.

The attorney for the deceased’s family had sequestered the subject forklift preventing immediate access to it. After an intense search, an exemplar was found. Although more than 30 years old, it was in almost new condition and had a patriotic Stars and Stripes paint job. We performed a series of static tilting platform stability tests that clearly showed the forklift easily met the requirements of ANSI B56.1, Test 4, Lateral Traveling Stability. The plaintiff’s attorney argued static tilt table tests were not representative of the accident, so our engineering manager assigned me the task to design a test fixture that would safely permit a dynamic turning test.

I designed a beefy, wheeled outrigger that allowed the truck to tip when turning at speed, but not let it hit the ground. With the lab staff looking on and me in my Bell Magnum racing helmet, I accelerated to full speed. With adrenaline pumping, I went into a series of turns at full speed. I started with very wide turns and then tighter and tighter turns without any sign of overturn. Now feeling fairly racy, I traveled several times at full-governed speed and whipped the steering wheel into left or right U-turns with solid stability. A week after the test ended, the recently retired manager of development engineering was called to act as in-house expert. This respected and beloved engineer was my first boss at this company and he had extensive knowledge of the vintage forklift we were testing.

He took one look at our test setup and almost laughingly asked why we had an outrigger on a tip-proof, foolproof forklift. He said this forklift was so limited in speed and turn radius that it wouldn’t tip laterally under our test conditions. Having said that, he drove the exemplar, now without its test outrigger and proceeded to calmly whip the forklift around U-turns at full speed. Afterward, the lab engineering staff and our expert all voiced the same query: why did the subject forklift tip over?

The subject truck was finally released and shipped to our lab. Now, most four-wheeled forklifts have an unsprung, center-pivoted steer axle that toggles to permit the wheels to traverse small debris found in industry. Lateral stability depends on a so-called stability triangle. Axle stops limit the magnitude of axle toggling and shift the triangle to a stability parallelogram, increasing resistance to lateral tipping. In contrast, the subject forklift utilized small coil springs at each side of its solid, beam-type steer axle to provide suspension and a stability parallelogram.

With the exemplar for comparison and crispy old vellum design drawings at hand, we inspected the underside of the subject unit. There was an “Oh My God” moment as we saw the left-hand coil spring was missing. But why take off this spring? We saw that its removal allowed a rigid galvanized water pipe to be run from the hydraulic fluid reservoir to the hydraulic pump at the fan-end of the engine. By removing the spring to make way for the pipe, some careless technician had eliminated one corner of the stability parallelogram, inviting lateral tip over.

The evidence gathered was presented to the jury by our in-house and outside experts and the jury found us, the manufacturer, not guilty of negligent design and manufacture. This was, sad to say, a classic example of how careless repairs and careless driving produced tragic results.