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The Case of the Lurching Lift Truck
Myron J. Boyajian, President, Engineering Consultants -- Design News, August 15, 2005
As a test engineer for a major lift truck manufacturer, each day was a real kick as I tested components and complete lift trucks. Lift trucks seemed like benign, muscular servants, but I learned of the harm that could befall a careless operator.
Particularly devastating were lateral overturns caused by improper turns and other driving errors that trapped the driver between the ground and the overhead guard. When ANSI standards made overhead guards mandatory to provide fall-on protection, and as overhead guard usage increased, an overturn accident that one might have walked away from became a serious accident with injuries.
Our most popular lift truck, a 4,000/5,000-lb capacity, engine-powered, pneumatic-tire unit, led statistics in overturn accidents per man-hours of operation. On this speedy truck, design changes reduced travel speed and slowed the steering, but didn't stop the lurching lift trucks from their deadly deeds. By then, company engineers believed mechanical or structural failure contributed to some of these overturn accidents.
Scene of the Crime
On assignment to inspect one of these forklifts that had overturned, I learned the driver repeatedly had driven it over two-by-fours left by other tradesmen. Traveling forward with no load on foot-high forks, he turned right to get a load while rolling over wood scrap with the right side drive (front) wheel. His truck lurched and tipped onto its left side. Trapped by the overhead guard, the driver suffered paraplegic injuries. An ER toxicology screen revealed recent marijuana use, and his careless driving over wood scrap seemed to be a major factor. But what actually caused the overturn?
The Investigation
During the inspection, I was given a steel bar, one-inch-square, one-foot-long with a smooth, large radius bend at one end. I recognized the bar as a steer axle toggle stop, a critical frame member and the object of suspicions of other company engineers. The bar had a non-designed, slight bend in the middle. Weld beads had poor penetration both on the bar and its point of attachment on the frame.
Typical four-wheel, sit-down, counterbalanced lift trucks have no suspension except for a longitudinal toggle pivot at the center of a rigid steer axle that mounts the steerable wheels. Toggling allows wheels to climb over typical factory debris like a wood two-by-four while all four wheels remain on the ground. This design yields a so-called triangle of stability formed by the contact patches of the drive wheels and the steer axle center pivot.
Unloaded, the truck C.G. is near the apex of this triangle. Carelessly operated, lateral force can move the C.G. outside the triangle resulting in an overturn. Under similar driving conditions, a loaded lift truck with the same vertical but more forward C.G. location is less likely to overturn. Side-to-side tipping is limited by the contact of either toggle stop with the top surface of the steer axle. That contact transforms the stability triangle to a "stability trapezoid."
The Smoking Gun
As the sketch shows, as long as the C.G. stays inside these stability boundaries, likelihood of an overturn is minimized. In this case, repeated banging over debris bent and broke the poorly welded toggle stop, and there was no transformation to a stability trapezoid. As the forklift lurched, its C.G. went outside its stability triangle and it overturned.
With time, more careless drivers were pushed by employers with time and labor constraints, and rougher operating conditions increased abuse on this early-design lift truck. Nevertheless, my observation of this belatedly discovered structural weakness reinforced the need for actions already started by engineering and corporate management. In this accident, my employer settled with the injured driver.
A recall campaign started that was (I believe) a model of stand-up corporate and engineering responsibility, offering free conversion to beefy cast steel toggle stops with carefully specified welding procedures including pre- and post-heat methodology. Already on then-current models, future designs had toggle stops integral to the frame. Almost all those early lift trucks have long since turned into iron oxide or been recycled into cars and buildings.
| Author Information |
| Myron J. Boyajian, P.E., ( ntesla@ieee.org ) is President of Engineering Consultants, a consulting service for forensic and design activities. Cases presented here are drawn from his actual files. |
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