Improper component assembly, service result in severe injury
An attorney representing a forklift manufacturer outlined a workplace accident where a mechanic received a severe and disfiguring injury while replacing a worn-out tire on one of the steerable (rear) wheels of a 4,000 lb capacity, pneumatic-tired forklift in a repair shop for other repairs.
As a new mechanic, he borrowed the only working shop impact wrench. Since there are no wheel brakes on the steer wheels of the forklift, the mechanic’s plan was to let the tire stay in contact with the ground to hold the wheel stationary so he could loosen the eight lug nuts. After the lug nuts were loosened, he would raise the forklift to remove the wheel/tire assembly to fix the flat.
The lug nuts yielded to the prodigious torque from the big impact wrench. As he loosened the eighth and last lug nut, he heard an explosion and was forcefully thrown back from the forklift. Other shop personnel ran to his aid and found him unconscious and bleeding heavily from a facial wound. First responders came quickly, stabilized the injured mechanic and transported him to a hospital. He eventually had to receive major reconstructive facial and dental surgery.
My task was to find out what caused this horrific injury, and how could such a thing be prevented? Notice of the accident came by way of a lawsuit months later. Because the accident occurred at a privately owned forklift repair shop, there was no immediate feedback to the forklift manufacturer that a factory facility could have provided. Not only had time transpired from the accident to the notice, the repair shop completed repairs on the forklift and delivered it to its owner.
Later, statements from the injured mechanic and witnesses pointed to a failure of the wheel itself, but with no evidence, accident reconstruction was to be a challenge.
Using an exemplar forklift, I began the accident reconstruction. From the private repair shop, I learned what kind of tire and wheel was on the forklift involved in the accident. I compared the configuration of the forklift against the equipment list furnished by the private repair shop. The truck was originally equipped with split rim wheels, but the wheel in this case was an aftermarket-sourced split rim wheel.
A split rim wheel is constructed of two identically stamped halves that are bolted together to allow mounting and removal of a tire and inner tube simply by unbolting the two halves of the wheel. I purchased a sample and found that the eight pressed-in studs on the after-market wheel were much longer than the studs on the OEM wheel.
At a factory-repair shop, I had a mechanic mount and demount the tire/wheel assembly from the steer wheel hub. We found that the OEM wheel could be installed and removed without difficulty. However, the after-market wheel would not mount to the hub because the longer studs hit the bottom of cast reliefs in the hub. By turning the after-market wheel around, the oval head of each stud set in each shallow relief, thereby allowing installation of the wheel. A visual comparison of mounted wheels revealed the possible sequence of events leading to the accident.
The Smoking Gun
With the OEM wheel correctly mounted, the mechanic is presented with the eight lugs that hold the wheel to the hub. However, with the incorrectly mounted after-market wheel, the mechanic is presented with 16 lug nuts - eight that hold the wheel to the hub and eight that hold the wheel halves together! Unscrewing the wrong lug nuts could, and in this case, result in injury.
The accident sequence now seemed clear. The mechanic failed to depressurize the tire before loosening the (wrong) lug nuts. Using the unfamiliar big impact wrench, the mechanic didn’t just loosen the lug nuts, he removed seven. As he loosened the eighth, the inner tube (inflated to 115 psi) and the tire blew off the partially separated rim, hitting and injuring the mechanic. This seemed to be a failure of the mechanic to follow safe tire demounting procedures, but the fact that a wheel and tire assembly could be reverse mounted seemed to be a faulty design. The company settled with the injured mechanic. More importantly, field warning bulletins and new safety decals were issued, and the hub reliefs were changed to allow proper wheel mounting.
Myron J. Boyajian, P.E. is president of Engineering Consultants, a consulting service for forensic and design activities. Cases presented here are from his actual files.