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The Case of the Manic Motor
Myron J. Boyajian, President, Engineering Consultants -- Design News, January 7, 2008
At a medical supplies warehouse, the operator of a stand-up rider forklift complained her truck would abruptly stop and start during sharp turns, severely jostling her. The forklift was removed from service for repairs. The truck was returned to service two days later. Happy to have her "regular" forklift she began her order-picking duties, but the truck again abruptly stopped and started in tight turns.
The Scene of the Crime
The operator then refused to drive the machine and asked for medical assistance, alleging the forklift injured her. The truck was again removed for repairs while the driver was placed on medical leave. Her doctor refused to clear her to return to work. The driver retained an attorney to sue the forklift manufacturer and its servicing dealer. Her attorney retained me to determine the cause of the jerky truck operation. I explained that I did not have medical knowledge to opine on the forces that would result in bodily injury, but I was confident I could determine a mechanical or an electrical cause of the jerky operation, if it existed.
The Investigation
Forklift repairs altered any physical evidence, but it was helpful to compare the chronology of the service records against the time line of the driver's complaints and alleged injury. Service notes showed the technicians replaced a speed control potentiometer on their first visit before releasing the truck for use. The jerkiness in sharp turns then again occurred. On their second visit, the service techs noted a severe grinding noise and jerky operation when turning. They found that a large diameter trunnion bearing (aka a slewing ring) that supported the steerable drive unit and wheel was breaking apart. They replaced this bearing and smooth operation was restored. OK, it seemed logical that a failing drive unit support bearing could cause rough or even jerky steering, but a question remained — could this bearing failure cause jerky forklift travel?
I compared an exemplar forklift's construction to illustrations in a service manual for the subject model and serial numbered forklift. The illustrations showed the armature of the vertically mounted drive motor was not supported by a motor bearing at the shaft output end, but only at the opposite end where a drum brake was located. The splined drive end of the shaft was inserted into, and axially supported by, a female splined plug on the drive unit input shaft. I surmised that this design was chosen to avoid the possibility of misalignment and binding due to having three bearings in close proximity on in-line shafts. That meant reliable drive system operation was dependent on reliability of the drive unit support bearing. I had my answer — failure of the support bearing let the drive unit shift about, forcing the motor armature to rub against the pole shoe surfaces and act like a powerful, but intermittent brake. The result — jerky truck motion in turns! My opinion was reinforced when I found service bulletins that described a revised drive train arrangement to eliminate the possibility of the armature dragging. However, no retrofit kit was made available, nor were there any product recall notices.
The Smoking Gun
At a dc motor rebuilding shop, the shop manager showed me a pile of motors of the exact part number for the subject truck. Every motor I saw had deep gouges and score marks on the surfaces of the armature and pole shoes. I learned that virtually every motor of that type, to varying degrees, had the same type failure I saw on the motors in the pile. Not having an intact subject forklift, I purchased some of the motors from the pile for illustrative purposes.
I presented my conclusions to my client, the plaintiff's attorney. He retained an expert in human factors and dynamics. This expert instrumented an exemplar forklift and measured acceleration forces by simulating the jerky travel when turning. He used the resulting data in his opinion report. In court, with my exemplar motor in hand (well, not quite in hand, as it weighed about 60 lb), I explained to the jury how a motor works and how a failed support bearing could cause the jerky operation. The human factors expert then explained his test results and the effects of the g-forces on the forklift operator. After deliberation, the jury found the defendant manufacturer not guilty of negligent design. After trial, a poll of the jury showed they accepted the idea that the motor did operate improperly and there was poor service, but agreed with the defendant's medical rebuttal experts that the jerkiness was not severe enough to cause injury.
| Author Information |
| Myron J. Boyajian, P.E., (mboyajian@sbcglobal.net) is president of Engineering Consultants, a consulting service for forensic and design activities. Cases presented here are from his actual files. |
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