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The Case of the Hot Wheels
Myron J. Boyajian, President, Engineering Consultants -- Design News, October 11, 2004
Engineers, designers, and manufacturers build machines and products used by the public—whose members have widely varying degrees of knowledge, skill, and care in the things they do with those products. This case provides a sad lesson of the serious consequences when an unsafe design is crossed with a careless user.
Out West on a clear November morning, a tractor pulling a dump-body semi trailer was moving at a steady, legal speed on a level, divided highway. The big rig suddenly began swerving from lane to lane as frightened auto drivers backed way off. The tractor veered to the right-hand median and slid along the Armco barrier. The big rig scuffed against the Armco and the now-horrified witnesses saw the truck driver bail out of his door followed by smoke and flames. He hit the ground, cleared the tractor's double wheels, but was fatally hit by the double wheels of the trailer. Calls quickly went out for help.
The police arrived, processed the scene, while EMTs cared for the driver and firefighters extinguished the still burning tractor. The tractor and semi-trailer were towed to a secure location for investigation.
The investigation
I was retained by the attorney who represented the plaintiff, the family of the deceased driver. Inspection began in the company of other experts and attorneys representing various tractor-trailer systems, component manufacturers, and a maintenance company.
Among the debris were heat-bulged and exploded cans of cleaners and lubricants. Some were scattered, but most were in a wire-frame container used to transport 1-gallon milk containers. Co-workers reported that the driver was careful in his work, and took first-rate care of his rig, and kept cleaners, polishes, and cleaning cloths in the basket. One of the loose cans was a bulged can of diesel starting fluid that consists mainly of highly flammable and volatile ethyl ether.
This can also had small holes that appeared to be the result of electrical arc burns and not mechanical punctures. This evidence pointed to an electrical arc that burned through the thin-gauge can, igniting the pressurized ether starting the cab fire. But, what initiated the arc burns?
Investigation showed that the dump-type trailer was equipped with hydraulically operated arms that deployed a rolled tarpaulin to cover the cargo area. The deployed tarp prevented dirt, gravel, or other loose materials from being wind-blown with the truck in motion.
Under the remains of the driver's seat was an electric-motor-driven hydraulic pump and solenoid valve unit that provided pressurized hydraulic fluid to deploy and store the tarp unit. A three-position (out-off-in) dashboard switch controlled the pump motor and the solenoid directional valve. A cable connected the truck battery positive terminal to an exposed terminal on a solenoid on the motor case.
This appeared to be the connection between the can of ether and a source of electrical current. The can had evidently lain against the live terminal on the solenoid. Once the thin label was scratched through by the bare terminal, an electrical path existed from the solenoid terminal through the can to electrical ground which was the metal surface of the cab floor or the motor itself.
The conclusion
My opinion report concluded the manufacturer of the motorized tarpaulin failed to provide a safe product by having exposed electrical terminals in the cab space, and did not adhere to SAE recommended practice. In addition to sleeving or booting the terminals, I also suggested an alternative construction to avoid live terminals inside the cab.
How did the ether get from secure storage area to the live terminal? Not a technical issue per se, this finding could impact on any jury verdict or settlement value. Investigators from the plaintiff's attorney's office learned that a fellow employee had borrowed the can of ether from the basket in the subject tractor to start his own rig on the chill November morning. In a rush, he casually tossed the can back in the subject tractor where it evidently rolled into contact with the exposed solenoid terminal. Thus, the driver of the burned rig became an innocent victim of improper equipment design and imprudent use of potentially dangerous material. The tarp unit manufacturer agreed to a settlement before trial started, and subsequently changed its motor pump unit design, but too late for the driver.
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