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Calamities: The Case of the Vile Valve
Myron Boyajian, President, Engineering Consultants -- Design News, August 14, 2008
The Scene of the Crime
A tractor-trailer driver dumped a load of pig iron at a steel processing company and, in some unknown manner, was run over by his own rig. I was retained to determine the cause of the accident.
The Investigation
According to the documentation, the driver dropped off his load of pig iron and went to lower the raised semi-trailer. He used a detented, three-position, lift-lower air valve on his instrument panel, but the air-operated hydraulic valve failed to lower the trailer. He then crawled under his tractor to manually operate the spool of the lift-lower valve. As he did so, the tractor rolled forward and its wheels crushed the driver's hips and legs.
At the trucking yard, I set up a video camera to record the tractor-trailer operation while the mechanics warmed the tractor's engine and built up pressure in its air reservoir. The trailer brakes were set, the lift-lower valve was actuated and hydraulic fluid flowed into the lift cylinder at the front of the frameless trailer. This type of trailer has a long A-frame pivoted midway along the belly of the trailer and terminated at the forward end in a structure that provided a base for the telescoping lift cylinder and a connection to the tractor's fifth wheel. As the trailer raised, its locked wheels kept the back end in a fixed location, while the tractor, with its parking brake not set, was pulled backward under the trailer, about 100 inches.
The mechanic then tried to lower the trailer, but it remained elevated, even after repeated operations of the lift-lower knob. After safely blocking the tractor wheels, the mechanic and I crawled under the tractor while an assistant actuated the control knob in the cab. As the lift-lower knob was operated, air from the tractor pneumatic supply was valved to one side or the other of a diaphragm-type actuator linked to the hydraulic valve spool. In the "Lift" position, the pneumatic actuator pushed the spool rearward to send hydraulic fluid to the trailer lift cylinder. In the "Neutral" position, the spool was centered and fluid was locked in the lift cylinder. However, in the "Lower" position, the pneumatic actuator was unable to pull the valve spool back into the valve body to allow fluid to flow out of the lift cylinder and permit lowering. The valve spool was coated with road grime, so after an OK from the attorneys present, I cleaned off the spool and the mechanics were able to lower the trailer by applying force to the valve spool.
I could now see the sequence of events leading to this accident. With the valve stuck, the driver attempted to mechanically free the valve spool, but had evidently left the dashboard control knob in the "Lower" position. As he worked on the valve, his legs were outstretched in line with the tractor wheels. Once he freed the stuck spool, the pneumatic actuator pulled the spool home letting fluid out of the lift cylinder. As the trailer lowered, it pushed the tractor forward and its wheels rolled over the driver. The sequence of events seemed clear, but why did the valve spool not shift to lower the trailer?
The Smoking Gun
The rig was pulled into the service garage where we could crawl under the tractor. With the valve spool extended rearward, I could see the smooth, polished surface of the spool and a sharp transition to the part of the spool that remained outside the valve body. The exposed part of the spool was scratched, pitted and corroded. The valve was exposed to splash, spray and stones or other debris thrown up by the tires of the dual rear axles. There was no shrouding or shield, nor any provision for a bellows or boot to protect the exposed spool surface. It seemed these sources, singly or in combination, resulted in the rough, pitted surface that prevented the spool from smoothly moving into the valve to allow the trailer to lower. My opinion report cited the poor valve location and spool condition while acknowledging the driver's carelessness in failing to block the wheels or neutralize the dashboard control. My client, the truck line, received a summary dismissal from the case, while the driver settled with the other defendants.
