JAMES BRUCE MCMATH AND ROBERT H. WENDT
"Union Pacific Manager Dies in Tragically Freak Accident." This was the headline in the Arkansas Democrat-Gazette on October 17; 1996. The article described a collision in which the outer wheel and tire assembly on the trailer of a large truck separated from the vehicle, crossed the median of a divided highway, and crashed into an oncoming vehicle. The crash killed the driver, Donald Huffman, and inflicted serious brain injuries on his passenger, Phillip Crow.
While this incident may have seemed "freakish," it was, unfortunately, not unique - the Ontario Trucking Association's Web site, www.ontruck.org, has a "statistics/facts" page discussing this problem.
In the June 1994 issue of Heavy Duty Trucking, the editor recounted the experience of one driver:
Late one night in 1986, California trucker R.J. Taylor was prone in the sleeper of his showpiece 1951-model Kenworth as it traveled at 55 m.p.h. His co-driver made an unscheduled stop and roused him from his sleep. "You better come check this out," he said, motioning to the rear of the truck. .. . Taylor ... was aghast to find only a single wheel where the right rear dual wheel assembly should have been. The inner cap nuts had failed and sent the outer wheel careening into obscurity. "I didn't notice it come off, even though my head was less than 10 feet away," he recalls. ...
The article went on to say:
Not everyone is that lucky ... Sandra Sinclair was killed by a wayward dual wheel assembly which crashed through her Mobile, Alabama, home as she lay sleeping in bed. Unaware that he had lost the wheels, the tractor-trailer driver continued on down the road. He later told police he didn't discover the wheels were missing until he inspected the vehicle after finishing his run. There have been even worse loss scenarios. Over a three-month period in 1991, a series of five wheel loss incidents have left seven people dead.
Andrew Ryder, Wheel Loss Nightmare, HEAVY DUTY TRUCKING, June 1994, at 64.
The discussion of truck wheel loss in industry publications seems to have been spurred by a 1992 National Transportation Safety Board (NTSB) investigation of wheel separation incidents. NTSB/SIR-92-04. The investigation was prompted by three wheel-separation occurrences in North Carolina in which five people were killed. In one incident, a wheel crashed through a school bus windshield, killing two children and a chaperone.
The NTSB estimated that there are between 750 and 1,050 wheel separations annually from large trucks, with an injury rate of 26 percent. The report also noted statistics from the Office of Motor Carriers documenting 26,805 cracked wheels found in spot vehicle inspections from 1989 to 1991. Despite this rather dramatic indication of widespread system failure, the NTSB did not consider that a design flaw may be responsible for the wheel cracking and wheel separation occurrences but instead blamed poor maintenance.
The investigation of the collision that killed Donald Huffman is further evidence that authorities often incorrectly assume that truck wheel losses are caused by faulty maintenance. After the crash, examination of the wheel revealed numerous fractures of the disk running from bolt hole to bolt hole. The Arkansas State Patrol, unaware of any design-related cause, attributed the separation to poor maintenance and considered criminal manslaughter charges against the truck owner and driver.
But an investigation by plaintiff's counsel revealed that a design defect existed. The wheel was a Firestone heavy duty extra service disk wheel that was manufactured in 1973. It incorporated a "stud-piloted" mounting design. In this system, also known as a "Budd" mounting, the wheel is centered and held onto the hub by nuts that have rounded profiles that interface with concave surfaces around the bolt holes in the wheel areas known as chamfers. This system has been commonly used on large trucks since the 1930s, when disk wheels began to replace the traditional spoke wheels.
An examination by metallurgist Alan Milner of Tucson, Arizona, a pioneer in many types of tire-related cases, revealed that fatigue caused the cracks. The Out of Service Manual, a publication of wheel manufacturers, attributes such cracks to loose nuts. Inspection also revealed that the chamfer holes were well-worn, another condition blamed on either loose or over-tightened nuts.
Further review of industry literature revealed the reintroduction of another wheel retention system in the 1980s, known as a hub-piloted system. In that system, the wheel disk has a hole that fits over a protrusion on the hub that centers the wheel. This frees the studs (bolts) and nuts so that their only function is to retain the wheel assembly on the hub. This eliminates the need for wheel chamfers and rounded nuts. It also reduces the number of nuts, because only a single nut per stud is used to retain both wheels, instead of the dual nut system used on the stud-piloted design in which there is an inner and outer nut on each stud holding the inner and outer wheels, respectively.
Sales promotion literature for this hub-piloted design - published by the Accuride Corporation, a successor to Firestone's steel products division - suggests stud-piloted wheels are inferior to hub-piloted wheels in several respects.
ACCURIDE PRODUCT CATALOG at 6. Examining these differences exposed a multifaceted design defect.
Briefly, the fastening system on large truck wheels, especially in the stud-piloted system, is problematic for reasons that apply to any bolted joint. The threads on a stud form an inclined plane that creates clamping force (preload) as torque is applied to the nut. This tension is maintained by stretching the stud, which effectively acts as a spring. Through various mechanisms, any bolted joint will tend to loosen under loads-especially cyclical loads-if not properly designed for the task. This can be prevented by achieving sufficient pre-load so that the components that form the joint are immobile relative to each other under load. The amount of pre-load required depends on the load placed on the joint and the coefficient of friction between the mated parts of the joint.
The problem with the stud-piloted system begins with the chamfer hole and curve-faced nuts. This arrangement turns the nut into a wedge that is driven into the chamfer with the potential of deforming the components. Consequently, the manufacturers have recommended an upper limit of 500 foot pounds of torque when tightening the cap nuts of the stud-piloted system.
THOMAS C. DILLINGHAM & ROBERT L DRURY, TWO-PIECE COATED FLANGE NUT FOR STUD MOUNTED DUAL STEEL WHEELS (SAE Technical Paper Series No. 851462). Dr. Milner's testing showed significant damage to the cap nuts and the chamfer holes even at the recommended limit of 500 foot pounds.
More significantly, the stud-piloted system induces only 16,000 pounds clamping force at the torque recommendation as opposed to the 45,000 pounds delivered by the hub-piloted system. The greatly reduced clamping force of the stud-piloted system results in nut loosening when the wheels are subjected to dynamic operating conditions involving loads, high speeds, vibration and uneven road surfaces. Even Firestone concedes that loose nuts lead to wheel failure and must be avoided.
Further, with repeated use, the amount of clamping force generated by a given level of torque can drop even more. As a result, the amount of clamping force required to immobilize the joint may only barely be achieved or may not be achieved at all at the recommended maximum torque. when the system "settles in,,, additional clamping force is lost and the joint begins to move, leading to further loosening, stress, and, ultimately - component failure. To make matters worse, an operator has no way of knowing the actual clamping force achieved during nut tightening.
In contrast, the hub-piloted system produces a clamping force three times greater than the stud-piloted system with no risk of damaging the wheel even at higher-than-recommended torque levels. Consequently, normal installation will generate larger damping force margins than the Budd system, and the possibility of damaging the components during tightening is virtually eliminated.
The stud-piloted system may have been reasonably functional when first used on highway trucks. However, its inherent shortcomings became manifest as the interstate highway system brought higher speeds that, in conjunction with radial tires and increased load limits, have placed greater stresses on truck wheel retention systems.
It is not known how long the industry has been aware of the problems associated with the stud-piloted design. However, industry literature has featured cracked wheels in Out of Service manuals for decades and has blamed loose nuts for the problem. The International Organization for Standardization promulgated a standard in 1979 recommending that future truck wheels have a hub-piloted design. ISO 4107-1979(E). A company document shows that Firestone knew of the problems as early as 1985, and reveals efforts to design a nut that would solve these problems.
FIRESTONE STEEL PRODUCTS AGENDA.
Rather than admit to a design flaw, the industry tried to shift responsibility to the user by demanding unreasonable maintenance, such as re-torquing after 100 miles of operation and periodically thereafter. HEAVY DUTY TRUCKING, supra, at 67. The impracticality of constant re-torquing is underlined by the internal Firestone project paper referenced above, which estimates that it would take two hours to re-torque all the nuts on an 18-wheel truck, assuming one had ready access to a torque wrench. The chairperson of the Truck Maintenance Council of the National Truckers Association and President of Roadway Tire, Peggy Fisher, has said that no one re-torques as recommended by the manufacturers. HEAVY D~ TRUCKING, supra, at 67.
Indeed, having become sensitive to the manufacturing allegation that loose nuts cause wheel cracks, and knowing the impracticality of re-torquing, many operators are probably over-torquing wheel nuts. Ironically, over-torquing damages the components and, in turn, leads to loose nuts. The wheel manufacturing industry has continued to use a fastening system for large trucks with an inherently flawed design. The frequency of wheel failures and wheel loss accidents have made this flaw increasingly apparent. Still, many plaintiffs' attorneys may be unaware that wheel loss accidents are related to a product defect and that loose nuts can equal lost lives.
James Bruce McMath is a member of the firm McMath Woods PA., in Little Rock, Arkansas. Robert H Wendt is a member of The Wendt Law Firm, PC, St. Louis, Missouri.
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