The wheel watch

May 1, 2005
IN ADDITION to having degrees in chemistry, physics, and mathematics, Ted Schorn has one in divinity. So perhaps it came as no surprise that in the course

IN ADDITION to having degrees in chemistry, physics, and mathematics, Ted Schorn has one in divinity.

So perhaps it came as no surprise that in the course his presentation, “Keeping Wheels On Trailers: Critical Factors For Success,” he once wondered whether he was “preaching too much,” prompting one audience member to jokingly ask if an offering would be taken.

No one, however, was complaining about too much “preaching,” given the complicated dynamics and critical nature of this issue — not to mention the increased attention it has received from the National Highway Traffic Safety Administration (NHTSA). And Schorn was the perfect man for it, given that he is chairman of the technical committee of the Trailer Safety Industry Coalition (TSIC), which was formed in September 2004 to respond to NHTSA's concerns.

The TSIC features leaders of the trailer industry and its trade groups — the National Association of Trailer Manufacturers (NATM), National Truck Equipment Association (NTEA), Recreation Vehicle Industry Association (RVIA), and National Marine Manufacturers Association (NMMA) — along with major wheel, axle, and trailer manufacturers.

After conducting an investigation into wheel-attachment technology and wheel-separation issues affecting trailers under 26,000 lb GVWR, the TSIC's technical committee developed a series of recommended actions, communicating them throughout the industry in an effort to eliminate or significantly reduce incidents of torque loss and wheel separation.

“When a wheel comes off, that's a safety concern,” said Schorn, who is responsible for corporate quality at Enkei America Inc, a supplier of aluminum wheels. “I need to emphasize that. The problem is not merely the potential for loss of control of the trailer or vehicle. It's what happens to the wheel when it leaves the trailer. Typically it ends up in another lane of traffic and accidents occur. There have been incidents where wheels landed in the windshield of oncoming traffic.

“NHTSA sees this as a serious safety concern. It cannot be ignored or dealt with lightly. They're taking a heavy hand with it. It's certainly a problem with customer dissatisfaction. They typically occur with low mileage or short time of usage. In reliability language, these incidents are an ‘infant mortality phenomenon.’ That means there's a much higher likelihood of failure during the first period of use, as opposed to an ‘old-age phenomenon,’ where they wear out and get tired and finally there's failure in the end.

“We're trying to earn our credibility there — particularly with a new customer to a trailer. So it's a big deal when it happens early on. In perhaps one-third of the 100 incidents, the wheel came off during the process of delivery to the dealer from the manufacturer while the trailer was in the hands of the transporter. The bulk of wheel-off situations have involved aluminum wheels, which haven't been around nearly as long as steel in this application. There is some question about the proper torque that should be applied and about the use and function of aluminum wheels and their approach to torque retention.”

Raising funds

He said the TSIC's technical committee informed the NATM, RVIA and TSIC that it needed $170,000 to fund research. The NATM board of directors has authorized $40,000 to be put into that fund, and Schorn said the RVIA is “on the verge of approving $80,000 for this work.”

The recommended practices were completed on volunteer time, but the fund will be directed toward three projects:

  • A study of the relationship between torque and tension.

    “As you torque down a nut, that doesn't create a strong force that holds the two pieces together,” Schorn said. “You may just be crushing paint. In order to hold something tight together, you have to lengthen the bolts so their desire to return to their original length holds the pieces together.”

  • A study that takes that scientific work and applies it directly to trailers.

    “We'll take trailers and fit them with accelerometers and load-sensing equipment right at the place of the wheel joint and run them through a duty cycle at the Bosch Test Track in Elkhart, Indiana,” he said. “We'll see what kind of clamp load is required to hold them together.”

  • The creation of a recommended practice for the process control of the assembly operation within the manufacturers of trailers.

“Surveys showed there is very little commonality between methods of control,” he said. “There is no standardization of equipment, method, or in terms of measuring a system control. There are very few who have written practices. Very few use calibrated gauges. Some don't check the torque at all. They just use an impact wrench.”

Schorn said the stud is affected by forces from the weight of the trailer (gravity) and the turning that is applied by friction with the road. When the trailer is moving in a straight line, vertical loads combined with braking put forces on the wheel that tend to twist the wheel against the studs. The clamping friction between the hub and wheel resist these twisting forces; the clamp load must be maintained or the studs will fatigue from cyclic bending and eventually break.

He said that during turning, friction between the road and the wheel work against the tendency of the trailer to follow its original line of travel. These forces act to pry the wheel lose from the hub. The forces within the bolt that draw the wheel and hub together are increased during cornering, making it very important to not over-torque the bolt. Rapid or tight cornering increases these forces.

Clamp load

How do wheels stay on? Schorn said the bolt is stretched by the action of the lug nut when it is tightened. This stretch in the fastener is what generates clamp load in the bolt. The wheel is clamped to the hub by the force of the bolt trying to return to its original (shorter) length. When a torque value is applied to the nut, that represents the resistance that is experienced to turning the lug. This should be related to the lengthening of the bolt (and the subsequent result: clamping force).

“As you twist on the lug nut, you're forcing that down into contact with the nut seam,” he said. “As you register torque as you experience difficulty in turning, you are crushing those two surfaces together. At some point toward the end of that torquing operation, you begin to reach a place where the bolt begins to stretch in response to continued turning. You're lifting by the thread that bolt so it stretches some small percentage out of its length. Its desire to return to its original length is what actually balls the two pieces together.”

What can go wrong? Schorn said attempts are made to generate the proper clamp load by measuring the applied torque, but in reality the applied torque is used in three ways:

  • Overcome friction at the lug nut/wheel interface.

    “We could waste torque by trying to overcome friction of the lug nut/wheel interface. If those surfaces have materials sandwiched in there to such a degree that torque is wasted by overcoming friction in those materials, then you're not going to get clamp load.”

  • Overcome friction at the lug nut/stud interface (threads).

    “Manufacturers of trailers, for cosmetic reasons, sometime apply additional paint to the axles prior to the assembly of wheels. They get paint on the studs and in the threads of the axles. That is extremely bad practice. Why? Because as you're turning the lug nut, torque is now working to squeeze the paint out from in and around the threads, and we're wasting torque on that interface as opposed to pulling the bolt to apply clamp load. There ought not to be any paint on the fastener interfaces, and certainly not on the threads of the studs that are supplied with your axle.”

  • Generate clamp load by stretching the fastener.

    He said that by measuring only the torque, we may be fooled into thinking we have the proper clamp load, but that may not be the case due to: high friction at the lug nut/wheel surfaces from excessive paint, contamination, foreign materials on the surfaces, and uneven, galled, or scarred surfaces; or friction at the lug nut/stud interface from cross-threaded nuts, excessive paint, contamination, or foreign materials on the surfaces.

“The philosophical question has been asked: Why is it that trailer manufacturers, almost to a man, and the industry have historically demanded its consumers to regularly and repetitively re-torque their wheels when such a condition is not normally expected and there isn't any overall requirement within the passenger car and truck industry?” he said. “The question that NHTSA asks us, and has done so in writing: Why can't you guys design a joint that will stay together without making the consumer maintain that joint to the point where you're relying on that to maintain the critical assembly in proper torque relationship?

“I think it's possible, and in fact it has been demonstrated, that you can create a joint that doesn't have to be repetitively torqued. And so, please do not take the position that it is always a consumer's responsibility when a wheel comes off: ‘You did not follow my maintenance procedure by torquing after (a prescribed number of) miles.’ NHTSA isn't buying it. They're basically dissatisfied with us hiding behind that requirement. They think we can design a joint better. So by observing some of these practices, maybe we can get to the place where we don't have to place such an onerous burden on our customers.”

Schorn said paint should not be added on surfaces that will “mate” in the wheel-hub joint. These surfaces should be kept free of excessive rust, grease, or other contamination. The parts should be specified to be used together by the manufacturer or supplier, particularly for replacement parts and beauty components (upgraded aluminum wheels or decorative center caps).

He said if too much torque is applied, the bolt can be stretched beyond its elastic limit, with a high potential of generating microcracks in the bolt — which will reduce the fatigue life of the fastener. Use of lubricants and oils (when not directed to do so) can make it easy to stretch the bolt. He said the bottom line is, follow torque recommendations and don't use lubricants unless directed by the manufacturer.

Schorn said that at trailer assembly, mistakes can be made and the right torque value not applied by: the unrestrained use of impact wrenches that can over-torque bolts; failing to follow a criss-cross pattern, which can lock the wheel down either off-center or off-flat; failing to secure the wheel with a gradual increase of torque, which can lock the wheel down off-center or off-flat; failing to initially hand-thread lug nuts; or failing to confirm final torque with a calibrated torque wrench, which can lead to inconsistent results.

Parts fit exactly

He said parts must be engineered to match in order for them to function properly together. The mating shapes must fit exactly. The strength and properties of the joint materials (wheel, nut, stud, hub) must be correct for each other.

“There is some degree of standardization, but not nearly enough to guarantee that you can pick any wheel and any axle, and put them together and make it work,” he said.

He said if a wheel does come off, an investigation is necessary to determine the cause. Help should be sought from the component suppliers, and parts should be saved by the service center for later engineering review. Parts should not be reused, especially the studs, lug nuts, and wheel — even if they appear undamaged.

“From NHTSA's point of view, the manufacturer of the trailer is the one they go after,” he said. “They don't go after the axle guy, the wheel guy, or the lug nut guy. They go after the manufacturer because he's that guy who put the vehicle on the road.”

He said NHTSA is serious, and everyone else should be serious.

“Any rogue manufacturer who does bad things smears their black mark on all of us, because we are part of the same industry,” he said. “That's why we're trying to cooperate and get this widely distributed.”