System integrity depends on proper torque loads. It's hard to imagine a circumstance more horrifying to truck drivers or the motoring public than helplessly watching a 500-lb. dual tire/wheel assembly that has separated from its hub careening wildly down the highway.
Statistically, it would appear that these are quite rare events. Indeed, the National Transportation Safety Board (NTSB) has projected that only 750 to 1,050 truck-wheel separations occur annually, accounting for about 40-50 fatalities a year. Most truck safety experts, however, say that wheel-offs are actually a more common occurrence. This discrepancy probably exists because fleets aren't likely to report wheel-off incidents that don't result in accidents.So it's probably not that wheel-offs are few and far between -- just underreported.
Shop mechanics and drivers are the industry's first line of defense in avoiding wheel-offs; they must be vigilant in checking for fastener tightness. In addition to reducing wheel separations, torque-checking measures can avoid wheel cracking and other costly damage.
As part of the pretrip inspection routine, drivers are required to check wheel fastener security. In the shop, technicians must be extra careful about following prescribed mounting procedures, and make certain that every wheel assembly is secured with the proper clamping force to eliminate movement at the wheel /hub interface. Whether hub-centered, stud-centered, or cast spoke, wheel systems depend on the force of properly torqued fasteners to produce the high-tensile load necessary to keep them secure.
According to Consolidated Metco/Con Met, insufficient clamping force is the prime reason for wheel stud fatigue and failure. In a recent technology bulletin, the company talks about maximizing wheel-stud life by making sure there's sufficient friction to prevent motion at the wheel/hub interface. This is usually achieved by torquing fasteners properly.
This assumes, of course, that wheel and attaching hardware are in good condition and that hub, drum, and wheel are correctly aligned. There are a number of reasons why fasteners can loosen, including the presence of foreign material or soft paint between components, damaged threads on studs and/or nuts, and bent, cracked, or corroded studs.
When a stud is damaged, Consolidated Metco suggests replacing the stud on either side; if two or more are damaged, all studs should be replaced.
Wheel fastener systems consist of high-quality parts -- not just studs, but flange nuts, or cap nuts, for example. For many years, however, there has been a low-level problem with counterfeit wheel fasteners.
Indeed, some junk fasteners have been implicated in high-profile accidents. As a result, Congress passed the Fastener Quality Act. This landmark law attempts to deal with the difficult subjects of lot traceability and standards of conformity for the high-strength fasteners. Since wheel hardware is critical, reputable manufacturers produce it to rigid specifications, in the prescribed hardness grade.
In response to the new law, Metform Corp. has taken improved the markings on its fasteners to include the "Securex" brand name and a special alphanumeric identification code.
Metform's two-piece flange nuts, which are used by many wheel OEMs, are designed to provide the requisite clamping force by allowing some rotation between the two hardened surfaces of the fastener as it is tightened to its torque value. This results in a bearing-type action that allows more torque to be transmitted directly into the clamping force. At the same time, this interaction provides for a much more precise torque-to-clamp force relationship, allowing t.he fastening system to be used at the optimum end of the load range.
Disc-Lock America has just won a British Institute of Road Transport Engineers' award for the greatest contribution to safety with its wheel fastener. This specially designed fastener is gaining acceptance, and is specified in the U.S. and Canada by a number of major bus municipal fleets. The company expects a further substantial boost to sales with the announcement that vehicle insurance-premium discounts are becoming available that help to offset installation cost.
A safety system that is said to eliminate the possibility of losing a wheel due to vibration or improper lug nut maintenance has recently been introduced by U-Lock Technologies. The system, which uses a set of special fastener caps linked with a thin cable, prevents adjacent lug nuts from loosening. Finger force is all that's required to push the U-Locks into place on wheel fasteners; the system is designed for easy installation and maintenance.
Other recent technical developments include the latest offering from Alcoa, which has incorporated an improvement that makes hub-piloted wheels easier to demount. A precision groove recessed into the wheel-hub bore reduces by 50% the area of wheel-hub contact where corrosion can build up, causing wheels to "freeze" in place.