IN THE FALL of 1991, five separate accidents killed seven people. In each of those accidents, a wheel had come off the truck or trailer.
The accidents sent shock waves in all directions — truck and trailer operators, trailer manufacturers and their suppliers, the mass media, and the governments of Canada and the United States.
For more than a decade, industry groups have been analyzing the issue and developing ways to make sure wheels of commercial vehicles remain safely on their axles. That cooperative effort has paid dividends, and the National Transportation Safety Board (NTSB) recently closed the books on most of the areas that the agency determined were contributors to the rash of accidents 10 years ago.
The accidents, occurring in close succession, triggered a special investigation by NTSB. Among the conclusions published in this special investigation:
The incidence of medium/heavy truck-wheel separation accidents is small, about 750 to 1,050 per year, compared to the total number of truck accidents, about 349,000 annually.
The leading causes of wheel separations from medium/heavy trucks are improper tightening of wheel fasteners and bearing failure; both are the result of inadequate maintenance.
Undertightening of wheel fasteners usually results from the failure to follow recommended wheel maintenance practices, such as always using a torque wrench, following proper tightening procedures, using only compatible components, and avoiding paint build-up, debris, oil, or rust between wheel fasteners, threads, and mating surfaces.
Overtightening can more easily result from using an air impact wrench instead of a torque wrench.
The trucking industry lacks uniform model guidelines for maintenance and inspection of all types of medium/heavy truck wheels.
Wheel bearing failure can result from inadequate lubrication, bearing misalignment, improper bearing nut adjustment, or overload.
To solve the problem, the NTSB special investigation called for a cooperative effort involving the American Trucking Associations, the National Wheel & Rim Association, the Motor Vehicle Manufacturers Association of the United States (now the American Automobile Manufacturers Association), the Truck Trailer Manufacturing Association, and the Society of Automotive Engineers. NTSB wanted to see action taken in several areas.
NTSB identified some specific areas that needed to be addressed and made recommendations aimed at improving wheel retention. In its special investigation report on medium/heavy truck wheel separations issued in 1992, the board requested the following from truck industry groups:
Develop and disseminate model guidelines for the inspection and maintenance of all types of medium/heavy truck wheels (Class II, Priority Action) (H-92-98).
Develop uniform recommended practices that specify how often truck wheel bearings should be examined. (Class II, Priority Action) (H-92-99)
Promote an educational program on proper wheel tightening procedures through carriers, manufacturers, and government. (Class II, Priority Action) (H-92-100)
Encourage manufacturers to provide a label on trucks that indicates the recommended torque for wheel fasteners, proper tightening sequence, and recommended frequency for retorquing fasteners. (Class II, Priority Action) (H-92-101).
Because of action taken by industry groups, NTSB has classified the safety recommendations “Closed — Acceptable Action.” In a letter that SAE Vice-President Raymond Morris received June 28, NTSB Chairman Marion Blakely said, “The Safety Board appreciates the SAE's responses to these recommendations…We look forward to continuing our partnership with the SAE to further improve highway safety.”
The Technical and Maintenance Council (TMC) of the American Trucking Associations now offers a number of reference items in the form of informational charts and pocket guides to assist mechanics. Additional TMC actions taken include:
Recommended Practice RP 231 Wheel System Maintenance Labeling Guidelines. These encourage manufacturers to install advisory labels on their vehicles that identify how important maintenance requirements need to be periodically addressed.
Recommended Practice, RP 644, Wheel End Conditions Analysis Guide provides instructions on how to detect wheel-end conditions and to evaluate wheel-end serviceability.
TMC updated RP 222, User's Guide to Wheels and Rims and released RP 640, Alternate Wheel Bearing Adjustment Systems.
Also, TMC Task Forces have provided educational programs and continued extensive efforts in training members at regularly scheduled seminars and technical sessions. In coordination with the SAE, the TMC encouraged the initiation of two new SAE subcommittees to produce new SAE Standards for improvement of wheel end installations on highway trailers.
The issue of preload
In early 1996, some TMC members perceived that a wheel bearing “preload” adjustment procedure would be needed to improve wheel end installations. At that time, the existing TMC Recommended Practice 618 advocated an adjustment of the range of endplay from .001" - .005". That change was an improvement from the .001" - .010' previously recommended.
Unfortunately, a controllable “preload” adjustment force able to be applied against the roller bearings was not possible with the wheel end fastener systems in use at that time.
A new SAE task force was formed in 1997 to address ways to improve the adjustment of wheel bearings on highway trailers. Composed of representatives of the tapered roller wheel bearing industry, the task force produced SAE Standard J2535, Setting Preload In Heavy Duty Wheel Bearings, in January 2002.
“This important document was the concerted effort by the major wheel bearing manufacturers making it possible to further improve the means of installing axle wheel end assemblies,” says Al Hagelthorn with Hub Nut Corporation. “SAE J2535 identifies the range of preload force allowable in the standard wheel assemblies used for trailer axles. In summary, the allowable internal force is limited to a range of 0 to 1000 pounds. This is produced by controlling the amount of torque applied to the spindle nut during the installation process.”
About the same time, another SAE Task Force began to develop a testing machine to analyze the performance capabilities of the various new designs of axle nuts anticipated for producing “preload” within the wheel end system. The idea was to develop a standard for comparing the performance of any new axle nut to the performance of the conventional three-piece jammed nut system that has been used over the past century.
“The development work has been completed for this spindle nut testing machine, but it remains for the industry to assist in completing the project,” Hagelthorn says. “There are a number of efforts underway to stimulate responses from potential industry partners and governmental agencies. With positive support, this project can be completed within the next year.”
Competing companies such as Timken Company, SKF, and NTN Bearing teamed up to produce the new standards. Task force leaders for these two SAE Standards activities were Tom Mohrdeick of NTN Bearing who encouraged the completion of the wheel bearing SAE J2535 standard and Bill Sphatt of Stemco Inc who spearheaded the spindle nut testing machine design program.
In spite of the recent NTSB approval of the groups' safety efforts, the TMC S.6 Task Force realized that additional preparation was also needed to satisfy the NTSB recommendation, “Promote an educational program on proper wheel tightening procedures through carriers, manufacturers, and government.”
The TMC Annual Meeting held in Fort Lauderdale, Florida, in March included a session to demonstrate spindle nut installation procedures. The idea was to show how to preload trailer wheel end systems. Stemco Inc provided two axle stands for the demonstration that included load sensors in the axle assemblies able to measure the amount of “preload” force applied against the wheel bearings. These readings were displayed on electronic digital indicators for the audience to see.
Four companies provided spindle nuts along with recommendations on how their product should be installed in order to comply with the industry requirement for “controlled preload.” This was a test using a variety of spindle nuts to show how trailer wheel bearings could be adjusted within the specified range of force published in SAE Standard J2535. Each of the four types of spindle nuts installed by the manufacturers' representatives demonstrated the capability of complying with the SAE specified requirements.
Trailer manufacturer options
The concern about wheel retention has led manufacturers of trailer axles and wheel end components to market new and improved versions of “preset” wheel assemblies able to maintain “preload.”
“With these new products, trailer manufacturers now have a greater assortment of better axle assemblies than were available ten years ago,” Hagelthorn says. “Clearly, great progress has been made, and this will contribute to further reduction of the wheel separations that continue to plague the nation's highways. One thing for sure, the challenge posed by the National Transportation Safety Board in 1992 has been met by a strong and worthy response from the industry. Additional work remains, but the essential needs have been fully addressed.
“In 2002, trailer manufacturers will face the task of deciding how to market trailers having the best solution for wheel bearing adjustment. The number of solutions now available and how to select the best for all purposes will be a challenge for all trailer engineers.”