Improved Preload Adjustment Methods

From personal observations while working in the engineering world, I have found that it is not unusual for engineers to explain complex technical requirements in ways that are not easily understood. Service technicians have often developed their expertise logically without adequate instructions from the engineers, and this has led to difficulties.

Problems can occur if engineers fail to realize that deviations of maintenance procedures might occur in a manner they did not anticipate.

It remains for all engineers to be constantly aware that the instructions they develop might either be improperly presented or misunderstood.

It is also a primary responsibility for all engineers to monitor product deficiencies and to provide worthy assistance to maintenance providers whenever necessary.

None of us are perfect and mistakes will sometimes happen. So, when unforeseen problems do occur, it is necessary that everyone communicate effectively to resolve the issue.

Having designed wheel-end systems for highway trailers, and being exposed to numerous wheel separation accidents over the past 25 years, I am aware that it is a very complex situation.

Because there are some obvious problems that remain unattended, I feel compelled to make recommendations to reduce the frequency of problems associated with inappropriate adjustment of tapered roller bearings.

Within the engineering community dealing with wheel-end systems on commercial highway vehicles, there has been general agreement that elimination of "endplay" in favor of a slight amount of "controlled preload" would be an advantage. However, there is a lack of specific instructions from the axle and wheel-end component manufacturers advising how to install preload consistently within a recommended range.

Endplay is the degree of freedom that provides for a measurable amount of lateral movement of the hub assembly on the axle spindle. Preload is an amount of applied force against the tapered roller bearing by controlled adjustment of the spindle nut assembly to eliminate endplay.

There are some problems that can be satisfied with application of newly developed concepts and with enhanced information. Consider the following:

• There are axle nut components currently available that can be used to assemble wheel-end systems with controlled preload. However, there is a lack of specific instruction from the manufacturers advising how to install preload consistently within a recommended range.

For safety, it is my recommendation that attempts to adjust tapered roller bearings with preload not be made unless the manufacturer involved issues specific instructions for that purpose.

• At the present time, the Recommended Practices (RPs) published by the Technology and Maintenance Council (TMC) do not describe how "controlled preload" can be installed by service technicians. Rather, the RPs state that the component manufacturers of axles, bearings, oil seals and axle nuts should be requested to provide appropriate instructions.

• There has not been any definitive information published to inform those involved in commercial vehicle maintenance how wheel-ends should be inspected. Nor have those who are responsible for preventative maintenance been instructed on how often the wheel-end components should be disassembled and visibly inspected.

Recently, I conducted a survey among maintenance personnel associated with major fleet carriers. Among the purposes of the research was to evaluate the interest in an axle nut that is capable of installing a desired amount of preload. Survey respondents were in total agreement on three issues:

• Oil seal longevity can be increased with the elimination of endplay.
• Preload adjustment of tapered roller bearings will provide improved performance.
• Maintenance can be simplified with improved practices.

With the results of this survey in mind, the following actions can be anticipated:
• Manufacturers of axle nut systems will develop and publish simple installation instructions to include identification of "an acceptable range of preload adjustment" and the description of how the process should be accomplished.
• Personnel involved in maintenance of wheel-end assemblies will be advised of when and how often periodic wheel-end inspections should be performed. (This recommendation was offered by the National Transportation Safety Board in a 1995 special investigation report, Medium/Heavy Truck Wheel Separations.)
• This year, there will be action on the part of some trailer component manufacturers to introduce improved versions of precisely adjustable wheel-end fasteners.
• There will be efforts by some component manufacturers to provide the improved maintenance instructions needed by mechanics involved in trailer maintenance to accurately install a minimum of end play within the wheel-end assemblies.
• A newly designed trailer safety nut having an automatic feature to assure precisely controlled preload adjustment using a simplified procedure will become available.
• Unfortunately, it is unlikely the National Highway Traffic Safety Administration will issue any regulations in the foreseeable future to change trailer wheel-end maintenance procedures for improvement of highway safety.

But, don't give up. Relief is on the way.

G. Allan Hagelthorn is a professional mechanical engineer with nearly 45 years of experience. Since 1982 he has owned and operated his own consulting business, Rather Engineering. He is a member of a number of engineering associations, including SAE and TMC. He has served on TMC committees developing new methods for improved trailer maintenance. As a member of SAE, he helped develop new standards applicable to trailer components. His inventions include equipment to facilitate measurement of camber and toe angularity of trailer axles. He holds patents covering axle spindle nuts for commercial tractor-trailer axles. These wheel-end systems provide superior methods for tapered roller bearing adjustment to assure controlled bearing preload for extended service operations.

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