Torque and Re-torque

Although we usually discuss tires and tire-related issues in this column, this month’s subject is the next closest thing – wheels. Specifically some important guidelines for remounting tire/wheel assemblies.

What’s so different or difficult, you may ask, about the simple task of mounting a wheel on a medium or heavy-duty highway truck? Not much, really. But as any parent will tell you about raising kids – tell them, re-tell them and re-tell them again.

Basic guidelines offered by vehicle and wheel equipment manufacturers usually include these important recommendations:

®′ Inspect used wheels and mounting hardware for any damage, excessive wear or corrosion, or any signs of abuse or neglected maintenance, and replace components as necessary.

®′ When replacement components are needed, use only known high-quality parts, preferably purchased from or approved by the wheel or vehicle manufacturer.

®′ Make sure that all wheel and mounting hardware components are compatible, as several wheel types and attaching systems are available for most applications. Parts of these different systems are generally not interchangeable and should not be mixed.

®′ Tighten lugs in a recommended sequence using a calibrated torque wrench to obtain the required torque setting.

Details of these basic but still vital recommendations and additional safety and servicing guidelines are readily available from just about every truck wheel supplier.

Several excellent sources are the National Wheel and Rim Association (904-737-2900), Alcoa Inc. (800-242-9898), and Accuride Corp. (888-626-7096).

In addition, free safety wall charts covering multi-piece rim matching, and mounting/demounting procedures for truck/bus tires are available from your local OSHA offices or from the OSHA Publications Office (202-523-9667). TIA’s Commercial Tire Service training program covers all these issues, and provides tremendous educational materials, as well.

All personnel in your shop involved with truck tire and wheel service should be trained and familiar with relevant safety and maintenance material.

Beyond the Basics

But the basics are, well, the basics. Astute commercial tire pros know there is a lot more involved, especially with current emphasis on optimizing vehicle performance (up-time) and with today’s legal atmosphere.

A few recent industry studies on common tire/wheel maintenance practices reveal a dire need to tell, re-tell and re-tell again. Here are two other vital recommendations your service staff should know by heart:

®′ Re-Torquing – Using a hand torque wrench, wheel and rim attaching nuts must be torqued to the manufacturer’s recommended values AND they must be re-torque to those values after approximately 50 to 100 miles of on-road operation.

®′ Lubrication of Flange Nuts – Two-piece flange nuts (commonly used with newer generation hub pilot single nut attaching systems) should be sparingly but positively lubricated with oil as recommended by the manufacturer prior to re-use.

Why Re-torquing?

Once the wheel attaching nuts have been properly tightened, why should re-torquing be necessary? According to Paul Levering, vice president of Webb Wheel Products Inc., this is due to "joint settling."

There are as many as 12 different mating surfaces in attaching a wheel – from the stud seat outward to the hub, drum, inner wheel, outer wheel, washer, hex nut, and finally to the stud threads. If each of these joints takes only a minimal set after initial tightening, some clamping force will be lost. If painted or coated wheels are used, additional compression may occur in these protective coatings, which could lead to loosening.

Industry studies have shown that a re-torque after the first 100 to 500 miles of service in either new or replacement situations is a very desirable maintenance practice and is recommended for both flange nut and ball seat mounting systems.

Is Lubrication Okay?

Other than to clean off any rust or corrosion, few would think that wheel fasteners should be lubricated. After all, wouldn’t a well-oiled nut easily work its way loose over time? The short answer is: The right lubricant used sparingly in the right places improves the clamping forces of a fastening system.

Actually, the subjects of torquing and lubrication are closely related by the common thread of an engineering term called "clamping force." The purpose of tightening bolts (for example, on a cylinder head, or lug nuts when attaching a wheel to a hub) is to create tension in the bolt or stud. This tension, in turn, creates clamping forces that hold the different pieces together.

The clamping force is actually friction at the interface that restricts movement of the joined surfaces relative to one another. Since clamping force is impractical to measure accurately in the field, torque of the fasteners is measured to approximate the desired clamping force.

This provides a practical fastening solution, since there is a close relationship between torque and the clamping force generated. However, that relationship can vary depending on the amount of friction between the bolt head or nut and the surface it is sliding against while being tightened.

The type or amount of lubricant applied to the threads and/or mating surfaces at the point of fastener contact – if any ®“ is why bolts torqued to the same value can deliver different clamping forces.

Good quality lug nuts are manufactured with coatings designed to deliver consistent clamping forces for given torque values. Typical coatings are Teflon phosphate and oil for the newer two-piece nuts used with hub-pilot mounting systems.

As these type nuts are tightened, the cone or washer seats on the wheel and the hex nut slides relative to the washer. It is this interface between the hex nut and the washer that manufacturers recommend be slightly lubricated at each reuse. The goal of lubrication is to restore the designed-in relationship between torque and clamping force. This clamping force relationship may have changed over time as the nuts have been exposed to moisture, salt, and other contaminants that invite corrosion at the sliding surface interface.

Most manufacturers advise against lubrication of the wheel or thread surfaces, but stress that the wheels should be clean and dry and that threads should be clean and free of any burrs or nicks. Also note that manufacturers generally do not recommend the use of any lubricant on the cap nut seating surfaces of ball seat mounting systems.

Be certain to follow the manufacturer’s recommendations for the particular service equipment you are using. Proper initial lug torque and maintenance of torque through the life of your customers’ vehicles can contribute significantly to the life expectancy of wheel end components and, most importantly, to highway safety.