ROCHESTER HILLS, Mich. (April 26, 2004) — Do you find that as you get older, you get more intolerant of things that never used to bother you before?
As a card-carrying member of the Baby Boomer generation, I find that crying babies really irritate me now. Stupid things that people do—like suing McDonald's for making them fat or trimming their hedges with their lawnmowers—really bother the heck out me, too.
I'm getting more and more like “Maxine,” the caustic old lady on Shoebox (a tiny little division of Hallmark) cards. She looks at life in a practical and irreverent way that I can really relate to.
For example, Maxine on driver safety: “I can't use the cell phone in the car. I have to keep my hands free for making gestures.” Maxine on body piercing: “I'd get my tongue pierced, but I still have a little bit of brain left in my head.” And Maxine on the technology revolution: “My idea of rebooting is kicking somebody in the butt twice.”
Are those words of wisdom, or what?
I've been noticing that the trucking industry is getting more “Maxine-like,” too, lately. It's probably because Baby Boomers make up the majority of its work force. Like me, they wonder why we still have rules for maintenance practices that nobody or very few people follow.
The industry requirement that irritated many fleets was the requirement to retorque lug nuts 50 to 100 miles after a wheel installation.
Fleets at the Technology & Maintenance Council (TMC) asked: Why do we have a retorque requirement when we know that literally hundreds of thousands of tire/wheel assemblies get changed per day in the U.S., nobody retorques the nuts due to operational constraints, but trucks don't lose their wheels by the thousands, either?
Tackling the task
With my age, wisdom and Maxine-like view of things, this was a perfectly logical question and issue to raise. So a task force was created to look into the need to retorque wheel lug nuts.
The task force conducted tests at several fleets to determine the incidence and cause of loss of nut torque on wheel systems. It found that when wheel system components were in good shape and well-maintained, the proper installation procedures were followed and the proper amount of torque was applied during installation, any loss of torque that resulted in the “settling in” of the system was still within the range of torque needed to keep the wheels on.
Fleets that had poorly maintained wheel components experienced greater losses of torque insufficient to keep the wheels on, which required that retorquing be done.
So it decided to create a Recommended Practice (RP)—RP237 Retorquing Guidelines for Disc Wheels—to help fleets develop an alternative preventive maintenance plan, for their use only, based on their vocations, equipment condition and wheel torque histories.
Now you have to understand that once a tire and wheel assembly has been installed on a vehicle, it goes through a period of “settling in.” This means the parts in the clamped assembly—including rust, dirt, paint and corrosion on wheel faces, nuts and stud threads—settle or move. What results is a wheel end that has lost some clamp load. The amount of clamp loss depends on the following factors:
* Installation practices used when installing wheel assemblies on vehicles. You have to get the wheels on straight, use the criss-cross pattern when tightening nuts so that the wheels are centered and affixed properly on the pilots if applicable. If the wheels aren't seated correctly, the system will become loose.
* The condition of studs. If studs have been over-tightened and have yielded, clamp load can never be attained.
* Fasteners, both studs and lug nuts. They have a finite service life and should be inspected and replaced when they are worn, damaged, exhibit cracks or the threads are stripped or corroded.
* The condition of studs, nuts, wheels, drums and hubs on installation. Burrs around bolt holes and center holes must be removed, and damaged components must be replaced. It is imperative that you clean all the mating parts on the axle end to ensure they are free of dirt, debris and corrosion. This junk inhibits proper clamp load attainment and results in torque loss during the “settling in” process when it gets worn away or compresses.
* Excessive or uncured paint on steel wheels. These can wear away in use and lead to a loss of clamp load/torque as well.
* Presence of paint. Paint must not be applied to the stud threads. When paint is present, the amount of clamp load may be reduced since it increases friction during the tightening process.
* Improperly seated brake drums and improperly installed studs. These also will affect torque retention just like improperly seated wheels. Brake drums must be fully seated on the hubs. Studs must be properly installed and fully seated when replaced, too. Stud heads can be bent from hammer blows, which will prevent the stud from seating properly against the back of the hub and can result in stud failure.
Only SAE 30-weight oil should be used on hub piloted wheel systems on the last two threads of the stud. Also put a drop between the nut body and the flange. Anti-seize compounds should be avoided on wheels and fasteners since product consistency varies, possibly resulting in overtorque.
Follow the guidelines
Fleets that have problems in these areas, or just don't apply the correct amount of torque to the fasteners to begin with, will have loose wheels.
RP237 explains how a fleet can establish a retorquing interval for its operation. It first must determine the amount of torque loss that routinely occurs on its wheel assemblies after installation. To do this, the fleet has to conduct a study and collect data on torque attainment and retention. To do this the fleet must:
* Follow recommended practices for wheel installation.
* Set the final installation torque with a calibrated torque wrench.
* Use a form provided in the RP to record condition of the hardware and the torque value as measured with a torque wrench for each nut on each wheel position.
* Drive the loaded vehicle on a route where it can make several turns on entrance/exit ramps for a minimum of five miles and return.
* Measure torque with a calibrated torque wrench and record data again.
A statistical sample (at least 30 wheel ends) or multiple checks should be made on a representative range of vehicles and operating conditions.
If retorque values are found to be abnormally low, the wheels must be removed to find and correct the problem. If any torque value is found to be below the acceptable torque range, retorquing is necessary and a retorque program is needed. If the retorque values are consistently within the required range for the wheel system, then the retorque interval may be extended.
Through this process a fleet can effectively determine the level of integrity of its wheel installations and the torque loss that normally occurs in its wheel systems.
TMC recommends that wheel systems be retorqued after wheel installation, but a fleet's own data will direct it in making its own decision about retorquing intervals.
According to this recommended practice, for those fleets that need to retorque soon after wheels are installed, retorquing may be done 50 to 100 miles after installation if the route is fairly straight highways or after five miles if the vehicle is loaded and can make several turns on entrance/exit ramps. The allowance of a five-mile route may be easier for fleets to comply with and may make retorquing a real possibility from an operational standpoint.
While a fleet may determine it can extend its retorque interval, it must still continue to collect data periodically to ensure its wheel system maintenance program remains sound. This can be done by spot checking on a regular basis or by collecting data throughout the year.
Spot checking should be done at a maximum interval of three months at each maintenance location on at least 30 randomly selected wheel ends.
RP237 Retorquing Guidelines for Disc Wheels presents a practical approach to wheel retorquing. It recognizes that some fleets do a great job in maintaining their wheel systems and axle ends while others do not and will have problems with torque retention that make retorquing necessary within a short distance. This RP helps fleets scientifically determine which category they fall into. It's a practical approach to the real world. One that Maxine would approve.
If you have commercial fleet accounts, let them know about this new industry recommended practice. If you have some that find they must retorque and then start whining to you, just lay a “Maxine-ism” on them: “If you need a shoulder to cry on, pull off to the side of the road.”