TORONTO, Ont. — The tolerable end-play for truck wheel bearings lies somewhere between the thickness of a human hair and a sheet of paper.

Various sources put the diameter of an average strand of hair at 0.001 inches and a sheet of standard office paper at 0.005 inches. In other words, it’s perfectly acceptable to adjust wheel bearings so that they are just a little bit loose, but not too loose.

The American Trucking Associations’ Technology and Maintenance Council’s (TMC) Recommended Practice 618B will ensure a standard wheel bearing is tightened to 0.001 – 0.005 inches of end play (also expressed as 1 – 5 mil or 0.025 – 0.127 mm) if the process is carefully followed and verified by a dial indicator.

It’s actually better for the bearings if they are a little bit tight, but since it’s impossible to actually measure tightness — known as preload — the industry developed the RP 618B adjustment procedure as a “good-enough” solution. While endplay is not the perfect wheel bearing condition, it can be accurately measured and repeated by skilled technicians who are willing to spend about 10 minutes per wheel end, getting the adjustment as close to ideal as possible.

Ideal, of course, is zero endplay or a slight preload. Bearing makers at Timken say slight preload can improve bearing, seal and tire life, but only if the entire process can be controlled and results in a consistent bearing setting range.

The difference between endplay and preload from the bearing’s perspective is significant. The potential for reduced bearing life at excessive preloads is far higher than life reduction at excessive clearance, says Divjot Singh, an applications engineer at NTN Bearing Corporation of America.

“Excessive preload will cause excessive friction and the bearing will run hot, compromising lubrication and eventually leading to flaking (material coming off) at the large end of the rollers/races,” he says. “On the other hand, adjusting the bearing too loose causes excessive looseness and vibration in the system. This could lead to a multitude of issues such as, but not limited to, wheel wobble and seal leakage along with ABS and braking problems. Eventually a too-loose setting will cause flaking at the small end of the rollers/races, which can lead to failure and possible wheel loss.”

With endplay, you’re essentially measuring a gap. From 0.001 to infinity, it’s still a gap. A gap means there is there is “zero” force applied against the bearing. The width of the gap matters, as Divjot explains, because it allows for movement in the bearing, which can produce undesirable results.

With preload, the two surfaces are in contact and the nut exerts force on the bearing. The problem lies in determining the strength of the force. There is no correlation between the amount of torque used to fasten the nut and the amount of force applied against the bearing. And the force applied to the bearing with a torque wrench is exponential rather than linear. In other words, once the gap is eliminated and the two surfaces are in contact, even a slight increase in torque applied to the nut will cause the force applied to the bearing to skyrocket, increasing the risk of excessive preload.

You could say that RP 618B or any other procedure that results in 0.001 to 0.005 of endplay is a compromise that errs on the side of caution to prevent excessive preload from damaging the bearing.

Pre-adjusted or Preloaded

TMC’s RP618B is widely used in trucking, but manually adjusting wheel bearings is a finicky procedure. Many fleets and most OEMs are now using pre-adjusted hubs and bearing sets from the likes of Com-Met and Stemco. These designs use an engineered spacer between the bearings to achieve the correct bearing settings by simply applying a certain amount of torque when installing the outer nut. The bearings are forced together by the tightening of the nut, but only as far as the spacer will allow. With the correct amount of torque, the bearings are set as close to zero endplay as possible or with very slight preload — the optimal condition for bearing life.

Pre-adjusted hubs relieve the technician of the need to measure endplay and generally save a great deal of installation time while providing greater accuracy and repeatability in bearing adjustment. Still, technicians need to be aware of the procedures associated with the various systems and use properly calibrated tools.

“Our 200 lb-ft torque requirement is the recommended minimum torque value. If an end user would prefer to torque the system to up to 250 lb-ft, that is permissible to account for torque wrench torque setting variations,” says Jim Rosema, category director for wheel end products at Stemco. “But if a technician inadvertently torques the Trifecta system to a higher value, the system will still operate as designed.”

With standard wheel end systems, different spindle nut types have different torque procedures. So  there’s risk there in the technician confusing one procedure for another.

“It’s important for technicians to be knowledgeable in order to not only identify the different spindle nuts styles, but to also be educated on the correct procedure for each type,” cautions Tony Ryan, technical service and training manager at SAF-Holland. “A common mistake made by technicians is using multiple extensions on torque wrenches while tightening the axle spindle nut. Extensions can compromise the accuracy of the torque wrench reading.”

The other alternative to RP 618B and pre-adjusted hubs is to set a slight preload on your bearing with a patented tool called Dr. Preload. It’s designed by Temper Axle and distributed by Meritor. It simplifies the process, cuts installation time to just a few minutes, and sets a measured and repeatable amount of preload on the bearings.

“SAE’s recommended procedure, J2535, provides target and maximum preload settings for Class 8 tractor and trailer axles as a guide to any supplier that would develop systems for setting wheel end bearings to preload,” says Ray Piascik, director of marketing and sales at Temper Axle, makers of the Dr. Preload system. “Our system was designed to meet those targets when installing bearings with slight preload.”

Among the other touted benefits of the Temper Axle system are longer seal life and reduced irregular tire wear due to bearing adjustment. Joel Morrow, head of research and development at Norwalk, Ohio-based Ploger Transportation, has been using the system for several years and says his 50-truck fleet hasn’t had a seal failure in over a year.

“In a fleet our size, five or six failures a year wouldn’t be unusual, but we haven’t had a single problem since we stated using Dr. Preload for bearing adjustment,” he says.

Jim Pinder, corporate fleet director at Erb Group of Companies, is an RP 618B man. It’s the way they have always done things under his leadership at Erb and the results are good.

“We stick to TMC’s RP 618B for the most part because it works for us,” he says. “We are very careful with the installation procedure and don’t take any short cuts. We also use only one type of seal here. Our people are familiar with it, they know how to install it. That helps minimize problems further out in time.”

He said he has experimented with the Dr. Preload tool, and the results have been good so far. He has also worked with unitized hubs and preset hubs, and found each to have their own pros and cons. More to the point, he says overall the number of seal and bearing failures has dropped in recent years, and he attributes at least some of that to better equipment.

“Seals usually don’t fail because the product is bad, it’s technicians not following procedures,” he says. “You also set the bearing up to fail if you don’t install it properly. So yes, there’s a lot riding on proper wheel-end maintenance, and we have trained our people to do it properly. I think that’s what makes the difference.”

Can wheel bearing adjustments affect fuel economy?

Compelling but anecdotal evidence suggests that wheel end tightness can affect fuel economy — sometimes significantly. Joel Morrow, head of research and development at Norwalk, Ohio-based Ploger Transportation, is utterly and completely obsessed with fuel economy, and regularly posts his trip results on Facebook. He’s currently averaging better than 10 mpg (23.5 L/100 km) and leaves no stone unturned in pursuit of ever-better fuel mileage.

He claims that the difference between factory-tightened tractor and trailer wheel bearings and those adjusted with a tool designed to apply a slight bearing preload can be as much as 1/10 of a mile per U.S. gallon per wheel end. Testing-and-tuning done prior to last year’s Run on Less fuel economy challenge sponsored by the North American Council on Freight Efficiency (NACFE) produced a before-and-after improvement of 1.4 mpg over the entire truck when the bearings were set with the Dr. Preload tool from Meritor.

It’s now standard practice at Ploger to strip off the factory-installed pre-set bearings and replace them with traditional sets using the tool and Temper-Loc spindle nuts to ensure proper adjustment.

“Bearing adjustment on new vehicles straight from the factory is very inconsistent,” Morrow says. “We recently took delivery of 10 new [tractors], and on two out of the 10, every wheel end on the truck was too tight. Most of the trucks had one or two wheel ends that were too tight. On two other tractors, all the wheel ends were just fine. Trailers are even worse than tractors.”

He suspects there may be some calibration issues with the tools used to install wheel hubs at the factory.

“When you take delivery of a new truck or trailer, jack it up, and spin the wheels by hand,” he suggests. “Some you can barely move, others free-spin nicely. That’s not very consistent.”

For Morrow and his team, the cost and inconvenience of disassembling and readjusting their wheel bearings is worth it. And he says he hasn’t had a seal failure since he began adjusting the bearings himself.

“Proper torque at the factory is critical to ensure the bearing adjustment is correct,” says Bryan Williams, ConMet’s vice-president of engineering. “If the torque applied was too high the bearings can be put into excessive pre-load, or if the torque was too low, the bearings can have excessive endplay, both of which could affect fuel economy and the performance of the bearings over time.

“Typically, wheel bearings have very limited impact on rolling resistance,” he adds. “However many alternative factors, including but not limited to wheel seals, lube, environmental factors, and the installation process, have a much more significant impact on the rolling resistance.”