Keep 'Em Rolling
Wheels may make the world go around, but any major problems in the wheel-end system can make a good day turn bad. Proper wheel-end maintenance is vital to keeping you from being stranded on the side of the road and late for a delivery.
One of the toughest things to do with the wheel as it turns is to hold it snugly in place on a vehicle without generating a lot of friction. This is the job of the wheel bearing. Tapered roller bearings have a cone shape, and they roll in cone-shaped bearing cups. This allows them to carry vertical loads and handle end thrust. A wheel-bearing assembly consists of two tapered roller bearings situated opposite one another and turned in opposite directions. While they work together in supporting the wheel and handling the friction, they work in opposite directions in handling the end thrust. One of the two bearings takes the end thrust generated when the wheel pushes toward the chassis in corners, and the other takes thrust when the wheel pushes away from the chassis. The distance the wheel can be shifted from side to side, from the point at which one bearing begins to resist the end thrust until the other does, is called “end-play.”
Adjusting the bearing assembly involves controlling how far apart the two bearings are when they are assembled. If the two bearings are forced closer and closer together they will eventually reach the point where both are pressed tightly against the bearing cup that they roll against. Such a situation would eliminate end-play and produce “pre-load,” a situation where a slight tension would be maintained on the bearings.
Under controlled manufacturing conditions, where bearing adjustment can be performed with absolute precision, the bearing parts may be adjusted up to the precise point that all the end-play is eliminated and a small amount of tension is maintained. This is considered an ideal situation. But it’s considered impractical to achieve on manually adjusted systems.
A wheel-end consists of the two bearings and their grease or oil, together with the seals that hold the lube in. Not only are wheel-ends difficult to maintain, but the consequences of failing to keep them in proper working order can be severe. Under extreme neglect and abuse, wheels may fall off while a truck is on the highway.
Wheel-end maintenance was made a little easier during the 1990s as more trucks and trailers were manufactured with outboard brake drums. These are drums bolted to the outside of a flange on the hub that contains the wheel-end. They can be unbolted and pulled off the hub without disturbing the wheel-end components. After outboard drums were introduced, only wheel-ends needed to be accessed to ensure proper lubrication, a big savings for fleets where brake jobs must be done frequently.
But you can’t just ignore your long-life wheel systems. ArvinMeritor experts stress that “Maintenance-free doesn’t mean inspection-free.”
An annual inspection replaces routine adjustment and “repacking” or regreasing. With all long-life systems, safely lift each wheel-end well off the road. Slowly rotate the wheel to check for any roughness in the bearing. Also, firmly tug the wheel in and out to check for significant end-play. On greased systems, remove the hubcap to check for grease leaks. If the bearing operates smoothly and without end-play and there’s no sign of leakage, you can run it for another year. If there is any question in your mind, get it checked by a technician to locate excessive end-play.
Steve Slesinski, product manager for drive axles and wheel-ends at Dana Corporation’s Commercial Vehicle Systems Division, recommend frequent inspection of conventional systems.
The fastening system for the Meritor Triad wheel-end system is used with a relatively simple and inexpensive traditional bearing and seal system that can easily be disassembled and serviced. Because of a patented spindle nut installation procedure, it works at an unusually tight tolerance and offers longer life than typical conventional wheel-ends.
Annual disassembly, repacking, replacement of seals, and readjustment is “the ideal,” in the view of the ArvinMeritor experts. They say that if you do brake jobs at 100,000-mile intervals, this wheel operation should be performed at least every second brake job, or 200,000 miles. You should consult the instruction book and go by any manufacturer-specified interval if your situation is different. Of course, a frequent check for leaks is essential, especially when using oil rather than grease. Leaks have to be fixed before oil leaks onto brakes or ruins the bearings. If you are routinely adding oil, replace the seals.
Whenever a wheel-end must be disassembled for any reason (including just to do a brake job if you have inboard drums) it needs to be cleaned, repacked with grease and given new seals.
The ArvinMeritor Triad system is good for three years before routine repacking, readjustment and replacement of seals.
Slesinski says that whenever dealing with a bearing lubed with oil that has a sight glass, check the color of the oil to make sure it’s not milky or chocolate-colored due to contamination. If it looks dirty, an immediate disassembly, replacement of seals, inspection and replacement of lube is in order.
Independent trucker Earl Evans of Canfield, Ohio, believes annual draining and replacement of bearing oil is just good maintenance. He’s found both bearings and seals then last much longer.
A do-it-yourself job?
Wheel-end maintenance requires a high skill level, which means it is expensive and tough to learn yourself.
The work is difficult because of several variables that are hard to control. The most critical is adjusting the end-play of the bearings properly. The second most critical item is installing the seals that retain the lube squarely and doing it without damaging them. The third, more of a worry with greased bearings, is maintaining proper cleanliness and completely filling the bearing with grease when relubricating. The new systems help control the variables better by assembling everything in a factory environment.
Suppliers have introduced some effective solutions to the problem of wheel-end maintenance. On many long-life systems, the end-play adjustment is entirely eliminated by inserting a precision-machined spacer in between the two bearings and then merely torquing the parts together so they’re held snugly in place.
Slesinski points out that the “unitized designs require a high amount of torque -750 pounds-feet – to install.” There are also a lot of special tools you’ll need, depending on the system, and Slesinski stresses the need to be sure you get all the right information for the system you are working on before you start.
If you decide to tackle the job, you’ll need: for all adjustable bearings, a dial indicator; the correct socket wrenches and more than adequate torque wrenches; seal installation plates and tools to operate them; and a bearing packer. The last item ensures the complete purging of old grease and solid packing with new grease without air voids. You’ll also need a dolly to support the hub while positioning it onto the spindle. You can’t just hang it there prior to installing the outer bearing cone and assembling and adjusting everything. And use a jack when removing or replacing dual wheels – don’t attempt this with your back.
There are ways to be sure to get the right information about your system’s operation and needs. Start with the Technology and Maintenance Council’s Recommended Practices (“RPs”) 618 – Wheel Bearing Adjustment Procedures; 622 – Wheel Seal and Bearing Removal, Installation, and Maintenance; and 640 – Alternate Wheel Bearing Adjustment Systems.
The Meritor Packaged Bearing wheel-end system bolts together and the nut is simply torqued rather than requiring a complex and sometimes inexact setting procedure.
The type of nut system and the adjusting technique for setting the bearing varies, and you need to understand fully how to do it right. You may even need to use encoded snap rings to set the tension correctly, says Slesinski. Also, visit the wheel-end maker’s, vehicle maker’s, or seal maker’s website to get further specifics. Seal installers are needed to ensure the seal is forced in place via the outer diameter so it won’t be damaged. It must be fully seated all around so it will not be cocked.
In setting end-play in conventional adjustable systems, bearings must first be seated in two stages, Torque the primary attaching nut first to 200 pounds-feet and then to 50 pounds-feet with the wheel rotating. The nut is then backed off a certain fraction of a turn, which depends upon the threads per inch on its attaching stud. RP-618 provides a chart, and the manufacturer will also provide information. The ArvinMeritor Triad system is somewhat different, so make sure to use the right technique. Long-life systems typically come as an installed package. The attaching nut is then torqued to a critical reading in pounds-feet to set pre-load.
The right choice
Equipment with conventional wheel-ends cost less initially. However, it will cost you more when it comes to downtime and, unless you are a very good technician, repair bills. Considering the fact that even the most careful work on the traditional system still offers limited life and reliability as compared with the long-life systems, can you think of any good reason not to specify a long-life system on your next new tractor, truck or trailer?
Comparing the Big 3 Systems
Conventional wheel bearing
Steve Slesinski, product manager for drive axles and wheel-ends at Dana Corporation’s Commercial Vehicle Systems Division, defines the conventional setup as, “A conventional standard tapered bearing and spindle consisting of two separate bearings as well as a conventional seal and adjustable nut system.”
Jim Grant, ArvinMeritor’s chief engineer of worldwide trailer axles, says the conventional system is basically “two tapered, single-row bearings set as close as possible to an optimal adjustment with a two-nut system and washer.”
The special case, the ArvinMeritor Triad system also is designed to make more precise adjustment possible, It comes ready for several years’ trouble-free service because of the way the adjustment is performed in original manufacture.
Cartridge or packaged bearing or pre-adjusted
With this type system, says Slesinski, the object is to “control the pre-load of the bearings via the machining of the components themselves or a precision-machined spacer. The nut system is torqued rather than adjusted.” Everything is kept in exactly the right state of adjustment – actually under tension – by installing and torquing a single retaining nut to hold parts snugly together.
Unitized hub assembly
Slesinski says this system is designed to work “integrally with the hub.” In other words, it’s the machining of the hub that actually determines the exact fit of all the parts and the pre-load (the Dana Spicer system is called the LMS Hub.) He adds that the “seal is incorporated in the bearing and is also integral to the hub.” The attaching nut is torqued.
Marketing Manager Varun Rao describes the ArvinMeritor TB system as “completely unitized with no separate bearing cup. The inside of the steel hub serves as the cup.” This means that, on the ArvinMeritor TB system, the bearings actually roll against the hub’s inner machined-hardened and ground surface instead of a pressed-in bearing cone. The bearing is factory set and effectively “all contained within the hub assembly.”
Which system should I use?
Slesinski says the tradeoff is simple: “Cost vs. maintenance.” Pre-adjusted or unitized systems cost more when you purchase the equipment. But, they eliminate a lot of maintenance headaches and cost. “Unitized systems have almost no failures. The biggest difference is reduced cost of maintenance.” So, cost per mile in most applications is likely to be a lot less with the more sophisticated systems.
Baroni says, “If you’re in an application where you need to do brake jobs every 25,000 miles, long-life systems may be less interesting.” In other words, if you need to bring the vehicle in and do brakes frequently anyway, you may not save as much in maintenance dollars as an over-the-road fleet where the truck runs for years between brake jobs. The conventional type is fully rebuildable. If a fleet has routinely brought in its vehicles for disassembly, repacking, and seal replacement of wheel-end parts, and wants to continue such procedures, the conventional system is the system they want. And the simple parts for the conventional setup are easier and quicker to get and cost less than replacing a long-life cartridge or unitized wheel-end. If you do not have to pull the wheel-end except for bearing and seal replacement, the long life cartridge or unitized system may be of value to you.
However, in terms of both overall cost and convenience, the long-life systems win hands down. The experts all say that unitized hubs typically last twice as long as conventional systems, and often give you 1,000,000 miles. Similarly, the packaged/pre-adjusted systems last twice as long as the conventional system. In a world in which a “wheel off” situation traceable to improper wheel-end work could land you in liability trouble, the long-life systems could also add peace of mind.
The pre-adjusted systems are also simpler to replace, although you may need the tools to create a lot of torque for the fastening nut. There’s no need to adjust end-play with a dial indicator, or to clean or grease parts. Slesinski says, “If you have one good technician or are one yourself, the conventional system is no problem. But the unitized systems eliminate most of the maintenance variables.”
A Word about Grease
Just as there is more than one way to skin a cat, there is more than one way to lube a wheel bearing. Oil bath bearings often used on front, or less often on trailer axles, use 50-weight transmission oil or, occasionally, engine oil, according to ArvinMeritor experts. Obviously, drive axle bearings share the axle lube with the gearing.
William Wallace, a senior research engineer with ChevronTexaco Global Lubricants, says there are advantages with each system:
“The advantages of semi-fluid and solid greases include the fact that there isn’t as much leakage as with oil. Oil leakage can damage brake linings or other parts. Also, because of less leakage, grease may last longer and in the long run be more economical. Finally, grease acts as a sealant to prevent contamination of the wheel-end.”
On the other hand, Wallace says, “Oil is easier to service.”
Warren Eckert, an engineer at ExxonMobil Lubricants & Specialties, says, “An oil bath wheel bearing application offers, overall, the least maintenance, long lubricant life, and lowest operating temperature, but is most susceptible to leakage in the hub seal area. A semi-fluid wheel bearing grease can offer the same long-life advantage of a liquid oil with less risk of leakage, but requires some additional maintenance time during the initial fill and in performing period level checks. A solid grease can provide the most protection to keep contaminants out of the bearings, but will operate slightly hotter and requires a more frequent replacement than a liquid oil or semi-fluid grease.”
Ed Fliss, industrial lubricants products manager, Shell Oil Products U.S., says, “Synthetics (greases) are a popular choice and economical over the long run because maintenance intervals can be extended a lot further. But, you still have to make sure the grease is appropriate for the type of design.”
For further information contact:
Shell Oil Products U.S.