WHEEL-END LUBE: NOT SO SIMPLE
From the July/August 1999 issue.
The business of keeping the brakes dry and the wheels attached is a challenge, for sure, but some fleets say it’s routine, no matter which seal or bearing lube you use. You just have to keep things clean during installation, fill the hub with lube properly, and drive the new seal in accurately. For
some maintenance shops, there’s just no reason to alter the tradition of using a gear-oil lubricant, commonly an 80W90 grade.
But for many others it’s been a different story altogether.
Many fleets have shifted away from oil as their wheel-end lubricant of choice toward a higher-viscosity semi-fluid grease, both mineral-based and
synthetic. And many of them are happy to be done with the leakage issues that oil seemed to create. Some say leakage is gone entirely and
wheel-end failures are down by 75% or more.
Fleets began switching from oil to semi-fluid grease lubricants with a vengeance in about 1995, though some of those greases have been on the market since 1987. At a meeting of The Technology & Maintenance Council (TMC) of the American Trucking Associations several years ago, one carrier’s maintenance chief described his success with the switch, and that effectively lifted the flood gates. Tired of dealing with leaking wheel seals, many others followed his cue.
As well as ending leakage, these semi-fluid greases were also said to offer longer seal life, superior suitability to temperature extremes (which is open to debate, some say), extended service intervals (also open to debate), and of course elimination of brake-shoe contamination. But they
also add a new twist.
“With an inadequate amount of semi-fluid lube, we began to see a lot of bearing failures on the tapered-spindle designs,” explains Mark Wagner, hub product manager at Consolidated Metco Inc. in Portland, Ore., “because the small end would be starved of lubricant.
“So this is the issue. Now that we have converted a lot of people to semi-fluid lube, they’re experiencing a significant number of more serious types of failure.
“The failure mode has changed. The failure mode is no longer oil leaking from a seal; the failure mode will be a bearing that fails, and this difference has been difficult for people to live with.”
Some fleets using semi-fluid grease have seen early failures — including wheel-offs — on trailer axles in just a few hundred clicks or as far out as 200,000. They might find rust on the outer end of the axle spindle, on the bearing-adjustment nuts, and even on the outer bearing itself. There may be evidence of excessive pressure and moisture in the hub assembly, the latter often caused by high-pressure washing of the wheel-end components. There have been reports of sufficient rusting or excessive heat, or both, to cause the elements of the outer bearing to seize together Here’s how Rolf VanderZwaag, maintenance and technical advisor to the Ontario Trucking Association, summarized the situation for us last year:
“In many of the reported failures there are indicators of poor lubrication of the outer bearing in a tapered-spindle application. This is evidenced by heat discoloration of the bearing elements and spinning of the inner bearing race or the bearing cup in their respective journals.
“Spindles that use inner and outer bearings of the same dimension have been identified as not being prone to failures associated with the use of semi-fluid lubricants.
You can spot lube contamination, VanderZwaag says, especially outside of the outer bearing, by looking for discoloration of the lubricant or a change in its consistency. You might also see changes in the appearance of the inner surfaces of aluminum hubcaps.
There are two key issues with the grease option, says Nicolas Samman of Petro-Canada Lubricants in Mississauga, Ont. One is contamination by the entry of water and dirt through the hubcap vent, with the result that rust and debris migrate to the outer bearing and bring failure.
The other issue, Samman says, and it’s the more critical one, is confusion as to what constitutes the correct fill level. How much is enough? And how do you know how much lube is in there?
“Many of the problems involved with failures on the road today are related to the process of installing lubes into the system,” agrees Con Met’s Wagner. “Especially the semi-fluids, where it’s very difficult to monitor the amount you’ve installed. And it’s also important to know where it is in the hub. You have some segregation of areas in the hub that can mean the outer bearing is blocked off from the center portion of the hub, or the
grease cavity as we call it. And then there’s a region near the seal. A semi-fluid lube, when it’s hot, can migrate between these bearings but it doesn’t do so readily.
“I’ve seen many bearings that have a lower level of fill, where the hubcap remains completely dry inside, so no lube is ever getting out there.”
Recommended fill levels usually refer to percentage of fill, but that can be tough to communicate to service personnel who are used to dealing with static fill-level indicators like those used for oil. And if fill levels are to be determined as a percentage of the volume of space within the hub, then technicians need to know more. If they don’t know the cavity’s volume, then they’ll either work blind or ask, reasonably, “percentage of what?”
“A fill-quantity recommendation would be more useful, either by weight or by volume, but this often requires taking into account a residual quantity of lubricant remaining in the hub,” says VanderZwaag.
Mobil’s recommendation had been a 30% fill, but that’s changed to 40%-50% with tapered bearings. TMC’s advice, part of Recommended
Practice #631, is 50%.
There’s another view on the fill question, and it’s an interesting one, from Jon Cox, vice president of product engineering at Stemco Inc. in Longview, Tx. He doesn’t necessarily agree that the culprit is the fill level.
“That’s what we’ve been told by the industry,” Cox says. “But what I have not seen, as an engineer, is a complete and thorough root-cause analysis to find out if that really is the root cause. I’m not sure if it is. There are people out there who say they filled their hubs appropriately and they still had burn-up issues.
“So the question that I have is, do we really know what happens as far as wheel-end burn-ups are concerned, or are we just blaming it on the
At this point, there’s no answer to his question.
Nor is there a totally straightforward answer to the question as to which lubricant is best suited to wheel-end duty. There are three basic choices: an oil, almost always an SAE 50 synthetic transmission fluid; a semi-fluid grease, either synthetic or mineral-based; or a #2 grade grease, which
is similar to chassis lube.
We haven’t discussed the latter solid grease, but there are those who consider it the optimum wheel-end lube in some situations. The chief
benefit is that it stays put, won’t migrate away from the bearings, and it certainly won’t leak. But it demands a very different fill procedure, and its
cold-weather performance demands careful spec’ing approach because some such greases will not bleed oil readily at low temperatures.
The downside with semi-fluids, as we’ve covered, concerns fill and inspection procedures for the most part, but they’ll never be as easy to deal with as oil. And as lubricant engineer Bob Tanis of Henkel Lubricant Technologies in Cincinnati, Oh., points out, if semi-fluid lubes demand as much installation and inspection rigor as seems to be the case, then their initial low maintenance promise is in question. Tanis chaired TMC’s RP 631 Task Force.
In fact, there’s no broad agreement that any one lube is the best option. Samman says semi-fluids are “the more forgiving lubricants,” but the trend toward their use was a matter of seal reliability, not an issue with oil itself. And now that a great deal of development work has led to improved seal technology, oil may be poised to make a comeback. Some say it’s the best choice but others, including Wagner, disagree. His vote goes to grease, with qualifications (see below).
Tanis has another theory on the switch to semi-fluids. “I think the reason that grease was given any credence at all was the fact that the seal
manufacturers were under the very strong opinion that truckers would not pay for a better seal,” he says. “Really, they took a step back in lubrication technology by 30 years. In the 1950s, they came out of grease and went to an oil and thought it was wonderful. Now we’re saying ‘grease is better?’ I guess I still have a question about that.”
So does Jon Cox of Stemco, though he won’t make specific recommendations.
“I believe the simplest, most cost-effective lubricating means is still oil,” he says. “It’s very easy to install, and most importantly it’s verifiable. That is becoming more and more an issue out there. As people go to higher-viscosity lubricants, now they find they have a lack of confidence as to whether or not they have enough in there.
“But our products are designed to work with any type of lubricant,” he adds. “If the fleet wants to use lubricant ‘A’ versus lubricant ‘B’ or lubricant
‘C’, we need to seal it reliably, regardless of its consistency.
“From about 1995, we had significant growth in the use of semi-fluid lubricants, but I personally think that the growth has stopped. I’m not saying that’s good or bad, just that it appears that other componentry such as the seals, and the installation practices, are now of a quality that people no
longer need to use a more expensive, more exotic lubricant.”
It may be, however, a matter of choosing the right horse for the course.
“We don’t think there’s one lubricant for everybody,” says Mark Wagner. “To choose the optimum lube, you have to look at your maintenance and
service environment, also at your application. There are cases where you’d want to have the flag, the warning of leaking oil, to try to minimize any further damage.”
Wagner also points out that severe-duty situations lend themselves to the use of oil and simpler seal technology because there’s already a lot of maintenance being done and long seal life isn’t a factor.
Both he and Cox talk about the prospect of million-mile seals, but while Wagner figures that grease technology will be necessary in that case, Cox says it isn’t yet obvious. Regardless of the lube to be used, that ‘dream seal’ isn’t far away.
“I think we’re going to continue to see new innovations in the wheel end as the industry tries to benchmark up to that million-mile mark,” says Cox.
“We hope to get there in about two years.”
THE UNITIZED OPTION
The ultimate solution to the problem of leaking seals, in some eyes, is a pre-packaged or unitized hub. The hub, seal, lubricant, and bearings are assembled together as a single unit which is set into the axle spindle, eliminating the need for bearing adjustment, seal installation, and
Con Met, Meritor, Stemco, and Dana’s Spicer Division have introduced such designs. Unitized hubs have about 20% of the steer-axle market
and some 10% of the trailer market, according to Meritor’s Kurt Burmeister.
The various makers all agree that ‘maintenance free’ doesn’t mean ‘forget about it.’
“Our field-maintenance manual says that when the user has a trailer in for normal maintenance, the mechanic is to take a look at those wheel
ends, ensure they rotate smoothly, and check for any excessive end play,” notes Jim Grant, chief engineer for Meritor trailer products. “The bearings are slightly pre-loaded, so there shouldn’t be any end play, but the point is, you do want to keep an eye on things anyway.”
Burmeister says the payback on the extra cost of a unitized hub occurs, typically, at about 250,000 miles when the first change of a conventional seal would likely be done. The estimated $250 cost of that job is about the upcharge for the unitized hub.
TMC RP 631
The Technology & Maintenance Council’s Recommended Practice #631 concerning wheel-end lubrication was re-written in 2000 to better
integrate semi-fluid grease more fully into the RP with the addition of updated lube-fill and inspection advice.
The re-write included a four-level inspection regime for grease-lubricated wheel ends, beginning with the driver’s pre-trip routine, suggesting he look for seepage and feel the hubs for excessive temperature.
Level 2 is a “detailed external inspection” to be done at PM intervals or at least annually. Among other things, it urges that the vehicle be raised to check for smooth rolling of the wheel and for excessive end play. The hub cap is not removed.
In Level 3, at intervals determined by the vehicle maker, lubricant quantity and condition are checked by pulling the outer bearing. There is simply no other way to get an accurate picture with semi-fluid grease (this stage would be skipped with hard grease). The presence of lube on the outer bearing is part of the check, of course, and re-filling to 50% is advised.
Level 4 is only to be reached, the RP says, if abnormal conditions have been found in the previous stage. It involves a complete system tear-down and suggests that prematurely failed parts and lubricant samples be kept for analysis.
During a TMC Task Force meeting a few years ago, an interesting suggestion came from the floor. Lorne Brock, senior technical specialist with Imperial Oil in Toronto, said Canadian cold-weather experience has proven the value of lubricating the spindle with a molybdenum-containing grease.
The purpose: to prevent fretting corrosion, which occurs in metal-to-metal contact when small particles of metal come adrift and form a very hard ferrous oxide crystal. It can be extremely damaging if it gets in the bearing raceway, Brock said in a subsequent interview, and it can form in as
little as two months.
“It’s not a time issue,” he said. “It’s a matter of type of service and [ambient] temperature.” And he added that it gets just as cold in Wisconsin and in several other states as it does in most of Canada.
In the absence of lube and/or water, this corrosion appears red, like ordinary rust. With lube present, it appears black. It forms on the bore of the bearing cone and on the spindle bearing journal, underneath the cone.
A moly grease is the only solution, Brock told the Task Force meeting. “We know it works,” he said. Responding to the suggestion that graphite would also work, he said he and his colleagues couldn’t find a graphite that was compatible with semi-fluid greases.
If you don’t already own TMC’s huge Recommended Practices Manual, contact them at 2200 Mill Rd., Alexandria, Va. 22314. Tel. 703/838-1753 or fax 703/684-4328.
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