packing bearings - PeachParts Mercedes-Benz Forum

Robert Davis · #1 10-06-2003, 09:19 AM

Techs out there - please advise!
I know this isn't a Mercedes but please advise. I have a Gulfstream RV on a Toyota chassis. It has duel rear wheels. I pulled the hubs, axles, and drums this weekend to find out where the grinding noise was coming from. Turned out to be a fried outer wheel bearing.
I know I cleaned out at least a pound of grease out of each hub. My question is is it necessary to fill the hub with that much grease, or just pack the bearings good and reassemble? Anyone have anything to say about Mobil 1 synthetic grease? (cool purple color)

Thanks all,
Robert Davis

Kestas · #2 10-06-2003, 10:21 AM

My Mobil I synthetic grease has a cool red color!

I hope you replaced the entire set when replacing the bearing. This means both races and rollers.

When repacking conventional wheel bearings, you have to play "goldilocks" with the amount of grease.... not too much and not too little. Too little will shorten the grease life. Too much will cause the bearings to overheat. I work for the bearing industry and our specifications not only have a min but also a max on the amount of grease.

First, all old grease must be cleaned out.

When repacking, the grease has to be fully packed into the roller assembly. This is commonly done by scraping the taper roller assembly against a glob of grease in the palm of your hand. Keep going around the roller assy until grease comes out the other side.

Next, all interior surfaces of the bearing hub cavity are buttered with a light coating of grease.

Place the roller assembly into the race and add some more grease to cover the roller sides (both sides). The axle is now properly greased. Though not specified, some people pack the cap with grease before installation. They say it provides a measure of protection should the bearing overheat for some reason.

Use new seals.

Make sure you properly set the preload on the bearing. In the absence of a specified procedure, first tighten the axle nut while rotating the wheel (about 40 ft-lbs). Then back off the nut and tighten finger-tight. Then install the cotter pin mechanism. If none of the cotter pin holes line up, back the axle nut off just a bit for the best fit.

Diesel 924 · #3 10-06-2003, 12:35 PM

That was a great how-to. The only thing I'd like to add is the dustcap is lightly greased to keep condensation from rusting the inside of the cap.

#4 10-06-2003, 01:20 PM

I agree, great how to man!!

Quote:

That was a great how-to. The only thing I'd like to add is the dustcap is lightly greased to keep condensation from rusting the inside of the cap.

I was under the impression that the dust cap was actually loaded up with grease to serve as a resevoir once it heats up and begins flowing like oil. When I did my wheel bearing in my diesel recently, I measured the correct amount out per corner, packed the bearings nicely, spread some on the inner surfaces, and used the rest as a resevoir in the cap. Mabye this is wrong?

I talked to a Mechanic recently who said it is a pretty un-precise procedure in most Mercedes... he never measures.

Kestas · #5 10-06-2003, 01:35 PM

None of our automotive or truck product specifies packing grease into the dust cap. Except for perhaps wasting some grease, I don't see any problem with it. In fact, like I said earlier, in dire circumstances it may help the car limp along a little farther. Properly specified grease doesn't heat up and flow like oil. After all, how many times have you taken a wheel apart and found the grease intact in the hub, just like it was left? If it flowed like oil you'd probably get good mixing (churning) of the grease with very few dead spots.

No two people grease alike. Some may butter the insides a little heavier. There's nothing wrong with putting the extra inside the cap.

I don't measure my grease either. I simply follow good regreasing techniques. I think it's very thoughtful of MB to specify a certain grease charge to prevent people from getting too enthusiastic about lubrication.

#6 10-06-2003, 01:40 PM

I believe that grease behaves very much like an oil when it is heated... here, let me find an old post.

This is from JimSmith in response to my post here: Wheel Bearing Removal

Quote:

Cazzzidy

The intent of packing a bearing with grease is to provide lubrication and cooling to the interface between the loaded balls and the races. Grease is used to avoid the need for a forced or pumped oil lubrication system, and is a very practical, robust and inexpensive solution when it is carried out according to plan.

Grease works by storing the base lubricationg oil in a suspension of thickeners, chemical stabilizers and other materials. The oil is released by increases in bulk temperature to replace the oil that is used by the bearing. "Using" the oil occurs when the oil molecules are heated to very high temperatures and subjected to very high pressures and mechanical loads, which occurs in the film between the ball and the race. Under these conditions the oil molecules are broken apart, oxidized and turned into stuff other than oil. Some also vaporize and condense elsewhere, often outside the area where they can be returned to duty. The stabilizers and "other materials" are intended to protect the oil molecules from being oxydized or mechanically ripped apart by the extreme conditions in that film between the loaded balls and the race. The science of tribology and these additives is a little like the dark magic of witchcraft - I am not convinced these things are "designed" as it seems they are mostly the result of lots of trials, tests, and, with the good formulas, a healthy dose of luck.

As the bulk temperature of the grease increases the oil comes out of suspension from the stored grease adjacent to the bearing and runs into the area where it is needed. This cools the bulk mass down and the grease stiffens up again, and stops bleeding oil. Until it is needed again.

So, you need enough grease, in the right places in the bearing and the bearing cavity, to fulfill this job for a significant period of time. There is also a role as a heat transfer media to draw heat away from the bearing and spread it to adjacent surfaces to help cool the bearing. Grease is generally much better than air at this function.

Normally as you run a bearing after packing it, the bulk of the grease you packed in the bearing itself is forced out of the bearing and comes to rest in the adjacent housing volume, where it can readily supply that oil as it is needed by the process noted above. If the bearing is packed correctly, and then the adjacent volume in the bearing housing is packed full of grease too, there is no place for the excess grease in the bearing to go. As a result it ends up being churned like butter as it is dragged around with the balls and cage. This creates excess drag, and the energy you put in to overcome the drag goes toward heating the larger volume of grease. In this situation there is no need for the oil in the bearing, as it is already full of grease and oil, so the oil runs to some other part of the housing (and usually leaks out the cap), which makes the remaining stuff thicker, creating more drag, more heat, and so on.

This condition eventually defeats the original "design" of the grease lubrication system and you get a rapid loss of lubricating oil, while the bearing itself is still full of thickeners and chemical additives. Since oil from the adjacent "fresh grease" cannot run into the bearing because it is blocked by these thickeners, as well as the overall grease pack inside the housing, there is no means to lower temperatures.

Most thickeners and additives are not lubricants (thickeners are typically fine clay particles or metal based soaps, neither of which is a lubricant by itself), and as, as noted, the oil all runs out so the bearing keeps getting hotter and fails. The key is not to overfill the actual housing. I have seen many automotive rolling element bearings packed solid with grease, and as long as the housing is not more 25% or so filled, the bearing works fine.

As engatwork noted, there is typically a volume, measured by weight of a specified type of grease, listed by the auto manufacturer for the bearing application. I am not aware of where this information is provided by Mercedes-Benz, but I would feel pretty comfortable packing the bearing itself "solid" and putting about the same amount of grease in the cavity or housing as I found when I opened it up. When handling the bearing it is best to go through great pains to keep it and the housing, as well as all the tools and the grease you use absolutely clean. This means you should use rubber gloves as well, as while you handle the bearing with bare hands you will off load a significant quantity of chlorides from your skin, as well as other acidic skin oils and greases. These chemical contaminants will rapidly deplete the additives in the grease, leaving the bearing vulnerable to rapid grease failure, and particulate stuff will quickly lead to mechanical failure of the races.

Good luck, and I hope this helps. Jim

#7 10-06-2003, 03:13 PM

Screeeeeeeeeeeeeeeeeeeech! Hold on everyone.
Robert, are you dealing with a "floater style rear differential? I bet you are because of the inner and outer wheel bearings that I think you described. A simple discription is one where you pull the axle out first and then the break drum comes off. That type of setup is lubricated from the differential lubricant. The lube runs out of the axle tube and fills the well between the wheel bearings so that the bearings run in that 90 weight gear lube.
Wheel bearings fail in this type of setup most often because the rear diff is allowed to run low. If it is just 1/2 inch low there will be no lube moved to the axle. Fry baby Fry!!!

#8 10-06-2003, 03:16 PM

By the way, that pound of grease you pulled out probably contributed to the bearing failure by keeping the differential lube out. Just for fun, tell me how low your diff level is. Use a piece of wire and stick in the fill plug. Oil should run out when you pull the plug of a full diferential.