PeachParts Mercedes-Benz Forum - View Single Post

JimSmith · #5 07-30-2003, 11:51 PM

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