Quote:
Originally Posted by Racekar
It's impossible to overpack bearings, the excess grease just squirts out the other side of the cage. Infact thats how you know they are packed properly and fully.
I am partial to synthetic grease, years ago I had a 280Z racekar that would eat front wheel bearings and spindles on a regular basis. After I tried synthetic the problem went away.
|
In reality it is not only possible to overpack bearings, but a frequent occurrence and leads to low life due to over heating. The grease works in a ball bearing by reaching a certain temperature and then releasing the oil it has in suspension. The oil forms a hydrodynamic film between the ball and the race, and is subjected to very high, very localized pressures. The film thickness for a given application is ideally thicker than the defects in the surface finish of the balls and races, and any of the grease thickening agents or other additives. The oil also acts to remove heat from the bearing rolling elements and contact areas of the races. As the oil cools the bearing the temperatures drop into a range of acceptable (in a balanced design) temperatures where the grease pack can resupply the oil being lost to evaporation and mechanical destruction on a molecular level, for a regular, repeatable time period.
Overfilling the bearing causes the balls to have to churn the extra grease as they spin, and it adds drag to the cage. This all makes additional heat and makes the normal flow oil less effective as it runs out of the working area and it evaporates sooner, leaving little more than the thickeners and additives, which are not lubricants in most cases. It is well documented that too much grease leads to premature failures from over heating.
Newer greases with better oils and thickeners are probably more tolerant of this condition, but they are not immune either. The tolerance comes from higher temperature grade oils and thickeners, all of which are usually more expensive, and that leads to using less rather than more grease in a bearing repacking more often than not. More is not better when it comes to packing bearings. The right amount is best, and that amount is determined by the bearing size, the bearing housing shape and the loads. Jim
__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
|