Northern Illinois

Wheel Bearing Maintenance

Recently, one of our Northern Illinois members experienced a wheel bearing failure, even after being serviced by the dealer. Here are a few tips for the more adventurous of us (who maintain their own bearings) or for those who just want to make sure the mechanic is doing the job properly. Most of our trailers use two tapered roller bearings per wheel. Inner bearing-Timken P/N 25580, outer bearing – Timken P/N LM67048, Grease seal – National oil Seals P/N 412920 (2.258×3.371x .375)

Typical causes of bearing failure:

Fatigue failure (spalling) is when a fracture originates below the surface of the races or roller bearings and the surface comes loose to form a very rough spotty surface. Upon disassembly, the grease may still be apparent (if the wheel was not run too long after failure). Even if only a few spall marks are present, the bearing and the raceways should be replaced. Spalling at the small end of the bearing means it was adjusted too tight. Spalling at the large end mean they were adjusted too loose.

Over Heating – This may be caused by the bearings being: adjusted too tight, improperly greased, improper type of grease, the load was too high, or the brakes overheated and burned the bearing. When disassemble, the bearing would at first appear to have been under lubricated, but that may not be the root cause of the problem, as the grease may have been melted due to the high temperatures. An over heated bearing will have a blue or brown discolored surface.

Contamination - Bearings may become contaminated by water or dirt or the original wheel bearing grease may have been contaminated. Bearing races will have scratches and a mildly rough surface.

Routine Observations
The driver should check the axle caps for bearing heat when filling the gas tank. A hot bearing should be serviced very soon. At the next stop, the suspect wheel should be jacked off the ground and rotated by hand to detect any roughness and to see that there is the proper amount of looseness (0.005 – 0.020 in.) or play in the wheel, by twisted from right to left.

Preventive Maintenance
At least once a year, the bearings should be cleaned, inspected, and relubricated (usually at the same time the brakes are checked). For real travelers, the bearings should be cleaned, inspected and repacked every 20,000 mile. You will need a grease seal puller to examine the inner bearing. There are several types of wheel bearing grease on the market. Not all of them are compatible, so you need to clean all the old grease off before repacking the bearings. Grease is made from an oil and a fatty soap (which provides the body to keep it in place). Older greases use sodium or calcium soaps but newer greases are lithium or synthetic based. Lithium greases have higher temperature ratings and are suitable for bearings that run hotter, such as on trailers or disk brakes. Airstream recommends we use a lithium grease meeting NLGI grade 2 rating per ASTM 265 (which should be marked on the grease container). The condition of the grease seal should also be checked and replaced (if necessary) at the time the bearings are repacked. To properly reset the bearing tightness, the retaining nut on the axle should be torqued with a wrench to12 ft. lb. while the wheel is being rotated. Then the nut should be backed off and retightened by hand until snug and then backed off to allow the cotter pin can be installed in the next available slot in the castle nut. The wheel should then be rocked and a small amount of looseness or play should be felt (0.005 to 0.020 inches when measured at the wheel rim). This is equivalent to the 0.001 to 0.010 in. of end play specified in the Airstream service manuals. Here is a link to a complete manual -
http://www.classicperform.com/tech_articles/Tapered-Wheel-Bearings/Tapered-Wheel-Bearings.htm

Dwight Dixon 11/28/07