It is widely accepted that a universal material for plain bearings does not exist because they are developed for specific applications. As engineers continue developing polymer materials for plain bearings, the choice of shaft material has become increasingly important. Shaft material is critical for proper performance and acceptable wear from bearing systems.
Surprisingly, the most expensive shaft material is not always the best choice. Smoother shafts are typically more expensive than rougher ones. Squeaking (a common problem for plain bearings) indicates stick-slip caused by excessively smooth shafts. Slightly rougher shafts help pull lubricants from self-lubricating plastic bearings. Smooth shafts adhere to plain bearings, which increases friction. The difference between static and dynamic friction also increases, and running characteristics become unstable. The conditions lead to premature wear of both bearings and shafts. Shaft surfaces that are too rough, on the other hand, result in abrasion. This increases friction, but not as much as excessively smooth shafts.
Designers have a choice between friction and life expectancy when choosing shaft surface roughness. When low friction is critical, shafts with average roughness of 32 to 64 rms work, but when long life is required, use shafts with average surface roughness of 20 rms.
This information supplied by igus, inc., Providence, R.I. For more information.
