Fighting friction with solid-film coatings

Low-friction coatings for bearings can improve bearing life and performance. Edited by Jean M. Hoffman Scuffing on bearing/wear surfaces, such as on the lobes of this rotor, is reduced or eliminated by the use of a low-friction coating. The bearing area of a knee brace is coated at the hinge to reduce friction and wear. Fluid lubricants would have to be used in sealed systems that would add weight to the brace. Coating the inside of a journal bearing prevents metal-to-metal contact with the mating shaft. Low-friction coatings are used on bearings for two reasons. The first is as a backup lubricant if the primary (fluid) lubricant fails or is insufficient to prevent surface-to-surface contact. Lubricant failure occurs when new equipment is in its break-in period, during machinery start-up, or during reversing operations. Occasionally, lubrication fails when a conventional hydrodynamic bearing is momentarily overloaded and the film of oil is partially pressed aside. In these conditions, there is mixed lubrication or, worse, boundary lubrication. Boundary lubrication refers to lubrication failure between two rubbing surfaces that don't have a full-fluid lubricating film between them. The second reason for using low-friction coatings is to handle applications that can't use fluid as the primary lubricant. Examples include nonmetallic bearings made from fiber, plastic, composites, or paper in instrument and aircraft applications. The coating also reduces friction in the bearing, which in turn reduces the heat and wear on the coating. Some bearing applications benefit more than others from the application of a coating. Here are some examples. in journal bearings with hydrodynamic lubrication should be greater than one. These types of bearings are found on line shafts, prop shafts, turbine shafts, pumps, machine tools, and some axle bearings. Side leakage from these bearings is not enough to cause total loss of the oil film.