A dry subject

Bearings sans liquid lubricants find use in everything from computer printers to rocket engines. Director, Center for Rotating Machinery Dry bearings, as the name implies, work with little or no liquid lubrication. Compared with rolling-element bearings, dry bearings are simpler to design, less expensive to purchase and maintain, and better resist contamination. Materials suitable for dry bearings include polymers and polymer composites, carbon graphite, ceramics, and wood. A basic understanding of the underlying physics of dry bearings should help with the selection process. Two bodies in dry, sliding contact, carry some of the total normal load, (MN) through asperities in the softer material. This so-called real contact area, ) can be estimated by: = yield pressure (MPa) for the softer asperities, which is typically about 3X higher than the material's tensile yield strength. Real contact area is a small fraction of nominal area. For example, an acetal bearing with an asperity yield pressure = 300 MPa under a bearing load of 3 MPa has an asperity contact area equal to 1.0% of gross projected area. (MN) needed to displace an asperity-of the softer material with shear strength, ) in dry sliding contact with a rigid body is: This asperity shearing can account for 90% or more of total friction force. Other factors that may contribute to friction force include the "lifting force" needed to raise asperities over roughness of mating surfaces, scratches by dirt and particles, internal material damping, and surface layers. , directly relates to shear strength and compressive strength, typically key to the same material properties such as lattice structure and bond strength, so often falls in a narrow range of about 0.20 to 0.35 for a variety of bearing materials varying several hundred fold in yield strength. Low-shear-strength surface layers of polytetrafluoroethylene (PTFE) or solid lubricants can lower