Lubrication Overview – Physical Aspect

Abstract

There are more than likely 50 points of contact in most homes that need lubrication, more if you include electrical receptacles. Many of these surfaces are lubricated simply to prevent them squeaking. Door hinges are one example. With continued usage noisy hinges become noisier, and then become hard to operate and eventually the parts seize together. The clearance fit between the pin and cylinder has apparently become an interference fit. Bumps are built up by plastic flow due to the high local traction stress. This buildup can be verified by using a surface roughness tracer over a region of the pin on which seizure had occurred. A drop of oil usually stops, or even reverses the progression toward complete seizure. The oil need only wet the surface in most cases. When a lubricant becomes drawn into the high-stress contact points, and if the resulting coefficient of friction reduces to about 0.3 for annealed steel (and likely for some other metals as well), the high points (the stretched asperities) are ironed down and seizure is averted. The situation is much different with steam turbines, jet engines, and automotive components. Thin adsorbed films of oil would not survive the rigors of high speed and high contact stress. Thicker films of lubricant must be pumped in or built up by hydrodynamic action. Many liquids will lubricate, such as gasoline, mercury, catsup, acids, mashed potatoes and more. The primary purpose of the lubrication of machine elements is to protect surfaces in contact against damage and wear and to reduce friction in most cases. Another auxiliary role of lubrication includes heat dissipation. Effective lubrication is usually achieved by the formation of a film between the surfaces in contact.