Oil and grease are not necessarily the best media for lubrication; in fact you will often find a lower coefficient of friction when using oilless bearings. The use of dry film lubricants or vapour deposited coatings is often a better way to go. It is possible for friction to be reduced below that of oil-lubricated systems. There are a number of compounds that are used in oilless bearings, and each has its individual advantages. Depending on the parameters to be filled there are four main compounds that may be used:
- Graphite has a thin flat crystalline structure that sticks strongly to metal surfaces and is a good conductor of heat and electricity. Graphite is relatively inert, being unaffected by acids, alkalis and solvents. As a lubricant it enables metal surfaces to resist seizure even at very high loads
- PTFE (polytetrafluoroethylene), better known as Teflon has a coefficient of friction of 0.1 Due to its low friction, it is used for applications where sliding action of parts is needed, such as oilless bearings, bushings, gears, slide plates, etc. In these applications it performs significantly better than nylon and acetal, and is comparable to ultra high-molecular weight polyethylene (UHMWPE), although UHMWPE is more resistant to wear than Teflon
- Molybdenum Disulphide (MoS2) is a black crystalline solid which shears easily and has a low coefficient of friction. It is resistant to solvents and chemicals and can be used at temperatures up to 250ºC. MoS2 has a coefficient of friction of < 0.1
- Tungsten disulphide (WS2) has a coefficient of friction of 0.03. Tungsten disulphide was developed by NASA as a lubricant for the Mariner deep space probes, where the high vacuum and temperature render conventional lubricants unsuitable. Since then it has found uses as a specialised engineering lubricant in oilless bearings and in the plastics industry as a permanent (coated on) release agent.