Some bearings use a thick grease for lubrication, which is pushed into the gaps betweenthe bearing surfaces, also known as packing. The grease is held inplace by a plastic, leather, or rubber gasket (also called a gland)that covers the inside and outside edges of the bearing race to keep the greasefrom escaping.
Bearings may also be packed withother materials. Historically, the wheels on railroad cars used sleeve bearingspacked with waste or loose scraps of cotton or wool fibersoaked in oil, then later used solid pads of cotton.
Bearings can be lubricated by a metalring that rides loosely on the central rotating shaft of the bearing. The ringhangs down into a chamber containing lubricating oil. As the bearing rotates,viscous adhesion draws oil up the ring and onto the shaft, where the oilmigrates into the bearing to lubricate it. Excess oil is flung off and collectsin the pool again.
Some machines contain a pool oflubricant in the bottom, with gears partially immersed in the liquid, or crankrods that can swing down into the pool as the device operates. The spinningwheels fling oil into the air around them, while the crank rods slap at thesurface of the oil, splashing it randomly on the interior surfaces of theengine. Some small internal combustion engines specifically contain specialplastic flinger wheels which randomly scatter oil around theinterior of the mechanism.
For high speed and high powermachines, a loss of lubricant can result in rapid bearing heating and damagedue to friction. Also in dirty environments the oil can become contaminatedwith dust or debris that increases friction. In these applications, a freshsupply of lubricant can be continuously supplied to the bearing and all othercontact surfaces, and the excess can be collected for filtration, cooling, andpossibly reuse. Pressure oiling is commonly used in large and complex internal combustion engines in parts of the engine where directly splashed oilcannot reach, such as up into overhead valve assemblies. Highspeed turbochargers also typically require a pressurized oil system to cool thebearings and keep them from burning up due to the heat from the turbine.
Composite bearings are designed witha self-lubricating polite trafluroethylene (PTFE) liner with a laminated metalbacking. The PTFE liner offers consistent, controlled friction as well asdurability whilst the metal backing ensures the composite bearing is robust andcapable of withstanding high loads and stresses throughout its long life. Itsdesign also makes it lightweight-one tenth the weight of a traditional rollingelement bearing.