Rolling-element bearings are used for the precise guidance of moving parts. Frictional losses are extremely small because the bearings rest on rolling elements, enabling very heavy loads to be adsorbed. Rolling-element bearings may be lubricated with greases or oils depending on the point of use. Lubrication is necessary to prevent direct metallic contact between rolling elements and bearing rings, thereby preventing wear due to welding, friction, material fatigue or corrosion.
The overwhelming majority of rolling-element bearings are lubricated with grease firstly, because of the absence of problems involved in sealing the bearing to prevent oil escaping; secondly, because of the sealing action of the grease itself in preventing penetration of foreign bodies, dust, etc.; and thirdly, not least because of the multitude of special greases available permitting the optimum choice for the particular application. Greases of NLGI consistency classes 1 to 3 can be used, depending on the rotational speed, service temperature range, running noise requirements, re-lubrication intervals, sealing, starting torque and environmental influences.
Up to 180°C/356°F and higher only greases based on synthetic oils such as Esters, Phenylethers or Silicones can be used. Should temperature even reach 250°C/482°F or above the only remaining base oil is Perflouropolyether (PFPE). From a grease thickener perspective simple Lithium Soaps should not be used above 200°C/392°F, but Lithium Complex Soaps, Polyurea or Polytetraflouroethylene (PTFE) present a good selection.
In those cases were speeds are low, greases with solid lubricants such as Molybenum disulfide(MoS2) should be preferred. Alternatively heavy duty thickeners such as Calcium Complexes can be used or sometimes a higher than the typical base oil viscosity of 100 mm2/s at 40°C/104°F.
One key application with heavy water ingress is in steel and paper mill applications where Calcium Complex, Polyurea or Lithium Complex thickened greases with good tackiness and adhesion are used.
Since raw materials available for use with food contact are limited mostly Aluminum Complex thickened greases are used, which for higher load applications can contain white solid lubricants.
Since shear forces are much higher as speed increases, the base oil viscosity for "high speed" greases should be considerably lower than the typical value of 100 mm2/s at 40°C/104°F. "high speed" greases should not contain any solid lubricants and the thickener system needs to be tailored for that application since a solid particle in the over rolling contact can cause problems such as noise or excessive wear.
Phenylethers, Flourosilicones and Perflouropolyether(PFPE) exhibit very good resistance against chemicals and are the preferred choice as base oils for greases used in rolling-element bearings along with the polymer based thickeners such as Polyurea or Polytetraflouroethylene(PTFE).
Particles of all nature contained in a grease generate noise in rolling element bearings. In general the smaller the bearing is the better homogenized and filtered the grease has to be. Only special Lithium Complex thickeners and Polyurea give such properties.
Similar to high temperatures only synthetic base oils can be used, since mineral oils are too viscous at temperature down to -40°C/-40°F and below, though at high temperatures those mineral oils would evaporate. The thickener system is not so critical at low temperatures, but base oil selection is limited to some low molecular weight Polyalphaolefines (PAO) and mainly Silicones. For very low temperatures Silicone base oils are used for greases as low as -73°C/-99°F.
This information has been sourced, reviewed and adapted from materials provided by Dow Corning.
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