Tag Links: Bearing Grease | Bearing Lubrication
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
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
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.
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.
Source: Dow Corning
For more information on this source please visit the Dow
Corning supplier profile on AZoM.com