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| Basic Technical Parameter |
PURPOSE OF LUBRICATION |
Lubrication increases bearing life in most operating environments. The positive effects of lubrication may be summarized as follows:
- A lubricant film reduces friction and wear between the balls and raceways of the bearing, extending fatigue life.
- Dissipates frictional heat and can provide additional cooling if lubricant is circulated.
- Guards against corrosion and may help prevent foreign material from entering the bearing.
In addition to these general benefits, a good lubricant should have the following intrinsic properties:
- Resists change in structure or consistency during prolonged service.
- Resists excessive viscosity changes at temperature extremes.
- Resists leakage, dripping or throw-off from rotating surfaces.
- Compatibility with materials used in bearings, particularly seals and retainers.
- Tolerate some contamination, including moisture, without loss of characteristics.
Other factors that are affected by lubricant choice include:
- Noise level
- Maximum operating speed
- Torque
- Storage time (shelf life)
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| LUBRICANT TYPES |
Grease: Grease lubricant is comprised of a base oil carried in a thickening agent, which delays migration and evaporation of the oil, lengthening its effective operational and shelf life. Additives may also be included in the basic grease formula to improve corrosion resistance, reduce oxidation, and otherwise improve the lubricant performance. Grease lubrication is recommended unless low torque or high speed is essential to the application.
Oil: In general, oil is used as a lubricant where grease is unsuitable, for example, where low torque or high speed are considerations. Typical oils used are the same as those used as the base oils in greases. The operational life of oils is usually much less than that of equivalent greases due to evaporative losses. One exception is the perfluorinated hydrocarbon family of oils that have extremely low saturated vapor pressure (SVP), giving greatly extended life. |
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| GREASE COMPONENTS |
| Base oil: Petroleum oils, silicone or synthetic diester/tetraester oil or fluorinated oils are mainly used as the base oil for grease. Although the lubricating properties of grease depend mainly on those of its base oil, the thickener used also has an effect. In general, low viscosity base oils are more suitable for high speed, low temperature applications and high viscosity base oils are more suitable for high temperature, heavy load applications. |
BASE OIL |
TEMPERATURE RANGE |
OTHER PROPERTIES |
Petroleum |
-25oF to 250oF
(-32o to 121oC) |
Excellent load capacities (high film stregth). Lower viscosity = high speed (dN) capability. |
Diester/tetraester |
-65oF to 350oF
(-54o to 177oC) |
Wide temperature range. Moderate film strength. Oxidation resistant. |
Silicone |
-100oF to 400oF
(-73o to 204oC) |
Widest temperature range. Less film strength than diester/tetraester. Tends to migrate giving less rust prevention. |
Fluorinated |
-30oF to 550oF
(-34o to 288oC) |
Stable at high temperature. Chemically inert. Ultra low vapor pressure (10-9 to 10-11 torr) gives long life. Can be viscous at low temperatures. |
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| Thickener: Grease thickeners can be grouped into several families. Types used include compounds of metallic soaps (lithium, sodium), inorganic (silica gel, bentonite), heat resistant organic (urea, flouric) - or a mixture of several types. Important grease characteristics influenced by the choice of thickener are the dropping point (temperature at which the grease becomes sufficiently fluid to drip, affecting high-temperature performance), and the water solubility. In particular, sodium soap emulsifies with high humidity and cannot be used as a thickener when moisture is present. |
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| GREASE CHARACTERISTICS |
Channeling: If a grease stays in place after it has been displaced by the balls during initial rotation, it is referred to as a channeling grease. This property is useful to reduce torque. A non-channeling grease will tend to migrate back into the raceways, which can promote better lubrication in heavy load applications.
Dropping point: The temperature at which a grease heated in a specified small container becomes sufficiently fluid to drip. Dropping points for common greases are listed in the table of commonly available lubricants. Above the dropping point, a grease will lose its essential characteristics.
Consistency: The consistency of a grease is characterized by its penetration, which is the depth in 1/10th millimeters that a standard cone sinks into the grease under prescribed conditions. High numbers indicate softer grease, since the cone has sunk further. Grease may change consistency once it is worked (kneaded), so penetration levels are referred to as worked or unworked. If a grease has a uniform consistency, then there will only be a small spread between the worked and unworked values. Channeling greases tend to be thicker, so will have lower penetration values. Specific penetration values are available from grease manufacturers (ranging from approximately 100 to 400 x 1/10th mm). |
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