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Lubricating oils

Straight mineral oils are generally the favoured lubricant for lubricating rolling bearings. Oils containing extreme pressure (EP) or anti-wear (AW) additives to improve lubricant properties are generally used only in special cases. The information covering EP and AW additives in grease (→ Load carrying ability) also apply to these additives in oils.
Synthetic versions of many of the popular lubricant classes are available. Synthetic oils are generally only considered for bearing lubrication in extreme cases, e.g. at very low or very high operating temperatures. The term, synthetic oil, covers a wide assortment of different base stocks. The main ones are polyalphaolefins (PAO), esters and polyalkylene glycols (PAG). These synthetic oils have different properties than mineral oils (table 1).
The thickness of the hydrodynamic film, which prevents metal-to-metal contact in a bearing, plays a major role in bearing fatigue life. The thickness of the hydrodynamic film is determined, in part, by the viscosity index (VI) and the pressure-viscosity coefficient. For most mineral oil based lubricants, the pressure- viscosity coefficient is similar, and generic values obtained from literature can be used. However, for synthetic oils, the effect on viscosity to increasing pressure is determined by the chemical structure of its base stock. As a result, there is considerable variation in pressure-viscosity coefficients for different types of synthetic base stocks. Due to the differences in the viscosity index and pressure-viscosity coefficient, it should be remembered that the formation of a hydrodynamic lubricant film, when using a synthetic oil, may differ from that of a mineral oil with the same viscosity. For additional information about synthetic oils, contact the lubricant supplier.

In addition, additives play a role in the formation of a hydrodynamic film. Due to differences in solubility, different types of additives are used in synthetic oils that are not included in mineral oil based lubricants.
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