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Thursday, 18 July 2013

The best bearing lubrication.

The best bearing lubrication practices to prevent premature failures.




Often the best bearing lubrication is difficult to accomplish. This may be due to operating environment or even the design and construction of the equipment. Therefore for the best bearing lubrication it’s advisable to fully understand what constitutes good bearing lubrication.

 

  What is the best bearing lubricating oil?

Straight mineral or synthetic oils are generally favoured for rolling bearing
lubrication.

Oils containing EP, antiwear and other additives for the improvement of certain lubricant properties are generally only considered as the best bearing lubrication in special cases.

Synthetic versions of many of the popular lubricant classes are available. Synthetic oils should be considered for bearing lubrication in extreme cases, e.g. at very low or very high operating temperatures.

The term synthetic oil covers a wide range of different base stocks. The main ones are polyalphaolefins (PAO), esters and polyalkylene glycols (PAG). These synthetic oils have different properties to mineral oils.

The best bearing lubrication should maintain a good lubricating film under pressure.

With respect to bearing fatigue life the actual lubricant film thickness plays a major role. The oil viscosity, the viscosity index and the pressure-viscosity coefficient influence the actual film thickness in the contact area for a fully flooded condition. For most mineral oil based lubricants, the pressure-viscosity coefficient is similar and generic values obtained from literature can be used without large error.

However, the response of viscosity to increasing pressure is determined by the chemical structure of the base stocks used. As a result of this there is considerable variation in pressure-viscosity coefficients for the different types of synthetic base stocks. Due to the differences in the viscosity index and pressure-viscosity coefficient, it should be remembered that the lubricant film formation, when using synthetic oil, may differ from that of a mineral oil having the same viscosity. Accurate information should always be sought from the individual lubricant supplier when determining the best bearing lubrication.

In addition, additives play a role in the film formation. Due to differences in solubility, different types of additives are applied in synthetic oils when compared with the mineral oil based counterparts.

The best bearing lubrication does not rely on EP/ AW additives.

Bearing life is shortened if the lubricant film thickness is not sufficient to prevent metal-to-metal contact of the asperities on the contact surfaces.

One option to overcome this is to use so-called EP (Extreme Pressure) additives. High temperatures induced by local asperity contact, activate these additives promoting mild wear at the points of contact. The result is a smoother surface, lower contact stresses and an increase in service life.

Many modern EP additives are of the sulphur/phosphorus type. Unfortunately these additives may have a negative effect on the strength of the bearing steel matrix. If such additives are used then the chemical activity may not be restricted to the asperity contacts. If the operating temperature and contact stresses are too high, the additives may become chemically reactive even without asperity contact. This can promote corrosion/ diffusion mechanisms in the contacts and may lead to accelerated bearing failure, usually initiated by micro pitting.

Therefore, for the best bearing lubrication it’s recommended to select less reactive EP additives for operating temperatures above 80 °C. Lubricants with EP additives should not be used for bearings operating at temperatures higher than 100 °C.

For very low speeds, solid lubricant additives such as graphite and molybdenum disulphide (MoS2) are sometimes included in the additive package to enhance the EP effect. These additives should have a high purity level and a very small particle size; otherwise dents due to overrolling of the particles might reduce bearing fatigue life.

AW (Anti-Wear) additives have a function similar to that of EP additives, i.e. to prevent severe metal-to-metal contact. Therefore EP and AW additives are very often not differentiated between. However, the way they work is different. The main difference is that an AW additive builds a protective layer that adheres to the surface. The asperities then slide over each other without metallic contact.

The roughness is therefore not reduced by mild wear as in the case of EP additives. Here too special care has to be taken when considering the best bearing lubrication; AW additives may contain elements that, in the same way as the EP additives, can migrate into the bearing steel and weaken the structure.

Certain thickeners (e.g. calcium sulphonate complex) also provide an EP/AW effect without chemical activity and the resulting effect on bearing fatigue life. Therefore, the operating temperature limits for EP additives do not apply for these greases.

For extreme conditions it’s recommended that you seriously consider synthetic lubricant for your equipment. Give Habot Synthetic Lubricants a call – we have the expertise and the products to supply you the best bearing lubrication.

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