Greasing Bearings – An Overview

Friction is a necessary evil and is here to stay, hence the importance of lubrication and applying grease. Bearing are used commonly in running machinery and some of the few reasons generally stated for the loss of a bearings are

• loss of lubricant
• incompatible grease
• in-correct greasing
• grease degradation
• excess of lubrication

However, the main reason, which has often been the cause of many bearing failures in the past, is improper greasing.

The task of applying grease to bearings is as difficult as selecting a grease for a particular application. Selecting a type of grease for a particular application mainly depends on the temperature of the part that the grease is to be applied to. Generally, low temperature grease is used for low temperature applications and high temperature grease is used for high temperature applications. This is done mainly to maintain a thin layer of grease between the mating surfaces.

If the right type of grease is not used it might lead to increase in temperature of the rubbing surfaces and reduction in the lubricating film. For e.g., if high temperature grease is used for a low temperature applications, then the grease will remain in semi-solid form and will not cover the rubbing parts completely, leading to increase in temperature and wear.

Normally the manufacturer of a machinery would recommend the type or grade of grease to be applied but what is not on record normally, is the quantity to be applied, so we will talk more about the best bearing grease application aspect in the subsequent section.

Now suppose that you have selected grease for a particular type of application, but then how would you decide how much grease is to be put. Putting the exact amount of grease is very important as applying improper quantity of grease is equal to not putting anything.

Also, excessive grease is dangerous as it leads to over heating of the rubbing parts. More the grease, more it will interfere with the moving parts, raising the temperature of that particular area. So what to do?

However, the basic rule that should be followed while applying grease is that though the bearings should be filled completely with grease, the free space in the housing should be filled partially. This left out space gives space to the grease to be ejected from the bearing when the moving part starts rotating. Moreover, a type of grease is also available that can be filled completely into the housing without allowing the temperature to rise. This grease is generally formed of lithium soap and has great stability even at high temperatures. This grease is helpful in applications where there are chances of impurities to enter the bearing.

Bearings can be divided into mainly two types – separable and non separable. No matter what type of bearing it is, the general practice is to fill up the spaces inside the inner race, outer race and the rolling components on each side of the bearing.

However, for re-lubrication the following formula should be followed.

G = 0.0005 DB

where

• G is the quantity of grease in grams
• D is the diameter of bearing
• B is the total breadth width of the bearing

Applying grease using this formula ensures that the grease remains at the rubbing surfaces for a longer time without coming off.

References

_{Gresham, R.M. & Totten, G.E. (2008) Lubrication and Maintenance of Industrial Machinery: Best Practices and Reliability. Florida: CRC Press}