Get a grip on grease

Selecting the correct bearing grease is essential to long life, smooth running and operational safety, says Ian Pledger

More than 80 per cent of all rolling bearings are lubricated with grease, so it is critical for engineers to select the most appropriate type of lubricant for the bearing, and the application in which it is operating. For correct selection will not only prolong the life of the bearing, enabling it to operate at its full capacity, but also ensure smoother running and improved operational safety.

But choosing a suitable lubricant can be a complex process. Grease is a type of oil with a thickening agent added to it, making it more viscous.

There are many different types of grease available, so engineers should take the time to make more informed selections to help ensure a long and trouble-free life for the bearing.

Greases can be used effectively to lubricate bearings over a very wide range of operating conditions. However, initial selection is based on two critical parameters.

First, the P/C ratio, which is a measure of the load P on the bearing in relation to the dynamic capacity C, which is calculated according to international standards and quoted by all leading bearing manufacturers in their technical literature.

In general, if the P/C ratio is greater than 0.15, the bearing is regarded as heavily loaded, and special lubricants with extreme pressure additives will be required to increase the load-carrying capacity of the bearing and prevent metal-to-metal contact, which leads to high wear and premature failure.

It is also important to keep the ratio above 0.02 as, in general, below this value the bearing will 'skid' rather than roll, again resulting in possible premature failure due to high wear and an increase in friction.

Second, the 'ndm' value. This is calculated by adding the shaft diameter and outside diameter of a bearing in mm and multiplying that 'dm' figure by the speed of the bearing 'n' in rpm. In general if the ndm value is below 400,000 then standard general-purpose bearing greases can be effectively used.

For ndm values in excess of this, special greases are required, which have relatively low base oil viscosities to reduce the amount of heat generated by the bearing when rotating at higher speeds. It is possible, with recent developments in both bearings and grease lubricants, to achieve ndm values of up to 2,000,000.

While grease lubrication is inherently simple, there are many bearing applications where oil is the only choice available, since it has the key attribute of being able to remove the heat generated in the bearing due to load and speed. It is possible, for example, that ndm values up to 4,000,000 are achievable for oil jet lubrication.

It is important to be aware of the importance of lubricants. Schaeffler has found that more than 40 per cent of all premature failures of rolling bearings are caused by inadequate or incorrect selection of lubricant.

So choosing the most appropriate grease and re-lubrication equipment for a bearing application is vital for companies looking to extend service and maintenance intervals, and minimise plant downtime.

Ian Pledger is a field service engineer at Schaeffler UK, a global manufacturer of rolling bearings and automotive components