More than 40 per cent of all premature failures of rolling bearings are caused by inadequate or incorrect selection of lubricant.

Choosing the most appropriate grease and re-lubrication equipment for a bearing application is therefore vital, said Ian Pledger of Schaeffler (UK).

More than 80 per cent of all rolling bearings are lubricated with grease.

It is therefore critical that engineers select the most appropriate grease for the bearing and the application it is operating in.

Not only will correct selection prolong the life of the bearing, enabling it to operate at its full capacity, but it also ensures smoother running behaviour and improved operational safety.

But, selecting a suitable lubricant for a rolling bearing can be a complex process.

Grease is basically a type of oil with a thickening agent added to it, giving it a more viscous form.

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

Grease 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 to prevent metal-to-metal contact, which leads to high wear and premature bearing failure.

Generally, a P/C of less than 0.15 for radially loaded bearings using standard lithium soap greases (normal operating conditions) is 400,000 min-1.mm.

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

To avoid slippage with caged ball bearings, engineers should work on a P/C of 0.01 and for caged roller bearings, a P/C of 0.02 is advised.

Second, the 'ndm' value - which is calculated from the speed of the bearing 'n' in rpm and the bearing mean diameter 'dm' in mm - will give an indication of the relative speed of rotation to the physical size of the bearing.

In general if the 'ndm' value is below 400,000 then standard general purpose bearing greases can be effectively used to lubricate the bearing.

For 'ndm' values in excess of this, special greases are required, which have relatively low base oil viscosities, in order to reduce the amount of heat generated by the bearing when rotating at higher speeds.

It is quite possible, with recent developments in both bearings and grease lubricants, to achieve 'ndm' values of up to 2,000,000.

Whilst 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 quite possible that 'ndm' values up to 4,000,000 are achievable for oil jet lubrication.

Schaeffler's Arcanol range of rolling bearing greases provides optimum conditions for achieving reliable bearings.

The company works closely with lubricant manufacturers to develop special greases that are suitable for rolling bearings.

Schaeffler conducts a series of stringent in-house tests on the lubricant to determine their service life, friction and wear characteristics.

Only the best-performing greases are then selected to undergo subsequent tests under simulated field conditions in far more complex rolling bearing test rigs.

If the grease fulfils Schaeffler's stringent tests, it is decorated with the Arcanol seal of quality.

In addition, each incoming batch of grease is 100 per cent inspected by Schaeffler to ensure the uniform quality of the product.

The Arcanol range includes 15 rolling bearing greases, each graduated to cover all application areas.

For example, Mutitop is a universal grease for ball and rolling bearings that is suitable for use in construction machinery, automotive engineering, rolling mills and grinding machine spindles.

VIB3 is a mineral-based grease with a polycarbamide thickener, which is suitable for use in blade adjusters in rotors for wind turbines, packaging machinery and rail vehicles.

The grease offers excellent resistance to high temperatures, high loads and oscillating motion.

Multi3 is a specially formulated grease for large electric motors, agricultural machinery and fans.

Food2 is a white oil-based grease for use with ball and roller bearings in applications where contact with food occurs.

In harsh operating conditions or aggressive environments, rolling bearings should be re-lubricated frequently via lubrication nipples.

Schaeffler therefore offers its FAG grease guns and matching reinforced hoses for this, which enable rapid manual re-lubrication of bearings.

Controlled re-lubrication of bearings is often required in order to extend service and maintenance intervals and minimise plant downtime.

Schaeffler offers three main products for this.

Motion Guard Compact is a single-point, electro-chemical lubrication system that comprises a housing filled with 120cm3 of grease, controlled via an activation screw.

Motion Guard Champion is a single-point, battery-operated lubricator that comprises either a 120cm3 or 250cm3 volume of grease, with drive unit and adaptor.

Motion Guard Concept6 is an electro-mechanical, single-point lubrication system that can be expanded to include up to six lubrication points.

FAG metering devices are also provided, enabling customers to supply grease to rolling bearings in metered quantities, from 10cm3 up to 133cm3.

By using a pneumatically-driven, double-direction piston pump, the lubricant is transferred from the grease drum (25kg or 180kg) via the metering valve to the point of application.

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