TechArchive

This article was originally written in the period 1995-2000

Grease is penetrating into high speed and high temperature applications where oil lubrication would once have been the only option. This from NSK-RHP

Of course, grease is less fluid than lubricating oils. This has meant that traditionally it has not been suitable for high speed operations. However, the convenience of the lubricating medium has led to greases being developed that offer superb lubrication performance even at high speeds exceeding 1.3 million DmN (DmN is the mean bearing diameter in mm multiplied by the rotational speed in rpm).

One of the most interesting developments over the last few years has been the re-emergence of polyurea-based greases. These have significantly improved average grease life at operating temperatures as high as 160degC.

Polyurea-based greases were used some years ago with limited success. Manufacturing techniques at that stage could not guarantee the stability of the grease. The result was that it went hard or soft depending upon the working conditions and this adversely affected bearing operation.

With modern manufacturing techniques, these problems have been overcome and polyurea has gained acceptance with a number of manufacturers in a relatively short time as a factory-packed lubricant for ball bearings.

Unlike metal soap thickened greases, polyurea can be used as a thickener with a number of different types of oil – mineral or synthetic (usually polyolester or PAO). It helps overcome the limitations of the oil at higher temperatures and also ensures a more structurally stable mixture. Polyurea also has better bleed characteristics and lasts longer and does not contain the solid particles – crystals of the soap – which cause noise.

Grease life has always been difficult to predict with any reasonable degree of accuracy. A number of systems are available for theoretical calculations but very little correlation or consistency exists between the various methods. It is for this reason that there is no substitute for actual rig testing.

Another area where grease technology is proving particularly important is for high speed machine tool spindles. High speed greases have been used in this area for some time, but the recent development in Japan of an ultra-high speed grease has meant that running speeds some 25% faster than was previously the case are now possible. By using greased hybrid bearings, speed increases of 15% to 20% can be achieved, depending upon the arrangement. In tests, the new grease has operated reliably for extended periods at shaft speeds up to 32,000 rpm. The only other way to achieve the extra speed with all-steel bearings in the spindles would be with expensive oil/air lubrication systems.

Moreover, it has also run significantly cooler than the best previously tested grease (26,000 rpm). The test bearing was a 7008 40mm bore unit. The greased hybrid bearings run at a significantly reduced operating temperature compared to all-steel bearings. This helps to increase the grease life.

In fact, in many applications, high speed operating conditions are made worse by the high temperatures at which the bearings work. This is particularly true of bearings in electrical prime movers and auxiliary equipment installed around engines. The deep groove ball bearings used in these applications may also have to contend with road salt and turbid water, not to mention high loads and vibration.

NSK has developed two high performance greases for these types of application. Both ENS and ENR greases use a polyolester base oil with a diurea thickener. Polyolester as a base oil is superior in oxidation, thermal stability and low temperature fluidity, while the urea compound is superior in heat and water resistance and shearing stability as a thickener.

Noise reduction of greased bearings is important in many applications, including the integral shaft bearings used in video cassette recorders (VCRs) and disk drives. It is also important in electric motors inside passenger vehicles and domestic equipment and in submarine motors.

NSK-RHP has successfully designed and used a grease noise tester that has provided the necessary feedback to grease manufacturers giving them the opportunity to develop low noise greases.

Noise is caused by particles in the grease and in the additive packages. By improving the cleanliness of the grease generally, and by using soluble rather than solid additives, substantial reductions in noise – 50% or more – can be achieved.

Low noise greases, such as polyurea-thickened types, appear to offer other benefits too. The results of tests indicate that their use actually improves bearing life. No doubt the cleaner structure of the grease is an instrumental factor in this respect.

Features of these greases include :

• Superior shearing stability, with less grease leakage during high speed rotation and outer ring rotation
• Superior high temperature durability with long grease life at temperatures as high as 160degC
• Good low torque performance: less abnormal noise in the bearings during cold starts
• Superior water resistance of the thickener, which makes softening and outflow difficult even when water may enter the bearing.

• NSK-RHP
• Tel: 0115 936 6464
• Fax: 0115 936 6400