Grease use slides into other areas
January 1st 2005

New grease technology provides more engineered solutions to lubrication of rolling element bearings

Grease, because it is convenient, effective and simple to use has always been the most frequently employed lubrication method for rolling bearings. Now its use is increasing further - even into high-speed and high temperature applications where oil lubrication would once have been the only option.

One of the major factors driving the increasing use of greases is the intense competitiveness of global manufacturing.

High speed applications Another area where grease technology is proving particularly important is in highspeed machine tool spindle applications.

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. In tests the new grease has operated reliably for extended periods at shaft speeds up to 32 000 rpm. 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.

To get the best utility from their investment many machine tool users demand more speed from their existing spindles. If all steel bearings were used in the spindles the only way to achieve the extra speed would be with expensive oil/air lubrication systems.

However, by using greased hybrid bearings, speed increases of 15% to 20% can be achieved, depending upon the arrangement. The grease hybrid bearings run at a significantly reduced operating temperature compared to all steel bearings. This not only increases the grease speed but also, very importantly, the grease life.

This has led to a move towards more integrated bearing structures, which simplify assembly and reduce costs.

Another factor has been the increasing use - in domestic electrical equipment for example - of bearings that are sealed for life with grease. These offer the advantage of reduced maintenance. They are also particularly useful for industries such as food where the lubricant must be easily confined within the housing.

New greases handle combined speed/temperature In many applications high-speed operating conditions are made worse by the high temperatures at which 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 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.

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 160°C • Good low torque performance: less abnormal noise in the bearings during cold starts 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 major development projects, between grease suppliers and bearing manufacturers.

These have produced greases 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).

The rationalisation of bearing structures has had its downside too. Components are smaller and lighter, but if anything are subjected to greater stresses, and have to operate at ever-higher speeds and temperatures with lower noise - vehicle alternators are a good example of this process in operation. Improvements in bearing materials have helped in meeting these new rigorous operating demands, but the importance of the latest developments in grease technology should not be understated.

High temperature operation 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 160°C. 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. The benefit of Polyurea is that it offers a more engineered solution to lubrication needs than 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 Poly Alpha Olefin [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 than metal soap thickened greases. Moreover it does not contain the solid particles - crystals of the soap, which in the latter, metal soap- thickened, greases 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 in ascertaining grease life.

Superior water resistance of the thickener, which makes softening and outflow difficult even when water may enter the bearing Noise reduction Over recent years a special emphasis has been placed on the noise reduction of greased bearings. Integral shaft bearings used in video cassette recorders (VCRs) and disk drives are part of the reason, also motors for defence equipment such as submarines. In addition there is also the increasing use of electric motors inside passenger vehicles and domestic equipment in general.

NSK has 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.

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