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Product category: Bearings
News Release from: NSK UK | Subject: 6206 C4 deep groove ball bearings
Edited by the Engineeringtalk Editorial Team on 11 June 2001

Bearing technology transferred to
compressor

NSK has used its global resources to transfer bearing technology initially developed for high acceleration motor cycle engines to a new and revolutionary design of oil-free piston compressor

NSK has used its global resources to transfer bearing technology initially developed for high acceleration motor cycle engines to a new and revolutionary design of oil-free piston compressor, produced by the well known German manufacturer, Boge Kompressoren The design solution provided by NSK is based upon the company's latest clean steel technology, a special treated cage and an advanced grease option

It has overcome the major problems which Boge were experiencing in terms of bearing life with its new patented design.

The severe load duties, and reduced cooling due to no oil lubrication, meant that in development runs some manufacturer's bearings were lasting only a few hours.

This compares to a required life of 10,000 hours, which NSK has been able to achieve in exhaustive testing.

Subsequent to these tests, the new compressor has been launched onto the market, and the first size in the range was shown at the Hannover Messe in April 2001.

Boge, based in Bielefeld, Germany, has been at the forefront of compressor design and manufacturing for over 70 years.

In keeping with its reputation for innovation, the company's latest project is a radical new design of oil free piston compressor.

This is an important development because it meets the needs both of industries that require large quantities of cheap compressed air, free of oil vapour, and also the environmental movement, which is looking for increasingly lower emissions of oil from compressors, generally.

The new design fully warrants the description "radical".

In addition to its oil free operation it employs a patented method of mechanical operation in which a single crankshaft is used to drive two opposing pistons, the first directly, the second via a innovative slave arm.

This simplifies the crank arrangement considerably and means that the second "slave" piston does not have to be deep in section (to resist side forces) thus enabling a shallower design.

The method which Boge has adopted to achieve oil free running is to line the cylinder walls with a "Teflon-type" coating.

This has been tried before and is very difficult to achieve, but Boge have perfected the technique.

However, this design, being oil free, causes major problems for the lubrication of the compressor bearings.

These now have to be grease lubricated and, hence, sealed, also.

The major implication of this is that the crankshaft main bearing does not benefit from the cooling action of oil (or rather the oil-rich air circulating), as is the case with conventional compressors.

Also, the need to seal in the lubricant (the grease) means that cooling air is actually prevented from flowing through the bearing.

Adding to the lubrication problems, the action of a crankshaft imparts severe centrifugal load cycles on the main bearing, while the frequent stop-start regime (required to keep systems pressurised) means that the bearing sees more severe acceleration cycles than is the case with most other types of machinery.

The corollary of this combination of severe load duties and lower cooling on the original types of bearings employed was a drastic reduction in bearing life, particularly as regards the bearing cage.

In fact, some manufacturers bearings were lasting only a few hours, compared to the required life of 10,000 hours - with just a single grease fill.

This situation was still unresolved when engineers from NSK became involved in the project.

Once fully conversant with the problems, the engineers enlisted the help of NSK's worldwide technology network in an effort to solve them.

This global search provided almost immediate results.

Previous work which NSK had undertaken with duty cycles similar to those of the Boge compressor had brought high levels of success in the Japanese motorcycle industry with 2 and 4 stroke engines.

The same technology had also been used successfully in engines for sports vehicles (mainly go-karts); to solve problems of broken cages, resulting from the high cyclic stresses induced by continual revving (an essential part of the gear changing method!) Like so many solutions to bearing problems, the Boge solution is a synergy of high technology materials, optimised mechanical design and lubrication technology.

It is based upon a matched pair of 6206 C4 deep groove ball bearings, each sealed on one side (i.e open on their mating sides).

The specification of these bearings, features inner and outer rings made from NSK's ultraclean EP steel; a two piece, pressed steel cage with special surface treatment, and a special high temperature grease, Kluber's Petamo GY193.

Also playing a key role in the specification is software, in the shape of NSK's New Life Theory, a specially designed calculation program for determining bearing life.

This provides real advantages to users by giving a more accurate estimate of the life of an NSK bearing than previous theories, some of which are more than 50 years old.

The EP steel (Extremely Purified Steel) material, as used for the inner and outer bearing rings, is the product of research by NSK which proved that there is a correlation between the number of oxide group inclusions in a steel and its resulting fatigue life.

Put simply: the higher the number of larger oxide inclusions, the lower the fatigue life.

NSK has addressed this problem with image analysis techniques that measure maximum oxide inclusion sizes over unit areas in a steel.

These techniques have been made available to steel makers and the result is EP steel, a material that offers up to five times longer life, improved wear resistance and higher reliability than existing "clean" bearing steels.

Steel is also the material preferred for the bearing cage (the original problem area) due to its strength and temperature performance.

Essentially the cage is standard.

It does, however, benefit from the special TUFFTRIDE TF1 surface coating process which increases its service life, resistance to wear and corrosion.

The final element, crucial to the bearings performance, is the grease lubricant.

This is designed to provide long life in dry installations where bearings are sealed - a specification that could have been written with the Boge application in mind.

Importantly, the GY193 grease is highly compatible with a range of elastomers and is suitable for high temperatures up to 160C.

The linkage of these two features is significant as the bearing seals are made with elastomers and keeping those effective at high temperatures is key to maintaining the life of the bearing itself.

In addition to the above, the all round specification of the GY193 grease provides a number of other notable features which are particularly useful for the Boge application.

These include: * The ability to handle high contact pressures: this is especially important in view of the high stress activity resulting from the severe accelerations and deceleration encountered in the compressor cycle.

* High resistance to corrosion - which is useful in Boge's dry operating conditions.

* Resistance to false brinelling - the result of induced vibrations which can damage the bearing raceways while the compressor is at standstill.

The successful application with Boge Kompressoren is the most recent example of NSK's collaboration with compressor manufacturers seeking to improve the performance and/or life of their products.

The Boge application relates to piston compressors, but NSK has also been active in providing solutions in another equipment area: Screw Compressors.

In additions to their usage in the production of compressed air, screw compressors are widely employed in other equipment such as refrigerators and gas compressors.

In all of these applications there is a strong demand from users for longer intervals between overhauls.

NSK has been able to meet this demand with cylindrical and angular contact bearings equipped with special PPS cages.

Designed for use at temperatures up to 190oC, PPS is a glass fibre reinforced plastic that offers cage strength and chemical stability characteristics which change little, even when the material is exposed to compressor oil, refrigerator oil or ammonia gas.

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