TechArchive

This article was originally written in the period 1995-2000

NSK-RHP’s Sealed Clean bearings have been developed which have special rubber seals on both sides to help them last longer than standard bearings.

Using a pioneering stress analysis method, NSK-RHP has been able to determine that the premature flaking failures of ball bearings used in the manual transmissions of passenger cars was due to abnormal surface originating fatigue, rather than the normal sub-surface originating fatigue, rather than the normal sub-surface originating fatigue. The cause is the existence of high hardness fine ferrous gear particles in the transmission oil.

Based on these findings ‘Sealed Clean’ bearings have been developed which have special rubber seals on both sides. Tests have proven that these bearings last several times longer than open-type standard bearings, or special bearings having steel balls of extra-large size.

The benefits of ‘Sealed Clean’ bearings have led to their widespread usage by motor manufacturers all over the world. Moreover the ‘Sealed Clean’ concept is being applied in other areas with remarkable success. Finally, the stress analysis method developed for the failure analysis has opened up new possibilities of predicting remaining life on ball and roller bearings.

Machine designers use common calculating methods for basic rating life based on ISO standard 281, when choosing a suitable bearing for each application, or in dealing with problems on bearing service life.

In many applications where operating conditions and environments are not extremely adverse, actual life spans of bearings are found to be somewhat longer than what the calculating method would predict. This is because we nowadays have better metallurgy and manufacturing technology than that which existed when the current standard was developed.

However, ball bearings used extensively in manual transmissions for passenger cars were an exception. Indeed, compared to other applications in cars, some of these transmission bearings lived rather shorter lives than predicted by ISO calculations. These units developed premature flakings on ball race grooves, causing undesirable operating noise and eventually transmission failure.

Investigation showed that these premature failures were not due to defective manufacture of the gearboxes concerned. Furthermore, fitment of the bearings was found to be good, and there was sufficient oil in the gearbox to lubricate them.

Because of premature failure, designers of new transmissions were forced to provide a more abundant allowance in bearing capacity than was called for by the life calculation. In practice this meant using ball bearings with higher load ratings; units with larger than standard steel balls or even double row types.

Another measure was to specify expensive steel of extra cleanliness, in terms of non metallic inclusions, or some special heat treatment on ball races. These measures were particularly important where high performance engines were concerned. They were expensive of course, and often did not deliver the improvements expected. Bearings with larger size steel balls for example, were found to operate effectively only for a small fraction of their calculated life, falling short of the required life level.

To analyse the problem a series of dynamometer test programmes was undertaken, using a typical transmission for the purpose of reproducing and studying the premature failures under various operating conditions. A special vibration sensor was installed adjacent to each ball bearing to detect any change in bearing rotation, a condition which would stop the machine immediately, so that failed bearings could be inspected in the early stages of flaking.

Subsequent inspection of the units revealed that all of their inner raceways were indented, with or without flakings. The indentations were a common feature as failed bearings retrieved from the field also revealed their existence and an excessive degree of flaking.

Having verified that lubricant was present, the contaminants in the transmission gear oil were then studied at various stages of usage from a brand new transmission to one which had covered 50,000km without an oil change.

While it was difficult to make exact comparisons between various cases on a quantified basis, the study did disclose that the following foreign matter was included in the transmission oils:

• Metallic chips or particles: Ferrous, copper, aluminium, zinc.
• Non-metallic particles: Rubber, silicon, plastics, fibre.

The above particles were present, even in oil flushed from a brand new transmission and these are run for only a few minutes during dynamometer tests and during the few kilometres to the point where the vehicle is shipped. And as the mileage accumulates, more foreign matter will be released from the meshing gears, synchroniser rings and other moving parts. Furthermore, as there is a limitation as to how much of this matter will be collected by the transmission’s magnetic trap, the majority will probably remain at large in the oil.

It was almost certain that the flaking occurring in the transmissions were of surface originating type which were caused by surface stress concentration, due to dents and the resulting protrusions around them on raceways.

In order to support the above, a ‘fatigue analysis’ method, which by then had been almost fully developed by the company’s research group for analysing and evaluating metal fatigue phenomena on a quantitative basis, was used.

This method clearly explained why and how the premature flakings occurred, and this led to a complete solution of the problem. In essence, the ‘fatigue analysis’ method consists of the following three techniques:

• Choosing and measuring some key physical properties relating to metal fatigue on ball tracks of inner races which are known to have a higher level of stress than outer races or balls do.
• Conducting the same measurements to the depth of the surface by removing a metal layer of some thickness repeatedly without affecting the fatigue properties at all.
• Combining these measurements for each layer of into one ‘Fatigue Index’ figure that would distinctly indicate each extent of fatigue including its limit level; that is, the flaking stage.

Fatigue index readings for over 500 tested bearings – large and small, ball and roller, under light and heavy loads – at early or late stages in their life cycle were subsequently plotted. Each band of plots for each category shows a steady increase of fatigue index until it reaches the flaking failure stage. Each dot represents the peak index value average of the number of specimens. Surface fatigue type reaches flaking stages around Index 2 and subsurface around Index 3.

The Fatigue Index can determine to what extent the fatigue is progressed on each bearing in relation to its ultimate life. This latter point is very important because it leads to the possibility o predicting the remaining life of any bearing is usage. This had not been possible before, the only life predictions previously available being on groups of new bearings and then strictly on a statistical basis.

Through the fatigue analysis method, it was established that premature failures of transmission bearings are caused by surface originating type flakings due to internal stress distribution as a result of the many dents caused by hard particles circulating between running balls and inner races. Also, surface type flakings occur much earlier than sub-surface type ones and the failures they cause are entirely different from those caused by the latter – the flaking type upon which the ISO method of life calculation is based.

Based on the tests and experiments which were conducted, ball bearings were developed which would not be affected by the presence of any foreign matter in the lubricant oil of gear box transmissions.

After sorting out numerous alternatives by tests, the design that was finally adopted is identical to that of standard ball bearings, but special rubber seals are added on both sides of the bearing, which are different from those now used in ordinary sealed bearings.

Whilst totally denying the ingress of fine hard particles the special seals fitted to ‘Seal Clean’ types do allow oil into the bearings, which eventually replaces the initial grease charge. In tests ‘Sealed Clean’ bearings have been shown to provide up to 7 times the life of standard bearings and even reduced size Sealed Clean variants have produced up to 4 times the life.

The benefits of Sealed Clean units are not limited to extra life. It is natural to expect that they should run more quietly than equivalent standard bearings, because of their complete freedom from surface dents and protrusions on the raceways. In fact tests show that there is as much as 20dB of difference in noise levels when measured against standard bearings. What’s more the standard bearings used in the test had not yet developed any flaking. Indeed, it was the mere presence of the small dents that caused the decibel increase.

• NSK-RHP
• Tel: 0115 936 6600
• Fax: 0115 940 5419