Bearings can live with dirty lubricant

A new bearing steel delivers up to 10 times the life of conventional bearings under conditions of contaminated lubricant.

The arduous conditions encountered in quarrying applications cause many rolling bearings to fail prematurely.

The causes of these failures, vibration, shock loads, dust or moisture ingress are well known and are accounted for in many bearing designs.

Not so well accounted for is another major cause of failure, contaminated lubricant.

Recently this situation has changed with the introduction by NSK of its Super-TF material, a new bearing steel that delivers up to 10 times the life of conventional bearings under the conditions of contaminated lubricant.

The ability of Super-TF to provide extra life in difficult operating environments has been proven in the steel and aluminium producing industries.

As a result of this success, NSK is making the material available for use in the quarrying and mining industries in its latest range of vibrating screen specification (VS) spherical roller bearings and new high capacity EM and EW series of cylindrical roller bearings.

NSK's development of Super-TF steel is an answer to the problem of premature bearing failures resulting not only from contaminated, but also from insufficient lubricant.

These failures occur because, under arduous and dirty operating conditions, bearing lubricant is easily contaminated with debris.

The debris particles are then indented on the bearing raceways causing stress concentrations at the indentation edges.

In addition, if the viscosity of the lubricant is not high enough, metal surfaces come into contact and small "peeling" damage occurs, leading to stress concentrations around the contact areas.

The result in both cases is that cracks form from the surface of the material at the stress concentration points, leading to an accelerated rate of fatigue damage and, ultimately, shorter bearing life.

Super-TF overcomes these problems with a unique microstructure, which is able to relax the stress concentrations on the edges of indentations.

This microstructure - a material with a high volume of retained austenite and a high hardness - produces a longer life even under conditions where there is an insufficient oil film and surface damage, such as peeling, easily occurs.

In comparison with bearings made of ordinary carburised steel, Super-TF material offers up to 10 times the life with contaminated lubrication and up to twice the life where the lubricant is clean.

Where the problem is related to insufficient lubrication, the life of Super TF is still impressive at around 5.5 times that of ordinary carburised steel.

Besides extending service life under conditions of boundary or contaminated lubrication, Super TF bearing steel also offer increased resistance to wear and seizure as a result of the dispersion of large number of fine carbides and nitrides in the bearing material.

In comparative tests with bearings made of conventional materials, such as AISI 52100 steel, the Super TF material structure has shown less than one third the rate of wear and a 40% increase in seizure resistance.

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