(January 14, 2011)

NSK has developed a technology that it claims can improve gearbox bearing wear life even under conditions of a contaminated lubricant.

Bearings are critical components in all types of gearboxes and can be subjected to very demanding operating regimes, not only from raised operating temperatures, as a result of high loads and operating speeds, but also from severe ambient conditions and lubricant, which may be contaminated.

Yet despite these operating conditions, the requirement on the bearing manufacturer is still to provide longer operating life, in some cases, with gearboxes used on wind turbines, as much as 15 to 25 years.

NSK has conducted an evaluation of all current gear designs - worm, hypoid bevel, epicyclics, spur and bevel types.

This has involved studying the numerous factors that affect bearing performance in gearboxes, such as load, speed, friction, environment, shaft arrangements, lubrication, assembly space and many more.

The result is a 'solutions-based' technology approach that stems from advanced materials technology, optimised mechanical design and life calculation software.

The technology package allies a state-of-the-art design of the spherical roller bearing with the Super-TF bearing material, which provides up to 10 times longer life in conditions of contaminated lubricant.

Completing the package are NSK's latest optimised methods and calculation procedures such as the Able-Forecaster (Advanced Bearing Life Equation) software, and Stiff, a program developed by NSK that considers parameters such as deformation of shafts and housings, misalignment, displacement of the bearing itself (and adjacent bearings), as well as internal operating cycles.

The materials component of NSK's technology package is Super-TF steel.

This material provides an answer to the problem of premature bearing failures resulting from insufficient or contaminated 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 identation 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 microstructure that 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 carburized steel, Super-TF material is said to offer 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 around 5.5 times that of ordinary carburized steel, said NSK.

In addition to extending service life under conditions of boundary or contaminated lubrication, Super TF bearing steel also offers increased resistance to wear and seizure; this is 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 SUJ2 steel, the Super TF material structure has shown less than one-third the rate of wear and a 40 per cent increase in seizure resistance.

Complementing Super-TF as the optimised mechanical design element in NSK's gearbox package is the high-performance HPSTM series of spherical roller bearings.

The HPS series is said to able to deliver operating benefits by reducing maintenance and energy costs, as well as enabling downsizing of product design.

The main features, however, are the maximum limiting running speed, which has been improved by up to 20 per cent, and overall performance, with twice the life compared with conventional bearings of the same size.

With its HPS design, NSK claims that it has clarified the mechanism that causes slippage and creates high friction on the bearing surfaces, leading eventually to damage due to surface fatigue.

NSK verified by theoretical analysis and bench testing that the friction can be reduced by controlling the motion of rotating rollers and that a special surface treatment of the outer ring is effective in achieving this reduction.

The result is HPS bearings that can minimise slippage, prolonging running life by more than twice that of conventional products, according to NSK.

HPS bearings also benefit from a special nitriding surface treatment to their high-precision, pressed steel cages.

When compared with conventional nitriding, NSK said that its treatment forms a finer and harder surface with more uniform hardness.

As a result, the problem of cage wear under conditions of continuous running at high speeds, which restricted any increase of the limiting speed of the bearing, has been solved.

This is evidenced by the 20 per cent increase in limiting speed achieved by the HPS series design.

The 'Rolling Bearings for Industrial Gearboxes' publication explains all factors in selecting bearings for gearboxes.

NSK said that it is making the process of selecting bearings for all gearbox types more straightforward, structured and transparent with the launch of this free publication.

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