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Gearbox bearings offer improved wear life under challenging conditions

Gearbox bearings offer improved wear life  under challenging conditions
A gearbox on a wind turbine has to last as long as 15 to 25 years, and this puts considerable strain on bearing manufacturers to assure long life. Conditions in many other gearbox applications are just as challenging, with bearings subjected to elevated operating temperatures, high loads and operating speeds, severe ambient conditions and lubricant that may be contaminated.

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 - factors such as load, speed, friction, environment, shaft arrangements, lubrication, assembly space, and many more. The result is a unique 'solutions-based' technology approach built upon advanced materials technology, optimised mechanical design and life calculation software. 

 The solutions-based technology package allies state of the art spherical roller bearing design with a unique bearing material, Super-TF, which NSK says provides up to ten 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 important 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 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.

Insufficient oil film
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 carburized 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 carburized steel.    

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

Complementing Super-TF as the optimised mechanical design element in NSK's gearbox package is the high performance HPS series of spherical roller bearings. The HPS series delivers significant operating benefits, by reducing maintenance and energy costs, and enabling downsizing of product design. The outstanding features, however, are the improved maximum limiting running speed, which has been improved by up to 20 %, and overall performance, with twice the life compared to conventional bearings of the same size. 

With its HPS design, NSK has clarified for the first time 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, which can minimise slippage, prolonging running life by more than twice that of conventional products.

HPS bearings also benefit from a special nitriding surface treatment to their high-precision, pressed steel cages. When compared to conventional nitriding, NSK's 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% increase in limiting speed achieved by the HPS series design.
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