Two major causes of premature bearing failure in equipment such as rolling mills are due to equipment operating under conditions of contaminated or inadequate (boundary) lubrication.
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.
Additionally, 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.
Research by NSK has revealed that bearing life in conditions of contaminated or boundary lubrication can be improved by changing the microstructure of a bearing steel in order to enhance its ability to relax 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 practice, more effective results are obtained not when either retained austenite or hardness is increased alone, but only when both properties are increased together.
In comparison with bearings made of ordinary carburized steel, NSK’s Super-TF bearings 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 bearings is around 5.5 times that of ordinary carburized steel.
Besides extending service life under conditions of boundary or contaminated lubrication, Super TF bearings also offer increased resistance to wear and seizure as a result of the dispersion of large numbers of fine carbides and nitrides in the bearing material.
In comparative tests with bearings made of materials such as SUJ2 steel, the Super TF structure has shown less than 1/3 the rate of wear, a 40% increase in seizure resistance and dimensional stability superior to that of SUJ2 or ASTM bearing steel.