NSK Super-TF Steel Saves €16,300 Annually For Cold Rolling Mill

NSK was called in to find a solution to the failure problem. This involved engineers from the company in analysing the mill bearings, and taking samples of the lubricant used (grease) to determine the actual operating conditions. At the conclusion of the analysis, NSK recommended using spherical roller bearings manufactured from the company’s Super -TF steel, a material particularly suited for use in environments subject to contamination from debris.

A unique bearing material developed by NSK, Super-TF provides up to ten- times longer life in conditions of contaminated lubricant, in comparison with bearings made of ordinary carburised steel. Where the problem is related to insufficient lubrication, the life of Super TF bearings is still impressive at around 5.5 times that of ordinary carburised steel.

In addition to extending service life under conditions of contaminated or boundary lubrication, Super TF bearings also offer increased resistance to wear and seizure. In comparative tests with bearings made of conventional materials, such as AISI 52100 steel, the Super TF has shown less than one third the rate of wear and a 40% increase in seizure resistance.

Following NSK’s recommendation, Super-TF spherical roller bearings were installed in the cold rolling mill. After 4-months in operation the bearings were disassembled & sent back to factory. The laboratory results found that the minimum residual life was between 29-50 months, compared to less than 4-months previously. To date the customer has replaced the bearings once, and that as part of a planned annual shutdown for scheduled maintenance.

Super-TF steel provides an answer to the problem of premature bearing failures resulting from contaminated or 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. This results in cracks forming at the stress concentration points, leading to accelerated rates 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.