Bearing material’s microstructure enables extended service life under conditions of contaminated or boundary lubrication
NSK’s asset improvement programme is saving the operator of a Cold Rolling Mill €16,300 annually as a result of solving a problem of bearing failure that was occurring three/four times per year, resulting in unplanned shutdowns and serious loss of production at the mill.
The mill operator was being forced to change bearings up to four times a year due to the harsh working environment in which the bearings operated. The bearings were failing prematurely due to contamination: ingress from dirt, debris and liquids was affecting the bearing lubricant and shortening bearing life.
NSK was called in to find a solution to the failure problem. This involved engineers from the company 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 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 per cent 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 and sent back to factory. The laboratory results found that the minimum residual life was between 29–50 months, compared with 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 its 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.
NSK has detailed its bearing products for the steel industry in a new publication. Running to 40 pages, the free booklet provides information regarding the choice of bearings for plant and equipment used in steelmaking processes. Free copies of the publication are available via NSK’s Freephone service Tel: 0500 2327464. Callers should quote the catalogue reference Cat STE/B/E/03.11 (English), and also the title of the publication: ‘Bearings for the Steel Industry’. For more information, visit the NSK website.