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NSK enables competitive gains with spherical roller bearings

NSK enables competitive gains with spherical roller bearings

NSK TL series spherical roller bearings are active across the world in a host of papermaking machinery applications. Set against conventional spherical roller bearings with standard heat treatment, the company says this advanced product is proving increasingly popular due to a number of distinct benefits that include enhanced inner ring strength, longer service life and dimensional stability at high operating temperatures.

The original development of TL (tough and long life) bearings was to solve a specific problem: inner ring fractures of spherical roller bearings with tapered bores, particularly when used with a relatively high interference fit. This issue can occur on dryer cylinders (including Yankee dryer rolls) and calender rolls, where super-heated steam passes through the hollow journals of dryer rollers and the centre of bearings. The subsequent temperature difference increases the interference fit, producing a tremendous increase in hoop stress (circumferential force) on the inner ring that enhances the potential for cracks.

NSK knew the key to providing a successful solution hinged on the development of a bearing that could better withstand hoop stress. The company therefore set about creating the TL specification based on a steel with a formulation for case-carburising, but using a special method of heat treatment. From a metallurgical standpoint, the raceways are harder than conventional bearing steel but the core remains soft. This contrast delivers the shock resistance needed to prevent fracturing, as well as the necessary surface hardness for long bearing life.

The resistance of inner rings that meet the TL specification is higher than the resistance of inner rings made from bainitic steel or standard bearing steel, as demonstrated in a series of benchmark fracture tests. The tests involved subjecting inner rings to a radial load of 2000 kg at a rotational speed of 1800 rpm, while simultaneously applying a measured hoop stress value.

Notably, the rings made from bearing steel, some with standard heat treatment and others with bainite treatment, fractured under hoop stress of 400 MPa after 1 x 107 revolutions and 7 x 107 revolutions respectively (on average). In contrast, the TL specification steel did not fracture under hoop stress of 430 MPa, even after 3 x 108 revolutions. The TL rings also marginally outperformed those made from carburised steel with dimension stabilising treatment.

Part of the reason for this high performance was the residual stress (after heat treatment) present in the crack-tested inner rings made from TL specification steel. Residual stress was notably higher in TL steel test pieces in comparison with those made from bearing steel featuring either standard heat treatment or bainite treatment. Residual stress works in the opposite direction to hoop stress, serving to supress this unwanted force.

In fatigue life tests, NSK reports that the TL series bearings all demonstrated far superior performance against the other bearing material types when comparing Weibull plots of the test results.

Another significant characteristic of the TL specification is higher surface hardness of the raceway in comparison with bearings made from bainitic steel or case-hardened steel. Measurements of TL specification steel revealed a hardness of 710-720 HV (60.5-61.0 HRc). This value compares favourably against bearing steel with dimensional stabilised treatment (690-700 HV, 59.5-60.0 HRc), bearing steel with bainite treatment (670-680 HV, 58.5-59.3 HRc) and carburised steel with dimensional stabilised treatment (680-700 HV, 59-60 HRc).

NSK’s proprietary material heat treatment technology also serves to support dimensional stability at high temperatures, as demonstrated by further tests. Bearings made from TL specification steel (at 150°C bearing temperature) showed a negligible rate of dimensional change (around 0.002%) after 10,000 hours – less than bearing steel with dimension stabilising treatment (at 130°C) and far less than bearing steel with standard heat treatment.

 

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