NSK is expanding its Tough Technology programme to include Water Tough WTF, a newly patented bearing material that is claimed to achieve approximately three times the service life of conventional bearing materials in operating environments contaminated by water.
NSK's Tough Technology programme is committed to providing users with cost savings as a result of achieving bearing life extensions. The programme is an ongoing one involving continuous research into industry-specific bearing problems. Recently the focus of this research has been roll neck bearings for use in steel mills. The operating conditions of these bearings are particularly tough: water-infiltrated lubrication and the ingress of foreign particles, such as scale, which can cause flaking of the rolling contact surfaces of the bearings, reducing their service life and often resulting in premature failure.
Flaking, according to accepted bearing fatigue life theory, originates from non-metallic inclusions (ie a material defect) slightly below the rolling contact surface of the bearing material. However, the question of where on the bearing raceways flaking occurs under the influence of water-infiltrated lubrication has not been determined. In an effort to resolve this situation, NSK built a testing machine capable of simulating bearing flaking under water-infiltrated lubrication in the field. Simultaneous tests were conducted on this machine using grease-lubricated taper roller bearings, and also ball bearings lubricated with clean and water-infiltrated oil. The results of both tests revealed that flaking occurs mostly in the load zones of the bearing outer rings.
Armed with this information, NSK proceeded to analyse how pits in the raceway surfaces occur with water-contaminated lubricant. This involved a number of fatigue life tests, the first using bearings whose rings were made of martensitic stainless steel and super-clean steel, to see if pits in the raceway surface were caused by corrosion or by non-metallic inclusions on the surface of the bearing material.
The tests showed that the fatigue life of highly corrosion-resistant martensitic stainless steel showed no improvements over that of standard bearing steel AISI521O0. This means that the pits in the raceway surfaces are not caused by corrosion.
In the second of the tests, the fatigue life of a bearing with 'super clean' steel rings was compared to that of a standard bearing with 52100 steel rings. The result of this test was conclusive in that the fatigue life of the super-clean steel was three times longer than that of the standard 52100 bearing steel. This provided the clearest indication that spalling of non-metallic oxide inclusions was the cause of formation of pits in the raceway surface.
The culmination of NSK's research into water-contaminated environments is Water-Tough WTF, a new super-clean bearing steel that benefits from tightly controlled levels of surface-retained austenite to ensure that fatigue life is improved under debris-contaminated lubrication. Proof of this improved life was highlighted in tests that performed using WTF bearings in an actual cold strip mill. These support the laboratory test findings on WTF; the super-clean material achieved a life approximately three times longer than that of bearings made from conventional case-hardened steel.