Improved machine life
March 1st 2008

Rolling bearings are indispensable to machinery and equipment; their function is crucial in the role of transferring rotational energy with minimal energy loss. At the same time bearings can play a role in the preservation of natural resources by providing enhanced operating life, and also through downsizing as a result of the combination of improved materials and optimised designs

The life of rolling bearings is usually determined by the severity of "flaking," a fatigue phenomenon where a part of the bearing surface peels off in small flakes as a result of repeated stress loads. Rolling bearing life can be determined by researching the causes of flaking and taking preventive measures accordingly. Flaking is a result of two types of bearing defects: internal defects (nonmetallic inclusions in bearing material) and bearing raceway surface defects (dents caused by foreign particles in the bearing lubricant). Flaking caused by internal defects can be improved with extra-pure and cleaner steels, such as the Z steel and EP steels developed by NSK.

Flaking caused by raceway surface defects can be improved by optimising the retained austenite content in bearing raceway surfaces, as is the case with NSK's TF, HTF RTF, and STF steels. This results in reduced stress concentration from dents and, once again, improved life. The corollary of this is less demand on the environment, in terms of raw material usage and preservation of natural resources.

Like new materials technology, surface optimisation plays a key role in reducing demands on the environment through extended bearing life. However, surface optimisation goes further by reducing the energy-consuming friction that ultimately leads to wear and, possibly, premature failure. For a surface design to ensure reduced friction and longer bearing life, machining processes (barrelling and honing) as well as surface roughness play important roles.

A good surface geometry leads to optimum oil film formation, while the machining processes ensure good compressive stress levels and high resistance to abrasion and excessive wear.

Complementing the role of surface optimisation, improved bearing design contributes greatly to energy conservation.

Reducing the frictional resistance of rolling bearings is an endless challenge. However, as part of its commitment to achieving this goal in automotive applications, NSK has developed a theory for frictional resistance for tapered roller bearings that run in rolling and sliding contact at their ribs and roller ends. This theory has been used to reduce frictional torque by as much as 40% in automotive power transmissions, without adversely affecting bearing life and rigidity.

Low torque designs that reduce friction have also been developed for more widely used ball bearings. These designs, which are complemented by new low torque contact seals and greases, are reducing torque by up to 50% in electric motors, a critical area of power consumption, accounting for 2/3 of all industrial usage.

Achieving low torque in bearings is also about lubrication, and in particular greases. NSK's experience with the automotive industry, which has had a historic requirement for low torque bearings to ensure efficient cold starts, has proved pivotal in this area. Originally developed for use on commutator motor shafts, NSK's EA3 grease is a poly-alphaolefine type, which is superior in lowtorque to maximise energy efficiency in the operation of rolling bearings.

In addition to developing special low torque greases, NSK has considered the problems of using grease lubrication per se. Grease is used widely for the lubrication of rolling bearings and ballscrews, and it can contaminate the soil, rivers, or sea if it leaks or is disposed of improperly. This could inflict considerable harm on the natural environment.

NSK has addressed this problem with its Excella Green grease, which is readily decomposed by bacteria, and contains no harmful substances. Excella Green is a long-life grease developed, for bearings in small-to-medium-sized general purpose motors, air conditioner fan motors, and IT devices. It is 60% or more degradable than conventional greases, as determined by a 28-day biological oxygen demand test in bacteria activated sludge, which is an effect evaluation index for an ecosystem.

Grease, of course, is retained within bearings by seals and shields, and these units also play a role in energy conservation: a seal or shield that is too tight a fit will lead to friction in a bearing and consequent energy loss. The challenge in the bearing market has been to develop high-speed seals, which minimise bearing friction.

This has led to the development of NSK's V type seal. The V seal is a design that seals effectively without an increase in torque or operating temperature. It has better sealing capability than a shield, and a speed capability comparable to that of a shielded bearing. The non-contact lip of the V design reduces drag in the bearing - an important advantage where power loss is critical, as in small electrical motors.

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