TechArchive

Original article date: November 1998

Electric motors are required to run more quietly and smoothly. Several techniques can be applied in combination to meet the objective. Tony Cuff of NSK-RHP explains.

As electric motors function at increasingly higher speeds and higher operating temperatures selecting the right bearings is crucial to ensuring long life. However this is not the only operating consideration. Other developments in materials production and lubrication technologies are also involved.

NSK-RHP uses SUJ-2Z bearing steel for ball and ring assemblies in its motor bearings. SUJ-2Z is an ultra-clean vacuum-degassed bearing steel which is much cleaner than the typical market level. With its near aircraft steel quality its oxygen content lower than 9ppm. SUJ-2Z dramatically increases the fatigue life of electric motor bearings.

Manufacturing a quiet bearing depends on a variety of factors that relate to the overall bearing quality levels. The raceways for NSK-RHP’s electric motor bearings are finished to a super-smooth condition using a special honing process developed especially for this purpose. Surface finishes of less than 1um facilitate increase the effectiveness of lubrication and extend bearing life. To ensure the smoothest possible operation NSK-RHP uses high grade steel balls manufactured by a proprietary process to give optimum “sphericity”.

Over recent years a special emphasis has been placed on noise reduction in greased bearings. One major reason is the increasing use of electric motors inside passenger vehicles and domestic equipment. Noise is caused in part by microscopic particles in the grease and in the additive packages. By improving the cleanliness of the grease generally and by using soluble rather than solid additives substantial reductions in noise – 50% or more – can be achieved.

Another way to achieve smooth and silent motor operation is by producing bearings with a reduced radial (CM) clearance. The reduced clearance controls more closely the gap between the rotor and the stator in the motor which results in a much quieter running motor. Two-piece riveted steel bearing cages are designed with extremely close tolerances in the ball pocket. This design approach is more effective at reducing friction by providing even distribution of the lubricant on the rolling elements. The benefit is a strong cage with a lower operating temperature and higher speed limits.

With many of the applications for electric motors being in dirty or dusty environment several sealing options are available to protect motor bearings. The popular metal Z shields for high speed applications protect against light contaminants such as dust and are coated with special anti-corrosion zinc. They perform across a wide temperature range.

In applications calling for even more protection DU contact seals or patented non-contact labyrinth V seals are available. The V seal seals effectively without an increase in torque or operating temperature. They have 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.

November 1998