Squeezing more efficiency from anti-friction bearings

May 1, 2010 12:00 PM,

Dan Statler Walter Verhaert SKF USA Downingtown, Pa.

Redesigned components and new lubricants lead to a breakthrough in bearing efficiency.

Design enhancements in the ultralow friction E2 bearing include an optimized internal geometry, lowfriction lubricant, and new cage.
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Bearings, by design, are energy-saving devices. They limit friction between moving parts in countless machines and, over hundreds of years, have continually evolved toward greater efficiency and reliability.

Given this history, it would seem further advances in bearing efficiency would be incremental at best. That's what makes a recent development by SKF, Göteborg, Sweden, all the more remarkable.

The company has developed a new performance class of bearings, termed the SKF Energy Efficient (E2), that reduces frictional losses in the bearing by at least 30% when compared with the company's already-efficient standard bearings. The E2 class today includes deep-groove ball bearings, double-row angular contact ball bearings, and cylindrical, tapered, and spherical roller bearings.

Designed for grease lubricated, light-to-moderate load applications, the ultra-low friction of E2 bearings lets OEMs build greater energy efficiency into their products and reduce operating costs for users. And because the bearings are dimensionally interchangeable with bearings that conform with ISO specifications, no equipment redesign is necessary.

With potential applications across many millions of machines — in electric motors and pumps, for instance — energy-saving bearings could make a significant contribution to global sustainability. How significant? If these deep-groove ball bearings were used on every industrial motor in the U.S. and E.U., energy savings would be about 2.5 billion kWh/year.

And the benefits go beyond energy savings. In most cases, the bearings run cooler than standard bearings at equivalent loads and speeds. They also consume less lubricant and potentially extend the lives of other components and machinery.

Bearing geometry

The concept behind the E2 was to make a bearing with a much longer service life under specific operating conditions than possible with standard bearings. And extending life largely depends on reducing friction and grease consumption inside the bearing. Researchers and engineers attacked the problem on three fronts.

SKF’s high-efficiency bearings are for applications with light-to-moderate loads, such as electric motors up to 50 hp. They reduce frictional losses by at least 30% and can last twice as long as conventional bearings.

First, they optimized the internal geometry of the bearing to minimize the contact area between ball and raceway. The aim was to make friction in the contact area between the balls and raceways as small as possible.

There was no change in the size of the balls. But consistency in size is extremely important. Balls that very precisely match provide better load distribution, reduce vibration, and run more smoothly, all of which reduce friction and operating temperatures.

Because there is always some variation when manufacturing, balls must be sorted by size. A set of balls for a typical bearing should not vary more than one micron in diameter. The same goes for the surfaces of the raceways. After they are hardened, ground, lapped, and honed, there are small but measurable variations between the diameters of the raceway's inner and outer rings. In production, all parts are measured and dimensions recorded, so inner and outer rings can be subsequently matched with properly sized balls to give the clearances needed to meet engineering specifications.

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