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Squeezing more efficiency from anti-friction bearings

Squeezing more efficiency from anti-friction bearings

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

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.

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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Lubrication

The second innovation was developing a new, low-friction bearing grease, which consists of a synthetic base oil and a lithium thickener, specifically for the new ball bearing. The grease has a rather low viscosity and generates less friction when the bearing operates, compared with standard bearing lubricants.

The key is that the lube has been specifically tailored to the physical characteristics of the E2 bearing and formulated for extremely low friction. It is also application dependent, because friction in the grease is a function of the load.

The grease is for applications with light-to-moderate loads, defined as:

C/P ≥ 8,

where P = bearing equivalent load and C = dynamic load rating. Loads greater than that increase friction inside the bearing, accelerate grease consumption, and shorten bearing life.

The grease is not suited for other types of bearings. For instance, energy-efficient spherical roller bearings used in high-speed fans need a different grease formulation. That's because mechanical components in these bearings behave differently than those in deep-groove ball bearings.

Cage design

Conventional bearings generally use steel cages to hold and guide the balls. Steel cages are the most economical types for deep-groove ball bearings, and manufacturers readily stamp them from sheet steel, form the required profile, rivet the two halves together, and insert and secure the balls.

The E2 deep-groove ball bearing, on the other hand, uses a polymer cage. It weighs less and generates lower inertial forces. And the fiber-reinforced polyamide material generates less friction than steel and is somewhat self-lubricating.

The polymer cage is not merely a copy of the steel version. It's redesigned to better hold each ball in place and retain and deliver grease where it's needed. This guides the balls more precisely and helps minimize bearing friction and rolling resistance.

Benefits

As mentioned above, SKF E2 deep-groove ball bearings are designed for operating conditions characterized by light-to-normal loads at relatively high speeds — generally above 1,000 rpm. Typical examples include electric motors up to about 37 kW (50 hp), as well as pumps, conveyors, and fans.

When used under these conditions, energy-efficient bearings operate with at least 30% lower friction moment than standard bearings. And, in some cases, the reduction is significantly greater. For example, in wind-turbine gearboxes, E2 cylindrical roller bearings operate with up to 80% lower friction than the bearings they replace.

The accompanying speed-versus-time curve illustrates the effectiveness of energy-efficient deep-groove ball bearings in a 3-kW electric motor. In this example, the motor was equipped with standard SKF deep-groove ball bearings and later with E2 bearings. The free running time after power was switched off was about 50% longer with E2 bearings.

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Likewise, the frictional moment of SKF 6306 energy-efficient bearings was measured under various operating conditions. When compared with standard SKF shielded deep-groove ball bearings, the E2 bearing showed, on average, approximately 50% less frictional moment. The Power Loss graphic illustrates typical energy savings as rotational speed varies.

Lower-friction bearings generate less heat and run cooler than standard bearings. The operating temperatures of energy-efficient bearings and standard bearings were measured at certain speeds, as detailed in the accompanying graph. Bearings ran for 24 hr at different speed intervals, at room temperature, with a 0.5 kN radial load. Tests show that compared with standard bearings, E2 bearings ran 5° to 15°C cooler, depending on speed.

Grease life

In light-to-normal load applications, metal fatigue is rarely an issue. So grease life becomes a limiting factor when determining the life of a sealed bearing. Lower operating temperature in a bearing helps extend grease life, and that means longer bearing service life. Compared with standard shielded deep-groove ball bearings, energy-efficient versions can more than double the mean time between failure, as shown in the Grease Life comparison chart.

The bottom line is energy-efficient deep-groove ball bearings can last twice as long as comparably sized standard bearings when properly applied. This can halve the number of bearings a machine requires over its lifetime.

E2 deep-groove ball bearings have the same operating characteristics as SKF standard deep-groove ball bearings. They are dimensionally interchangeable with standard bearings made to ISO dimensional specifications and are available in common bore sizes from 5 to 60 mm.

Though open versions are available, the energy-efficient deep-groove ball bearings are normally shielded on both sides. Shielded bearings are primarily intended for applications where the inner ring rotates. If the outer ring rotates, there is a risk that grease will leak at high speeds. Bearings are lubricated for life and are maintenance-free, but are not suited for washdown conditions or temperatures above 80°C.

Although they are somewhat more expensive than standard bearings, energy-efficient bearings will cut overall costs over the life of a machine by reducing energy and lubricant consumption. And significantly longer bearing service life means less downtime, lower maintenance costs, and fewer replacement parts.

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