Bearing technologies offer benefits for power transmission designs

Paul Boughton
Rotary bearings are often considered as commodity items, especially when the commonly used sizes are available from stock for what can seem very low prices. Of course, buyers have to take care if purchasing low-cost bearings, as imports from outside Europe can sometimes be manufactured from poor-quality materials and to tolerances that are somewhat slack, and even bearings that are boxed and branded as coming from one of the major European manufacturers sometimes turn out to be counterfeit.

While specifiers and buyers can do a certain amount to ensure that the right bearing is purchased for a particular power transmission system, design engineers have the most important role to play in the overall process. Indeed, engineers today have a wider choice of bearings than ever before, enabling them to influence the design in such a way as to reduce costs, enhance performance, extend product lifetime and minimise maintenance requirements.

Broadly speaking there are three main aspects of a bearing's design that manufacturers can fine-tune: geometry, materials and finishes. Needless to say, there are other areas to consider beyond the bearing if a product's performance is to be maximised; these range from the design and precision manufacture of the mating components, bearing seals, lubricants, installation techniques, correct maintenance and, to avoid unplanned stoppages, condition monitoring is very useful. It is also worth considering whether a rolling element bearing is still the right answer, now that plain bearing materials are so highly developed.

Given the long history of rolling element bearings, it might seem surprising that there are still significant new developments in something as basic as bearing geometry. On the other hand, it has to be recognised that modern computational, simulation and analysis techniques are providing new opportunities for bearing manufacturers to optimise their designs and try more radical alternatives than would have been feasible in the days when all testing had to be undertaken with physical prototypes.

One of the most dramatic developments in recent years is Schaeffler's 'ball roller' energy-efficient bearings that utilise truncated spherical elements. This unusual design also necessitated the implementation of new assembly methods. The result is a range of ball roller bearings that offer the load handling capabilities of fully spherical ball bearings yet they enable the overall bearing width and mass to be reduced by around 20 per cent, plus friction is lower.

In essence, the ball roller design removes the areas of conventional rolling element balls that are not under load. This means that 15 per cent of the ball's diameter is removed from each side, making the flattened ball 30 per cent narrower than a conventional ball. This not only saves valuable design space, but means that the slim shape of the ball roller, in combination with new assembly methods, enable the number of rolling elements to be increased. This, in turn, means the filling capacity of the bearing can be increased. Using more rolling elements in the same space results in higher load ratings and longer service life, thereby providing designers with opportunities to downsize products and assemblies.

With the 'redundant' material removed from the sides of the balls, control of the rolling elements at start-up became critical, requiring a new type of cage to be designed. However, the rolling elements become self-locating once the bearings are moving under conditions of speed and load. In addition, having digressed from a fully spherical shape, it is possible to give the rollers a logarithmic profile, since the rotational axis is always perpendicular to the variable contact angle. The osculation conditions - the 'kiss' between the roller and the bearing groove - therefore do not change. If the load ratio changes from axial to radial and the contact angle changes as a result, the osculation 'creeps' in an optimum manner with the change in load.

Schaeffler's standard 6207 ball bearing (DIN 625) incorporates nine rolling elements, which equates to a fill capacity of around 60 per cent. In comparison, the new BXRE207 ball roller bearing contains 14 rolling elements in the same space, so the filling capacity is increased to 90 per cent and the bearing life is 2.4 times longer.

Double-row ball rollers

Taking the concept a stage further, Schaeffler's BXRO range of ball roller bearings are double-row bearings with the rolling elements in an 'O' arrangement so that the bearings can support both radial and axial loads. Compared to Schaeffler's series 32 ball bearings, up to 50 per cent more rolling elements can be fitted in the ball roller. The advantage of the BXRO over series 33 bearings (which have two inner rings) is in the simplified installation of the inner ring on the shaft. The optimised raceway geometry of the single-piece inner ring results in increased bearing accuracy. Applications for these bearings include automotive manual transmissions and textile machinery.

The BXRT ball roller series is a double-row design with the rows of ball rollers in a tandem arrangement. In this case the rolling elements can be guided using one or two cages. The two-cage design enables the optimal adjustment of the two raceways independently of each other, thereby ensuring maximum load support with minimum size and reduced friction. Compared with tapered roller bearings, friction is reduced by around 30 per cent due to the elimination of rib friction. This means that energy consumption can be significantly reduced, leading to applications in manual transmissions, differentials, hydraulic motors and pumps.

Faster fitting during assembly and maintenance

Designers are also keen to identify ways to save time during assembly and maintenance operations. To satisfy this need, SKF recently launched its latest generation of Concentra roller bearing units that can be mounted 20 minutes faster than conventional split housings. Additionally, these units enable more accurate mounting, resulting in enhanced bearing performance and longer life.

Concentra roller bearing units use SKF's proprietary locking technology to facilitate mounting and dismounting. Once the unit is on the shaft, it is locked in position by tightening three screws to a predetermined torque by using the 'smart' Allen key provided. This method ensures the bearing is locked concentrically around the shaft to improve bearing and machine performance. Additional savings in installation time result from the units being delivered pre-lubricated with one of four greases to suit the application; this feature also reduces the risk of premature bearing failure due to over-greasing or the ingress of contaminated grease into the bearing during maintenance. The bearing series covers shaft diameters from 35-100 mm.

For applications involving extreme operating conditions and low to medium speeds, NKE has introduced the NNF series of double-row, full-complement cylindrical roller bearings. These feature a compact design, high load capacity and double seals. Typical applications include rope wheels and pulleys. The increased radial load carrying capacity of the NNF series bearings is achieved through the full-complement design. In addition to radial loads, these bearings can also accommodate axial forces and tilting moments in both directions. Lubrication holes in the inner and outer rings enable easy relubrication via the shaft and housing. For enhanced corrosion protection, the bearings are also offered with a RoHS-compliant, anti-corrosion coating known as SQ171. A black oxide coating can also be applied to optimise the tribological characteristics. If required, the bearings can be manufactured with custom clearance and tolerance classes.

NSK is another company that has been taking a close look at bearing geometry in order to develop innovative products. For example, at the 2009 Tokyo Motor Show, the company was exhibiting miniature cages and rollers for use in high-speed planetary gears. Utilising what is claimed to be the world's smallest roller (at 1.5 mm diameter) and retainer, the compact and light bearings are designed to meet the needs of increasingly diverse and faster transmissions such as belt-type continuously variable transmissions, automatic transmissions and hybrid automobile transmissions.

While the foregoing illustrates what manufacturers can achieve by reviewing the bearing geometry, they are simultaneously working on new materials. For example, NSK has developed ES1 (Excellent Stainless Steel) for use in high-humidity, chemical and hygienic environments, where corrosion can impair the reliable long-term operating life of rolling bearings. ES1 is claimed to provide superior corrosion resistance and rolling contact fatigue life compared with conventional AISI 440C steel in wet and aggressive environments; this improvement helps to reduce maintenance, downtime and replacement bearing costs.

Until the development of ES1, AISI 440C had been the general choice for use in bearing applications where corrosion resistance was a primary concern. Because this steel contains coarse eutectic carbides that act as crack initiators under rolling contact stress, its performance had not always been satisfactory. ES1, however, has no eutectic carbides, but instead consists of fine carbides, nitrides and strong martensite that improve resistance to corrosion and fatigue. NSK reports that ES1 outperforms other stainless steels in a five per cent aqueous sodium chloride immersion test, a salt-water spray test, 5-N sulphuric acid and 5-N hydrochloride solution immersion tests, and life tests in water.

Schaeffler has also introduced a high-performance stainless steel, Cronitect, that provides enhanced corrosion resistance under extreme operating conditions, including dry running applications or when bearings come into contact with aggressive media such as water, salt spray, acids and cleaning chemicals. Cronitect steel is a high-grade martensitic hardening steel that is based on the consistent refinement of Nirosteels. Through a combination of its composition and a new thermo-chemical surface treatment process, Cronitect offers extremely high hardness. Even after 600 hours of salt spray testing in accordance with DIN 50021 SS, the material is reported as showing no signs of corrosion.

Surface treatments improve tribological performance

Bearing manufacturers have also developed coatings and surfaces finishes to improve both corrosion resistance and tribological performance. Not surprisingly, in some cases it has been necessary to create specialist coatings for particular application areas. For instance, Schaeffler's Durotect B is a coating for FAG cylindrical roller bearings used in wind turbine gearboxes. Field tests have demonstrated that significantly lower failure rates in wind turbine gearboxes are experienced if bearings with the Durotect B coating are used. This coating is a matt black, mixed iron oxide layer that is produced in a dip solution. After application, the coating is said to be highly resistant to bending and stress.

As well as offering excellent anti-corrosion protection, Durotect B also increases resistance to slippage and sliding, as well as performing well in the alternating, low-load conditions that are common in wind turbine (and tidal stream energy) applications. In a wind turbine, the rotor shaft bearings are subjected to highly dynamic loads and harsh conditions. This means that the design and specification of the rotor shaft, as well as the bearings that support it, are critical; typically a locating/floating arrangement of spherical, cylindrical and tapered roller bearings will be specified. For the spherical roller bearings, Schaeffler often recommends its Durotect P coating. This is a 10-20 micrometre PTFE coating that reduces friction and provides improved anti-wear protection.

Nitriding treatment for steel cages

As well as surface treatments for bearings rings, NSK has also been working on treatments for cages. The company says it has applied a special nitriding surface treatment to the high-precision pressed steel cage of its HPS series spherical roller bearings, which can significantly reduce wear in harsh operating environments.

Compared with conventional nitriding processes, NSK's treatment is said to form a finer and harder surface, with more uniform hardness. Since the special treatment is conducted at high temperatures, it might be expected that the dimensional accuracy of the cage would be affected. However, NSK has found a way of ensuring high accuracy of the cage after treatment.

One of SKF's recent developments is its NoWear bearings that feature a wear-resistant diamond-like carbon (DLC) coating. This is claimed to extend the life expectancy of the bearings by between five and ten times compared with conventional components. DLC-treated bearings are therefore suitable for applications with inadequate or low lubricant viscosity and where repetitive small movements or machine vibration could cause fretting corrosion in normal bearings. Applied to the bearings' component surfaces using a chemical vapour deposition process, it is thermally stable up to 200°C.

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