Power Transmission
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.
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
Fig. 3. For bearings used in high-humidity, chemical and hygienic environments, NSK says its ES1 stainless steel offers better corrosion resistance and fatigue life than AISI 440C.
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
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 (Fig. 2). 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
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