Chopped spheres make bearing breakthrough

Tom Shelley reports on a totally new type of rolling element bearing - the biggest step forward in the field in a century



Bearing elements that are spherical - but have had their sides cut off - maintain axial load handling, despite a 20 per cent reduction in bearing width and mass.

An alternative, within the same design envelope, is to carry greater loads and provide a larger grease reservoir and more space for improved sealing.

The bearings, developed by INA and available in the UK from Schaeffler, are likely to find use in all branches of mechanical engineering - especially in automotive transmissions and wheel bearings.

In a normal ball bearing, only 70 per cent of the ball width is used, so it makes sense to remove the outer 15 per cent to the left and right of the ball diameter. Because the ball rollers must not rotate relative to their axes, the new bearings - designated 'KXR' - have a novel cage. The pocket bases of the cage are designed so that the ball roller under load can align itself freely as a function of the contact angle.

Apart from the obvious advantages stemming from being able to make bearings thinner, or the same thickness and able to carry the same load, it is possible to get more bearing elements into the same sized bearing. These are loaded by positioning the inner race eccentrically with respect to the outer race. Because the ball rollers have their sides chopped off, it is possible to get 11 ball roller elements into a 6207 basic bearing instead of only nine full balls.

Furthermore, having got away from a completely spherical shape, it is possible to give the roller a logarithmic profile since the rotational axis is always perpendicular to the variable contact angle. The osculation conditions, that is 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. All computer simulations and running tests show that the rollers rotate about their intended rotational axes. Due to their moments of inertia, the rollers are more quickly stabilised than fully spherical balls by gyroscopic effects, similar to those that make pedal cycles more stable at higher speeds.

The new bearings already exist in a number of specific forms and specific applications have already been identified for them.

Single row KXR bearings such as 6206KXR offer either higher performance (for the same outside diameter and bore), or or narrower size but with the same performance as their conventional counterparts. Or, alternatively, they can be made with higher performance and the same width but with an enhanced grease reservoir or sealing. Or, they can give an equal performance but be made smaller radially as well as narrower than their standard equivalents.

KXRT bearings have been developed to replace taper roller bearings. They have the same load carrying performance but are narrower. They also offer lower friction since there is no sliding friction on ribs. They are seen as the logical development of tandem ball bearings used, for example, in pinion bearing arrangements with two rolls of balls. Bearing adjustment and lubrication is simpler and more robust for KXRT semi-locating bearings, claims the company.

A four row KXR4 bearing replaces double row tapered roller bearings and four row ball bearings of the same bore and outside diameter. It can also replace double row ball bearings with a reduced outside diameter.

KXR bearings also offer advantages for virtual shafts in gearboxes. In a twin shaft gearbox, the conventional locating bearing on shaft 1 may be replaced by a single row KXR bearing of equal performance - but of smaller dimensions and narrower cross section. The two semi-locating bearings on shaft 2 may be substituted by double row KXRT bearings. Centre distances and bearing spacings can be reduced and the overall design envelope reduced. This is particularly advantageous for front wheel drive vehicles with transverse mounted engines with small mounting dimensions. Tests under load and speed in which contact angles change from plus to minus for each ball roller on each revolution show that the rollers 'creep' with the correct angle in accordance with theory. Dynamic tests under axial and radial load conditions show that the KXR bearings fulfil theoretically anticipated lives. Drive tests show that in manual gearboxes, they function correctly under jump starting and drive and coast conditions.

The only mystery in the product design is that the company is unwilling to reveal exactly how the ball rollers are made. Since the original business was founded on a better technology to make steel balls for ball bearings, we can expect that it will do its best to keep the production technologies secret as long as is possible.

Schaeffler (UK)

Eureka says:

This looks to be the biggest breakthrough in rolling element bearing design in the last century

Pointers

* For an unchanged load case, less space is required, the bearings are of lower mass and offer lower friction and reduced energy consumption

* For the same design envelope, they offer higher load carrying capacity, and/or larger grease reservoirs, and/or more space for improved sealing

Author
Tom Shelley

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