A BEARING RACE

This invention relates to rolling-element bearings, and particularly to the design of outer races for such bearings.

A known problem with rolling-element bearings (especially those that operate at high speeds but at low loads) is skidding. The rolling element, instead of rolling on the inner track of the bearing, slides along it. This causes the rolling element to heat up at the contact point. If the sliding element breaks through the lubricant film, damage may be caused to the rolling elements or to the tracks. This phenomenon is known as skidding.

A known way to reduce skidding in lightly-loaded bearings is deliberately to make the outer race non- circular so that an interference exists across the bearing at two or more points. In use, the outer race will then bend due to this interference until a force equilibrium is reached, and a resultant load will be present at two or more positions across the bearing. The magnitude of the force can be controlled (by suitable choice of the magnitude of the non-circularity) to prevent the bearing from skidding.

A disadvantage with such bearings is that they require a squeeze film housing, or some other type of housing with enough clearance to allow the outer race to bend. If it is not possible to incorporate such a housing (for example, because of space or weight restrictions or oil feed issues) then it will be necessary to use a clear housing or a flanged outer race. In high speed applications a clear housing may be prone to fretting and so a flanged outer race is preferable. Another reason that a flanged outer race may be preferred is that such a race allows tighter

control of the alignment of the shaft carried by the bearing.

A problem is that a flanged outer race is usually much stiffer than other types of outer race. The solution to the problem of skidding set out above, using a non-circular outer race, relies on the flexibility of the outer race and consequently it is unsatisfactory to use a flanged outer race in this situation.

This invention presents an alternative solution to the problem of skidding in lightly-loaded bearings, which is suitable for high-speed bearings requiring flanged outer races. It may also be applied to other configurations of rolling-element bearing.

According to the invention, a race for a rolling- element bearing is characterised in that the race comprises at least one circumferentially-extending region having a reduced stiffness compared with the remainder of the race.

The or each circumferentially-extending region may be thinner than the remainder of the race, and may further comprise a hole or a slot.

Preferably, there are two such circumferentially- extending regions, each extending around an arc of approximately 60 degrees in the circumferential direction.

The race may be an outer race, and may be flanged. The race may be a part of a rolling-element bearing. The rolling elements in such a bearing may be rollers.

An embodiment of the invention will now be described, by way of illustration, with reference to the accompanying drawings in which Figure 1 is a perspective view of a race for a rolling-element bearing according to the invention; and

Figure 2 is a sectional view of the race of Figure 1, on the line II-II.

Referring to Figure 1, a bearing race for a rolling- element bearing of a gas turbine engine is shown generally at 11. The outer race 13 has a flange 15, and a plurality of bolts 17 (not all are shown) secure the outer race to other structure (not shown) of the engine.

On the radially inner face of the outer race 13 is a circumferentially-extending track 19, on which run a plurality of rollers 21, held in a cage 23. Radially inwards of the cage is an inner race (not shown) . Figure 2, a cross-section of Figure 1 on the line II-II, shows the relationship of these features more clearly.

At two points around the circumference of the outer race 13, thinned regions 25 and cut-outs 27 are provided. Each of these features 25, 27 extends around approximately 60 degrees of the circumference. Their effect is to reduce the stiffness of the outer race 13 at these places on the circumference, compared with the unthinned regions 29. This localised reduction in stiffness causes the outer race to deform into a non-circular shape in use, with the majority of the deformation coincident with the thinned regions 25 and cut-outs 27, and an insignificant deformation at the positions 29. This deformation causes an interference with the rollers 21 at the positions of the features 25, 27, and the consequent force on the rollers 21 prevents any sliding which may lead to skidding. Despite this, however, the rigidity and dimensional accuracy of the outer race 13 in the region of the flange 15 is unaffected, because the deformation is substantially limited to the thinned and cut-out regions 25, 27. It will be appreciated that certain modifications may be made to the embodiment described here, without departing from the concept of the invention. For example, the thinned regions and cut-outs may each occupy less or more than the 60 degrees of the embodiment described. More than two such

regions may be provided. By such modifications, the stiffness variation around the circumference of the race may be tailored for a particular application.

Although it is particularly suited for applications in which a high-speed, lightly loaded roller bearing needs to be provided with a flanged outer race, it could advantageously be applied to other types of bearings, or to bearings operating under different speed or load conditions .