GB2065241A - Rolling contact bearings - Google Patents

Abstract

A rolling element bearing includes means 22 attached to the radially inner race whereby the rotational speed of the bearing cage is adapted to be controlled by the inner race. <IMAGE>

Description

SPECIFICATION Improvements in or relating to rolling element bearings This invention relates to rolling element bearings and more particularly to such bearings suitable for use in high speed applications such as for example gas turbine engines.

It is well known to use rolling element type bearings in gas turbine engines; either to support a rotatable engine component from a stationary structure or alternatively used as an intershaft bearing to support a rotatable engine component from a further rotatable component. Such bearings usually comprise a radially inner race, a radially outer race and a plurality of rolling elements arranged therebetween. The rolling elements being retained in their respective locations between the races by means of a bearing cage.

All such rolling element bearings can under certain operating conditions suffer a disadvantage in that the rolling elements may lose contact with a bearing race which results in skidding of the rolling elements. Such skidding is obviously detrimental to the effectiveness of operation of the bearing as it results in both high bearing race and rolling element wear and also leads to overheating of the bearing.

There are several factors which may lead to this condition. For example, it can be caused by the radial growth of the radially outer bearing race due to the centrifugal forces acting upon it. Or alternatively in the case of a ball bearing race which may be used to accommodate axial thrust loads as well as radial loads the balls may lose contact with one of the races when thrust loads are reduced due to changes in the direction of thrust acting upon the bearing.

In the case of an intershaft bearing in which the radially outer bearing race runs at a higher speed than the inner race and in the same direction of rotation, the rolling elements are held in firm contact with the outer race due to centrifugal force to a greater degree than a static outer race.

This results in the balls and cage tending to run at the outer race speed and some sliding occurs between the rolling elements and radially inner race.

The well known solution to this problem is to ensure that there is a closer clearance between the cage and the inner race, than between the cage and the outer race. By introducing oil into the gap between the radially inner race and the cage it is possible to set up a differential viscous drag between the bearing cage and the inner and outer race which favours the inner race. There is thus a net force which tends to slow the cage down to a value which is nearer the true epicyclic rolling speed of the balls and hence reduce the speed of sliding between the rolling elements and the inner race.

Whilst the above described simple solution may be adequate in some applications it is becoming increasingly apparent that some applications call for a greater degree of drag on the cage to control skidding than this simple arrangement can readily provide. The limitation of this arrangement is the minimum radial clearances which can be safely used without causing a high rate of wear or even catastrophic seizure of the cage on the radially inner race.

An object of the present invention is to provide a rolling element bearing in which the aforementioned disadvantages are substantially overcome.

According to the present invention a rolling element bearing comprises a radially inner bearing race, a radially outer bearing race, and a plurality of rolling elements arranged within a bearing cage situated between the races, the radially innermost bearing race including drive means whereby the rotational speed of the cage is adapted to be controlled by the inner race.

The drive means comprises a plurality of segments which are pivotably mounted upon the inner race such that a portion of each segment may be flung radially outwardly under the influence of centrifugal force such as to reduce the clearance between a portion of the bearing cage and the segments and this controls the rotational speed of the cage by means of frictional drag.

Alternatively the segments may be attached to a further member on which the inner race is secured.

Furthermore each segment is provided with an abutting portion such that the overall diameter of the plurality of segments cannot exceed a predetermined value and thus completely eliminate the clearance between the radially inner race and the bearing cage.

For better understanding of the invention an embodiment thereof will now be more particularly described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a pictorial view of a ducted fan type gas turbine engine having a broken away casing portion disclosing a diagrammatic embodiment of the present invention, Figure 2 shows an enlarged cross-sectional view of the embodiment shown diagrammatically at Figure 1, Figure 3 shows a view of the segments taken on line 3-3 of Figure 2.

Referring to the drawings a gas turbine engine shown generally at 10 comprises in flow series, a fan 12, an engine compressor section 13, a combustion section 14 and a turbine section 1 5.

The compressor and turbine sections of the engine consisting of separate low, intermediate and high pressure working sections which are mounted upon a common concentrically arranged multishaft arrangement. The engine's shafts are each mounted for rotation within bearings one of which is shown diagrammatically at 1 6.

Figure 2 of the drawings shows a crosssectional view in greater detail of the bearing shown diagrammatically at Figure 1. The bearing comprises a radially inner race 17, a radially outer race 1 8 and a plurality of ball bearings one of which is shown at 19 situated between the two bearing races. The ball bearings are equally spaced within the gap defined by the two races 17 and 18, and are maintained in their preferred locations by means of the bearing cage 20.

An annular member 21 is secured (by means not shown in the drawings) to the radially inner race 17, and this member serves to carry a plurality of pivotably mounted segments 22 which are equally spaced about the periphery of the member 21 and associated bearing race 17. The pivotably mounted segments are secured to the annular member 21 by means of axially extending pins one of which is shown at 23.

During operation of the bearing the free ends of the segments 22 are flung radially outwards until their further outward movement is restrained by their end portions 24 abutting a portion of the next adjacent segment. The radially outward movement of the segments serves to reduce the clearance between a portion 25 of the bearing 20.

It will be appreciated that it is necessary to supply the bearing with oil to both lubricate and cool the bearing and therefore by adjusting the clearances between the inner bearing race 17 and the cage 20 to a predetermined value it is therefore possible to control the speed of the cage by means of the viscous drag caused by the oil flowing through the clearances between the respective inner and outer races 17, 18 and the bearing cage 20.

The segments may be shaped such that when they are located in their radially outermost position their radially outermost curved surface defines a cylinder. In this case a parallel gap will be defined between the segments and the adjacent surface provided on the cage portion 20a. Therefore a tangential drag force will be created by shearing of the coil within the clearance and no radial force is present, the full weight of the segments is carried by the pivots 23.

Alternatively the outermost curved surface of each segment 22 may be shaped such as to define a converging clearance with its adjacent portion of the bearing cage 20. In this way the converging wedge of oil formed within the clearance will effectively transfer some of the weight of the segments to the cage and thus reduce the forces acting upon the pivot 23. The magnitude of the hydrodynamic lift produce is a function of the relative rubbing speed between the segments and the bearing cage, the oil properties, and the geometry of the oil wedge.

It will be appreciated by those skilled in the art that although the above described example particularly relates to ball bearing type races, the same invention is equally applicable to roller bearings.

Claims (5)

1. A rolling element bearing comprises a radially inner bearing race, a radially outer bearing race, and a plurality of rolling elements arranged within a bearing cage situated between the races, the radially innermost bearing race including drive means whereby the rotational speed of the cage is adapted to be controlled by the inner race.

2. A rolling element bearing as claimed in claim 1 in which the drive means comprises a plurality of segments which are pivotably mounted upon the inner race such that a portion of each segment may be flung radially outwardly under the influence of centrifugal force such as to reduce the clearance between a portion of the bearing cage and the race and this controls the rotational speed of the cage by means of frictional drag.

3. A rolling element bearing as claimed in claim 1 in which the segments may be attached to a further member on which the inner race is secured.

4. A rolling element bearing as claimed in claim 2 in which each segment is provided with an abutting portion such that the overall diameter of the plurality of segments cannot exceed a predetermined value and thus completely eliminate the clearance between the radially inner race and the bearing cage.

5. A rolling element bearing as claimed in any preceding claim and substantially as hereinbefore described by way of example only and with reference to the accompanying drawings.