GB2258699A

GB2258699A - Shaft bearing arrangement

Info

Publication number: GB2258699A
Application number: GB9117355A
Authority: GB
Inventors: Michael John Raine
Original assignee: Rolls Royce Business Ventures Ltd
Current assignee: Rolls Royce Business Ventures Ltd
Priority date: 1991-08-10
Filing date: 1991-08-10
Publication date: 1993-02-17
Also published as: GB9117355D0

Abstract

The shaft (10) of a small gas turbine engine is supported against radial loads by a journal air bearing (15) and supported against axial loads by an electromagnetic thrust bearing (28). The journal air bearing provides the required degree of stiffness but has poor damping properties so the thrust bearing is arranged so the flux (3) of the bearing (28) passes through the shaft (10) in such a way as to provide radial damping of the shaft (10). <IMAGE>

Description

SHAFT BEARING ARRANGEMENT This invention relates to a bearing arrangement for a rotatable shaft. It is particularly relevant to small, high speed shafts such as those used in small gas turbine engines.

Small gas turbine engines are conventionally of very simple construction. Typically they have a central rotatable shaft which carries a centrifugal compressor impeller at one end and a radial inflow turbine rotor at the other end. The shaft could of course, by carried by conventional journal and thrust ball or roller bearings.

However the high rotational speeds of such shafts (typically up to 100,000 rpm) result in such bearings providing a significant amount of power absorption.

High bearing power absorption could be avoided by the use of electro-magnetic bearings. However electro-magnetic bearings, and in particular electro-magnetic journal bearings require the use of active control systems in order to maintain shaft concentricity. Such control systems are not desirable, however, in view of their complexity and cost.

As an alternative, air bearings, either aerostatic or aerodynamic, could be used as the main journal bearings.

While such air bearings would be cheaper and capable of providing the necessary degree of stiffness and load capacity, they have poor damping properties. Such poor damping properties provide difficulties in the maintenance of shaft concentricity under certain engine operating conditions.

It is an object of the present invention to provide a rotatable shaft having a bearing arrangement which substantially avoids the aforementioned problems.

According to the present invention, a shaft rotatable about its longitudinal axis is supported radially against radial loads by at least one journal gas bearing and supported axially against thrust loads by at least one thrust magnetic bearing, the magnetic flux of said thrust magnetic bearing passing through said shaft with components ranging between axial and radial.

The present invention, will now be described, by way of example, with reference to the accompanying drawing, which is a sectioned side view of a portion of a small gas turbine engine having a shaft bearing arrangement in accordance with the present invention.

Referring to the drawing, the main shaft 10 of a small gas turbine engine is rotatable about its longitudinal axis 11. At its upstream end (with respect to the general flow of gas through the engine) the shaft 10 carries a centrifugal compressor impeller 12. The impeller 12 is of conventional construction and comprises a plurality of air compressing vanes 13. Air flowing in the general direction indicated by the arrows 14 is compressed by the rotating impeller 12 before being directed to combustion apparatus (not shown) where it is mixed with fuel and the mixture combusted. The resultant combustion products are then directed to drive a conventional radial inflow turbine (not shown) which is located on the opposite end of the shaft 10 to the impeller 12. The turbine thus serves to rotate the shaft 10 and thereby drive the impeller 12.

The gases exhausted from the radial inflow turbine may be used to drive a power turbine (not shown).

During rotation, the shaft 10 is subject to both axial and radial loading. To facilitate this loading, the shaft 10 is supported by both journal and thrust bearings.

The shaft 10 is supported radially against radial loads by two journal bearings, one of which 15 is adjacent the impeller 12 as can be seen in the drawing. The other journal bearing (not shown) is situated adjacent the radial inflow turbine (not shown).

The journal bearing 15 is of the aerostatic gas type although it will be appreciated that it could be of the aerodynamic type if so desired. It comprises a hollow cylindrical body 16 which is supported from static engine structure 17 by two axially spaced apart annular seals 18.

The hollow cylindrical body 16 in turn coaxially encloses three axially adjacent ring members 19,20 and 21. The ring members 19,20 and 21 have an internal diameter which is slightly greater than the external diameter of the shaft 10 portion which they surround.

The ring members 19,20 and 21 are axially spaced apart from each other by a small amount. The spaces so defined between them are fed with pressurised air delivered through a duct 22 in the static engine structure 17. Air from the duct 22 is delivered to an annular groove 23 in the external surface of the hollow cylindrical body 16. A number of small passageways 24 in the body 16 direct the pressurised air from the groove 23 to the gaps between the ring members 19,20 and 21. The air passes through these gaps to establish an air film between the ring members 19,20 and 21 and the shaft 10. It is this air film which the shaft 10 rides upon, and is radially supported by, as it rotates.

Additional seals 25 between the static engine structure 17 and the hollow cylindrical body 19 and either side of the groove 23 ensure that there is minimal air leakage as the air flows from the duct 22 and into the passages 24.

In order to ensure that the operation of the aerostatic journal bearing is not adversely affected by air leakage from the impeller 12, an annular-seal 26 is provided between the downstream portion the impeller 12 and static engine structure 27.

The shaft 10 is supported axially against axial loads primarily by an electro-magnetic thrust bearing 28. The electromagnetic bearing 28 comprises two axially spaced apart poles 29 and 30 which respectively cooperate with the annular radial faces of triangular cross-section flanges 31 and 32 on the shaft 10. When the electromagnetic bearing 28 is activated, the magnetic flux between the poles 29 and 30 passes through the shaft 10 as indicated by the interrupted lines 31. The flanges 31 and 32 are thereby attracted to the poles 29 and 30. This force of attraction, which is in the same direction as the general flow of gas through the gas turbine engine, opposes the axial force in the opposite direction caused by the operation of the impeller 12.

The axial thrust of the electro-magnetic thrust bearing 28 is additionally opposed by an additional electro-magnetic thrust bearing 32 located immediately upstream of and acting upon the end of the shaft 10 and the impeller 12. A balance is thereby achieved between the opposing attractive forced provided by the electromagnetic bearings 28 and 32. It will be appreciated, however, that alternative means could be employed to balance the attractive force of the electro-magnet 28. For instance, the shaft 10 could be mounted in a vertical position so that its weight and the weight of the components which it carries opposes the attractive forces of the electro-magnetic 28.

As mentioned earlier, while the aerostatic journal bearing is effective in providing adequate stiffness and load capacity, it does not provide a great deal of radial damping of the shaft 10. The shaft 10 is thereby prone to radial or whirling movement. Such movement is, however, countered by the magnetic flux 31 associated with the electromagnetic bearing 28. The flux 31 has components which range between radial and axial. Any radial or whirling movement by the shaft 10 thereby results in the flux 31 generating eddy currents within the shaft 10. These eddy currents then dissipate the energy of shaft 10 radial or whirling movement as heating of the shaft 10. This thereby provides damping of radial or whirling movement of the shaft 10 and hence stabilises the shaft 10.

Claims

Claims:

1. A shaft rotatable about its longitudinal axis, said shaft being supported radially against radial loads by at least one journal gas bearing and supported axially against thrust loads by at least one magnetic thrust bearing, the magnetic flux of said thrust magnetic bearing passing through said shaft with components ranging between axial and radial.

2. A shaft as claimed in claim 1 wherein said at least one gas bearing is an aerostatic gas bearing.

3. A shaft as claimed in claim 1 or claim 2 wherein said gas bearing is an air bearing.

4. A shaft as claimed in any one preceding claim wherein said shaft is provided with two or more axially spaced apart flanges, said at least one thrust magnetic bearing acting upon said flanges to exert said thrust loads upon said shaft.

5. A shaft as claimed in claim 4 wherein each of said flanges is of generally triangular cross-sectional configuration to define an annular radial surface which confronts said thrust magnetic bearing.

6. A shaft as claimed in any one preceding claim wherein at least one further thrust bearing is provided to exert axial loads upon said shaft which act to oppose said thrust loads exerted upon said shaft by said at least one thrust magnetic bearing.

7. A shaft as claimed in claim 6 wherein said at least one further thrust bearing is positioned so as to act upon the end of said shaft.

8. A shaft as claimed in claim 6 or claim 7 wherein said at least one further thrust bearing is a magnetic thrust bearing.

9. A shaft as claimed in any one preceding claim wherein any one of said magnetic thrust bearings is an electromagnetic thrust bearing.

10. A shaft as claimed in any one preceding claim wherein said shaft carries a compressor impeller.

11. A shaft substantially as hereinbefore described with reference to and as shown in the accompanying drawings.