BEARING ASSEMBLY INCLUDING ACTIVE VIBRATION CONTROL

Abstract

A bearing assembly for a rotatable shaft, the bearing assembly comprising: a bearing housing; a bearing located within the bearing housing having an axis of rotation and arranged to receive a rotatable shaft; a first spring bar that couples the bearing to the bearing housing, the first spring bar being configured to tune vibrations of the rotatable shaft; and a first piezoelectric actuator disposed between the bearing housing and the first spring bar, the first piezoelectric actuator being configured to extend in a first direction, wherein extension of the piezoelectric actuator in the first direction displaces the first spring bar relative to the bearing housing.

Claims

1. A gas turbine engine having a bearing assembly for a rotatable shaft, the bearing assembly comprising: a bearing housing; a bearing located within the bearing housing having an axis of rotation and arranged to receive a rotatable shaft; a first spring bar that couples the bearing to the bearing housing, the first spring bar being configured to tune vibrations of the rotatable shaft; and a first piezoelectric actuator disposed between the bearing housing and the first spring bar, the first piezoelectric actuator being configured to extend in a perpendicular direction to the axis of rotation of the bearing, wherein extension of the piezoelectric actuator in the first direction displaces the first spring bar relative to the bearing housing.

2. The gas turbine engine according to claim 1, wherein the first spring bar is configured to act as a lever between the bearing housing and the bearing.

3. The gas turbine engine according to claim 1, further comprising: a second spring bar that couples the bearing to the bearing housing at a position on the opposite side of the axis of rotation of the bearing to the first spring bar, the second spring bar being configured to tune vibrations of the rotatable shaft; and a second piezoelectric actuator disposed between the bearing housing and the second spring bar, the second piezoelectric actuator being configured to extend in a second direction, wherein extension of the second piezoelectric actuator in the second direction displaces the second spring bar relative to the bearing housing.

4. The gas turbine engine according to claim 3, wherein the second direction is anti-parallel to the first direction.

5. The gas turbine engine according to claim 3, further comprising: a third spring bar that couples the bearing to the bearing housing, the third spring bar being configured to tune vibrations of the rotatable shaft; a third piezoelectric actuator disposed between the bearing housing and the third spring bar, the third piezoelectric actuator being configured to extend in a third direction; a fourth spring bar that couples the bearing to the bearing housing, the fourth spring bar being configured to tune vibrations of the rotatable shaft; a fourth piezoelectric actuator disposed between the bearing housing and the fourth spring bar, the fourth piezoelectric actuator being configured to extend in a fourth direction, wherein extension of the third piezoelectric actuator in the third direction displaces the third spring bar relative to the bearing housing, wherein extension of the fourth piezoelectric actuator in the fourth direction displaces the fourth spring bar relative to the bearing housing, and wherein the third spring bar and the fourth spring bar couple the bearing to the bearing housing at positions opposite one another with respect to the axis of rotation of the bearing.

6. The gas turbine engine according to claim 5, wherein the fourth direction is anti-parallel to the third direction.

7. The gas turbine engine according to claim 5, wherein the first spring bar, the second spring bar, the third spring bar and the fourth spring bar are equally spaced around the bearing.

8. The gas turbine engine according to claim 7, wherein the gas turbine engine further comprises: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades; and a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft.

9. The gas turbine engine according to claim 8, wherein the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft; the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft.

10. A method of minimising out-of-balance vibrations in a bearing assembly of a gas turbine engine, the bearing assembly comprising: a bearing housing; a bearing located within the bearing housing having an axis of rotation and arranged to receive a rotatable shaft; a first spring bar that couples the bearing to the bearing housing, the first spring bar being configured to tune vibrations of the rotatable shaft; and a first piezoelectric actuator between the bearing housing and the first spring bar, the first piezoelectric actuator being configured to extend in a first direction; the method comprising the steps of: rotating the bearing within the bearing housing; detecting the magnitude of vibrations received at the bearing housing due to rotation of the bearing; and extending the piezoelectric actuator in the first direction to displace the first spring bar relative to the bearing housing to reduce the magnitude of the vibrations received at the housing.

11. The method according to claim 10, wherein the bearing assembly comprises: a second spring bar that couples the bearing to the bearing housing at a position on the opposite side of the axis of rotation of the bearing to the first spring bar, the second spring bar being configured to tune vibrations of the rotatable shaft; and a second piezoelectric actuator disposed between the bearing housing and the second spring bar, the second piezoelectric actuator being configured to extend in a second direction; the method further comprising the step of: extending the second piezoelectric actuator in the second direction to displace the second spring bar relative to the bearing housing to reduce the magnitude of the vibrations received at the housing.

12. The method according to claim 11, wherein the bearing assembly comprises: a third spring bar that couples the bearing to the bearing housing, the third spring bar being configured to tune vibrations of the rotatable shaft; a third piezoelectric actuator between the bearing housing and the third spring bar, the third piezoelectric actuator being configured to extend in a third direction; a fourth spring bar that couples the bearing to the bearing housing at a position on the opposite side of the axis of rotation of the bearing to the third spring bar, the fourth spring bar being configured to tune vibrations of the rotatable shaft; and a fourth piezoelectric actuator disposed between the bearing housing and the fourth spring bar, the fourth piezoelectric actuator being configured to extend in a fourth direction; the method further comprising the steps of: extending the third piezoelectric actuator in the third direction to displace the third spring bar relative to the bearing housing to reduce the magnitude of the vibrations received at the housing; and extending the fourth piezoelectric actuator in the fourth direction to displace the fourth spring bar relative to the bearing housing to reduce the magnitude of the vibrations received at the housing.

13. The method according to claim 10, wherein the step of detecting the amplitude of vibrations received at the bearing housing due to rotation of the bearing is performed by an accelerometer coupled to the bearing housing and arranged to provide an output proportional to the magnitude of vibrations received at the bearing housing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0061] FIG. 1 is a sectional side view of a gas turbine engine;

[0062] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0063] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

[0064] FIG. 4 is a cross-sectional view of an example bearing assembly; and

[0065] FIG. 5 shows the cross-section Z-Z shown in FIG. 4.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0066] Aspects and embodiments of the present disclosure will now be discussed with reference to the corresponding drawings. Other aspects and embodiments will be apparent to those skilled in the art.

[0067] FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

[0068] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the core exhaust nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

[0069] An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0070] Note that the terms low pressure turbine and low pressure compressor as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the low pressure turbine and low pressure compressor referred to herein may alternatively be known as the intermediate pressure turbine and intermediate pressure compressor. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

[0071] The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0072] The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

[0073] It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

[0074] Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

[0075] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0076] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20 meaning that the flow through the bypass duct 22 has its own nozzle 18 that is separate to and radially outside the core exhaust nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

[0077] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

[0078] FIG. 4 schematically shows a cross sectional view of a bearing assembly 41.

[0079] The bearing assembly 41 comprises a bearing housing 42. A bearing 43 is located within the bearing housing 42, the bearing 43 comprising an inner race 44, having an outer groove, an outer race 46, having an inner groove, and ball bearings 48 received between the inner race 44 and outer race 46 in the inner and outer grooves. While ball bearings 48 are used in the embodiment of FIG. 4 it should be noted that any suitable type of bearing could be used in practice.

[0080] A rotatable shaft 50 is received in the bearing. The inner race 44 is received over and attached to the rotatable shaft 50.

[0081] A first spring bar 52 couples the bearing 43 to the bearing housing 42. An end of the first spring bar 52 is connected to the outer race 46. An opposite end of the first spring bar 52 is connected to the bearing housing 42.

[0082] The inner race 44 is arranged to rotate relative to the outer race 46 by virtue of the ball bearings 48, and so the bearing has an axis of rotation. As the inner race 44 is received over and attached to the rotatable shaft 50, and the outer race 46 is connected to the bearing housing 42 via the first spring bar 52, it can be seen that the rotatable shaft 50 is arranged to rotate in the bearing 43 relative to the bearing housing 42. The axis of rotation of the bearing 43 is arranged along a horizontal direction in the orientation shown in FIG. 4.

[0083] A first piezoelectric actuator 54 is disposed between the bearing housing 42 and the first spring bar 52. The first piezoelectric actuator 54 is configured to extend in a first direction. In the embodiment shown in FIG. 4, the first direction is perpendicular to the axis of rotation of the bearing 43, and in the orientation shown in FIG. 4 is a downward direction. Extension of the first piezoelectric actuator 54 causes displacement of the first spring bar 52 relative to the bearing housing 42.

[0084] The first piezoelectric actuator 54 comprises an inline force sensor 56 at its interface with the bearing housing 42. The first piezoelectric actuator 54 is coupled to a mounting ring 58 at its interface with the first spring bar 52. As can be best seen in FIG. 5, the mounting ring 58 circumferentially surrounds the spring bars. Each piezoelectric actuator exerts a force on the mounting ring 58 when it extends or retracts.

[0085] A preload spring 60 is disposed around the first piezoelectric actuator 54. The preload spring 60 is coupled to the bearing housing 42 and exerts a predetermined force on the first spring bar 52. As the preload spring 60 is wound around the first piezoelectric actuator 54, the predetermined force is exerted on the first spring bar 52 against the first direction. The predetermined force exerted by the preload spring 60 is set so as to compress the piezoelectric actuator 54 to a neutral point, wherein an application of voltage will, depending on polarity, either cause an extension towards its original length or retraction towards its minimum acceptable length. This ensures that the piezoelectric actuator 54 normally remains in compression, which extends the life of the piezoelectric actuator 54.

[0086] A squeeze film 62 is provided between the first spring bar 52 and the bearing housing 42. The squeeze film 62 acts to dampen vibrations caused by the rotation of the rotatable shaft 50.

[0087] A similar arrangement is provided between the bearing 43 and the bearing housing 42 at a position diametrically opposite the first spring bar 52.

[0088] A second spring bar 64 couples the bearing to the bearing housing 42. An end of the second spring bar 64 is connected to the outer race 46. An opposite end of the second spring bar 64 is connected to the bearing housing 42.

[0089] The inner race 44 is arranged to rotate relative to the outer race 46 by virtue of the ball bearings 48. As the inner race 44 is received over and attached to the rotatable shaft 50, and the outer race 46 is connected to the bearing housing 42 via the second spring bar 64, it can be seen that the rotatable shaft 50 is arranged to rotate in the bearing 43 relative to the bearing housing 42. The axis of rotation of the bearing 43 is arranged along a horizontal direction in the orientation shown in FIG. 4.

[0090] A second piezoelectric actuator 66 is disposed between the bearing housing 42 and the second spring bar 64. The second piezoelectric actuator 66 is configured to extend in a second direction. In the embodiment shown in FIG. 4 the second direction is also perpendicular to the axis of rotation of the bearing 43, but is opposed (i.e. anti-parallel) to the first direction. Thus, in the orientation shown in FIG. 4 the second direction is an upward direction. Extension of the second piezoelectric actuator 66 causes displacement of the second spring bar 64 relative to the bearing housing 42.

[0091] The second piezoelectric actuator 66 comprises an inline force sensor 68 at its interface with the bearing housing 42. The second piezoelectric actuator 66 is coupled to the mounting ring 58 at its interface with the second spring bar 64. A preload spring 72 is disposed around the second piezoelectric actuator 66. The preload spring 72 is coupled to the bearing housing 42 and exerts a predetermined force on the second spring bar 64. As the preload spring 72 is wound around the second piezoelectric actuator 66, the predetermined force is exerted on the second spring bar 64 against the second direction. The predetermined force exerted by the preload spring 72 is set so as to compress the piezoelectric actuator 64 to a neutral point, wherein an application of voltage will, depending on polarity, either cause an extension towards its original length or retraction towards its minimum acceptable length. This ensures that the piezoelectric actuator 64 normally remains in compression, which extends the life of the piezoelectric actuator 64.

[0092] A squeeze film 73 is provided between the second spring bar 64 and the bearing housing 42. The squeeze film 73 acts to dampen vibrations caused by the rotation of the rotatable shaft 50.

[0093] An accelerometer 100 is coupled to the bearing housing 42 and arranged to detect vibrations of the bearing housing 42. The accelerometer 100 is configured to produce an output which is proportional to the magnitude of the detected vibrations. The accelerometer 100 is communicatively coupled to a controller 102. While a coupling is a wired connection in the embodiment shown in FIG. 4, a wireless connection could equally be used. The operation of the controller 102 will be described in more detail below.

[0094] FIG. 5 shows the cross-section Z-Z shown in FIG. 4. Like reference numerals have been retained where appropriate.

[0095] In the view of FIG. 5, two further piezoelectric actuators are visible, i.e. a third piezoelectric actuator 76 and a fourth piezoelectric actuator 86.

[0096] A third spring bar 74 couples the bearing 43 to the bearing housing 42. An end of the third spring bar 74 is connected to the outer race 46. An opposite end of the third spring bar 74 is connected to the bearing housing 42.

[0097] The inner race 44 is arranged to rotate relative to the outer race 46 by virtue of the ball bearings 48. As the inner race 44 is received over and attached to the rotatable shaft 50, and the outer race 46 is connected to the bearing housing 42 via the third spring bar 74, it can be seen that the rotatable shaft 50 is arranged to rotate in the bearing 43 relative to the bearing housing 42. The axis of rotation of the bearing 43 is into, or out of, the page in the orientation shown in FIG. 5.

[0098] A third piezoelectric actuator 76 is disposed between the bearing housing 42 and the third spring bar 74. The third piezoelectric actuator 76 is configured to extend in a third direction. In the embodiment shown in FIG. 5 the third direction is a rightward direction. Extension of the third piezoelectric actuator 76 causes displacement of the third spring bar 74 relative to the bearing housing 42.

[0099] The third piezoelectric actuator 76 comprises an inline force sensor 78 at its interface with the bearing housing 42. The third piezoelectric actuator 76 is coupled to the mounting ring 58 at its interface with the third spring bar 74.

[0100] A preload spring 82 is disposed around the third piezoelectric actuator 76. The preload spring 82 is coupled to the bearing housing 42 and exerts a predetermined force on the third spring bar 74. As the preload spring 82 is wound around the third piezoelectric actuator 76, the predetermined force is exerted on the third spring bar 74 against the third direction. The predetermined force exerted by the preload spring 82 is set so as to compress the piezoelectric actuator 76 to a neutral point, wherein an application of voltage will, depending on polarity, either cause an extension towards its original length or retraction towards its minimum acceptable length. This ensures that the piezoelectric actuator 76 normally remains in compression, which extends the life of the piezoelectric actuator 76.

[0101] A fourth spring bar 84 couples the bearing to the bearing housing 42. An end of the fourth spring bar 84 is connected to the outer race 46. An opposite end of the fourth spring bar 84 is connected to the bearing housing 42.

[0102] The inner race 44 is arranged to rotate relative to the outer race 46 by virtue of the ball bearings 48. As the inner race 44 is received over and attached to the rotatable shaft 50, and the outer race 46 is connected to the bearing housing 42 via the fourth spring bar 84, it can be seen that the rotatable shaft 50 is arranged to rotate in the bearing 43 relative to the bearing housing 42. The axis of rotation of the bearing 43 is into, or out of, the page in the orientation shown in FIG. 5.

[0103] A fourth piezoelectric actuator 86 is disposed between the bearing housing 42 and the fourth spring bar 84. The fourth piezoelectric actuator 86 is configured to extend in a fourth direction. In the embodiment shown in FIG. 5 the fourth direction is a leftward direction, i.e. anti-parallel to the third direction. Extension of the fourth piezoelectric actuator 86 causes displacement of the fourth spring bar 84 relative to the bearing housing 42.

[0104] The fourth piezoelectric actuator 86 comprises an inline force sensor 88 at its interface with the bearing housing 42. The fourth piezoelectric actuator 86 is coupled to the mounting ring 58 at its interface with the fourth spring bar 84.

[0105] A preload spring 92 is disposed around the fourth piezoelectric actuator 86. The preload spring 92 is coupled to the bearing housing 42 and exerts a predetermined force on the fourth spring bar 84. As the preload spring 92 is wound around the fourth piezoelectric actuator 86, the predetermined force is exerted against the fourth spring bar 84 in the third direction. The predetermined force exerted by the preload spring 92 is set so as to compress the piezoelectric actuator 86 to a neutral point, wherein an application of voltage will, depending on polarity, either cause an extension towards its original length or retraction towards its minimum acceptable length. This ensures that the piezoelectric actuator 86 normally remains in compression, which extends the life of the piezoelectric actuator 86.

[0106] As can be seen in FIG. 5, the first, second, third and fourth spring bars 52, 64, 74, 84 are evenly spaced around the circumference of the bearing and evenly spaced around the interior of the bearing housing 42 when viewed along an axial direction of the rotatable shaft 50. This is an especially useful configuration as the first, second, third and fourth piezoelectric actuators are arranged in two opposing pairs. The first and second piezoelectric actuators are arranged to extend in opposite directions to move the bearing up and down within the bearing housing 42. The third and fourth piezoelectric actuators are arranged to extend in opposite directions to move the bearing left and right within the bearing housing 42. By providing opposing pairs of actuators, each piezoelectric actuator needs to apply a substantial force only when it extends. It does not also need to apply a substantial tension force as it retracts, as this can be provided by the other actuator in the actuator pair. This force can be at least partially transmitted over the mounting ring 58. The mounting ring 58 transfers forces between the piezoelectric actuators and the bearing along the respective extension/retraction axes of each piezoelectric actuator.

[0107] In the configuration shown in FIG. 5, the bearing 43, and hence the rotatable shaft 50 can be moved in two dimensions in the Z-Z plane, i.e. an orthogonal plane to the axis of rotation of the bearing. If the centre of mass of the rotatable shaft 50 does not initially align with the axis of rotation of the rotatable shaft then the bearing can be moved in the Z-Z plane until the centre of mass of the rotatable shaft 50 does align with the axis of rotation of the bearing. This results in reduced vibrations arising from the rotation of the rotatable shaft.

[0108] As described above with reference to FIG. 4, an accelerometer 100 is coupled to the bearing housing 42. The accelerometer is configured to produce an output which is proportional to the magnitude of vibrations of the bearing housing 42. The accelerometer 100 is communicatively coupled to a controller 102.

[0109] The controller 102 is coupled to the first piezoelectric actuator 54, the second piezoelectric actuator 66, the third piezoelectric actuator 76 and the fourth piezoelectric actuator 86, and is configured to be able to operate each of the actuators to cause them to extend or retract along their respective directions. The controller 102 comprises a memory storing logic, wherein said logic includes a feedback loop which checks the output of the accelerometer 100, extends or retracts one or more of the actuators, and then re-checks the output of the accelerometer 100. Using this loop the controller 102 acts to minimize the output of the accelerometer 100. The minimum of the output of the accelerometer corresponds to the centre of mass of the rotatable shaft 50 being positioned at the axis of rotation of the bearing.

[0110] The bearing assembly described with reference to FIGS. 4 and 5 has particular effectiveness where the rotatable shaft 50 comprises a high pressure rotor of a gas turbine engine, such as the gas turbine engine described with reference to FIGS. 1-3.

[0111] The bearing assembly may make use of resonant vibration modes occurring within the operational range of the engine. When the engine operates within a resonant vibration mode the stiffness of the spring bar is reduced, which, in turn, reduces the force required to displace the spring bar. Consequently, a smaller piezoelectric actuator can be used. The use of a smaller piezoelectric actuator reduces the packaging size of the actuator, as well as the weight of the actuator.

[0112] The bearing housing described above may be a housing of a gas turbine engine, such as an intermediate casing. While the housing 42 is shown as a single housing that supports the spring bars, the actuators and the squeeze films, in practice each of these components could be supported by a separate housing, or some subsets of the components could be supported by common housings.

[0113] While the above example includes four piezoelectric actuators, any number of piezoelectric actuators could be used.

[0114] Additionally, while in the example described above each actuator acts on a respective discrete spring bar, in some embodiments a unitary squirrel cage component could be provided between the bearing and the bearing housing. In this case, the squirrel cage effectively comprises a number of spring bars extending between annular end portions that join the ends of the spring bars together. Each actuator may act on a spring bar of the squirrel cage. The cage could comprise perforations to increase its flexibility.

[0115] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.