Abstract:
A bearing arrangement includes: first and second thrust bearings, arranged on a shaft and including respective first and second pressure faces; and a hydraulic connection, connecting the first and second thrust bearings and having a non-compressible fluid. Applying a shaft thrust load axially moves the first thrust bearing so that the first pressure face displaces the non-compressible fluid from the first thrust bearing to the second thrust bearing so as to apply a reaction force to the second pressure face, in order that the thrust load is shared between the first and second thrust bearings.

Description:
BACKGROUND 
     The present invention relates to a bearing arrangement, in particular a load sharing bearing arrangement. 
     Rotating shafts, such as those used in gas turbine engines, typically require bearings to support relatively moving or rotating components. Where light weight and minimum power loss from friction are required, roller element bearings are common and may be used to react both radial and thrust loads. 
     SUMMARY 
     Since single bearings inevitably have a limited thrust capability, two or more bearings may be arranged adjacent one another to share the thrust load. In these so-called “stacked” bearing arrangements, small variations (of the order of a few microns in some cases) in the geometry of the sets of rolling elements or bearing races between the bearings can lead to one bearing taking more of the load than the other(s). Furthermore, under-loading of one set of rolling elements may result in “skidding” of that set, which may cause damage, debris release and bearing failure. For these reasons, the geometry of the rolling elements and bearing races of the different bearings needs to be carefully controlled and matched so that the load may be shared (ideally equally) between the bearings. This requirement to precision-engineer and match bearings in pairs (or other multiples) incurs costs in the manufacturing and supply chain. 
     Furthermore, the materials of the rolling elements and/or bearing races may expand in use due to heating, which can exacerbate further the geometrical variations and lead to a “runaway” effect in which one bearing takes progressively more of the load, potentially resulting in bearing failure. This may occur even if matched bearings are selected and installed because even very small geometrical variations between the bearings may be magnified under the severe environmental operating conditions in gas turbine engines. 
     In addition, it is difficult or even impossible for the engine operator to determine the loads imposed on the bearings in operation, leading to uncertainty with regard to service life and maintenance schedules. 
     It is an object of the invention to alleviate the problems of the prior art at least to some extent. 
     The invention is set out in the accompanying claims. 
     According to an aspect, there is provided a bearing arrangement, comprising: first and second thrust bearings, arranged on a shaft and including respective first and second pressure faces; and a hydraulic connection, connecting the first and second thrust bearings and comprising a non-compressible fluid; wherein applying a shaft thrust load axially moves the first thrust bearing so that the first pressure face displaces the non-compressible fluid from the first thrust bearing to the second thrust bearing so as to apply a reaction force to the second pressure face, in order that the thrust load is shared between the first and second thrust bearings. 
     The hydraulic connection enables the axial thrust load to be shared between the first and second thrust bearings, such that geometrical variations and/or differential expansions between the thrust bearings may be tolerated. Hence, the requirement for careful control and matching of bearings is eliminated, or at least relaxed. 
     The non-compressible fluid may comprise a gel, a grease, or a liquid, for example an oil. 
     The hydraulic connection may comprise first and second cavity spaces between the respective first and second thrust bearings and a surrounding component, the first and second cavity spaces being coincident with the respective first and second pressure faces and connected to one another, optionally by a passage which extends through outer races of the first and second thrust bearings. 
     The first and second cavity spaces may be bounded by resilient elements of the surrounding component, the resilient elements being arranged to allow axial displacement of the thrust bearings in order to share the thrust load between the thrust bearings in the event of a failure of the hydraulic connection. 
     The first and second pressure faces may have substantially the same surface area such that the thrust load is substantially equally shared between the first and second thrust bearings. Or, the first and second pressure faces may have substantially different surface area such that the thrust load is unequally shared between the first and second thrust bearings. 
     The bearing arrangement may include a sensor arranged to detect the pressure of the non-compressible fluid. 
     The first and second thrust bearings may be axially spaced on the shaft. A spacer element may be disposed between the first and second thrust bearings to define the axial distance there between. 
     The bearing arrangement may comprise three or more hydraulically-connected thrust bearings. The thrust bearings may be ball bearings, tapered cylindrical roller bearings, hydrostatic bearings, or hydrodynamic bearings. 
     According to another aspect, there is provided a gas turbine engine, comprising a bearing arrangement as described herein above. 
     According to another aspect, there is provided a bearing arrangement, comprising: first and second thrust bearings, disposed on a shaft and including respective first and second pressure faces; and a hydraulic connection, connecting the first and second thrust bearings and comprising a non-compressible fluid; wherein applying a shaft thrust load axially displaces the first thrust bearing so that the first pressure face exerts a force on the non-compressible fluid, and the said force causes the non-compressible fluid to be displaced between the first and second thrust bearings to exert a reaction force on the second pressure face, in order that the thrust load is shared between the first and second thrust bearings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described by way of example, with reference to the accompanying figures in which: 
         FIGS. 1 and 2  are schematic illustrations of a bearing arrangement according to the invention, in an unloaded condition; 
         FIG. 3  shows the bearing arrangement of  FIG. 1  in a part-loaded condition; and 
         FIG. 4  shows the bearing arrangement of  FIG. 1  in a loaded condition. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a half-section of a portion of a bearing housing  101  of a gas turbine engine. In this embodiment, the engine is a three-shaft gas turbine engine of an aircraft. A shaft  201  of the engine extends through the generally-annular bearing housing  101  and is supported by a location bearing arrangement  301 . In this embodiment, the shaft connects a high pressure turbine and a high pressure compressor (not shown) of the engine. 
     In this embodiment, the location bearing arrangement  301  comprises first and second bearings  401 ,  501 , each of which includes an inner race  401   a ,  501   a  and an outer race  401   b ,  501   b , between which a plurality of rolling elements  401   c ,  501   c  is disposed in a cage (not shown). In this embodiment, the rolling elements  401   c ,  501   c  are balls. In this embodiment, each of the first and second bearings  401 ,  501  is a single-row bearing of the deep-groove type. In this embodiment, the inner races  401   a ,  501   a  and the outer races  401   b ,  501   b  comprise M50NiL steel and the rolling elements  401   c ,  501   c  comprise M50 steel. 
     Referring now to  FIG. 2 , in this embodiment, the inner races  401   a ,  501   a  of the first and second bearings  401 ,  501  are mounted on the shaft  201  and are separated by an axial gap  601  which has an axial dimension of 5 mm. In this embodiment, a spacer  603  is disposed in the axial gap  601 , as will be discussed further herein below. In this embodiment, the inner races  401   a ,  501   a  are in fixed axial relationship with one another and with the shaft  201 . In this embodiment, each of the outer races  401   b ,  501   b  comprises an axial end surface, or pressure face  401   d ,  501   d.    
     The outer races  401   b ,  501   b  of the first and second bearings  401 ,  501  are free to slide axially within the bearing housing  101 . In this embodiment, seals  701  are disposed between each of the outer races  401   b ,  501   b  and the bearing housing  101 . In this embodiment, the first and second bearings  401 ,  501  include stops  401   e ,  501   e  for limiting their axial travel relative to the bearing housing  101 . 
     A cavity is provided between the first and second bearings  401 ,  501  and the inner surface of the bearing housing  101 . In this embodiment, the cavity comprises a first cavity space, or chamber  801   a  adjacent (coincident with) the pressure surface  401   d  of the first bearing  401  and bounded by a first support element  101   a  of the bearing housing  101 . The cavity further comprises a second cavity space, or chamber  801   b  adjacent (coincident with) the pressure surface  501   d  of the second bearing  501  and bounded by a second support element  101   b  of the bearing housing  101 . In this embodiment, the first and second support elements  101   a ,  101   b  are generally cone-shaped. In this embodiment, the first and second chambers  801   a ,  801   b  are connected by a conduit, or passage  801   c . In this embodiment, the passage  801   c  extends through the outer races  401   b ,  501   b  of the first and second bearings  401 ,  501 , and also through an axially-movable communication duct  801   d  which connects the first and second chambers  801   a ,  801   b.    
     The cavity contains a substantially non-compressible fluid, in this embodiment a liquid, in particular an oil  801   e . The seals  701  prevent the oil  801   e  from escaping from the cavity. Thus the fluid-containing cavity comprises a reservoir which hydraulically connects the first and second bearings  401 ,  501 , and in particular provides a hydraulic path between the respective first and second pressure faces  401   d ,  501   d  thereof. 
     In this embodiment, a portion of a pressure sensor  901  is disposed in the passage  801   c  such that the pressure of the oil  801   e  therein may be detected. 
     In each of the first and second bearings  401 ,  501  there exists a clearance gap  401   f ,  501   f  between the rolling elements  401   c ,  501   c , and the inner race  401   a ,  501   a  and the outer race  401   b ,  501   b . In an unloaded condition (as shown in  FIGS. 1 and 2 ) the clearance gaps  401   f ,  501   f  provide free “play” or axial in the first and second bearings  401 ,  501 . In this embodiment, the clearance gap  401   f  in the first bearing  401  has a maximum size of 1.00 mm and the clearance gap  501   f  in the second bearing  501  has a maximum size of 1.01 mm. That is, there is a difference of 10 microns between the clearance gaps  401   f ,  501   f , which is caused by geometrical variations (manufacturing tolerances) in the first and second bearings  401 ,  501 . It will be understood that in the drawings of the Figures the size of the clearance gaps  401   f ,  501   f  has been exaggerated for the sake of clarity, particularly in the radial direction. 
     The operation of the location bearing arrangement  301  will now be described, at first with particular reference to  FIG. 3 . The shaft  201  is being driven by the high pressure turbine to rotate about its longitudinal axis. In addition, the shaft  201  is moving axially (from right to left as indicated by the arrow), relative to the static bearing housing  101 , under a net aerodynamic force between the high pressure turbine and the high pressure compressor. As the shaft  201  slides axially the inner races  401   a ,  501   a  of the first and second bearings  401 ,  501  (which in this embodiment are mounted to the shaft  201  and are in fixed axial relationship there with) come into contact with the respective rolling elements  401   c ,  501   c  at respective contact points  401   g ,  501   g . As the shaft  201  continues to slide, in the first bearing  401  the rolling elements  401   c  come into contact with the outer race  401   b  at contact points  401   h . Thus, an axial thrust load is exerted on, and transmitted through, the first bearing  401  by the shaft  201 . Due to the aforementioned difference in the size of the clearance gaps  401   f ,  501   f  in the first and second bearings  401 ,  501 , the clearance gap  501   f  of the second bearing  501  is not yet closed but is reduced in size (to 10 microns). 
     Referring now to  FIG. 4 , at a certain magnitude the force exerted at the contact points  401   g , by the rolling elements  401   c  on the outer race  401   b  of the first bearing  401 , is sufficient to overcome the friction resistance of the seals  701 , and the outer race  401   b  of the first thrust bearing  401  is axially displaced along with the moving shaft  201  (leftwards as shown in  FIG. 4 ). 
     At the same time, the (10 micron) clearance gap  501   f  in the second bearing  501  is closed as the rolling elements  501   c  of the second bearing  501  are brought into contact with the respective outer race  501   b  at contact points  501   h . Thus, the axial movement of the first bearing  401  enables the second bearing  501  to take up a share of the axial thrust load imposed by the shaft  201 . In this condition, the second bearing  501  may be sufficiently loaded that skidding of its rolling elements  501   c  may be prevented. 
     Still referring to  FIG. 4 , as the shaft  201  continues to slide (leftwards), the axial thrust load is transmitted to the oil  801   e  in the first chamber  801   a  via the pressure face  401   d  of the outer race  401   b  of the first bearing  401 . Since the oil  801   e  is substantially non-compressible, some portion of the oil  801   e  is displaced, from the first chamber  801   a  into the second chamber  801   b , via the passage  801   c  (from left to right in  FIG. 4  as indicated by the arrows). That is, at least some of the oil  801   e  is transferred in the axial direction from the first bearing  401  to the second bearing  501 . The displaced oil  801   e  exerts a reaction force, on the pressure face  501   d  of the outer race  501   b  of the second bearing  501 , to increase the load on the second bearing  501 . In this way, the axial thrust load is substantially equally shared between the first and second bearings  401 ,  501 . 
     It will be understood that the bearing arrangement  301  will behave in the same way if the axial thrust load is applied in the opposite direction to that described herein above, i.e. from left to right in  FIGS. 3 and 4 . 
     The ability of the hydraulic connection to distribute the axial thrust load between the first and second bearings  401 ,  501  means that relatively large geometrical variations and/or differential expansions between the bearings  401 ,  501  may be tolerated. Furthermore, the provision of a non-compressible fluid means that the loads imposed on the first and second bearings  401 ,  501  may be conveniently and reliably derived from pressure measurements taken by the sensor  901 . Knowledge of these loads can be exploited by the engine operator to better predict the service life of the bearings  401 ,  501 , which knowledge can in turn be fed back to the design process to improve the design of the bearings  401 ,  501 . 
     In the embodiment described herein above, the spacer  603 , which is disposed in the axial gap  601  between the inner races  401   a ,  501   a  of the first and second bearings  401 ,  501 , is fitted during assembly of the bearing arrangement  301 , after installation on the shaft  201  of the inner race  401   a  of the first bearing  401  and prior to installation on the shaft  201  of the inner race  501   a  of the second bearing  501 . The thickness T of the spacer  603  is selected to be sufficient to bring the rolling elements  401   c ,  501   c  of each of the first and second bearings  401 ,  501  into engagement with the respective inner races  401   a ,  501   a  at contact points  401   g ,  501   g  and the respective outer races  401   b ,  501   b  at contact points  401   h ,  501   h . That is, the spacer  603  ensures the closure (or at least part-closure) of the clearance gaps  401   f ,  501   f  to take up the free play or axial float in the first and second bearings  401 ,  501 , prior to the application of a significant thrust load from the shaft  201  during engine operation. In this way, the likelihood of skidding of unloaded (or lightly loaded) bearings  401 ,  501  may be reduced. Also, the provision of the spacer  603  may ensure that some load is maintained on the bearings  401 ,  501  in the event of a hydraulic failure. It will be understood that while the spacer  603  may optionally be employed to take up the free play or axial float in the first and second bearings  401 ,  501 , the spacer  603  is not necessary for the hydraulic load share function of the bearing arrangement  301  as described herein above. 
     In an embodiment, each of the support elements  101   a ,  101   b  of the bearing housing  101 , which bound the respective first and second chambers  801   a ,  801   b  of the cavity, comprises a flexible or resilient element, for example a diaphragm, which is arranged to be displaced or deformed under the axial thrust load imposed by the shaft  201  so that the bearings  401 ,  501  may be axially displaced. In this way, the bearings  401 ,  501  may remain loaded, and the load shared between the bearings  401 ,  501 , even in the event of a loss of hydraulic fluid. 
     In an embodiment, the inner races  401   a ,  501   a  are disposed on the shaft  201  such that the first and second bearings  401 ,  501  are far apart. There is no particular limit to the axial spacing between the first and second thrust bearings  401 ,  501 , other than with regard to practical tolerances and thermal expansion which will increase with distance. For example, in some embodiments the axial gap  601  has an axial dimension of anything up to about 25 mm, while in other embodiments the axial dimension exceeds 25 mm. The bearings  401 ,  501  could be separated such that they are in different respective bearing housings, so long as the hydraulic connection is provided between the bearings  401 ,  501 . 
     In an embodiment, the inner races are made integral with the shaft, for example machined on the shaft  201 . Furthermore, it will be understood that it is not essential to the load sharing function of the bearing arrangement  301  that an axial gap  603  is provided between the inner races  401   a ,  501   a  of the first and second bearings  401 ,  501 . 
     While the bearing arrangement  301  described herein above comprises ball-type roller bearings, it will be understood that the invention is generally applicable to all types of bearings which take a thrust load. For example, roller bearings comprising inclined or tapered cylindrical rolling elements, and hydrostatic or hydrodynamic thrust bearings. Furthermore, it will be understood that the invention is not limited to embodiments comprising two thrust bearings but may comprise any number of thrust bearings, for example three or four thrust bearings. 
     In an embodiment, the pressure faces of the first and second bearings are of dissimilar surface area such that they transmit different magnitudes of force, thereby providing unequal load share between the first and second bearings. For example, the pressure faces may be differently sized in order to achieve a load share of 60:40. 
     While the above-described embodiment comprises a reservoir, in which a passage extends through bearing outer races to connect two chambers, it will be understood that the bearings could be hydraulically connected in various different ways, so long as a non-compressible fluid may be displaced by a load which is exerted on the fluid by the first bearing, and the fluid may exert a reaction force on the second bearing, in order that the load is shared between the first and second bearings. Furthermore, the non-compressible fluid need not be in direct contact with the respective pressure faces of the first and second bearings in order for the load to be shared between the bearings. For instance, the applied load may be transmitted to, and the reaction force may be transmitted from, the non-compressible fluid via some intermediary element, for example a spacer. 
     In an embodiment, the non-compressible fluid is a gel or a grease, for example a high-temperature grease. 
     It will be understood that the invention has been described in relation to its preferred embodiments and may be modified in many different ways without departing from the scope of the invention as defined by the accompanying claims. 
     Although a bearing arrangement in a three-shaft gas turbine engine has been described, the invention is equally applicable to a two-shaft or a single-shaft gas turbine engine. As will be apparent to the skilled reader, the invention is appropriate for gas turbine engines used for other purposes than to power an aircraft, for example industrial gas turbine engines or marine gas turbine engines. Furthermore, the invention is not only relevant to gas turbine engines but has wider utility. The invention is suitable for application in any rotor, for example of a type used in oil and gas drilling, where an axial load is close to or beyond the capabilities of a single bearing.