Abstract:
The present invention relates to a bearing assembly ( 1 ) for a rotatable shaft ( 2 ). The bearing assembly comprises a bearing housing ( 10 ); a bearing ( 20 ) located within the bearing housing ( 10 ) and arranged in use to receive a rotatable shaft ( 2 ); and a damper ( 14 ) that couples the bearing ( 20 ) to the bearing housing ( 10 ) for damping vibrations of the rotatable shaft ( 2 ). The damper ( 14 ) comprises a shape memory alloy ( 40 ). This arrangement allows the bearing assembly to damp vibrations of the rotatable shaft.

Description:
BACKGROUND 
     The present invention relates to a bearing assembly, in particular, a bearing assembly comprising a damper. 
     In gas turbine engines it is necessary to control the vibrations of the rotor in order to reduce the vibrations transmitted to the housing. This is especially important in the case of gas turbine engines for jet engines where it is necessary to reduce the vibrations transmitted to the aircraft structure. 
     With reference to  FIG. 1 , in one previously considered arrangement  100  the vibrations of the rotor shaft  108  are controlled by providing a squeeze film damper (SFD)  106  in between the bearing  102  and the bearing housing  104 . The squeeze film is a small oil-filled clearance between the bearing  102  and the bearing housing. The oil film damps the radial motion of the rotating assembly and the dynamic loads transmitted to the bearing housing, thereby reducing the vibration level of the engine. Squeeze film dampers are very effective in reducing the moderate vibration that is still present after the rotor has been balanced. 
     SUMMARY 
     However, squeeze film dampers are passive because the damping force that they exert remains constant. This means that they are designed as a compromise for a range of conditions. The performance of squeeze film dampers can also be affected by the temperature of the oil, the pressure of the oil, the age and condition of the oil, and the thermal expansion of the bearing and bearing housing. All of these parameters vary during operation of the gas turbine engine which result in squeeze film dampers not controlling the vibration of the rotor sufficiently under all conditions. 
     According to one aspect of the present invention there is provided a bearing assembly for a rotatable shaft, comprising: a bearing housing; a bearing located within the bearing housing and arranged in use to receive a rotatable shaft; and a damper that couples the bearing to the bearing housing for damping vibrations of the rotatable shaft; wherein the damper comprises a shape memory alloy (SMA). 
     Preferably the damper comprises an elongate member having a portion of shape memory alloy attached thereto. In one embodiment, the shape memory alloy is attached to the elongate member at first and second opposing ends. 
     Preferably the bearing assembly comprises a mounting portion that couples the damper to the bearing housing. 
     The bearing may be a ball bearing or a roller bearing or a needle bearing. 
     In a preferred embodiment there are a plurality of dampers that each extend at least partially in the axial direction of the rotatable shaft which the bearing is arranged to receive. Each of the dampers may have a generally square cross section and the sides of the dampers may be parallel to one another. The plurality of dampers may be circumferentially arranged around the axis of the rotatable shaft which the bearing is arranged to receive. 
     There may be provided temperature control means for controlling the temperature of the shape memory alloy of the or each damper. The temperature control means may be arranged to maintain the temperature of the shape memory alloy at just above its transition temperature such that the shape memory alloy behaves as a superelastic material. 
     According to a further aspect of the present invention there is provided a method of controlling the stiffness of a damper of a bearing assembly, the bearing assembly comprising: a bearing housing; a bearing located within the bearing housing and arranged in use to receive a rotatable shaft; and a damper that couples the bearing to the bearing housing for damping vibrations of the rotatable shaft; wherein the damper comprises a shape memory alloy; and wherein the method comprises controlling the temperature of the shape memory alloy. In a preferred embodiment the temperature of the shape memory alloy is maintained at just above its transition temperature such the shape memory alloy behaves as a superelastic material. 
     According to another aspect of the present invention there is provided a gas turbine engine for a jet-engine comprising a bearing assembly according to any statement herein. 
     The invention may comprise any combination of the features and/or limitations referred to herein, except combinations of such features as are mutually exclusive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  schematically shows a previously considered bearing and damper arrangement; 
         FIG. 2  schematically shows a perspective view of a cage bearing according to the present invention; 
         FIG. 3  schematically shows a side view of a cage bearing according to the present invention; 
         FIG. 4  schematically shows a front view of the bearing of  FIGS. 2 and 3 ; 
         FIG. 5  schematically shows the cross-section A-A of  FIG. 4 ; 
         FIG. 6  schematically shows the cross-section B-B of  FIG. 3 ; 
         FIGS. 7A and 7B  schematically show an enlarged view of an individual damper; 
         FIG. 8  schematically shows the loading-unloading stress-strain curve for a shape memory alloy at a temperature above the transition temperature; 
         FIG. 9  schematically shows a damper including a means for controlling the temperature; 
         FIG. 10  schematically shows the shape of a deflected damper; and 
         FIG. 11  schematically shows a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIGS. 2 and 3  a bearing assembly  1  for a rotatable shaft  2  according to the present invention comprises a bearing housing  10  having a bearing  20  located within it. The bearing  20  is attached to the bearing housing  10  by an annular flange  12 , acting as a mounting portion. The annular flange  12  is connected to the bearing  20  by a plurality of dampers  14  in the form of webs. 
       FIGS. 4 and 5  show the bearing  20  alone. In this embodiment the bearing  20  is a ball bearing comprising an outer race  22  and an inner race  24 . The outer race  22  comprises an annular groove  23  on the inner surface and the inner race  24  comprises an annular groove  25  on the outer surface. A ring of balls  26  is located between the outer race  22  and the inner race  24  and sits within the respective grooves  23 ,  25 . The inner race  24  can therefore rotate with respect to the outer race  22 . The inner race  24  comprises an axial opening  28  within which the rotatable shaft  2  can be located. This allows the rotatable shaft  2  to rotate freely with respect to the outer race  22 . The webs  14  that attach the flange  14  to the bearing are attached to the outer race  22  and therefore the rotatable shaft  2  can rotate freely with respect to the flange  12 . As will be readily apparent to one skilled in the art, other types of bearings such as roller bearings may be used. 
     As shown in  FIG. 6 , the flange  12  comprises a plurality of mounting holes  13  for attaching the bearing  20  to the bearing housing  10 . The dampers  14  that attach the flange  12  to the bearing  20  are hollow and have a generally square cross-section. Each damper  14  is welded at a first end to the flange  12  and at a second end to the bearing  20 . The dampers  14  extend in the axial direction  3  of the rotatable shaft  2  which in use the bearing  20  supports. The dampers  14  are evenly spaced around the circumference of the flange  12  and the outer race  22  and are orientated such that their sides are parallel. 
       FIG. 7A  shows an enlarged view of a damper  14  that connects the flange  12  to the outer race  22  of the bearing  20 . Each damper  14  comprises an elongate web  15  and each end of the elongate web  15  comprises a portion of a shape memory alloy  40  (hereinafter referred to as SMA) on the outer surface. The edges of the SMA portions  40  are chamfered in order to avoid stress concentrations at their edges. 
     As shown in  FIG. 7B , a portion of SMA  40  is bonded to each of the sides of the web  15 . The SMA used may be Nitinol, for example. However, other suitable SMAs may be used. In this embodiment, each damper  14  comprises a portion of SMA  40  bonded to each side of each end of the web  15 . However, other arrangements may be possible such as manufacturing the webs  15  out of a SMA or applying portions of SMA to alternate webs  15 , for example. 
     A SMA is an alloy that can be easily deformed when cold and then when heated attempts to regain a previously defined shape. This effect is due to a solid-state phase transformation which causes the micro-structure of the SMA to change. The lower temperature phase is commonly known as the martensitic phase and the higher temperature phase is commonly known as the austenitic phase. The martensitic phase of the SMA has a modulus of elasticity that is lower than the austenitic phase. 
     In the martensitic phase the SMA can be easily deformed and when the SMA is heated, the micro-structure of the SMA changes to the austenitic phase and the SMA returns to a “memory” shape. When the SMA is cooled again, the martensitic phase is once again formed. The temperature-change over which the SMA solid-state phase-change occurs is known as the transition temperature range. This can be as low as 2° C. The phase-change can also be induced by stress. 
     SMAS can exhibit pseudoelastic (or superelastic) properties. If the SMA is held just above the transition temperature then when stress is applied, the austenitic phase transforms to the martensitic phase. As shown in  FIG. 8 , since the martensitic phase has a lower elastic modulus, a substantially flat stress-strain curve (A) results with very large strains of up to 18%. When the stress is removed from the SMA then the martensitic phase reverts back to the austenitic phase (B). Although there is a full strain recovery, the loading-unloading cycle is in the form of a hysteresis curve. This hysteresis curve is much larger than for most conventional materials. The area enclosed by the hysteresis loop of the SMA characterises the energy dissipation of the material. 
     In embodiments of the present invention, the temperature of the portions of SMA  40  that are bonded to the webs  15  of the bearing assembly  1  is controlled. 
     In this embodiment, the temperature of the portions of SMA  40  that are bonded to the webs  15  is maintained at a temperature that is just above the lower end of their transition temperature range. This may be done by feeding temperature controlled air, or other fluid, towards the SMA  40 . As will be readily apparent to one skilled in the art, other means of controlling the temperature may be used such as providing heating elements in the region of the SMA  40  or, as shown in  FIG. 9 , using heating coils  50  wrapped around the SMA  40 . Further, the temperature may not necessarily be specifically controlled. The ambient temperature of the fluid surrounding the SMA portions  40  may be sufficient to maintain the SMA  40  at just above the transition temperature. 
     In use, and as shown in  FIG. 3 , a rotatable shaft  2  is located within the opening  28  of the bearing  20 . The vibrations of the rotatable shaft  2  are damped by the bearing assembly  1 . When the rotatable shaft  2  vibrates, the dampers  15 , comprising the webs  14  and the SMA  40 , deflect.  FIG. 10  shows an enlarged, exaggerated view of a deflected damper  14 . The axial centre of the damper  14  undergoes little, if any bending, and this is why portions of SMA  40  are only applied to the ends of the web  15 . 
     When the dampers  14  are caused to bend due to the vibration of the rotatable shaft, a bending stress is induced in the SMA portions  40 . Since the SMA  40  is held at a temperature that is just above the transition temperature, it behaves as a superelastic material, as shown in  FIG. 8 . This means that the portions of SMA  40  act to dissipate the energy of the vibration. The hysteresis of the loading-unloading cycle of the SMA means that the energy dissipation of the SMA is particularly high when compared to conventional materials. The bearing assembly  1  therefore damps the vibrations of the rotatable shaft and reduces the vibrations transmitted to the rest of the structure. 
     In a further embodiment, as opposed to holding the SMA  40  at just above the transition temperature so that it behaves as a superelastic material, the temperature of the SMA portions  40  is controlled so that the SMA  40  can be changed from a martensitic phase to an austenitic phase. As described above, the martensitic phase has a modulus of elasticity that is lower than the modulus of elasticity of the austenitic phase. This allows the overall stiffness of the dampers  14  to be altered. 
     For example, in certain situations, such as when the rotatable shaft  2  is vibrating violently, it may be desirable for the stiffness of the dampers  14  to be low in order to allow other mechanisms (not shown) to steady and lock the bearing position. This can be done by reducing the temperature of the SMA portions  40  to below the transition temperature, thus causing the SMA to be in the martensitic phase. When the vibration of the rotatable shaft  2  has reduced, then it may be desirable to increase the stiffness of the dampers  14 . This can be done by increasing the temperature of the SMA portions  14  to above the transition temperature, thus causing the SMA to be in the austenitic phase. If the transition temperature range is narrow, for example 2° C., then the phase change can occur quickly and with ease. The temperature may be controlled by directing air of a suitable temperature into the vicinity of the SMA portions  40 , for example. 
     If the SMA temperature control mechanism fails, then some damping will still be provided. 
     In alternative embodiments of the invention, the dampers  14  may have a different cross-sectional shape (for example, circular, oval or hexagonal). They may be solid, though hollow webs are likely to provide the most weight efficient solution. The dampers  14  are described as being welded to the flange  12 , but may of course be joined by any suitable means. 
     If there is a natural bias in the forces on the bearings, then this may be compensated for by spacing the dampers unevenly around the circumference of the bearing. Alternatively or additionally, the dampers in different positions may have different properties (for example, diameter, thickness, strength or stiffness) to compensate for this natural bias. This ensures that during running, the offset loads will be evenly damped. 
     A second embodiment of the present invention is shown in  FIG. 11 . In this embodiment the ball bearing  20  is coupled to the bearing housing  10  using a damper assembly  30 . The damper assembly  30  comprises a bearing abutment portion  32  and a spring portion  34 . The bearing abutment portion  32  is attached to the outer race  22  of the bearing  20  and the spring portion  34  is attached to the bearing housing  10 . A portion of SMA  40 , such as Nitinol, is bonded to the surface of the spring portion  34 . The temperature of the portion of SMA  40  is maintained at a temperature just above the transition temperature of the material. This means that the portion of SMA  40  behaves as a superelastic material, as shown in  FIG. 8 . 
     When the rotatable shaft  2  vibrates the spring portion  34  of the spring assembly  30  deflects and this causes a stress to be induced in the portion of SMA  40 : Since the portion of SMA  40  acts as a superelastic material it dissipates some of the energy of the vibration and therefore damps the vibration of the rotatable shaft  2 . As will be readily apparent to one skilled in the art, the SMA  40  could be changed from the martensitic phase to the austenitic phase in order to change the stiffness of the damper. 
     The damper assembly  30  may be a continuous annular assembly extending around the entire circumference of the bearing  20 , or it may comprise a plurality of discrete dampers spaced around the circumference. In the latter case, the dampers would generally be symmetrically spaced around the bearing  20 . 
     Although the embodiments described herein have referred to a ball bearing, it will be appreciated that the invention is equally applicable to other types of bearing, for example roller bearings or needle bearings.