Abstract:
A damping treatment is applied to the ligaments of a squirrel cage type bearing support. The damping treatment reduces the vibrations transmitted to the engine casing as the engine passes through various resonance frequencies of the various parts of the gas turbine engine. A squeeze-film damper damps engine vibrations, but the oil in the squeeze-film damper can act as a stiffener during certain operational ranges, which increases the transmission of vibrations to the engine structure. Applying a damping treatment to the ligaments of the squirrel cage provides effective vibration damping.

Description:
BACKGROUND 
       [0001]    The present invention relates to gas turbine engines. More particularly, the present invention relates to vibration damping for a bearing support assembly for a gas turbine engine. 
         [0002]    Squeeze film damping can be used in a bearing support assembly that supports a rotatable shaft of a gas turbine engine. The bearing is contained within a stationary housing. A bearing centering spring, also known as a squirrel cage, supports the bearing. The squirrel cage is connected to the stationary housing such that the squirrel cage does not rotate relative to the stationary housing, yet the squirrel cage is flexible enough to bend in response to loads experienced from the bearing. A film of liquid, such as oil, is supplied to an annulus disposed between the stationary housing and the squirrel cage. The film of oil reduces or damps engine vibration and the transmission of vibrations from the bearing to the engine structure. 
         [0003]    When the compressor or turbine shaft rotates, imbalance or rotor instabilities can cause excessive whirling of the shaft and vibration of the engine, engine mounts and airframe. As a speed of the shaft increases, the rotating shaft passes through a variety of critical speeds, which occur when the speed of the rotating shaft traverses a resonant frequency of one of the various structures of the engine. Due to the multitude of structures that make up any given engine, the rotating shaft passes through several critical speeds as the rotating shaft accelerates. As the rotating shaft rotates, the rotating shaft may be displaced from a centerline of the engine. The rotating shaft then whirls about a centerline of the engine. As the bearing moves with the rotation of the shaft, the squirrel cage vibrates within the stationary housing and squeezes the oil in the annulus. Squeezing the oil in the annulus creates a pressure wave that may comprise a damping force, when the oil shears at an angle orthogonal to the displacement of the shaft from the engine centerline, or a stiffness force, when the oil shears along the direction of displacement or opposite the direction of displacement. When the oil acts as a stiffener, the oil actually increases the transmission of vibrations to the engine and airframe structure. In such a case, the vibrations experienced by the bearing assembly must be damped in another manner. 
       SUMMARY 
       [0004]    According to an embodiment of the present disclosure, a squirrel cage includes a first end, a second end, a plurality of ligaments extending between and connecting the first end and the second end, and a damping layer disposed on at least one of the plurality of ligaments. 
         [0005]    According to another embodiment of the present disclosure, a damping assembly includes a bearing, a squirrel cage disposed radially outward of and supporting the bearing, and a housing disposed radially outward of the squirrel cage. The squirrel cage includes a first end, a second end supporting the bearing, a plurality of ligaments extending between and connecting the first end and the second end, and a damping layer disposed on at least one of the plurality of ligaments. 
         [0006]    According to yet another embodiment of the present disclosure, a method of damping vibration in a gas turbine engine includes damping engine vibrations with a damping layer disposed on at least one ligament of a squirrel cage, the squirrel cage supporting a bearing outer race; supplying oil to an annulus disposed between the squirrel cage and a housing; damping engine vibrations with the oil; and switching off an oil supply to the annulus when the engine speed is at least at idle speed of the engine, whereby the engine vibrations transmitted through the bearing outer race are damped exclusively by the squirrel cage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a cross-sectional view of a bearing support assembly. 
           [0008]      FIG. 2  is a cross-sectional view of a squeeze-film damper bearing assembly. 
           [0009]      FIG. 3  is a perspective view of a squirrel cage of the bearing support assembly in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  is a cross-sectional view of bearing support assembly  10 . Bearing support assembly  10  includes squirrel cage  12 , stationary housing  14 , bearing assembly  16 . Squirrel cage  12  includes first end  18 , second end  20 , and ligaments  22 . First end  18  includes mounting flange  24 . Second end  20  includes piston ring grooves  26   a  and  26   b , annulus  28 , and race housing  30 . Ligaments  22  include damping layer  32 . Bearing assembly  16  includes bearing element  34 , inner race  36 , and outer race  38 . Bearing support assembly  10  further includes piston rings  40   a  and  40   b  and oil supply passage  42 . 
         [0011]    Ligaments  22  extend between and connect first end  18  and second end  20 . Mounting flange  24  extends radially outward from ligaments  22 . Mounting flange  24  includes a plurality of fastener openings therethrough, the fastener openings are configured to receive fasteners for securing squirrel cage  12  to stationary housing  14 . Piston ring grooves  26   a  and  26   b  extend circumferentially about an outer surface of second end  20 . Annulus  28  similarly extends circumferentially about an outer surface of second end  20 , and annulus  28  is disposed between piston ring groove  26   a  and piston ring groove  26   b . Race housing  30  extends circumferentially about an inner surface of second end  20 . 
         [0012]    Squirrel cage  12  is supported by stationary housing  14 . A fastener, such as a bolt, extends through stationary housing  14  and mounting flange  24  to secure squirrel cage  12  to stationary housing  14 . During engine operation, squirrel cage  12  does not rotate about an axis of the engine. Ligaments  22  extend between first end  18  and second end  20  and provide flexible support to allow squirrel cage  12  to vibrate within stationary housing  14 . In this way, squirrel cage  12  is cantilevered and supports bearing assembly  16 . 
         [0013]    Race housing  30  supports outer race  38  of bearing assembly  16 . Bearing element  34  is rotatably disposed between inner race  36  and outer race  38 . Bearing assembly  16  can support a rotatable shaft (not shown), such as a compressor or turbine shaft of a gas turbine engine. Passage  42  supplies oil to annulus  28 . Bearing assembly  16  is generally a thrust bearing, which is a bearing that can support both a radial and axial load. While bearing assembly  16  is preferably a thrust bearing, bearing assembly  16  may be a roller bearing or any other suitable bearing for supporting the rotating shaft. In addition, while bearing assembly  16  is illustrated as including a spherical bearing element  34 , bearing element  34  could be a roller or any other suitable shape for supporting the load of the rotating shaft. 
         [0014]    Piston rings  40   a  and  40   b  are disposed on either side of annulus  28  in piston ring grooves  26   a  and  26   b  respectively. Passage  42  extends through stationary housing  14  and is fluidly connected to annulus  28  to supply oil to annulus  28 . Passage  42  includes a valve (not shown) for selectively controlling the supply of oil to annulus  28 . The oil in annulus  28  is held between squirrel cage  12 , piston rings  40   a  and  40   b , and stationary housing  14 . In this way, bearing support assembly  10  includes a squeeze-film damper, whereby the oil disposed in annulus  28  damps vibrations to reduce the rotor vibration and the transmission of vibrations from bearing assembly  16  to stationary housing  14 . As the shaft rotates, vibrations experienced by the shaft are transmitted to stationary housing  14  through bearing assembly  16 . Due to the motion of the shaft, squirrel cage  12  vibrates within stationary housing  14 . The motion of squirrel cage  12  squeezes the oil in annulus  28 , and the oil in annulus  28  reduces the vibrations experienced from the rotating shaft. 
         [0015]    As the rotating shaft accelerates, the rotating shaft passes through several critical speeds. A critical speed is experienced when a rotational frequency of the rotating shaft traverses a resonance frequency of one of the various structure of the engine. Due to the variety of structures that make up any given engine, the rotating shaft can pass through a multitude of critical speeds during acceleration. As the rotating shaft rotates, a centerline of the rotating shaft may be displaced from a centerline of the engine. When displaced, the rotating shaft whirls about the centerline of the engine, thereby squeezing the oil disposed within annulus  28 . Squeezing the oil generates a rotating pressure wave, which may comprise a stiffness force and a damping force. A shear angle of the oil determines the damping or stiffening properties exhibited by the oil. The shear angle is an angle of the rotating pressure wave to the direction of displacement between the centerline of the rotating shaft and the centerline of the engine. The shear angle is affected by parameters such as the annulus diameter, the annulus length, the clearance between the outer race and the housing, and the oil viscosity. To effectively reduce or damp engine vibrations, the oil disposed within annulus  28  preferably has a shear angle of about 90 degrees. The shear angle is at about 90 degrees when the rotating pressure wave is orthogonal to the direction of displacement between the centerline of the rotating shaft and the centerline of the engine. 
         [0016]    However, the squeeze-film damper can generate stiffness where the shear angle is closer to about 0 degrees or about 180 degrees. The shear angle is about 0 degrees or about 180 degrees where the rotating pressure force is aligned with the direction of displacement or opposed to the direction of displacement, respectively. When the shear angle is about 0 degrees or about 180 degrees, then the oil acts as a stiffener, which causes the squeeze-film damper to enhance, rather than damp, the vibrations transmitted from the rotatable shaft. To reduce transmission through the oil film, the valve on passage  42  can optionally be switched to an off position, whereby the supply of oil through passage  42  is ceased. Eliminating the supply of oil to annulus  28  prevents the squeeze-film damper from transmitting vibrations. However, the rotating shaft continues to generate vibrations that are transmitted through squirrel cage  12 . 
         [0017]    Damping layer  32  is disposed about at least one of the plurality of ligaments  22 . The rotating shaft rotates and ligaments  22  are strained as squirrel cage  12  vibrates within the bearing compartment. As ligaments  22  strain, damping layer  32  provides damping to squirrel cage  12 . Because the vibrations are not transmitted through the squeeze-film damper, due to the valve being switched to an off position to cease the supply of oil to annulus  28  through passage  42 , the vibrations are transmitted through squirrel cage  12 . As the vibrations pass through squirrel cage  12 , ligaments  22  are strained and damping layer  32  provides damping. 
         [0018]    Damping layer  32  preferably includes a viscoelastic material covering at least one ligament  22 . It is understood, however, that damping layer  32  may include viscoelastic material, such as a constrained layer viscoelastic material, a damping tape, a plasma spray coating, of any other damping material suitable for dissipating the vibrations through ligaments  22 . For example, the damping tape may optionally include a viscoelastic damping material having a metallic backing. Additionally, damping layer  32  may be applied to ligaments  22  in any suitable manner, such as by wrapping a damping tape around ligament  22 , spray coating damping layer  32  on ligaments  22 , masking one or both ends of squirrel cage  12  and dipping squirrel cage  12  in the damping material to coat ligaments  22  in damping layer  32 , or by any other suitable manner. 
         [0019]      FIG. 2  is a zoomed-in cross-sectional view of the squeeze-film damper of  FIG. 1 .  FIG. 2  illustrates squirrel cage  12 , stationary housing  14 , and bearing assembly  16 . Second end  20 , which includes piston ring grooves  26   a  and  26   b , annulus  28 , and race housing  30 , of squirrel cage  12  is illustrated. Bearing assembly  16  includes bearing element  34 , inner race  36 , and outer race  38 . Piston rings  40   a  and  40   b  are also illustrated. As illustrated, squirrel cage  12 , stationary housing  14 , and bearing assembly  16  form a squeeze-film damper. 
         [0020]    Squirrel cage  12  supports bearing assembly  16 , and squirrel cage  12  is supported by stationary housing  14 . Bearing assembly  16  supports a rotatable shaft, such as a compressor or turbine shaft of a gas turbine engine. Piston ring grooves  26   a  and  26   b  extend circumferentially about an outer surface of second end  20 . Annulus  28  is disposed between piston ring grooves  26   a  and  26   b  and similarly extends circumferentially about an outer surface of second end  20 . Race housing  30  extends circumferentially about an inner surface of second end  20 . Race housing  30  supports outer race  38  of bearing assembly  16 . Piston rings  40   a  and  40   b  in piston ring grooves  26   a  and  26   b  respectively and provide a seal between squirrel cage  12  and stationary housing  14 , defining annulus  28 . 
         [0021]    Passage  42  extends through stationary housing  14  and fluidly connects an oil supply to annulus  28 . As previously discussed, a valve controls a supply of oil provided to annulus  28  through passage  42 . The oil is retained in annulus  28  by squirrel cage  12 , piston rings  40   a  and  40   b , and stationary housing  14 . As the shaft rotates, vibrations experienced by the shaft are transmitted to stationary housing  14  through bearing assembly  16 . The oil is squeezed within annulus  28  by the motion of squirrel cage  12 , which cause the oil to damp the vibration of the rotatable shaft. To damp vibrations, the oil shears within annulus  28 , preferably having a shear angle of about 90 degrees. During operation the oil slowly leaks out of annulus  28  through gaps in piston rings  40   a  and  40   b . As such, a constant supply of oil must be provided to annulus  28  to maintain the squeeze-film damper. 
         [0022]    During acceleration, the rotating shaft passes through several critical speeds. A critical speed is experienced when the rotating shaft rotational frequency traverses a resonance frequency of one of the various structure of the engine. Due to the multitude of structures that make up any given engine, the rotating shaft can pass through several different critical speeds during acceleration. The oil disposed within annulus  28  shears as the rotating shaft rotates to damp vibrations. To effectively damp vibrations, the oil preferably has a shear angle of about 90 degrees. However, the oil acts as a stiffener when the shear angle is about 0 degrees or about 180 degrees. When the shear angle is about  0  degrees or about 180 degrees, then the oil can cause vibrations to be transmitted to stationary housing  14  and other engine structures. To reduce the transmission of vibrations through the oil film, the valve disposed on passage  42  is switched to an off position and the supply of oil to annulus  28  is ceased. Once the supply of oil is shut off, vibrations are no longer transmitted through squeeze-film damper. Instead, the vibrations are transmitted through squirrel cage  12 . 
         [0023]      FIG. 3  is a perspective view of squirrel cage  12 . Squirrel cage  12  includes first end  18 , second end  20 , and ligaments  22 . First end  18  includes mounting flange  24 . Second end  20  includes piston ring grooves  26   a  and  26   b , annulus  28 , and race housing  30 . Ligaments  22  include damping layer  32  (best seen in  FIG. 1 ). 
         [0024]    Ligaments  22  extend between and connect first end  18  and second end  20 . Mounting flange  24  extends radially outward from ligaments  22 . Mounting flange  24  includes a plurality of fastener openings extending therethrough, the fastener openings are capable of receiving fasteners, the fasteners securing squirrel cage  12  to stationary housing  14  (best seen in  FIG. 1 ). Damping layer  32  is disposed on at least one ligament  22 . Piston ring grooves  26   a  and  26   b  extend circumferentially about an outer surface of second end  20 . Annulus  28  similarly extends circumferentially about an outer surface of second end  20 , and annulus  28  is disposed between piston ring groove  26   a  and piston ring groove  26   b . Race housing  30  extends circumferentially about an inner surface of second end  20 . Squirrel cage  12  is preferably metallic, and is preferably made from titanium, a titanium alloy, nickel, or a nickel alloy. During operation, squirrel cage  12  is bathed in oil that is typically about 350° F. (177° C.), and squirrel cage  12  is thus made of a material that can withstand such temperatures. 
         [0025]    As the rotatable shaft vibrates, vibrations are transmitted to squirrel cage  12  through bearing assembly  16 . As previously discussed, the vibrations can be damped by a squeeze-film damper, whereby supply of oil provided to annulus  28  acts as a damper. However, the oil may act as a stiffener and transmit vibrations to an engine casing during certain operational speeds of the engine. When the oil acts as a stiffener, the supply of oil provided to annulus  28  is shut off and vibrations are transmitted directly through squirrel cage  12 . For example, the squeeze-film damper can damp vibrations until the engine reaches an idle speed, at which point the supply of oil is shut off. However, the supply of oil may be resumed at any point during operation of the engine where the squeeze-film damper provides damping, not stiffness. As such, squirrel cage  12  must damp the vibrations transmitted from the rotatable shaft when the supply of oil to annulus  28  is ceased. 
         [0026]    As the rotating shaft rotates, ligaments  22  are strained as squirrel cage  12  vibrates within bearing compartment  10 . As ligaments  22  strain, damping layer  32  provides damping of any vibrations experienced through ligaments  22 . The vibrations pass through squirrel cage  12  and damping layer  32  provides damping to reduce the transmissibility of vibrations to the engine structure. 
         [0027]    Damping layer  32  preferably includes a viscoelastic material covering at least one ligament  22 . While damping layer  32  may include a viscoelastic material, such as a constrained layer viscoelastic material, damping layer  32  may also include a damping tape, such as a damping tape having a viscoelastic adhesive and a metallic or composite backing, a plasma spray coating, or any other damping material suitable for dissipating the vibrations through ligaments  22 . Additionally, damping layer  32  may be applied to ligaments  22  in any suitable manner, such as by wrapping a damping tape around ligament  22 , spray coating damping layer  32  on ligaments  22 , masking one or both ends of squirrel cage  12  and dipping squirrel cage  12  in the damping material to coat ligaments  22  in damping layer  32 , via electron beam physical vapor deposition, or by any other suitable manner. Squirrel cage  12  is bathed in oil that is typically about 350° F. (177° C.) during operation. Damping layer  32  thus is made of a material capable of damping vibrations at such temperatures. 
         [0028]    Discussion of Possible Embodiments 
         [0029]    The following are non-exclusive descriptions of possible embodiments of the present invention. 
         [0030]    A bearing centering spring includes a first end, a second end, a plurality of ligaments extending between and connecting the first end and the second end, and a damping layer disposed on at least one of the plurality of ligaments. 
         [0031]    The bearing centering spring of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
         [0032]    A constrained layer viscoelastic material. 
         [0033]    A plasma spray coating. 
         [0034]    A damping tape. 
         [0035]    A viscoelastic polymer adhesive and a backing material attached to the viscoelastic polymer adhesive. 
         [0036]    A metallic backing. 
         [0037]    A damping assembly includes a bearing; a squirrel cage disposed radially outward of and supporting the bearing, the squirrel cage including a first end, a second end, the second end supporting the bearing, a plurality of ligaments extending between and connecting the first end and the second end, and a damping layer disposed on at least one of the plurality of ligaments; and a housing disposed radially outward of the squirrel cage. 
         [0038]    The damping assembly of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components. 
         [0039]    A first piston ring disposed between the squirrel cage and the housing, a second piston ring disposed between the squirrel cage and the housing, an annulus defined by the squirrel cage, the housing, the first piston ring, and the second piston ring, and an oil feed to the annulus. 
         [0040]    A valve attached to the oil feed. 
         [0041]    The valve remains in an open position during engine start up. 
         [0042]    The valve is switched from the open position to a closed position after an engine speed reaches an idle speed. 
         [0043]    The bearing is a thrust bearing. 
         [0044]    A constrained layer viscoelastic material. 
         [0045]    A plasma spray coating. 
         [0046]    A damping tape. 
         [0047]    A viscoelastic polymer adhesive and a metallic backing. 
         [0048]    A method of damping vibrations in a gas turbine engine includes damping engine vibrations with a damping layer disposed on at least one ligament of a squirrel cage, the squirrel cage supporting a bearing outer race; supplying oil to an annulus disposed between the squirrel cage and a housing; damping engine vibrations with the oil; and switching off an oil supply to the annulus when the engine speed is at least an idle speed of the engine, whereby the engine vibrations transmitted through the bearing outer race are damped exclusively by the squirrel cage. 
         [0049]    The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components. 
         [0050]    Damping engine vibrations with a damping layer sprayed onto the squirrel cage. 
         [0051]    Damping engine vibrations with a damping tape wrapped around at least one ligament of the squirrel cage. 
         [0052]    Damping engine vibrations with a damping layer applied to the squirrel cage by masking at least one end of the squirrel cage and immersing the squirrel cage in a material having damping properties. 
         [0053]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.