Abstract:
A damper ( 2 ) for damping a blade movement of a turbomachine ( 1 ), and to a method for producing the damper ( 2 ). The damper ( 2 ) has at least one side surface ( 21, 21 ′) which can be brought into frictional contact with a friction surface of the turbomachine ( 1 ) in order to damp a blade movement. The side surfaces ( 21, 21 ′) are asymmetrically convex in shape.

Description:
The present invention relates to a damper for damping a blade movement of a turbomachine, and to a method for producing the damper. 
     BACKGROUND 
     Turbomachines, in particular gas or steam turbines, have a rotor and blades which are coupled to the rotor and distributed around the circumference thereof. The blades must be designed to resist a plurality of stresses during operation of the turbine. Such stresses include, for example, centrifugal forces, erosion-corrosion, and vibrations. 
     The vibratory stresses to which the present invention relates may result from a combination of the medium passing through the turbine and the forces acting on the blades. In the long term, blade vibrations can cause a change in the microstructure of the blade material, which may eventually lead to a fatigue fracture. Therefore, it is necessary to damp the vibrations of the blades. In the prior art, a plurality of damping means for blade vibrations are known. 
     In German Patent Document DE 103 40 773, a damping means for a rotor blade of a turbine is disposed in a pocket of a rotor blade platform. The damping means has a triangle-like shape in a cross section normal to the axis and has rounded longitudinal edges. The longitudinal edges each have a symmetrically convex shape between the corners. During operation of the turbine, the damping means contacts an inner wall of the pocket and a friction surface of another rotor blade platform in order to damp movement of the rotor blade. 
     A disadvantage of the prior art damping means of symmetrically convex shape lies in the manner in which the region of frictional contact with a friction surface of the turbomachine is defined due to the symmetrically convex configuration of the longitudinal edge of the damping means. If the friction surface of the turbomachine has a particular shape, this may result in poor contact of the damping means with the friction surface of the turbomachine. For example, the friction surface of the turbomachine may be configured such that the damping means contacts the friction surface of the turbomachine only in a small region of frictional contact. As a result, the frictional heat produced during damping of the blade movement can only be dissipated through the small region of frictional contact, which may result in damage to or wear of the damping means and/or the corresponding friction part of the turbomachine. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to improve the damping of a turbomachine blade. 
     The present invention provides a damper that has at least one asymmetrically convex side surface intended to damp a blade movement of a turbomachine. When the blade vibrates during operation of the turbomachine, the vibrational movement of the blade is damped by friction of the side surface of the damper with the friction surface of the turbomachine in the region of frictional contact. 
     In the process, the asymmetrically convex side surface allows the region of frictional contact between the friction surface of the turbomachine and the side surface of the damper to be shifted to a more convenient region. Preferably, the region of frictional contact is shifted such that the region of frictional contact between the side surface of the damper and the friction surface of the turbomachine is enlarged. The frictional heat produced during damping of the blade movement can be dissipated into a friction part of the turbomachine through the enlarged region of frictional contact. This reduces the risk of the damper and/or the friction part being damaged or worn during damping of the blade movement. 
     Another advantage of using an asymmetrically convex side surface is that when the side surface of the damper becomes worn by friction and/or heating, the friction region of the damper, and thus the region of frictional contact between the damper and the friction surface of the turbomachine, is enlarged more than proportionally. A larger region of frictional contact allows frictional heat to be dissipated more rapidly, thereby reducing the risk of damage to and/or wear of the damper and/or the friction part of the turbomachine. Yet another advantage of an asymmetrically convex side surface is that friction-induced wear results in an adaptation of the friction region of the damper to the respective friction surface of the friction part of the turbomachine. In this manner, manufacturing accuracies of the friction part are compensated for. Ultimately, this reduces the manufacturing effort required to produce the friction part of the turbomachine. 
     The term “convex” is understood in the context of the present invention to mean a convexly curved surface. The term “asymmetrical surface” is understood in the context of the present invention to be a surface having two separated regions that cannot be transformed into each other by reflection in an axis or plane. Therefore, an “asymmetrically convex side surface” is understood in the context of the present invention to be a convexly curved side surface that has two differently shaped zones. The zones are so shaped that there is no plane of symmetry normal to the side surface, with respect to which the zones separated by the plane of symmetry would be mirror-symmetric. 
     The term “region of frictional contact” is understood in the context of the present invention to be a region in which friction occurs between the friction region of the damper and the friction surface of the turbomachine during blade movement. 
     A “friction part” is understood in the context of the present invention to be any part of a turbomachine that is in frictional contact with the damping means to damp blade vibration. A friction part may, in particular, form part of a blade, or may be coupled to a blade. In the context of the present invention, a friction part may be, for example, a blade platform, a shroud segment of a blade, and a positioner for positioning the rotor blade in the axial direction of the rotor. The aforementioned friction parts will be described in more detail below. 
     In a preferred embodiment, the asymmetrically convex side surface may be capable of slightly rotating about a damper axis. The friction of the side surface with the friction surface of the turbomachine causes a frictional force to act on the damper. This frictional force may cause a slight rotation of the damper. The asymmetrically convex side surface is configured such that the friction region of the damper, and thus the region of frictional contact between the damper and the friction part of the turbomachine, is enlarged upon slight rotation of the damper. 
     The side surface may have at least two zones of different radii of curvature. At least one zone which is radially farther away from the rotor axis may have a smaller radius of curvature than a zone that is radially closer to the rotor axis. The damping of blade vibration is improved when the friction surface of the turbomachine contacts the zone of the side surface that has the aforementioned small radius of curvature. The improvement in damping results because the zone of frictional contact forms in a region of the side surface of the damper that is distal from the rotor axis. In this connection, it holds that the farther away the zone of frictional contact is from the rotor axis, the greater is the frictional torque which is caused by the frictional force occurring in the zone of frictional contact and which damps the vibration of the blade. 
     In an advantageous embodiment of the present invention, the damper has a main body that has a triangular or polygonal shape in a cross section normal to the axis. The side surfaces of the triangular or polygonal main body may each have rounded corners of the main body. The zone of the asymmetrically convex side surface that has a smaller radius of curvature and that is disposed at the end of the side surface which is distal from the rotor axis may be located in each instance adjacent to a respective one of the rounded corners. 
     The damper may have attached thereto an anti-rotation means and/or a fastening means. The anti-rotation means prevents or limits rotation of the damper about a damper axis within a pocket of the turbomachine. During operation of the turbomachine, the damper is moved in a direction away from the rotor axis due to centrifugal force. The damper is acted upon by a rotational force which causes the damper to rotate about the damper axis until the anti-rotation means abuts against an abutment surface provided on the turbomachine. The anti-rotation means is preferably designed such that it abuts against the abutment surface when the damper is rotated into a position where the above-mentioned zone having the small radius of curvature is in frictional contact with the friction surface of the turbomachine. 
     When the side surface becomes worn by the friction, the shape of the damper, and thus the position of its center of gravity, change. The position of the center of gravity can be adjusted by suitably designing the side surfaces of the damper. This makes it possible to improve the dynamic properties of the damper. 
     The fastening means serves to prevent or limit movement of the damper, in particular in the direction of the rotor axis of the turbomachine. Thus, the fastening means ensures that the damper cannot leave the pocket of the turbomachine. 
     The turbomachine may be a gas or steam turbine, and, in particular, an aircraft engine. The turbomachine has a rotor and stator and rotor blades which are distributed around the circumference of the rotor and arranged in succession in the direction of gas flow. The rotor is provided with grooves which are distributed around its circumference and extend parallel to the rotor axis. The blade, in particular a rotor blade, may have a shroud segment, an airfoil, a blade platform, and a blade root. The blade is positioned by the blade foot in the groove in radially fixed relationship to the rotor axis. Fixing of the blade in the axial direction of the rotor may be accomplished by a securing plate provided in the groove and/or by a positioning means separately provided on the rotor. 
     Blade vibration can occur in a blade relative to the rotor and/or between two or several blades. For purposes of damping blade vibration, the damper may be disposed at different locations in the turbomachine. 
     The shroud segment of a blade may have a pocket which at least partially defines an, in particular closed, cavity and in which the damper is disposed. For example, the cavity may be defined by the pockets of two shroud segments of adjacent blades. The damper is disposed in the pocket such that when the turbomachine is operating, one side surface of the damper is in frictional contact with a friction surface of the pocket of one shroud segment, and another side surface of the damper is in frictional contact with a friction surface of the other shroud segment. 
     Alternatively or additionally, a damper may be disposed in a pocket of a positioning means which secures the position of the blade in the axial direction of the rotor. In this case, the damper is disposed such that one side surface thereof is in frictional contact with a friction surface of the positioning means. Another side surface of the damper is in contact, in particular frictional contact, with a blade surface. 
     Alternatively or additionally, a damper may be disposed in a pocket of a blade platform. The blade platform is located between the blade root and the airfoil. The damper is disposed in the pocket such that when the turbomachine is operating, one side surface of the damper is in frictional contact with a friction surface of the blade platform in which the pocket is formed, and another side surface of the damper is in frictional contact with a friction surface of an adjacent blade platform. 
     The damper, which has least one asymmetrically convex side surface, may preferably be manufactured by primary shaping, forming and/or machining techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages will become apparent from the dependent claims and the exemplary embodiment. In the drawings, 
         FIG. 1  is a schematic view of a damper in a cavity according to an embodiment of the present invention; 
         FIG. 2  is an enlarged view A-A from  FIG. 1  of a region of frictional contact according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Damper  2  shown in  FIG. 1  has a main body  20  having a substantially triangular shape in a cross section normal to the axis. Triangular main body  20  has a supporting surface  25  and two side surfaces  21 ,  21 ′, which merge into one another via rounded ends. Side surfaces  21 ,  21 ′ each have an asymmetrically convex shape. The asymmetrically convex shape of the individual side surfaces  21 ,  21 ′ results because side surfaces  21 ,  21 ′ each have three zones having different radii of curvature R 1 , R 2 , R 3 . Damper  2  further has an anti-rotation means  24  which is attached to main body  20  and extends via supporting surface  25  in a radial direction with respect to the rotor axis. 
     Damper  2  is disposed in a cavity defined by two pockets  11  of adjacent blades  10 ,  10 ′ of a turbomachine  1 . The cavity has a triangular profile in a cross section normal to the axis. The individual cavity walls are longer than the respective side surfaces  21  and supporting surface  25  of damper  2 . Damper  2  is disposed in the cavity such that it is contact with the cavity walls of both blades  10 ,  10 ′, regardless of the operating condition of turbomachine  1 . 
     Both side surfaces  21 ,  21 ′ of damper  2  have a first zone having a first radius of curvature R 1 , a second zone having a second radius of curvature R 2 , and a third zone having a third radius of curvature R 3 . Moreover, in both side surfaces  21 ,  21 ′, the second zone having the second radius of curvature R 2  is disposed between the first zone and the third zone and is longer than the first zone and the third zone. Third radius of curvature R 3  has a smaller value than first radius of curvature R 1  and second radius of curvature R 2 . Moreover, first radius of curvature R 1  has a smaller value than second radius of curvature R 2 . 
     In first side surface  21 , the first zone having the first radius of curvature R 1  is disposed at the end of side surface  21  that is radially proximal to the rotor axis. The third zone having the third radius of curvature R 3  is disposed at the end of side surface  21  that is radially distal from the rotor axis, and is in frictional contact with the respective cavity wall in a region of frictional contact  22 . 
     In second side surface  21 ′, the first zone having the first radius of curvature R 1  is disposed at the end of side surface  21 ′ that is radially distal from the rotor axis. The third zone having the third radius of curvature R 3  is disposed at the end of side surface  21 ′ that is radially proximal to the rotor axis, and is in frictional contact with the respective cavity wall in a region of frictional contact  22 . 
     Blade  10  of turbomachine  1  is configured to have a recess  14  through which anti-rotation means  24  extends radially with respect to the rotor axis. Recess  14  is bounded by the walls of recess  14  and an abutment surface  12 . Abutment surface  12  is provided on the blade  10 ′ that is adjacent to the blade  10  having recess  14 . Recess  14  is configured such that damper  2  cannot fall out from the cavity therethrough when the turbine is at rest. When turbomachine  1  is in a condition of rest (not shown), supporting surface  25  of damper  2  rests against the respective cavity wall, and anti-rotation means  24  extends through recess  14  in a radial direction with respect to the rotor axis. 
     During operation of turbomachine  1 , damper  2  is moved radially away from the rotor axis due to centrifugal force until side surfaces  21 ,  21 ′ abut against the cavity walls. During this movement toward the cavity walls, damper  2  is rotated about a damping axis. 
     Damper  2  is rotated until anti-rotation means  24  abuts against abutment surface  12  of the one blade  10 ′. Ultimately, the two side surfaces  21 ,  21 ′ of damper  2  are in frictional contact with the cavity walls in a respective region of frictional contact  22 . When one or both of blades  10 ,  10 ′ move radially and/or axially, the blade movement can be damped by the frictional contact of damper  2  with the cavity walls. 
     The positioning means P and fastening means F described above are shown schematically. 
       FIG. 2  is an enlarged view A-A from  FIG. 1  of a region of frictional contact  22 . As can be seen in  FIG. 2 , the third zone having the third radius of curvature R 3  of first side surface  21  is in frictional contact with the cavity wall. The second zone of first side surface  21 , which has a radius of curvature R 2  greater than radius of curvature R 3 , is not in frictional contact with the cavity wall.