EP0918139A2

EP0918139A2 - Friction Damper

Info

Publication number: EP0918139A2
Application number: EP98309280A
Authority: EP
Inventors: Kenan Yuce Sanliturk
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1997-11-25
Filing date: 1998-11-12
Publication date: 1999-05-26
Also published as: EP0918139A3; GB9724731D0

Abstract

A blade to blade vibration damper 30,34 for a gas turbine engine includes a first friction surface 28 for contacting a first face 22 associated with a blade of the gas turbine and a second friction surface 28 for contacting a second face 22 associated with an adjacent blade of the gas turbine. The damper is formed in at least two parts such that the first friction surface can move relative to the second friction surface. Relative radial movement of the two friction surfaces can take place thereby allowing the friction damper to respond to relative radial movement of adjacent turbine blades, while maintaining the first and second friction surfaces in contact with the first and second faces respectively.

Description

The invention relates to a blade to blade friction damper particularly but not exclusively for a gas turbine engine.
Gas turbine engines typically include a plurality of blades mounted around the periphery of a rotor disc. Each blade includes an aerofoil which projects into a working fluid flowing axially through the turbine. The flow of the fluid across the aerofoils causes the blades to rotate in the circumferential direction, thus rotating the rotor disc. The blades are closely spaced around the circumference of the rotor disc and include inner root sections mounted on the disc in such a way that expansion or contraction of the materials due to changes in temperature can be accommodated. This may be by use of a dovetailed joint and slot arrangement as described in US 5478207.
During operation of the turbine, the blades are subject to vibrations which result in fatigue of the blades and can produce cracks in highly stressed regions. This may result in premature failure of the blades. To minimise these vibrations, it is known to locate dampers, known as friction dampers, under platform dampers or cottage roof dampers, between adjacent turbine blades. The dampers are positioned such that during operation of the turbine, centrifugal forces draw inclined friction faces of the dampers into contact with complementary inclined faces on platforms associated with the blades. The platforms are located on and extend circumferentially from the blades, between their aerofoil and root sections. Vibration of the blades causes relative movement of the friction faces of the dampers and the faces of the blade platforms, causing these faces to slide against one another. The work done in overcoming the associated frictional forces dissipates the vibrational energy and thus reduces resonant response levels and suppresses excess vibration of the turbine blades.
A commonly used prior art vibration damper design is essentially wedge shaped with friction faces angled at approximately 45° to the radial direction of the blades and 90° to one another. The faces bear against angled faces of the blade platforms, faces of adjacent blade platforms forming a recess of approximately inverted V shape. Such wedge shape designs suffer from the drawback that they do not always achieve an exact fit in the inverted V shaped recess formed by the platforms of adjacent blades. This can result in one or both pairs of faces being in poor contact and in the damping action being correspondingly reduced.
Various designs have been proposed to overcome this problem. EP 509838 discloses a wedge shaped damper having raised pad surfaces on the two faces of the damper normally in surface to surface contact with the inclined faces of the turbine blade platforms. The raised surfaces are located so as to reduce tilting of the dampers and keep the raised surfaces in sliding contact with the platform faces.
US 5478207 discloses a damper which is generally wedge shaped but which has an off-set centre of gravity, intended to improve stability of the damper and to maintain planar contact between damper faces and the platform faces of the blades.
The vibration of turbine blades is complex and includes many different modes of vibration. While prior art dampers are effective for some of these modes, it has been found that current designs of dampers are not very effective when the mode of vibration is predominantly in-phase bending. Such bending causes relative radial motion of adjacent platforms. Studies have indicated that prior art dampers tend to roll rather than slide when subjected to this type of vibration. Such rolling movement brings one pair of faces out of contact and significantly reduces damping. Existing dampers can thus be considered inadequate for those modes with low nodal diameter vibration patterns.
According to the invention there is provided a blade to blade vibration damper for a gas turbine engine, the damper including a first friction surface for contacting a first face associated with a blade of the turbine and a second friction surface for contacting a second face associated with an adjacent blade of the turbine; wherein the damper is formed in at least two parts such that the first friction surface can move relative to the second friction surface.
Preferably the parts may undergo relative movement in the radial direction of the blades.
Preferably the parts of the damper engage a further friction surface. This further friction surface may be on another part of the damper. Preferably the damper includes a friction interface between two parts, the interface extending substantially in the radial direction of the blades.
Preferably the damper is substantially wedge shaped. Preferably the first friction surface is disposed at an angle of approximately 90° to the second friction surface. Preferably the first and second friction surfaces are located in use at an angle of approximately 45° to the radial direction of the blades.
At least one groove may be provided in the first friction surface or in both friction surfaces.
The first and second faces are preferably formed on platforms extending tangentially from the blades of the turbine.
According to the invention there is also provided a gas turbine engine including a vibration damper as previously defined.
Embodiments of the invention will be described for the purposes of illustration only with reference to the accompanying drawings, in which:
Fig. 1 is a fragmentary diagrammatic cross-section illustrating two adjacent turbine blades mounted on a rotor disc and provided with prior art friction dampers;
Fig. 2 is a diagrammatic cross-section of a prior art friction damper;
Fig. 3 is a diagrammatic cross-section illustrating rolling of a prior art friction damper in response to relative platform motion having a radial component;
Fig. 4 is a diagrammatic cross-section illustrating momentary contact separation and rolling of a prior art damper in response to relative platform motion having a radial component;
Fig. 5 is a diagrammatic cross-section illustrating the action of a damper according to the invention in response to relative platform motion having a radial component; and
Fig. 6 is a diagrammatic cross-section of an alternative embodiment of a friction damper according to the invention.
Referring to Fig. 1, a turbine section of a gas turbine engine includes a plurality of turbine blades 10 mounted on a rotatable disc 12. Each turbine blade 10 includes an aerofoil 14 which projects into a working fluid flowing axially through the turbine. The flow of the fluid across the aerofoils causes the blades 14 to rotate in the circumferential direction, thus rotating the disc 12. The blades 10 are mounted on the disc 12 by means of dovetailed root portions 16 which fit into correspondingly shaped dovetailed recesses 18 in the rotatable disc 12. These mountings are able to accommodate small changes in the material dimensions due to thermal expansion and contraction.
Located between the aerofoil 14 and root portion 16 of each blade 10 is a platform 20 having angled faces 22 on its radially inner side. The angled faces 22 of two adjacent blades form an inverted V shape which defines the upper boundary of the damper cavity 24. Each damper cavity 24 houses an approximately wedge shaped friction damper 26 having angled friction surfaces 28 of complementary shape to the inverted V formed by the angled faces 22. The friction damper 26 shown in Fig. 1 is of conventional "cottage roof" design.
When the turbine blades 10 rotate, centrifugal forces urge the friction damper 26 radially outwards so that its friction surfaces 28 are forced against the angled faces 22 of the platform 20. If a blade 10 undergoes vibration, this causes the friction surfaces 28 to slide against the angled faces 22, thus dissipating the vibrational energy and reducing the vibration.
This basic damper design is effective in reducing certain modes of vibration. For example, if circumferential vibrations are set up, whereby the circumferential distance between adjacent blade platforms 20 varies as the blades vibrate, the centrifugal forces acting on the damper 26 cause it to move radially inwards and outwards as the blades 10 vibrate, the friction surfaces 28 generally remaining in contact with the angled faces 22. However, where the mode of vibration for adjacent blades is predominantly in-phase bending, studies have indicated that conventional dampers are not very effective. They may roll, as illustrated in Fig. 3, rather than slide or one side may stick while contact separation occurs momentarily on the other side, as illustrated in Fig. 4, or a combination of both may occur, particularly when the blade vibration results in platform motion having a radial component.
Fig. 5 shows a damper 30 according to the invention, subjected to radial platform motion. The damper 30 is split down its centre to provide an extra central friction interface 32. When adjacent platforms undergo relative radial movement, centrifugal forces cause the two halves of the friction damper to slide relative to one another as illustrated in Fig. 5. Thus, rolling movement is prevented, and the friction surfaces 28 are maintained in contact with the faces 22.
An alternative damper 34 according to the invention is illustrated in Fig. 6. This damper has grooves 36 located approximately in the central region of its friction surfaces. Such grooves help to prevent rolling encouraged by unpredictable distribution of contact areas if the mating surfaces are not exactly flat. There is no contact in the grooved region and therefore the areas of contact are defined more precisely than in the first embodiment of the invention.
There is thus provided a friction damper which allows sliding rather than rolling motion even when subjected to radial platform movements.
Certain modifications may be made to the above design whilst still falling within the scope of the invention. The friction faces 22 and the friction surfaces 28 of the damper 30,34 may be angled at any suitable angle to the radial direction of the blades 10, not just at approximately 45° as is typically the case. The friction surfaces 28 of the damper 30,34 accordingly may also not necessarily be at 90° to one another. The angle of the respective friction surfaces 28 and friction faces 22 to the radial direction of the blades 10 may also be different for the respective surfaces 28 of a single damper 30,34 and respective faces 22. The splitting may be asymmetric, and the damper may be split into more than two pieces. The two or more pieces of the damper 34 may therefore be non-identical and/or non symmetric. The splitting effect can be extended in other directions, for example along the damper longitudinal axis, and the friction interfaces formed by the splitting may be inclined relative to the radial direction. The contact and non-contact surfaces of the damper need not be flat. The contact and non contact surfaces and faces could be curved whilst still allowing sliding of the respective surfaces, faces and parts of the damper 30,34. Where grooves are provided on the damper friction surfaces, more than one groove may be provided, and the grooves may be provided in any location, including on the friction interfaces formed by the splitting.
Although the invention has been described with respect to gas turbines it is applicable to any system having rotating blades subject to unwanted vibrations.
Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

A blade to blade vibration damper (30,34) for a gas turbine engine, the damper including a first friction surface (28) for contacting a first face (22) associated with a blade (10) of the turbine and a second friction surface (28) for contacting a second face (22) associated with an adjacent blade (10) of the turbine; characterised in that the damper (30,34) is formed in at least two parts which are arranged to slide relative to each other such that the first friction surface (28) can move relative to the second friction surface (28).
A damper according to claim 1 wherein in use the parts may undergo relative movement in the radial direction of the blades (10).
A damper according to claim 1 or claim 2 wherein the parts of the damper engage a further friction surface (32).
A damper according to claim 3 wherein the further friction surface (32) is on another part of the damper (30,34).
A damper according to claim 4 including a friction interface (32) between two parts, the interface extending in the radial direction of the blades (10).
A damper as claimed in any of claims 3 to 5 wherein the further friction surface is flat.
A damper as claimed in any of claims 3 to 5 wherein the further friction surface is curved.
A damper according to any preceding claim wherein at least one of the first or second friction surfaces are flat.
A damper according to any of claims 1 to 9 wherein at least one of the first or second friction surfaces are curved.
A damper according to any preceding claim wherein the damper (30,34) is substantially wedge shaped.
A damper according to any preceding claim wherein the first friction surface (28) is disposed at an angle of approximately 90° to the second friction surface (28).
A damper according to any preceding claim wherein the first and second friction surfaces (28) are located in use at an angle of approximately 45° to the radial direction of the blades (10).
A damper according to any preceding claim including at least one groove (36)in at least one of the friction surfaces (28).
A damper according to claim 13 wherein the first and second friction surfaces (28) each include at least one groove (36) .
A gas turbine including a vibration damper according to any preceding claim.
A gas turbine according to claim 11 wherein the first and second faces are formed on platforms (20) extending tangentially from the blades (10) of the turbine.