Friction damper

A damper ring is mounted in frictional engagement with a radially inwardly facing surface of a circumferential groove defined in a rotary part of a gas turbine engine. Energy dissipation is provided via sliding friction of the ring in the groove. Pressure relief dimples are provided around the outer diameter of the ring for locally reducing contact pressure at the outer diameter below a value at which the damper ring locks in the groove by friction forces when subject to centrifugal loads.

TECHNICAL FIELD

The application relates generally to gas turbine engines and, more particularly, to a frictional damper arrangement for damping vibrations transmitted to a rotor.

BACKGROUND OF THE ART

Gas turbine engines contain rotating parts (e.g. turbine or compressor rotors, discs, seal runners, etc. . . . ), which are in some cases subject to high vibrations and therefore require mechanical dampers to reduce vibratory stresses to provide adequate field life. Conventional dampers are typically provided in the form of a wire ring installed in a corresponding groove defined in the rotating part. Such ring dampers are subjected to centrifugal load that creates a reaction force between the damper and the mating rotor part. In high speed applications, this force could be enough to stick the damper to the rotor by friction so that no relative sliding is maintained and damper effectiveness is lost because it deforms together with the rotor as one solid body. This phenomenon is referred to as damper lock by friction. When the damper effectiveness is lost, energy dissipation by the damper is significantly reduced resulting in rotor vibratory stress increase that reduces service life and could result in in-flight engine failure.

SUMMARY

In one aspect of an embodiment, there is provided a damper ring mountable in a groove defined on a circumferentially inner surface of a rotor of a gas turbine engine to provide a friction damper assembly, the damper ring comprising: an outer circumferential surface configured to be centrifugally loaded against a radially inwardly facing surface of the groove, a plurality of circumferentially spaced-apart pressure relief dimples defined in the outer circumferential surface of the damper ring, the pressure relief dimples being configured to locally reduce the contact pressure at the outer circumferential surface of the damper ring below a threshold value at which friction forces lock the damper ring against movement in a circumferential direction relative to the rotor.

In a further aspect, there is provided a gas turbine engine rotor mounted for rotation about an axis, the rotor comprising: a body defining a circumferential groove having a radially inwardly facing surface, at least one damper ring mounted in the circumferential groove, the at least one damper ring having a plurality of pressure relief dimples formed at spaced intervals in an outer circumferential surface thereof and leaving circumferentially extending lands therebetween, in use, the at least one damper ring being displaceable under a centrifugal load from a first position, in which the lands are in contact with the radially inwardly facing surface of the circumferential groove while the pressure relief dimples are spaced radially inwardly therefrom, to a second position, in which the pressure relief dimples are deformed under the centrifugal load in contact with the radially inwardly facing surface of the circumferential groove.

In a still further general aspect, there is provided a method of providing frictional damping for a rotor of a gas turbine engine, the rotor having a circumferential groove with a radially inwardly facing surface, the method comprising: providing at least one damper ring configured to be centrifugally loaded against the radially inwardly facing surface of the circumferential groove of the rotor when the rotor is rotatably driven; and adjusting a contact pressure between an outer circumferential surface of the at least one damper ring and the radially inwardly facing surface of the groove below a threshold value at which the at least one damper ring locks against movement in a circumferential direction relative to the rotor when subject to centrifugal loads of a magnitude corresponding to centrifugal loads encountered during normal engine operation, wherein adjusting comprises providing pressure relief dimples at circumferential intervals in at least one of the radially inwardly facing surface of the circumferential groove and the outer circumferential surface of the at least one damper ring.

DETAILED DESCRIPTION

FIG. 1illustrates a gas turbine engine10of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan12through which ambient air is propelled, a compressor section14for pressurizing the air, a combustor16having a combustion chamber21in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section18for extracting energy from the combustion gases.

FIG. 2illustrates a rotary part or rotor20of the engine10. The rotor20can take various forms. For instance, the rotor20can be a compressor or turbine disk, a seal runner, a turbine cover or any other rotary parts requiring vibration damping.

As shown inFIGS. 3 and 4a, a friction damper, including at least one damper ring22, may be mounted in an associated circumferential groove24defined in an annular flange26projecting axially from one face of the rotor20. As shown inFIG. 5a, the ring22may be split to allow the same to be contracted to a smaller diameter in order to facilitate its installation in the rotor groove24, as known in the art. Once positioned in the groove24, the ring22springs back towards its relax state against the bottom wall of the groove24, thereby retaining the ring22in place in the absence of centrifugal loading (i.e. when the engine is not running). In use, the centrifugal load firmly urges the damper ring22in contact with the radially inwardly facing surface28(i.e. the circumferentially extending bottom wall) of the groove24. Energy is absorbed via sliding friction. The friction generated between the relative motion (i.e. the slippage in the circumferential direction between the damper ring22and the rotor20) of the two surfaces that press against each other under the centrifugal load is used as a source of energy dissipation. However, for the damping system to effectively work, some relative vibratory slippage between the damper ring22and the rotor20must be maintained even when subjected to high centrifugal loads such as those encountered when the engine10is operating at high regimes. For high speed applications, like small gas turbine engines, the centrifugal force may become so high that the friction forces tend to lock the damper ring22in place in the groove24, thereby preventing relative vibratory slippage in the circumferential direction between the ring22and the rotor20. At high rotation speeds, the friction forces may become so high that the damper ring22basically sticks to the rotor20. When the damper ring22sticks in the rotor groove24, the rotor20and the ring22becomes like one solid body. In such a case, no more vibration damping is provided.

Such lock by friction phenomenon can be avoided by appropriately reducing the contact pressure and, thus, the frictions forces, between the ring22and the groove24. For instance, as shown inFIGS. 5aand6, circumferentially spaced-apart pressure relief dimples30could be defined in the outer circumferential surface of the ring22. The term “dimple” is herein intended to refer to any suitable type of depression or discontinuity in the ring outer circumference and is, thus, not limited to regularly shaped circular depressions. As will be seen hereinafter, the shape and configuration of the dimples can be optimized to locally reduce the contact pressure to a value which is less that a threshold value at which the ring is friction locked in the groove.

The dimples30leave therebetween inter-dimple lands32or high points on the outer diameter of the ring22. These lands32provide a circumferentially discontinuous primary contact surface with the groove24to react the centrifugal load.

When assembled to the rotor20, the ring22will contact the radially inwardly facing surface28of the groove24at the lands32(or high points) only. Accordingly, in this state, contact forces are solely transmitted at the lands32. That is in the initial or “non-loaded” state, there will be no contact between the ring22and the groove24at circumferential locations corresponding to the pressure relief dimples30. Indeed, the recessed surface of the dimples30will be spaced radially inwardly from the radially inwardly facing surface28of the groove24and, thus, no contact forces will be transmitted in the dimple areas. However, upon accelerating the engine to operational speeds, the rotation of the rotor20will cause the damper ring22to deform under the centrifugal load. At one point, the radial bending of the ring22at each pressure relief dimple30will cause the ring22to contact the radially inwardly facing surface28of the groove24even in the dimple areas. However, the contact pressure will be smaller in these areas in comparison to conventional rings because part of the centrifugal (CF) force will be compensated by the stiffness of the damper (i.e. the force required to deform it). The reduction of contact pressure generally corresponds to the force required to deform the ring22so that that the recessed surface of the pressure relief dimples30contacts the bottom28of the groove24. In this way, the reduction of contact pressure may be calibrated to preserve the required vibratory slippage between the ring22and the groove24even at high rotation speeds where a conventional ring would tend to stick to the bottom of the groove (damper lock by friction).

Optimal dimple shapes could be achieved, for example, by finite element (FE) contact analysis of a numerical model of a damper ring installed in the rotor groove and subjected to a specified centrifugal load. By using computer simulation, each damper ring could be specifically designed for its intended application. To do so, the FE analysis or other suitable numerical analysis should consider the radial bending of the damper in the dimple areas and provide resultant contact pressure (that is reduced because of this bending). An iterative approach can be taken to establish the dimple configuration needed to obtain the desired contact pressure reduction to avoid locking of the ring for a given centrifugal (CF) load condition. The outcome of the optimization allows to define the shape that the dimples must have so that the line contact pressure [lb/in] (contact force per unit length of the damper circumference) in the dimples area is below the pressure that is required to lock the damper by friction. The threshold value line contact pressure [lb/in] required to lock the damper by friction could be calculated by FE transient dynamic analysis (with taking in account friction forces) or analytical method, as known by person skilled in the art. In general analytical method of calculation of the pressure that is required to lock the damper by friction at any given point on circumference is based on the equation of the forces equilibrium in circumferential direction of the infinitely small element of the damper (equilibrium occurs when damper element sticks to the bottom of the groove by friction and no sliding occurs (so called damper lock by friction phenomenon). As schematically depicted inFIG. 7, the forces to be considered in this equation are the internal tensile or compressive vibratory force acting on the damper element from each side and friction force applied to the damper element at the contact surface with the groove. Equation of equilibrium states that sum of the friction force and internal forces acting on this infinitely small element should be equal zero:
ΔFfriction lock+Fa(φ2)−Fa(φ1)=0
ΔFfriction lock=−(Fa(φ2)−Fa(φ1)
φ2−φ1=Δφ→0  [1]

WhereΔFfriction lock, [lbf] is the friction force acting on the damper element at the contact with the grooveFa(φ), [lbf] is the internal tensile (positive sign) or compression (negative sign) vibratory force acting in circumferential direction at the given section of the damper defined by circumferential coordinate φFa(φ1) is the internal vibratory force at the left side of the damper element (at section φ=φ1)Fa(φ2) is the internal vibratory force at the right side of the damper element (at section φ=(φ2)

Equation [1] defines what friction force is required to lock the damper element in circumferential direction. If actual friction force (based on CF pressure and friction coefficient) is less than the right side of the equation [1], then equilibrium will not be achieved and this element of the damper will slide in circumferential direction.

Contact pressure Plock, [lbf/in] that is the contact force (caused by CF load) per unit length of the damper outer circumference at any given point on the circumference, required to lock the damper by friction at this point, can be derived from the equation [1]. It will be the function of the following parameters:
Plock(φ)=Plock(A(φ),E,{dot over (ε)}o(φ),{dot over (ε)}o bending(φ),μ,Ro)  [2]

Whereφ, [rad] is the angular coordinate that defines the point on the damper circumferenceA(φ), [in{circumflex over ( )}2] is the area of the damper cross-section at any given point on circumference defined by angular coordinate φ (i.e. πd2/4 for the round damper wire with the cross-section diameter d) that in general case will vary over circumference because dimples will cause cross-section area variationE, [psi] is the modulus of elasticity of the damper{dot over (ε)}o(φ) is the circumferential vibratory strain gradient (strain rate) in circumferential direction (calculated as the first derivative versus circumferential coordinate φ) at the bottom of the rotor groove where it contacts with the damper{dot over (ε)}o bending(φ) is the circumferential vibratory bending strain gradient (strain rate) in circumferential direction (calculated as the first derivative versus circumferential coordinate φ) of the damper outer surface where it contacts with the rotor groove. This strain rate is due to damper bending only (bending is caused by radial vibratory displacement of the rotor groove where damper is installed)μ is the friction coefficient (between damper and rotor groove)Ro, [in] is the radius of the outer surface of the damper when installed in the rotor groove (basically it is equal to the rotor groove inner radius)

The shape of the pressure relief dimples30can be configured by iterative FE analysis to ensure that contact pressure at any point on the circumference in the dimple area is below the value defined by formula [2] when the center of the dimple coincides with the peak of the differential strain MAX (|{dot over (ε)}o(φ)−{dot over (ε)}o bending(φ)|). By doing this, it can be ensured that when the traveling vibratory wave of the rotor part passes in circumferential direction across the dimple30, at the moment when peak of the circumferential differential vibratory strain (differential between the damper bending strain and the groove strain) MAX (|{dot over (ε)}o(φ)−{dot over (ε)}o bending(φ)|) coincides with the center of the dimple30, sliding between the damper ring22and the rotor groove24will occur in circumferential direction across the whole dimple length. Optimum ratio between Plockand actual contact pressure could be selected in the dimples area in order to further improve the design and maximize the friction work for the given vibration mode shape and amplitude. This optimization could be done by experimental rig tests, FE transient contact analysis with friction and sliding or by further developing the analytical methods.

The length (L) of the dimples30in the circumferential direction is selected to maintain adequate bending stress of the damper ring22in the dimples areas. As shown in the embodiment illustrated inFIG. 6, the dimples30have a shallow profile. The depth (d) of the dimples30is smaller than the length (L) thereof. As shown inFIG. 5a, the dimples30may define a flattened sinusoidal-like profile at the outer diameter of the ring22. The smooth blending transitions between the dimples30and the lands32provides for a smoother stress distribution around the circumference of the ring22.

While the dimples30shown inFIG. 5ahave a same and unique shape, it is understood that the dimples30could have different shapes around the circumference of the ring22to target smaller and higher vibration amplitudes. Also the dimples30could have a regular pattern (good in most cases for engine rotor parts where vibration is a travelling wave) as shown or an irregular pattern to provide added damping efficiency for a standing wave type vibration. Also, as shown inFIGS. 4band 5a, the friction damper could comprises several damper rings22,22′ with different dimple shapes for installation in the rotor groove24in order to specifically target small and high amplitude vibrations and different mode shapes.

The pressure relief dimples30can be precisely machined on a CNC grinder while the damper ring22is held by a fixture clamping down against opposed flat sides of the ring22. Other suitable manufacturing processes are contemplated as well.

The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For instance, while maybe less practical from a manufacturing point of view, it is understood that the pressure relief dimples could be defined in the bottom surface of the rotor groove instead than in the outer circumferential surface of the damper ring. Other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.