Blade platform with friction damping interlock

Abstract

A turbomachinery rotor blade for disposition within a fluid flow path in cooperation with a rotatable disc is provided with a platform for partially defining the flow path of the fluid. The platform is disposed between the airfoil of the blade and preselected means for mounting the blade to a rotatable disc. The platform is provided with means for resisting circumferential and radial vibratory displacement by the action of frictional damping. The damping is supplied by frictional engagement with platforms of laterally adjacent blades. To enhance this function, the circumferential extremities of the platforms are provided with two diagonal edges axially spaced from one another along the same axial line, the edges having predetermined angular offsets from the radial direction. The two edges along each circumferential extremity have anuglar offsets of opposite senses, and adjacent platforms effectively interlock with one another.

Government Interests

The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the U.S. Department of the Air Force.

Description

BACKGROUND OF THE INVENTION

The present invention relates to blades for use in turbomachinery and, more particularly, to frictional vibration-damping systems therefor.

For the purpose of defining an aerodynamically efficient flow path for the working fluid within a gas turbine engine, airfoil blades are provided with platforms which combine with abutting similar platforms of adjacent blades to separate the fluid flow path from associated rotor cavities. High-speed rotation of rotor blades combines with the impingement upon the platforms and associated airfoils of a moving fluid to produce vibrational excitation which can do severe damage to the blade. More particularly, certain of these excitations may create various vibrational modes within the aforementioned platforms which can result in the dismemberment of the platforms by the breaking off of portions thereof.

Particularly susceptible to such breakage are the angular corners of platforms of typical design. Elimination of such angular corners might provide assistance in solving this problem. However, aerodynamic efficiency requies a substantially cylindrical path definition in this area which, when translated into terms of adjacent abutting platforms requires such angular corners.

Other damage to the rotor blades through vibration can occur. For example, certain modes of vibration often result in damage to portions of the airfoil surfaces thereof by breakage and loss of corners or by cracking and splitting in various directions. All of this vibrational damage is harmful to the useful life of the blades and is therefore advantageously eliminated.

Prior attempts at eliminating such vibration has included attempts to "tune" resonant frequencies of the various vibrational modes within the blades in order to remove them from the operating regime of the engine. These attempts have been successful to only a limited extent for the reason that such engines are operable within a wide range with the result that certain resonant frequencies cannot be eliminated. In recognition of this problem, prior attempts have been made to damp the amplitudes of vibrations which remain within operating ranges.

Damping has commonly taken the form of frictional engagement of a portion of the blade by a member added for that purpose. In particular, one prior attempt has involved the circumferential lengthening of blade platforms and the further addition of circumferentially extending overlapping plates between blades which members cooperate to damp vibrational amplitudes. Unfortunate characteristics of this and similar devices include the weight and expense required in the addition of elements which serve no function other than damping. Furthermore, lack of an effective interlock between blades and engaging dampers allows the damping elements to become separated over extended use with the possibility of no longer performing their damping function.

The present invention remedies these and other objections to blade damping members of the prior art by the provision for an interlocking blade platform which performs efficient frictional damping without the necessity of increasing the weight or size of the platform.

The invention relies, in part, upon recognition that, since typical blade airfoils are mounted substantially diagonally of the blade platforms, the corners proximate the leading and trailing edges of the airfoil will be stiffened thereby and experience vibrational amplitudes substantially lower than the unreinforced corners remote from the airfoil.

BRIEF SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide a frictional damping system for turbomachinery rotor blades which effectively damps vibrations while maintaining an effective interlock for reliable extended use and which does not necessitate the addition of costly and heavy materials to perform this function.

In order to accomplish this and other objectives, which will become apparent from the detailed description which follows, the present invention provides a rotor blade having a laterally extending platform disposed along its length between its diagonally disposed airfoil and the means for mounting the blade upon an associated rotor disc. The platform has a circumferential extremity disposed to either side of the airfoil, and this extremity is provided with diagonal edges which are canted from the radial direction by predetermined angles. Each extremity has two such edges, the aforementioned angles of which are of opposite sense. The diagonal edges of each platform are positioned to engage in an interlocking abutting relationship opposed edges of adjacent blade platforms. The edges thus formed provide frictional damping for vibration tending to cause relative radial motion between adjacent platforms as well as relative circumferential motion therebetween.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will become more easily understood upon reading the following detailed description in combination with the appended drawing wherein:

FIG. 1 is a perspective view of a plurality of rotor blades according to the present invention in their operating position upon an associated disc;

FIG. 2 is a perspective view of an individual rotor blade according to the present invention;

FIG. 3 is a top view of a rotor blade according to the present invention;

FIG. 4 is a side view of such a blade;

FIG. 5 is a cross-sectional view along line 5--5 of FIG. 1;

FIG. 6 is a cross-sectional view along line 6--6 of FIG. 1 and illustrating cooperation between opposed platform edges of adjacent blades, and

FIG. 7 is a view similar to that of FIG. 6 but illustrating cooperation between different platform edges.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, a rotor designated generally by the numeral 10 is shown to include a rotatable disc 12 carrying a circumferential groove 14 in the periphery thereof which is adapted to receive and retain dovetail mounting means 16 by which the individual blades 18 are attached to the disc. This particular variety of disc includes a loading slot 20 for accommodating the blades within the groove 14 of the disc 12. The dovetail 16 extends radially into conjunction with a platform 22 of each blade, which combines with adjacent platforms to partially define a fluid flow path 23 within the engine and to which the present invention particularly pertains. Extending radially outwardly from the platform is an airfoil 24 which is particularly configured to cooperate efficiently with a fluid stream directed therepast.

In operation, this rotor functions substantially similarly to typical rotors of this variety in that the disc 12 is attached to a shaft (not shown) about which it rotates in response to the interaction of the fluid stream with the blades 18 or in response to torque applied to the shaft by similar blades elsewhere disposed (depending upon whether the blades 18 shown are part of a turbine or a compressor). At any rate, the airfoils 24 of the various blades serve to transmit energy between disc 12 and the fluid stream. During this process, the rotational velocity of the blades, as well as aerodynamic disturbances within the fluid flow, create vibrations of various frequencies which tend to excite the blades at various frequencies. As mentioned hereinabove, such excitation may be undesirable since damage to the airfoils, platforms or other blade structure may result. In order to reduce the destructive amplitude of resonant frequencies in the blade material, which cannot be tuned out of the operating range of the engine, the present invention provides for frictional damping between adjacent blade platforms 22.

Referring to the remaining figures, the particular configuration by which this damping is accomplished is disclosed. It can be seen that each platform 22 includes two lateral extremities 30 and 32, respectively, which are provided with interlocking as well as damping means which will be described hereinafter. The platform also includes an upstream end 34 and a downstream end 36.

The platform of the present invention includes both radial damping means for frictionally damping radial relative movement between adjacent platforms, and circumferential damping means for frictionally damping circumferential relative movement between adjacent platforms. Generally these damping devices depend upon engagement by each platform with the platform of the circumferentially adjacent blade. To this end, each platform extends circumferentially outwardly of each airfoil and into substantial abutment with the adjacent platforms.

To enhance damping as well as to provide an effective interlock between platforms, the present invention provides unique treatment for each circumferential extremity 30 and 32 of each blade 18. More particularly, extremity 30 can be seen to include a first diagonal edge 40 which comprises a surface disposed at an angle .theta. .sub.1 from the radial direction, as represented by line 41. In addition, extremity 30 includes a second diagonal edge 42 which is comprised of a canted surface offset by an angle .theta. .sub.2 from the radial direction. It can be seen that angles .theta. .sub.1 and .theta. .sub.2 are of the opposite sense.

For simplicity of manufacture, diagonal edges 40 and 42 lie axially upstream and downstream relative to one another and substantially along a common axial line. (However, a similar arrangement might be made wherein the edges are radially spaced from one another and engage opposed edges of adjacent platforms.) It can also be seen that in the present embodiment, the extremity 30 is divided approximately into halves axially by the presence of the diagonal edges with the first of the halves comprising the edge 40 and the second of the halves comprising the edge 42.

Referring now to the second circumferential extremity 32, it can be seen that this extremity incorporates a similar pair of diagonal edges 48 and 50 which are canted from a radial line 52 by angles .theta. .sub.3 and .theta. .sub.4, respectively. These angles are respectively of the same sense as are angles .theta. .sub.1 and .theta. .sub.2 stated above. In the present embodiment it can be seen that angles .theta. .sub.1, .theta. .sub.2, .theta. .sub.3 and .theta. .sub.4 are all substantially equal in magnitude with one another. However, for proper functioning of the platform this is not necessarily so. In order to achieve proper abutting cooperation between the flat surfaces of the adjacent edges, angles .theta. .sub.1 and .theta. .sub.3 are kept substantially equal, and angles .theta. .sub.2 and .theta. .sub.4 are substantially equal. Also, the surfaces of the diagonal edges 40, 42, 48 and 50 may be provided with a frictionally enhanced surface to increase damping, but this also is optional.

Thus described, the damping system of the present invention functions as follows. Having been loaded by means of slot 20 into circumferential groove 14 and brought into abutting relationship with one another, the blades 18, according to the present invention, are retained within the groove and rotate in unison upon rotation of disc 12. During operation of the gas turbine engine, the disc 12 rotates about an associated shaft as described hereinabove. Interaction between the airfoils 24 and the flow of fluid through the engine flow path 23 induces vibrations of various frequencies, which are transmitted through the blade material to the platform 22 of each blade. In addition, vibration induced to the blade dovetails 16 is transmitted through the blade material to the platforms 22. Also the platforms may be excited directly by periodic fluid pressure fluctuations on the platform surface. As a result, the platforms as well as the airfoils tend to vibrate in various modes.

As already indicated, the resonant frequencies of the blades are "tuned out" of the operating ranges of the engine to the extent possible. Nevertheless, certain of the resonant frequencies of the blades remain within the operating regime of the engine. The present invention operates to reduce the amplitude of such vibrations, as well as in some instances to further tune the associated blades.

Typical vibrational modes can occur wherein the platforms 22 vibrate in such a way that the platforms are placed in radial relative motion with respect to adjacent platforms. Other vibrational modes might place the platforms in circumferential motion relative to one another. The present invention functions to damp each of these relative movements by means of frictional energy dissipation.

As stated hereinabove, the blade platforms 22 are formed with four corners for the purpose of defining a relatively cylindrical and thus aerodynamically efficient fluid flow path. For example, in FIG. 3, the corners are designated A, B, C and D. As further stated above, those corners of the blade platform which lie relatively closely to the leading and trailing edges of the airfoil 24 will be reinforced and stiffened thereby against steady state centrifugal as well as vibratory motion. In the Figure, corners A and C exemplify such corners. On the other hand, the remaining corners, labeled B and D in FIG. 3, are not stiffened by the airfoils and thus tend to vibrate with greater amplitude. The present invention takes advantage of this characteristic by bringing into abutment blade platforms in such a fashion that the reinforced corners A and C cooperate closely with mating unreinforced corners B and D of adjacent blades, and vice versa. As a result, vibration of each corner A, B, C or D of one blade platform meets with frictional resistance provided by adjacent blade platforms.

More particularly, with reference to the present configuration, each corner A, a stiff corner, abuts a more flexible corner D of an adjacent blade, as shown in FIG. 6. Similarly, each corner C, a stiff corner, abuts a flexible corner B. Under the influence of a given vibratory stimulus, corners A and C will tend to vibrate with an amplitude smaller than corners B and D. Hence, each corner, throughout its vibratory motion will be rubbing against an adjacent corner having a different amplitude of vibration. As a result, each corner tends to damp the vibration of its abutting mate and vice versa. This is the basic mechanism by which the present invention operates to effectively damp vibrational amplitudes, both in the radial and circumferential directions.

To further enhance this action, the stiff corners (A and C) are associated with inwardly facing surfaces 48 and 42 respectively. Similarly, flexible corners B and D are associated with outwardly facing surfaces 50 and 40. As a result, the steady state centrifugal force upon the platform at any given RPM results in mating surfaces being brought into a more intimate contact, thus increasing the frictional damping effect of the present invention.

In addition, this mechanism is enhanced by the particular configuration of the blade platforms of the present invention. To this end, the interlocking dual edges of each circumferential extremity 30 and 32 have been provided. As stated, the angles of departure of .theta. .sub.1 and .theta. .sub.3 of the surfaces 40 and 48, respectively, from the radial direction are equal in magnitude and of the same sense. Similarly, angles .theta. .sub.2 and .theta. .sub.4 by which surfaces 42 and 50 depart from the radial are also equal in magnitude and of the same sense. As a result, when adjacent blades are brought into abutment, surfaces 40 and 48 of adjacent blades overlie one another in substantially flush cooperation across their entire surfaces. The same is true of adjacent surfaces 42 and 50. The effect of this configuration is, in part, to provide surfaces having frictional engagement with one another both in the radial and circumferential vibratory modes. In addition, the contacting surfaces vibrating in each direction are maximized in area to likewise maximize frictional force. Furthermore, the fact that each circumferential extremity incorporates two edges of opposite sense which engage two edges of likewise opposite sense of an adjacent blade provides for an interlocking mechanism for retaining the platforms against tendencies to separate from one another. This is a distinct improvement over prior art dampers which tend to become non-functional after lengthy service has resulted in material deformations. Another advantage of this configuration is that the interlocking platform function minimizes the separation of adjacent platforms under the influence of various vibrations and thus increases damping affect.

In addition to damping the amplitude of the various vibrations, the present invention offers means for further tuning the resonant frequencies of the blades as may be desired. It is well known that resonant frequency is a function of the mass in vibration as well as the spring constant associated therewith. Each platform involves a predetermined mass, a portion or all of which can be in vibration at any given time. In addition, each platform incorporates a material and predetermined thickness which jointly lead to the spring constant thereof. By bringing adjacent blade platforms into interlocking cooperation with one another, it is possible to change the vibrating mass as well as the spring constant of the mass in vibration without adding or subtracting material from the platform itself. More particularly, when a given portion of one platform is placed in vibration and rubs against an adjacent platform tending to draw the adjacent platform into the same vibrational mode, the effective mass in vibration may be greater or less than that where the first platform is not engaged by the second. Similarly, the spring constant of the two vibrating masses may be different and may supplement one another. These interactions may be utilized to adjust the resonant frequencies of vibration of platforms utilizing the present invention while the frictional damping means of these devices are used to reduce vibrational amplitude as described hereinabove.

Thus may be described one embodiment of the present invention. It is readily apparent that those skilled in the art may make substantial variations of the structure presented herein without departing from the spirit of the present invention. For example, the particular configuration of the circumferential extremities of the present blade platforms may be varied substantially but their function maintained. One such example was mentioned hereinabove wherein it was stated that the oppositely angled edges of one extremity might be radially displaced from one another rather than axially. Equalivantly, a tongue and groove cooperation might be established between adjacent blade platforms which would serve to both frictionally damp the amplitude of vibrations and partially tune the resonant frequencies thereof. Such variations are intended to be comprehended within the scope of the following claims.

Claims

What is claimed as new and intended to be secured by Letters Patent of the United States is:

1. A turbomachinery rotor blade for disposition within a fluid flow path in cooperation with a rotatable disc, the blade comprising:

an airfoil;

mounting means extending radially from the airfoil for attaching the blade to the disc; and

a platform disposed between the airfoil and the mounting means, the platform comprising first and second circumferential extremities and first means for abutting and interlocking said platform radially, axially and circumferentially with a similar adjacent platform to partially define the flow path and for frictionally damping radial and circumferential relative movement between said platform and said adjacent platform, said first means including first and second diagonal edges on said first extremity of said platform, said first diagonal edge being canted from the radial direction by a predetermined first angle, said second diagonal edge being canted from the radial direction by a predetermined second angle of opposite sense from said first angle, said first and second diagonal edges positioned to abut and interlock with an edge of the adjacent platform in sliding frictional engagement therewith and further positioned to effect damping of radial and circumferential relative movement between said diagonal edges and said adjacent edge during said frictional engagement.

2. The blade of claim 1 wherein said first and second diagonal edges lie axially upstream and downstream relative to one another.

3. The blade of claim 2 wherein said first and second diagonal edges lie along a substantially common axial line.

4. The blade of claim 3 wherein said first circumferential extremity is divided approximately into halves axially, one of said halves comprising said first edge, and the second of said halves comprising said second edge.

5. The blade of claim 1 wherein said first means includes third and fourth diagonal edges on said second extremity, said second extremity being disposed to the side of the platform laterally opposite said first extremity, said third diagonal edge canted from the radial direction by a predetermined third angle and said fourth diagonal edge canted from the radial direction by a predetermined fourth angle of opposite sense from said third angle.

6. The blade of claim 5 wherein said first and third angles are substantially equal and said second and fourth angles are substantially equal.