Blade Constraining Structure

Abstract

In an axial flow apparatus including stator and rotor elements, means are included for constraining axial movement of individual blade members relative to the rotor axis when the rotor is subjected to high centrifugal loading. The constraining means include a ring which is circumferentially expandable under centrifugal loading. Part of the outward radial force causing the expansion of the ring is directed axially against the blade members to provide an axially constraining force which is proportional to the centrifugal loading on the rotor. The invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of the Army.

Description

This invention generally relates to a blade constraining structure for use in an axial flow apparatus of the type having stator and rotor elements and more particularly to structure for constraining axial movement of blades under high centrifugal loading in axial flow compressors.

In axial flow compressors, blades are commonly retained by inserting root portions into either circumferentially or axially extending dovetail slots around the periphery of rotor elements. It is well known to retain the blades in the forward stages of a compressor by means of axially extending dovetail slots while the smaller blades in the aft stages are retained by means of circumferentially extending dovetail slots. The reason is simply that circumferential dovetail slots, due to inherent strength limitations, are not as well suited for retaining the larger and heavier forward stage blades under high centrifugal loading as are the axially extending dovetail slots.

Problems arise, however, from the use of axially extending dovetail slots. Axial translation of the blade root portions is extremely difficult to constrain, particularly under high centrifugal loading and any change in the axial position of the blade alters the rotor balance leading to severe vibrational damage. On today's high speed engines the problem becomes multiplied due to the more critical nature of the rotor balancing.

Therefore, it is an object of this invention to provide a new means for constraining the axial movement of blades relative to a rotor axis when the blades and rotor are subjected to high centrifugal loading.

It is also an object of this invention to provide a new means for imparting an axially constraining force on rotor blade elements, such that the force applied is directly proportional to the centrifugal loading on the rotor and blade elements.

The blade constraining structure of an axial flow compressor includes rotor and stator elements. A first rotor element has axially opposed faces together with a plurality of open ended generally radial slots extending from one face to the other. Each slot has a pair of side walls which are undercut to provide inwardly directed abutment faces. A second rotor member is axially spaced from the first rotor and includes an axially extending, circumferential flange. The tip of the flange is closely spaced opposite the slots of the first rotor and is axially undercut to provide an inwardly directed surface.

A wedge ring has an outer annular surface which is axially overcut to provide an outwardly directed surface which abuts the inwardly directed surface of the flange. Under centrifugal loading the wedge ring circumferentially expands, and the overcut surface slides over the undercut surface in both an outward radial direction and an axial direction, effecting both a radial and axial translation of the wedge ring.

A plurality of blades having root portions are inserted into the slots so that the forward inserted face of each root abuts the wedge ring. Each root has a thickened portion to provide outwardly extending shoulders which abut the inwardly extending shoulders of the slots, thereby radially retaining the blades under centrifugal loading. After insertion, locking means constrain retraction of the other face of each root so that under centrifugal loading, the axial translation of the wedge ring is directed against the abutting root face and cooperates with the locking means for further restrict axial movement of the blades. The wedge ring provides an additional advantage of serving as an extremely effective air seal between rotor elements.

While the specification concludes with claims distinctly claiming and particularly pointing out the invention described herein, it is believed that the invention will be more readily understood by reference to the discussion below and the accompanying drawings in which:

FIG. 1 is a partial cross-sectional view of an axial flow compressor embodying the invention;

FIG. 2 is a section view taken along the line 2--2 of FIG. 1;

FIG. 3 is a section view taken along the line 3--3 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a partial cross-sectional view of two intermediate stages of an axial flow compressor of the type that is suitable for use in a gas turbine engine. Referring now to both FIGS. 1 and 2, two intermediate stages of an axial flow compressor are shown generally at 10. The axial flow compressor includes an annular outer casing 20. A rotor assembly is shown generally at 11 and includes first and second disc like rotor elements 12 and 14 which are rotatably disposed around a central axis 15.

Rotor disc 12 has opposed faces 13 and 17 and includes a plurality of open-ended, uniform dovetail cross-sectional slots 16 in its periphery for retaining blades 24. The slots 16 extend from the face 13 to the face 17 of the rotor disc 12 and each has side walls 21 as shown in FIG. 2 which are undercut to provide inwardly directed abutment faces 18 on the side walls 21.

Each of the blades 24 has a platform 29 and a root portion 26 which is thickened at the inner radial end to provide outwardly extending shoulders 19 which abut the inwardly directed faces 18 of the dovetail slot, thereby radially retaining the blades under centrifugal loading.

Referring to FIG. 1 only, there is shown an adjacent rotor disc 14 which includes a first axially extending flange member 31, the end of which mates with rotor disc 12 to prevent circumferential slippage between the rotor elements. The mating structure may include a series of interlocking male splines 51 and female splines 52 as shown in the limited cross-sectional view of FIG. 3. An air seal may be provided between the interlocking male and female splines by an annular resilient plastic ring 53 inserted into abutting relation with the inside surfaces of the interlocking splines. The plastic ring 53 is maintained in fixed relation to the splines during rotation by the centrifugal forces which urge outward circumferential expansion of the ring.

Rotor disc 14 includes a plurality of circumferentially spaced blades 25 disposed about its periphery. The blades 25 may be radially and axially retained by means of a circumferential dovetail groove 37 extending around the outer periphery of the rotor disc 14. Each blade 25 includes a dovetail root section 27 for engagement with the opposing surfaces of the dovetail groove 37 and must be inserted through a notch (not shown) in the dovetail groove 37 and circumferentially slid around the slot into abutting relation with an adjoining blade. When the last blade 25 has been inserted through the notch in the dovetail slot so that all root sections 27 are in abutting relation, the notch may then be locked in a manner well known to the art.

An intervening row of stator vanes 22 is shown disposed between blades 24 and 25 and is fixedly attached to compressor casing 20. The vanes may also be rotatably attached to casing 20 in a manner well known to the art which is described in U.S. Pat. No. 3,216,700 filed by R. H. Bostock Jr., and assigned to General Electric Company.

Rotor disc 14 includes a second axially extending, circumferential flange member 32, the annular tip portion 35 of which is in close proximity to the trailing edge of the blade platform 29 in order to establish a smooth flow path and minimize flow losses. The outer tip portion of the flange member 32 is axially undercut to provide an inwardly directed surface 34.

Blades 24 are individually inserted into their respective dovetail slots 16 from a forward axial direction and retained from outward radial travel under centrifugal loading by the interaction of the blade roots 26 and the dovetail slots 16 in the manner previously described. Axial movement of the blades, however, is also critical and any shift of the axial blade position would alter the rotor balance, causing vibrational damage. Accurate axial blade retention even under high centrifugal loading is accomplished in the following manner.

Before insertion of the individual blades 24, a wedge ring 30 is stationed in abutting relation to the inwardly directed surface 34 which axially undercuts the outer tip 35 of the flange member 32. An outside surface 36 of the wedge ring is axially overcut to provide an outwardly directed surface for sliding contact with the inwardly directed surface 34. The undercut and overcut surfaces of the flange 32 and wedge ring 30 are preferably nominally planar, although curvilinear surfaces could also be utilized. The wedge ring may be formed of any resilient, plastic-like material which allows the circumference of the ring to be expanded slightly without fracture. Although the wedge ring must exhibit resiliency, it must also be of sufficient stiffness to resist axial cross-sectional compression without substantial deformation. A highly suitable plastic for use in the wedge ring of this invention is polyimide sold under the trade name of Vespel.

After the wedge ring is seated, the individual blades 24 are inserted into their respective dovetail slots 16. The blades 24 are inserted so that the forward inserted face 39 of the root portion 26 of each blade abuts against the wedge ring 30, thereby prohibiting further forward axial movement of the blades. Withdrawal of the blades 24 from the dovetail slots 16 is prevented by a locking ring 44. The rotor disc 12 is provided with a plurality of circumferentially spaced apart projections 40, axially extending from the face 13 thereof. Each projection 40 has a downward extending lip 42 for receiving and retaining the locking ring 44. The projections 40 are spaced radially outward to coincide with the spaced apart dovetail slots 16 so that locking ring 44 abuts the root portions 26 of the individual blades, thereby constraining axial movement.

Upon rotation of the rotor assembly 11, centrifugal force operates on the wedge ring 30, causing it to circumferentially expand. Circumferential expansion of the wedge ring causes the overcut surface 36 of the wedge ring to slide outward along the abutting undercut surface 34 of the flange 32. The abutting undercut surface of the flange 32 restrains the outward radial movement of the wedge ring 30, imparting an axial component thereto, so as to increase the axial constraining force operating on the individual blades 24.

Therefore, a portion of the centrifugal force operating on the wedge ring is directed to the forward inserted faces of the blade roots and cooperates with the locking ring to further restrict axial movement of the blades upon rotation of the rotor. The wedge ring also serves as an effective air seal between rotor elements preventing high pressure compressor air from leaking out of the main flow path into the cavities between the rotor elements.

Although the invention has been described in relation to an axial flow compressor, it is readily understood to have broader application. It may be useful for maintaining the axial integrity of axially inserted blades in any high speed rotating part such as the fan assembly of a turbofan engine.

Claims

Having thus described one embodiment of the invention, what is desired to be secured by letters patent is as follows:

1. In an axial flow apparatus having stator and rotor elements, a blade constraining structure comprising: a first rotor element having axially opposed faces and a plurality of open-ended generally radial slots extending from one face to the other, with each slot having side walls which are undercut to provide inwardly directed abutment faces; a second rotor member axially spaced from said first member and including an axially extending circumferential flange, the tip of which is in closely spaced opposing relation to said slots and is axially undercut to provide an inwardly directed surface; a wedge ring whose outer radial annular surface is axially overcut to provide an outwardly directed surface abutting said inwardly directed surface of said flange, such that under the influence of centrifugal loading said wedge ring circumferentially expands and said overcut surface slides over said undercut surface in both an outward radial direction and an axial direction, effecting an axial translation of said wedge ring; a plurality of blades having root portions inserted into said slots so that the forward inserted face of each root abuts said wedge ring, wherein each root is thickened to provide outwardly extending shoulders which abut the inwardly extending shoulders of said slots so as to radially retain said blades under centrifugal loading; locking means constraining the other face of each root to inhibit axial movement of said roots such that under centrifugal loading the axial translation of said wedge ring is directed against the abutting root faces and cooperates with said locking means to further restrict axial movement of said blades.

2. The structure of claim 1 wherein said second rotor member includes a plurality of circumferentially spaced blades disposed about its periphery wherein said blades are radially and axially retained by means of a circumferential dovetail groove around the outer periphery of said second rotor member.

3. The structure of claim 1 wherein said wedge ring provides an air seal between the rotor elements.

4. The structure of claim 1 wherein said wedge ring is a plastic-like resilient material which allows the circumference of the ring to be expanded slightly without fracture, yet is of sufficient stiffness to resist axial cross-sectional compression without substantial deformation.

5. The structure of claim 4 wherein said wedge ring is polyimide.

6. The structure of claim 4 wherein said locking means includes a locking ring abutting the other faces of said blade roots and maintained in abutting relation by a plurality of projections axially extending from said first rotor with each projection including an inward extending radial lip overlapping said locking ring.

7. The structure of claim 6 wherein said first and second rotor members are mated by a series of axially extending interlocking male and female splines so as to prevent circumferential slippage between the rotor elements.

8. The structure of claim 7 including an annular resilient plastic ring, the outside surface of which is in abutting relation with the inside surfaces of the interlocking splines to provide an air seal therebetween.