GB2301867A - Supporting unison rings in pivotable vane actuating mechanisms - Google Patents

Abstract

A gas turbine engine has a geared unison ring 20,(20a)(Fig.2) which is common to an array of pivoting vanes and drives them via levers 14,(14a) in the form of gear segments. The unison ring 20,(20a) is separated from the associated engine casing 10,(10a) by an intermediate support ring 28,(28a) which engages the unison ring 20,(20a) through anti-friction bearings 24,(50) and the engine casing through radially sliding joints 36,(46), so isolating the unison ring from the effects of differential thermal expansion between the unison ring and the casing. The sliding joints may comprise spigots 36, sliding in holes 32, or dogs (46) sliding in slots (45), and the antifriction bearing may comprise rollers 24 or pads (50).

Description

IMPROVEMENTS IN PIVOTABLE VANE ACTUATING MECHANISMS The present invention relates generally to actuator mechanisms for variable angle vane arrays of the kind used in axial fluid flow machines, such as aerofoil-bladed turbines and compressors. The vane angles are varied by making the vanes pivotable about their major axes, so as to enable variation in their angle of fluid delivery to an associated rotor stage.

In mechanisms for achieving pivoting of the vanes, it is usual to connect each vane via respective levers to a common actuating or unison ring surrounding, and mounted for rotation around, the machine casing. Hence, when the unison ring is rotated about the machine axis, the vanes pivot in unison to either increase or decrease the fluid flow deflection angle.

In one known type of mechanism, the lever arms comprise gear segments, one end of the lever arm including the gear circle centre, which is fixed to a respective vane, the other end comprising the toothed rim of the gear segment. The gear segment teeth mesh with a toothed unison ring to enable simultaneous pivoting of the vanes.

Problems can arise when this type of mechanism is used in situations where different parts are at substantially different temperatures. Hence, in the example of a turbine casing of a gas turbine engine, the unison ring must necessarily be supported from the casing it surrounds. For best actuation of the gear segment lever arms, the unison ring should be supported concentrically with respect to the casing.

For its rotation around the casing, support through rollers, friction pads, or the like is conventional. Since turbine operating temperatures are high, a large clearance is required between the rollers or friction pads and the ring when the casing is cold, before start-up of the turbine. This large clearance causes the ring to lie eccentrically of the turbine casing axis when the turbine casing is cold. Consequently, during the early stages of operation, movement of the unison ring causes uneven pivoting of the vanes with respect to their circumferential position around the casing, i.e., there is variation in the magnitudes of relative pivotal movement between the vanes; flow delivery to the turbine is thus uneven which results in vibration being generated in the turbine rotors downstream of the vanes.

Resolution of this problem by simple reduction of the cold clearance can cause seizure of the ring, or deformation thereof, by virtue of the rollers or friction pads being forced into it on expansion of their seatings. This latter fault again generates uneven vane pivoting and the abovementioned vibration.

The present invention seeks to provide an improved pivotable vane actuating mechanism.

According to the present invention, an actuator mechanism for a variable angle vane array in a fluid flow machine comprises; a fluid flow machine casing, a unison ring connectable to lever means for applying pivoting movement to each vane in the array of vanes, the unison ring surrounding the casing in spaced coaxial relationship for rotation circumferentially of the casing, and intermediate support structure supporting the unison ring though anti-friction means, wherein the intermediate support structure is connected to the casing through at least three radially slidable joints, the intermediate support structure supporting the unison ring substantially concentrically of the ' casing for radial expansion and contraction of the unison ring and the casing relative to each other within predetermined limits.

Preferably, the unison ring comprises a gear ring and the lever means comprises a gear segment attached to each vane in the array. In such a case, bias means is advantageously provided to urge the unison ring into engagement with the gear segments. This ensures firm meshing of gear teeth on the gear segments with the unison ring.

Conveniently, the bias means comprises a plurality of spring mechanisms acting directly between the intermediate support structure and the unison ring.

The anti-friction means may comprise, for example, rolling element bearings or pads of an anti-friction material.

In one embodiment of the invention, the radially slidable joints comprise spigots extending radially outwardly of the casing into corresponding holes in the intermediate support structure. However, in a preferred embodiment, the radially slidable joints comprise dogs extending radially inwardly from the intermediate support structure into corresponding slots in a flange structure of the casing.

The invention will now be described, by way of example and with reference to the accompanying drawings, in which: Fig 1 is a cross-sectional part view through a vane actuation mechanism in accordance with the present invention, the view being on a plane containing the axis of an associated gas turbine engine casing, and Fig. 2 is a cross-sectional part view similar to Fig. 1, but showing a further embodiment of the present invention.

Referring to Fig 1, a radially outer end 2 of each pivotable vane 4 in an annular array of such vanes arranged within an engine casing 10, is provided with a cylindrical spindle 6. This is held for pivoting movement of the vane by a spherical bearing 8 within a housing 9 bolted to the casing 10. The outer end of the spindle 6 is provided with a taper feature 12 and a threaded stud 13, by means of which a vane actuation lever 14 is secured thereto so that there can be no relative movement between the lever 14 and the spindle 6.

Each lever 14 is in the form of a gear segment having a toothed rim with gear teeth 16. For further details of a shape and configuration of this lever, our copending

application numbers NC 30/94/and NC 31/94 filed as British patent applications on the same day as the present application, should be consulted and are incorporated herein by reference.

Each lever's teeth 16 mesh with further gear teeth 18 on a unison ring 20, which surrounds the engine casing 10 and is supported coaxially of the casing for limited rotation relative thereto. As well known in the industry, an hydraulic ram mechanism (not shown), or the like, may be used to obtain the required circumferential movement of the unison ring 20 relative to the casing 10. Thus, rotation of the ring 20 by the ram mechanism will, through the levers 14, pivot each vane 4 about an axis 22 of the respective spindles 6.

The vanes together comprise a variable area nozzle for a power turbine rotor stage (not shown) downstream of the vanes, and the pivoting of the vanes as above therefore changes the throat area of the nozzle.

The unison ring 20 is supported about the axis of the casing 10 as follows. The outer circumference of the unison ring 20 is formed as the inner track 23A of a roller bearing, a number of rollers 24 being located between track 23A and an outer track 23B formed on an outer flange 26 of a support ring 28, the outer flange 26 projecting forwardly towards the vanes 4. The ring 28 has an inner flange 30 formed thereon which projects rearwardly, away from the vanes 4. To enable appropriate fixing of the support ring 28 to the casing, inner flange 30 is provided with at least three, and preferably many more, radial holes 32 therethrough, which are equiangularly positioned about the axis of casing 10. These comprise bearing bushes (not shown) which receive hollow dowels or spigots 36 as a sliding fit in order to locate the support ring 28 relative to a rearwardly extended flange 34 of the casing 10, from which the spigots project in the radial direction, the flange 34 extending parallel to the underside of the unison ring. This construction constrains the support ring 28 axially with respect to the casing, so that it participates in any expansion or contraction of the casing flange 34 in the axial direction, while allowing it to slide radially on the spigots to accommodate radial expansion and contraction of the casing 10, unison ring 20 and the support ring 28 relative to each other.

Assembly of the structure described above is by first loading the rollers 24 into a bearing cage (not shown) on the unison ring 20, sliding the ring 20 into the bore defined by the flange 26 on the support ring 28, then aligning the resulting sub-assembly coaxially with the axis of the casing 10 and sliding it forward towards the vane actuating levers 14 over flange 34 until a rear lip 35 of the support ring's flange 30 contacts the rear of the casing flange 34.

The spigots 36 are then inserted through holes 32, 38 in the support ring 28 and flange 34 respectively, and fixed therein, e.g., by nuts and bolts (not shown), for which the spigots 36 act as spacers. Alternatively, the spigots may be retained in the holes 38 by welding, or screw threads, or an interference fit, or by peening or swaging the lower ends of the spigots onto the underside of the flange 34.

When the turbine casing 10 is cold, the spigots 36 will maintain the sub assembly of unison ring 20 and support ring 28 in coaxial alignment with the casing, by effectively preventing relative movement between the support ring 28 and flange 34 in any direction which has a component which is normal to the radial lines on which the spigots lie. The phrase "effectively prevent" is used in acknowledgement that the limits to which the spigots 36 are manufactured will allow minimal relative movement between them and the bushes fixed in holes 32. However, manufacturing tolerances are magnitudes smaller than deliberately defined cold clearances and a consequence of this is that if relative movement away from the co-axial relationship occurs between the subassembly and the flange 34, its effect on vane movement is negligible.

During operation of an engine which includes the structure of the present invention, the casing 10 expands radially of the ermine axis. In so doing, the flange 34 expands therewith, resulting in the spigots 36 sliding through the holes 32 in the support ring 28. Ring 28, and therefore the unison ring 20, are thus unaffected in the radial sense by the aforementioned expansion and consequently do not cause asymmetric pivoting of the vanes 4. Note that radial expansion of the casing 10 with respect to the support ring 28 is allowed for by judicious sizing of the clearance xl between the radially inner side of the support ring 28 and the radially outer side of the casing flange 34.

To ensure firm meshing contact between the gear segment teeth 16 and the unison ring teeth 18, it is necessary to have some means of biassing the unison ring 20 forwardly against the gear segments 14, such as a spring arrangement similar to that shown and described in relation to Fig. 2, below.

Referring now to the alternative embodiment of Fig. 2, the engine casing 10a comprises axially successive portions 10b, 10c, which are bolted together at their respective radially outwardly turned flanges 40, 48. The bolts 49 are regularly spaced around a bolting diameter of the flanges.

Sandwiched between the flanges 40, 48, and penetrated by the same bolts 49, is a spacer 47. This has at least three rectangular slots 45 cut into its outside circumference, and preferably many more - say, 24 for a 1.2 metre diameter flange. The slots 45 are equi-angularly spaced about the engine axis.

When the casing flanges 40 and 48 are bolted together, the internal faces of the flanges form the front and rear sides of the slots 45, which receive radially inwardly projecting rectangular dogs 46 provided on a support ring 28a.

Dogs 46 are equal in number and spacing to the recesses or slots 45 and proportioned so as to be a radial sliding fit therein. They are thereby axially retained in the recesses 45.

It should be noted that Figure 2 is a composite sectional view, showing the slots 45 and the bolts 49 as though they occupy the same circumferential positions on the spacer 47, whereas they are circumferentially separated features; the bolts 49 do not pass through the spaces occupied by the dogs 46 in the slots 45.

The support ring 28a, through friction pads 50, supports a unison ring 20a, which in turn is connected via gear teeth 16a, 18a to vane pivoting gear segments 14a, only one of which is shown. Friction pads 50 are secured to the unison ring 20a and bear against a track 51 formed on the support ring 28a.

Unison ring 20a is retained on the support ring between predefined limits of axial (forward or rearward) movement.

In the rearward direction unison ring 20a is retained by a backplate 52, which is bolted onto the support ring, and in the forward direction by means of a lip 53 on the support ring which interacts with a similar complimentary lip 54 provided on the unison ring. Radial expansion of the casing 10a with respect to the support ring 28a is allowed for by judicious sizing of the clearances x2 between the radially inner ends of the dogs 46 and the radially inner ends of the recesses 45.

As for the Fig. 1 embodiment, positive engagement of the unison ring 20a with the toothed rims of the gear segments 14a is necessary to maintain consistent vane angles throughout the vane array at all engine conditions. To this end, several equiangularly spaced spring mechanisms, each comprising a coil spring S loading a plunger P within a housing 55, are provided in the backplate 52 of support ring 28a so that thea plungers P bear against the rear face of the unison ring 20a at all times to urge the teeth 16a,18a of the segments and ring respectively into meshing contact with each other. Other sorts of spring bias arrangements could also be used, e.g., leaf springs or the like.

Whether the engine casing 10a is hot or cold, the interaction between the slots 45 and the dogs 46 will maintain the unison ring 20a in substantially constant axial registration with the engine. When the casing 10a is hot, its integral flanges 40 and 48, with spacer 47, will expand relative to the support ring 28a and the dogs 46 formed thereon, thus providing the same advantages as the embodiment of Fig 1.

The person skilled in the art will realise that the rollers 24 of Fig. 1 may be substituted by the friction pads 50 of Fig. 2 and vice versa.

Taking an overview of the above described embodiments, at least the following general features may be summarised as common to both Figs. 1 and 2. A toothed unison ring (20, 20a) is carried by bearing elements (24, 50) from an intermediate support ring (28, 28a), which in turn is held concentric to the turbine casing (10) through radially sliding joints (36, 46). These utilise the principle of cross-key location to allow radial expansion or contraction of the casing relative to the unison ring while maintaining concentricity between the unison ring and the casing. To allow limited axial movement of the unison ring (20, 2ova) on the support ring (28, 28a), while ensuring firm meshing of the unison ring with the gear segments (14, 14a), thereby minimising backlash during axial expansion and contraction of the casing, the unison ring is biassed into engagement with the gear segments by a spring mechanism (S,P) acting on its rear side.

These measures allow correct scheduling of the vane angles, and hence of the variable nozzle throat area, at all engine conditions.

Claims (10)

Claims:

1. An actuator mechanism for a variable angle vane array in a fluid flow machine, comprising; a fluid flow machine casing, a unison ring connectable to lever means for applying pivoting movement to each vane in the array of vanes, the unison ring surrounding the casing in spaced coaxial relationship for rotation circumferentially of the casing, and intermediate support structure supporting the unison ring though anti-friction means, wherein the intermediate support structure is connected to the casing through at least three radially slidable joints, the intermediate support structure supporting the unison ring substantially concentrically of the casing for radial expansion and contraction of the unison ring and the casing relative to each other within predetermined limits.

2. A mechanism according to claim 1, in which the unison ring comprises a gear ring and the lever means comprises a gear segment attached to each vane in the array.

3. A mechanism according to claim 2, in which bias means is provided to urge the unison ring into engagement with the gear segments to ensure firm meshing of gear teeth on the gear segments with the unison ring.

4. A mechanism according to claim 3, in which the bias means comprises a plurality of spring mechanisms acting directly between the intermediate support structure and the unison ring.

5. A mechanism according to any preceding claim, in which the anti-friction means comprise rolling element bearings.

6. A mechanism according to any one of claims 1 to 4, in which the anti-friction means comprise pads of an antifriction material.

7. A mechanism according to any preceding claim, in which the radially slidable joints comprise spigots extending radially outwardly of the casing into corresponding holes in the intermediate support structure.

8. A mechanism according to any one of claims 1 to 6, in which the radially slidable joints comprise dogs extending radially inwardly from the intermediate support structure into corresponding slots in a flange structure of the casing.

9. A mechanism substantially as described in this specification with reference to Fig. 1 of the drawings.

10. A mechanism substantially as described in this specification with reference to Fig. 2 of the drawings.