GB2128686A

GB2128686A - Turbine overspeed limiter

Info

Publication number: GB2128686A
Application number: GB8326777A
Authority: GB
Inventors: Derek Aubrey Roberts; Greensted Roy Simmons; Graham John Jeffery
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1982-10-06
Filing date: 1983-10-06
Publication date: 1984-05-02
Also published as: GB2128686B; GB8326777D0

Abstract

The turbine of a gas turbine engine is provided with a mechanism 25 designed to tilt the nozzle guide vane segments 26 into the path of the turbine rotor blades 65 to decelerate the rotor if a shaft connecting the turbine rotor to a compressor rotor breaks. In one arrangement the segments 26 are supported on structure 42. When the shaft breaks, the rearward movement of the rotor unlatches the segments 26 from structure 42 and causes them to tilt into the path of the turbine rotor blades 65. The segments 26 and the turbine rotor blades 65 destroy each other and the resulting debris is ejected out of the engine jet pipe destroying further downstream turbine stages. <IMAGE>

Description

SPECIFICATION Turbine overspeed limiter for turbomachines This invention relates to a mechanism for preventing a turbine rotor of a gas turbine engine rotating at an unsafe speed.

A primary requisite in the design of gas turbine engines is that a failure of any component of the engine should not jeopardise the safety of the aircraft to which the engine is fitted, no matter how remote the likeliness of such a failure may be.

This invention addresses itself specifically to the problem of the failure of a shaft which connects a turbine rotor to a compressor or fan rotor.

During normal running the compressor and turbine rotors run at speeds up to predetermined maximum. The aerodynamic forces on the blades of the turbine drive the compressor, and the aerodynamic forces on the compressor oppose the rotation of the turbine rotor. Similarly, the axial load on the turbine is largely balanced by the axial load on the compressor. If a shaft connecting the turbine rotor to the compressor rotor were to break the aerodynamic loads on the turbine rotor accelerate it very rapidly (within a few milliseconds) as there is no opposition provided by the compressor rotor. Consequently, the turbine rotor can accelerate to a speed at which the disc or drum retaining the turbine blades bursts. The blades and disc fragments are then released and subject to an extremely high centrifugal force which can propel them through the engine casings.To provide structure to ensure that in these extreme, and unlikely conditions, all the ejected blades and disc fragments are contained within the engine casings would be very heavy and costly. There is, therefore, a risk that one or more of the blades or disc fragments could damage the aircraft.

The design of the attachments of the compressor rotor to its driving turbine and to the thrust bearing supporting the shaft may be such that if the shaft fails, the turbine rotor is not supported in the thrust bearing but is free to move axially, under the influence of its axial load, and is no longer balanced by the compressor.

It can be shown that simply allowing the rotor to run against a fixed stator structure downstream of the rotor will have no appreciable effect in slowing the rotor down because the heat generated by friction would melt the surfaces of the rotor and the stator vane structures and provide liquid metal lubrication of the rotor for a greater time than it takes for the disc to burst.

The present invention resides in the appreciation that it is possible to design a structure which makes use of the axial movement of the rotor to initiate deceleration of the rotor to safe speeds at which the blades are less likely to be ejected through the engine casings.

An object of this invention is to provide a mechanism for preventing a turbine rotor of a gas turbine engine exceeding a predetermined speed if a shaft, connecting the turbine rotor to a compressor rotor, breaks and releases its torsional and axial constraint on the turbine rotor.

The present invention, as claimed, makes use of the rearwards axial movement of the turbine rotor when the shaft breaks to initiate the release of segments of a stator vane assembly immediately downstream of the turbine rotor and cause them to collide with the turbine rotor blades to decelerate the rotor by destroying the rotor blades in a controlled manner.

The invention will now be described, by way of an example, with reference to the accompanying drawings in which, Figure 1 illustrates, schematically, a gas turbine engine incorporating a mechanism 25, constructed in accordance with the present invention, for preventing a turbine rotor 14 exceeding a predetermined speed if the shaft connecting the turbine rotor to a compressor rotor breaks, Figures 2, 5 and 8 are radial cross sectional views of the turbine 14 of the engine of Figure 1 and show, in more detail, various mechanisms 25, during normal engine running, Figures 3 and 6 are radial cross sectional views of the turbine 14 of the engine of Figure 1, and show, in more detail, the mechanism 25 of respectively Figures 2 and 5 after the shaft has broken and the rotor has moved rearwards, Figures 4 and 7 illustrate alternative ways to that shown in Figures 2 and 6 of monitoring the NGV segments, and, Figures 9 and 10 illustrate alternative ways to that shown in Figure 8 of mounting the NGV segments.

Referring to Figure 1 there is shown a two spool gas turbine engine of the by-pass type. The engine comprises, a low pressure compressor fan 12 driven by a low pressure turbine 14, a multistage axial flow high pressure compressor 1 6 driven by a high pressure turbine 18, a combustion chamber 20 and a jet pipe 22.

The mechanism for preventing the turbine 14 exceeding a predetermined safe speed in the event of the shaft 24 (which connects the turbine rotor 14 to the compressor fan 12) breaking, is shown by the reference numeral 25. For convenience, only the turbine 14 is shown as incorporating the mechanism 25 although it is to be understood that the turbine 18 may also incorporate a similar mechanism to that shown by the numeral 25 if desired.

Referring specifically to Figures 2 and 3, the turbine 14 is, in this example, a two stage turbine.

The mechanism 25 for preventing the turbine rotor overspeeding is constituted, in part by the inter-stage nozzle guide vane assembly 26. The NGV assembly 26 comprises a plurality of segments 26 each consisting of one or a plurality of stator vanes 28 extending between inner and outer shrouds 30, 32 respectively.

The outer casing 34 of the turbine is provided with an inward projecting flange 36 that forms an abutment face against which the outer shrouds 32 bear, and constrains the segments 26 against bodily movement rearwards.

The radially inner ends of the segments 26 are each provided with two axially spaced flanges 38, 40 projecting radially inwards. Each of these flanges 38, 40 is provided with a hook 41, 43 that locates in recesses in inner structure 42 of the engine.

The inner structure 42 comprises a cylindrical member 45 which has two flanges 44, 46 projecting radially outwards. The flanges 44, 46 are relatively flexible in bending in an axial direction.

The first flange 44 is located at an upstream region of the structure 42 and is constructed to provide a radially outward facing abutment face 48 and a forward facing abutment face 50 against which the flange 38 bears. The second flange 46 is located at a downstream region of the structure 42 and is constructed to provide a radially inward facing abutment face 52 and a rearward facing abutment face 54 against which bears the flange 40. In this way the flange 44 forms a strut and the flange 46 forms a tie that opposes the axial turning moment on the nozzle guide vane assembly 26 due to gas loads. The principle axial constraint of the segments 26 is provided at the outer ends of the segments 26 by the flange 36. The axial load on structure 42 in normal running is transmitted to the segments 26 by the flange and face 54 and is reacted by the outer casing flange 36.Bending moments on the segments 26 are resisted by the faces 48 and 52 in flanges 44 and 46 respectively. Torsional gas loads on the stator vanes 28 are reacted by dogs 56 on the outer ends of the segments which contact stops 58 on the outer casing 34.

The segments 26 are provided with a forward facing abutment face 60 (in the form of circumferential recess) and the outer casing 34 is provided with a rearward facing abutment face 62 (again in the form of a circumferential recess).

Circumferentially spaced bridging members 64 are located between the abutment faces 60, 62 and serve to hold the outer ends of the segments in position against the flange 36. The members 64 also provide a fulcrum at their downstream ends when it is required for the segments 26 to tilt into the path of the rotor blades 65, as will be explained later.

To prevent the flanges 44, 46 unlatching and releasing the inner ends of the segments unintentionally due to gas pressures acting on the upstream side of flanges 44, the flanges 44 are provided with a vent 58 to allow pressurised gas to enter the interior of the hollow box defined by the flanges 44, 46 and the segments. The shapes of the turbine rotor and structure 42 are such that when rearwards movement occurs the first substantial contact takes place on the flange 44, for example by providing a cylindrical projection 66 on the turbine disc. The flange 44 is also provided with a cylindrical projection 68. These projections 66, 68 ensure that when the rotor moves axially as a result of shaft 24 breaking, the projection 66 is the first part of the turbine to strike the structure 42.

Rearward movement of the turbine rotor 14 thereby pushes the flange 44 rearwards to a position where it no longer acts as a strut and the segments collapse inwards (as shown in Figure 3). This causes the radially outer upstream ends of the segments 26 to tilt into the path of the blades 65 of the turbine rotor breaking them into smaller pieces, which are ejected rearwards into the next stage of the turbine rotor. The debris ejected into downstream stages of the turbine 14 destroys the blades of the downstream stages of the turbine rotor (and other turbines in those engines with further turbines downstream). In addition, the downstream inner ends of the segments 26 collide with the second stage of the turbine rotor 14 to destroy its blades as well.

Consequently the aerodynamic efficiency of the turbine is destroyed and the loss of power together with the physicai entanglement decelerates the rotor to a safe speed and prevents it bursting.

Referring to Figure 4 the flange 44 of the structure 42 on which the segments 26 are mounted, is constructed as separate flange which is bolted on to a short radial flange 70 on the cylindrical part of the structure 42 by bolts 72.

The flange 70 and the inner region of the flange 44 are scalloped so that they may be assembled by a bayonetting action and indexing the flange 44 around the flange 70 to align the bolt holes.

Referring now to Figures 5 to 7 there is shown a second embodiment of the present invention. In this embodiment there is again shown an interstage nozzle guide vane assembly 26 of the two stage turbine 14 of the engine of Figure 1.

Each segment 26 is provided at its radially inner end with two axially spaced flanges 142, 144, which project radially inwards. The inner ends of the segments 26 are held in place by a releasable means in the form of a locking member 146 which also constitutes the static part of a labyrinth air seal.

The segments 26 are provided with lugs 138 and the casing 34 is provided with lugs 136 adjacent the outer upstream end of each segment 26. The segments 26 are pivotally mounted on the casing 34 by means of hinge pins 140 which pass through the lugs 136, 138. The hinges 140 lie tangentially to each segment 26.

The locking member 146 is cylindrical and has at its upstream end a circumferential outer surface that constitutes a radially outward abutment face 148, on which the flanges 142 abut. The locking member 146 has a shoulder which constitutes a forward facing abutment face 1 50 against which the flanges 142 of the segments 26 bear. The locking member 146 is provided at its downstream end with a radial flange 1 52 projecting towards the segments 26.

The flanges 1 52 has a recess 1 54 facing in a forwards direction. The recess defines a hook 1 56 at the free end of the flanges 1 52.

The flange 144 of each segment is provided with a cylindrical portion which forms a hook 1 58 pointing rearwards. The hooks 1 58 locate in the recess 1 54. In operation, the gas loads on the NGV segments 26 impose a turning moment which tends to push the flanges 142 radially inwards, and pull the flanges 144 radially outwards. Therefore, the hook 1 56 on the locking member provides a radially inward facing abutment face 1 59 and a forward facing abutment face.

The locking member 146 is provided with a plurality of circumferentially spaced helical screw thread forms 1 60 which face towards the segments 26. The thread forms 160 mesh with complementary shaped thread forms 1 62 on each of the segments 26. To prevent the locking member 146 rotating relative to the segments 26 by accident, shear pins 1 64 are provided through the hooks 156 and 158.

The rotor 14 has an engagement means 166 in the form of a plurality of projections or serrations facing towards the NGV assembly 26. The segments 26 are also provided with projections positioned so that if the shaft 24 breaks and the rotor 14 moves rearwards to strike the locking member 146, the engagement means 1 66 engages the locking member and rotates it relative to the segments 26. This causes the shear pin to fracture and screws the locking member axially rearwards to disengage the hooks 1 58 from the recess 1 54. The segments are prevented from rotating relative to the outer casing 1 34 by the hinge pins 140 and circumferentially spaced dogs on the outer casing.

When the hooks 1 58 are released the gas loads on the segments 26 cause them to pivot about the hinge pins 140 and thereby swings the inner ends of the segment 26 rearwards and outwards.

This opens up a gap between the outer shrouds 32 of the segments. Rearward movement of the rotor causes the tips of the rotor blades to strike the segments and break off. The debris is contained within the outer casing and ejected rearwards down the jet pipe and destroys downstream stages of the turbine.

in addition the rear edge of the radially inner ends of the segments 26 move into the path of the second stage rotor and destroys its aerodynamic efficiency.

Referring now to Figure 7 the locking member 146 comprises a hollow cylinder with two radial flanges 1 68,1 70 respectively at the upstream and downstream ends of the member 146. The flange 1 68 forms a forward facing abutment face against which the flanges 142 of the segments bear. A circumferential surface of the member 146 at its upstream end forms a radially outward facing abutment surface 1 72 on which the flanges 142 of the segments bear.

The rear flange 170 is slotted (slots 172) and provided with an inward facing recess so that the flange 1 70 effectively forms a plurality of spaced hooks 1 74 which impose an inwards and rearwards constraint on the hooks 1 58 of the segment 26. The confronting faces 176, 178 of the hooks 1 58, 1 74 lie in helical planes so as to form a course screw thread.

In operation, when the rotor strikes the locking member 146 the locking member is rotated so that the hooks 158 move into the spaces between the hooks 174 and allow the locking member 146 to be pushed axially. This releases the hooks 1 58 allowing the segments to swing about the hinge pins as desribed above in connection with Figures 5 and 6. Shear pins 180 are provided to prevent the locking member 146 rotating unintentionally until it is struck by the rotor.

Referring to Figures 8 to 10 there is shown a third embodiment of the invention. Again in this embodiment there is shown an interstage nozzle guide vane assembly 26 of the two stage turbine 14 of the engine of Figure 1.

Each segment of the NGV assembly 26 is provided at its radially inner end with two members 238, 240 which project radially inwards. A first of the members 238 is provided at an upstream region of the segment 26 and comprises a radial flange 242 and a portion 244 which proje,ts in a forwards direction to form a hook 246. The second of the members 240 is provided at a downstream region of each segment 26 and comprises a radial flange 248 and a portion 250 which projects rearwards to form a hook 252.

Slots 254 are formed in the rear hooks 252 either by slotting the portion 250 or by providing gaps between the portions 250 of adjacent segments 26.

The inner ends of the NGV segments 26 are mounted in structure 256 of the engine which includes two components 258, 260. The first component 258 comprises a hollow cylinder having a radial flange 262 projecting towards the segments 26. The flange 262 has a circumferential recess 264 formed in a rearward facing face of the flange 262 to provide abutment faces 266, 268 which face rearwards (266) and radially outwards (268). The front hooks 246 of each segment 26 locate in the recess 264 and the component 258 thereby imposes an axially outwards and rearwards constraint on the segments 26. The components 258 also has a forward facing flange 270 which co-operates with the first stage rotor 14 to form an air seal.

The downstream end of the first component 258 is slotted so as to fit into the slot 254 in the hooks 246 to prevent the component 258 rotating relative to the segments 26.

The second component 260 comprises a hollow cylinder which carries the static part of a labyrinth interstage air seal 272. The component 260 has a radial flange 274 projecting towards the segments 26. This flange 274 is provided with a circumferential recess 276 in a forward facing face of the flange 276 to provide abutment faces 278, 280 which face radially inwards (278) and forwards (280). The hooks 252 of the second members 240 locate in the recess 276 and bear against the abutment faces 278, 280. The component 260 thereby imposes a radially inwards and axially forwards constraint on the second members 240 of the segment 26.

The first component 258 is provided with a helical thread form 282 on its inside circumferential surface. Similarly the second component 260 is provided with a thread form 284 on its outer circumferential surface, which meshes with the thread form 282 on the first components 258.

The first stage rotor 14 is provided with an engagement device 286 which is in the form of a conical surface 288 which, when the rotor 14 moves rearwards, engages a complementary shaped surface 290 on the second component 260 and forms a simple friction clutch.

If a shaft 24 breaks, the rotor 14 moves rearwards and pushes the first component 258 rearwards to maintain its constraint on the front hooks 246 of all segments 26. Simultaneousiy, the conical surfaces 288, 290 engage and the second component 260 is screwed axially rearwards relative to the first component 250 to release all the rear hooks 252. The gas loads on the stator vanes segments 26 cause them to rock about the fulcrum formed by the recess 264 in the flange 262. The downstream outer ends of the segments 26 open up circumferentially allowing the upstream regions of the segments 26 progressively to destroy the turbine blades 292. The aerodynamic efficiency of the turbine rotor is thus greatly diminished and the rotor decelerates to a safe speed.The debris from the rotor blades 292 is ejected rearwards and contained within the turbine casings 34 and destroys downstream stages of the turbine 14 and the turbine 18.

It can be arranged that as the segments 26 tilt into the path of-the first rotor stage, their inner downstream ends move rearwards into the path of the second stage rotor blades.

Referring now to Figure 9 there is shown a modification to the structure 256 on which the inner ends of the NGV segments 26 are mounted.

The structure comprises two components 294, 296. One of the components 294 provides a radially outward facing abutment face 266 and a rearward facing abutment face 268 against which the first member 238 bear. The component 294 comprises a hollow cylinder which has a radial flange 298 at its upstream end to provide the abutment faces 266, 268. The periphery of the flange 298 is slotted (slots 300) to impart flexibility to the flange 298. The other component 296 consists of two parts 302, 304 which are slotted together. The upstream part 302 comprises a hollow cylinder with a radial flange 306 by which it is bolted to the downstream part 304. The upstream part 302 has a radial flange 308 which has a flangible region 310.The flange 308 extends radially outwards and has a cylindrical lip 312 which encompasses the flange 298 of the component 294 and the outer side of the hooks 246 of the first members 238. The extremity of the lip 312 is slotted (siots 314) to impart flexibility to the lip 312.

The downstream part 304 of the second component 296 has a radial flange 316 which extends towards the segments 26. A recess 31 8 is provided in the flange 31 6 to provide a forwards facing abutment face 278 and a radially inwards facing abutment face 280 against which the hooks 252 of the second members 240 locate.

The downstream end of the component 294 is slotted to fit into the slots 254 in the hooks 252.

The first stage rotor 1 4 has, on its downstream side, a tungsten carbide tipped cutting tool 320 positioned to engage the frangible region 310 of the flange 308 when the shaft 24 breaks and the rotor 1 4 moves rearwards. At the same time as the flange 308 is cut through, the components 294, 296 are pushed rearwards and retain the front hooks 246 of the segments 26 and release the rear hooks 252. The gas loads on the nozzle guide vanes segments 26 produces a turning moment which pushes the upstream inner end of the segments 26 inwards and pulls the downstream end outwards. Consequently the gas loads tilts the segments 26 into the path of the turbine blades 292. The slotted flanges 298, 308 are flexible enough not to retrict the tilting movement of the segments 26 into the path of the turbine rotor blades 292.

Referring to Figure 10 the hooks 246 of the segments 26 are held by a unitary component 322 which is a hollow cylinder with two radial flanges 324, 326. The front flange 324 has a rearward facing recess 328 whereas the rear flange 326 has a forward facing recess 330. The component 322 is dimensioned so that axial movement of the component 322 retains the front hooks 246 but releases the rear hooks 252.

To prevent accidental unlatching of the rear hooks 252, shear pins 332 are provided through the front hooks 246 and the component 322. These shear pins 322 are designed to break only when the structure 256 is stuck by the rotor 14 as it moves rearwards when the shaft 24 breaks. To assist in the loading of the segments 26 into the recesses 328, 330, and also provide flexibility to the front flange 324, so as not to restrict the tilting movement of the segments 26 whilst still providing a fulcrum, the flange 324 may be slotted (slots 338). The segments 26 are loaded by inserting the rear hooks 252 into the recess 320 and dropping the front hooks 246 through the slots 338 into the recess 328. The segments 26 are then rotated around the recess 328 to move the hooks 246 out of alignment with the slots 338.

In the above described embodiments, the segments 26 are prevented from rotating relative to the outer casing 34 by circumferentially spaced lugs 340.

Claims

1. A mechanism for preventing a turbine rotor of a gas turbine engine exceeding a predetermined speed if a shaft, connecting the turbine rotor to a compressor rotor, breaks and releases its torsional and axial constraint on the turbine rotor, the mechanism comprising a segmented statorvane assembly downstream of a stage of the turbine rotor, a constraining means operable to constrain the radially outer ends of the segments of the assembly against displacement bodily in a downstream direction, and static structure of the engine on which the radially inner ends of the segments are mounted, the structure including a releasable means which is operable, when struck in an axial direction by the rotor as a result of the shaft breaking, to release the inner ends of the segments and allow the segments to tilt into the path of the turbine rotor blades and thereby destroy the blades and decelerate the turbine rotor.

2. A mechanism according to Claim 1 wherein the structure on which the segments are mounted includes a releasable catch which holds the segments in position downstream of the rotor stage, andes operable when struck in an axial direction to release the radially inner ends of the segments.

3. A mechanism according to Claim 1 wherein the releasable means comprises one or more struts and the, or each strut, in use of the engine, exerts a radially outwards force on an upstream region of the inner ends of the segments, and the or each strut is constructed so that it is rendered inoperative as a support for the segments when the structure is struck by the rotor in an axial direction as a result of the shaft breaking, and the upstream radially inner ends of the segments are caused to collapse radially inwards as a result of the gas loads on the stator vanes and thereby tilt into the path of the turbine rotor blades.

4. A mechanism according to Claim 1 wherein the releasable means further includes one or more ties and the, or each, tie in use of the engine exerts a radially inwards force on a downstream region of each segment, and the, or each, tie is constructed to release the segments simultaneously when the, or each, strut is rendered inoperative as a support for the segments.

5. A mechanism according to Claim 1 wherein the releasable means comprise a hollow cylindrical member having a first circumferential flange extending towards an upstream region of the inner ends of the segments and the first flange constituting a strut, in use of the engine, and being constructed to provide a radially outward facing abutment face and a forwards facing abutments face, the cylindrical member having a second circumferential flange extending towards the downstream inner ends of the segments and constructed as a tie in use of the engine to provide a radially inward facing abutment face and a rearwards facing abutment face, and each of the segments being provided respectively at their upstream and downstream inner ends with first and second members that bear against respectively the abutment faces provided by the first and second flanges.

6. A mechanism according to Claim 5 wherein the first and second flanges of the releasable means together with the segments define a hollow annular box and a vent is provided to connect the interior of the box on the downstream side of the first flange with a space on the upstream side of the first flange.

7. A mechanism according to Claim 1 wherein the constraining means at the radially outer ends of the segments comprises: an outer casing for the turbine having two abutment faces, a first of which is located adjacent the downstream edge of each segment against which the segments abut to restrict their axial movement in a downstream direction, and a second of the abutment faces facing in a downstream direction; an abutment of the segments adjacent a downstream region of its outer end, these abutment faces being positioned to face in an upstream direction, and a plurality of circumferentially spaced bridging members each of which extends between the second abutment face on the outer casing and the abutment face on one of th - segments.

8. A mechanism according to Claim 1 wherein each segment has first and second members projecting radially inwards, respectively at an upstream and downstream region of the inner end of each segment, and the static static structure of the engine is constructed and arranged relative to the turbine rotor so that it engages the first and second members and provides radially outwards and axially rearwards constraint on the first members and radially inwards and axially forwards constraint on the second member, and when the shaft breaks and the turbine rotor strikes the structure in an axial direction, the structure constrains the first member of each segment and provides a fulcrum about which the segments rotate and simultaneously releases its axial and radial constraint on the second member of each segment thereby, in use, causing the gas loads on the segment to tilt an upstream outer region of the segments about the fulcrum into the path of the turbine rotor blades.

9. A mechanism according to Claim 8 wherein the first member comprises a forward pointing hook defined by a radial flange projecting radially inwards from the segment and a portion which extends forwards from the free end of the flange, and the structure has a rearward facing recess into which the hook locates.

10. A mechanism according to Claim 9 or Claim 10 wherein the second member comprises a hook which points rearward and the hook is defined by a second radial flange projecting radially inwards from the segment and a second portion which extends rearwards from a free end of the second flange, and the structure has a forwards facing recess into which the second hook locates.

11. A mechanism according to any one of Claims 8 to 10 wherein the structure comprises two components, a first of the components being provided to co-operate with the first members and a second of the components being provided to co-operate with the second members, and a releasable means is provided which, in the event of the rotor striking the structure in an axial direction, operates to cause the second component to move rearwards relative to the first component to release the second members.

12. A mechanism according to Claim 11 wherein the releasable means comprises a helical thread form on the first component, a means for preventing the first component rotating, a helical thread form on the second component which meshes with that of the first component, and engagement means on the turbine rotor operable to engage the second component, when the turbine rotor strikes the structure and rotate the second component relative to the first component thereby to effect relative axial displacement between the first and second components.

13. A mechanism according to Claim 11 wherein the releasable means comprises a frangible component which is constructed and positioned so as to be machined away by a projection on a rearwards facing face of the turbine rotor when the turbine rotor strikes the structure in a rearwards axial direction, and the frangible component fixes the second component axially relative to the first component until the frangible component breaks.

14. A mechanism according to Claim 13 wherein the frangible component is an integral part of the first or the second component.

1 5. A mechanism according to Claim 10 wherein the structure includes an axially displaceable unitary body, which is provided with a rearwards facing recess at an upstream end thereof for receiving the first hooks and forwards facing recess at a downstream end thereof for receiving the second hooks.

1 6. A mechanism according to Claim 1 5 wherein the upstream recess in the unitary body is embodied in a slotted flange.

1 7. A mechanism according to Claim 1 wherein each of the segments is pivotally mounted on the outer casing at a region adjacent a radially outer upstream end of each segment, and the static structure includes a releasable means which when the structure is struck by the turbine rotor is operable to release the innermost ends of the segments and allow them to swing rearwards and outwards about the pivotal attachment of the segments to the outer casing whiist retaining the upstream outer ends of the segments in the path of rotation of the blades of the turbine rotor so that they collide with the blades and decelerate the turbine rotor.

1 8. A mechanism according to Claim 17 wherein the releasable means comprises a locking member which in a first position secures the inner ends of the segments, and the turbine rotor is provided with a device that co-operates with the locking member when the turbine rotor moves rearwards to move the locking member to a second position to release the inner ends of the segments.

19. A mechanism according to Claim 18 wherein the locking member is provided with a forward facing abutment surface and each segment is provided with a rearward facing abutment surface which engages the abutment surface of the locking member.

20. A mechanism according to Claim 1 8 wherein the locking member is rotatably mounted so that it is rotated from said first position to said second position and the rotor is provided with engagement means for engaging the locking member and rotating it to the second position, and means are provided to prevent the locking member unintentionally rotating to said second position.

21. A mechanism according to Claim 18 wherein each segment is provided with a helical thread form facing the locking member, the locking member is provided with a helical thread form that meshes with the thread form on each segment and a stop means is provided to restrict the torsional movement of each segment about the axis of rotation of the turbine rotor, and the thread forms are arranged so that rotation of the locking member from the first positon to the second position advances it axially rearwards along the thread forms of the segments and thereby disengages the hooks from the locking member.

22. A mechanism according to claim 18 wherein the stator vane assembly is provided with a plurality of circumferentially spaced hooks, and each segment has at least one hook, the locking member has a plurality of circumferentially spaced recesses in which each hook engages so that in said first position of the locking member the locking member provides a radially inwards and axially forwards constraint on the inner ends of each segment, the regions of the locking member circumferentially between the said recesses being constructed so that when the locking member is rotated about it's axis of rotation to the second position as a consequence of the turbine rotor moving rearwards and striking the locking member, the hooks are disengaged from the recesses and the radial and axial constraint on the inner ends of the segments is released.

23. A mechanism according to claim 22 wherein the hooks are provided at the free ends of flanges that project radially inwards from the downstream region of the inner ends of the segments of the stator vane assembly and are defined by segments of a hollow cylinder extending in an axial direction and a radially outward facing first groove, the recesses in the locking member are constituted by a radially inward facing second groove in the inner circumferential wall of circumferentially spaced segments of a hollow cylinder at the free end of a radially outward projecting flange of the locking member, and in the first position of the locking member a side wall of the first groove contacts a side wall of the second groove.

24. A mechanism according to Claim 20 wherein the engagement means that co-operate with the locking member to move the locking member is a serrated face on the turbine which faces towards the locking member and the locking member has a serrated face confronting that on the turbine rotor.

25. A mechanism according to Claim 4 wherein the means for preventing the locking member rotating unintentionally is one or more shear pines designed to shear when the turbine rotor strikes the locking member in the event of the shaft breaking.

26. A mechanism for preventing a turbine rotor of a gas turbine engine exceeding a predetermined speed in the event that a shaft connecting the turbine rotor to a compressor rotor of the engine breaks and releases its torsional and axial constraint on the turbine rotor substantially as hereindescribed with reference to the accompanying drawings.