GB2111639A - Shaft assembly for a gas turbine engine - Google Patents

Abstract

If the shaft connection between fan (12) and turbine (16a) breaks, turbine overspeed results, with consequent throw off of blades. Extensive peripheral damage then occurs. The invention provides a torque tube (26) which in one example is directly connected to the shaft (18) so as to screw one end away from the other end if relative rotation occurs through shaft breakage and, in another example, screws a bolt away from a shoulder on the shaft and allows the shaft to move downstream under gas loads. In both cases the turbine (16a) collides with fixed structure, breaks up the blades and stops rotating. <IMAGE>

Description

SPECIFICATION Rotor assembly for a gas turbine engine The present invention concerns a rotor assembly for a gas turbine engine i.e. an engine of the kind which has a turbine driven by combustion gases and which in turn drives a fan stage or compressor, via connecting shafting.

Such engines are commonly used as the propulsion units of aircrafts.

In the event of engine breakdown during operation, any damage which results from the break down must be minimized so as not to jeopardise the flight cabability of the aircraft.

One cause of damage over and above that which initiates the break down is the high speed rotation of the turbine. Indeed, if a shaft breaks and the flow of combustion gases is not stayed, the turbine will rapidly overspeed, with consequent risk of throwing off its blades.

The continued gas flow does however, provide one useful function, in that it tends to move the turbine, downstream. (It should be understood, that the terms upstream and downstream, are used in the context of the direction of flow of gases through a gas turbine engine during normal operation thereof.

If the turbine moves downstream far enough and quickly enough, it will collide with fixed structure e.g. outlet guide vanes or exhaust bullet struts, and the blades will be broken up into small pieces before the blades in their entire form, can be shed. It follows, that the construction of turbine, fan and/or compressor and their connecting shafting must enable such an event.

The present invention seeks to provide an improved turbine, fan or compressor and shaft assembly for a gas turbine engine. Accordingly, the present invention comprises a fan or compressor, a turbine stage and a shaft which connects said fan or compressor and turbine stage for co-rotation at a common speed and a torque tube, wherein one end of said torque tube is effectively locked to said shaft at its upstream end so as to prevent relative rotation therebetween and the other end is connected via means which engages said shaft at a position downstream of the upstream end thereof, such that on relative rotation occurring between said ends of said shaft, through breakage thereof at any point between said torque tube connections, said torque tube exerts a torque which reacts on said means to move it axially in a downstream direction and so enable the downstream end of the shaft to make a similar movement.

The means may comprise a screw threaded land on the interior of the shaft.

The shaft may comprise two stub shafts splined together in coaxial alignment and the means may comprise a collar which abuts a shoulder on the downstream stub shaft and is in screw threaded engagement with a fixed hollow bolt and in splined engagement with the torque tube, for achieving reletive rotation with the bolt and downstream stub shaft and therefore, axial movement in a downstream direction.

Alternatively, the shaft may comprise two stub shafts splined together in axial alignment and the means comprise a hollow bolt which has a flanged downstream end by which the hollow bolt engages a shoulder on the downstream stub shaft and a screw threaded upstream end, a nut on the screwthreaded upstream end which engages the torque tube via splines for the purpose of achieving relative rotation between the nut and hollow bolt, whereby to move the hollow bolt downstream and to enable the downstream stub shaft to move in like manner.

Preferably the pitch angle of the screwthreads is at least 45".

The invention will now be described by way of example and with reference to the accompanying drawings in which: Fig. is a diagrammatic view of a gas turbine engine incorporating a front fan and including an embodiment of the invention.

Fig. 2 is an eniarged part view of fig. 1.

Figs. 3 and 4 are part views of the gas turbine engine of fig. 1 and each depicts alternative embodiments of the invention.

In fig. 1, a gas turbine engine 10 includes a front fan 12, a compressor 14 and a two stage turbine 16.

The low pressure turbine stage 16a is connected via a shaft 18, to front fan 12.

The high pressure turbine stage 1 6b is connected via a shaft 20 to compressor 14.

Shaft 20 is a one piece member. Shaft 18 however, comprises two shaft stubs 18a, 18b, which are joined by way of overlapping, spiined ends 18c.

A torque tube 22 lies coaxially within shaft 20 and is fastened at a position adjacent the upstream end of shaft 20. The downstream end 24 of torque tube 22 is screwthreaded and engages a complementary thread in a thickened portion of shaft 20.

A torque tube 26 is rigidly fastened by one end, to and within fan shaft 18b, at its upstream end, and is screwthreadedly engaged with shaft 18a at a position downstream thereof.

The function of torque tubes 22 and 26 is ex plained hereafter, in connection with torque tubes 26, through it should be understood that torque tube 22 operates in the same way thereas.

The expansion of hot gases from combustion equipment 28, through turbine 16, exerts a rotary load on low pressure turbine 16awhich is transmitted via shaft 18, to fan stage 12. As there are no stepping or idler gears in the connection, there results a rotation of turbine stage 16a, shaft 18 and fan 12, in a common direction and at a common speed.

Torque tubes 26 co-rotates with shaft 1 8b by virtue of its rigid connection thereto. In such circumst ances, no loads are applied to the screwthreaded portions of tube 26 and shaft 18a.

If shaft 18 breaks at any point between the fixing plane of torque tube 26 and its threaded portion, rotation of fan 12 will immediately slow and quickly stop and turbine stage 1 6a which will no longer have the braking effect of fan 12 acting upon it, will immediately overspeed. Relative rotation now occurs, between the portion of shaft 18bwhich is still attached to fan 12, and the portion of shaft 18a which is still attached to turbine stage 16a. There results the generation of a torque load by torque tube 26, which is transmitted thereby, to the screwthreaded portions.This is sensed by shaft portion 18a as an axial load and the hand of the screw threads being such asto apply the load in a downstream direction, shaft portion 18a along with associated turbine stage 16a is moved downstream a distance sufficient to bring about collision with struts 28.

Turbine 16a overspeeds extremely rapidly. The pitch angle of the threads is therefore of such magnitude as to impart a large axial movement to turbine stage 16, over a small portion of a rotation of the threaded portion of torque tube 26. For example, the thread pitch angle could be about 45".

The collision of turbine stage 16a, with struts 28 breaks the turbine blades into small pieces and eventually halts rotation entirely.

Brief mention is here made to fig. 2, which merely shows the shapes and relationships of parts 20,22 and 24.

Referring now to fig. 3 which like parts have like numerals. Thus, shaft portions 1 8a and 1 8b are joined via splines 18c for the transmission of rotary drive.

Fan stage 12 is bolted to a flange 30 at the upstream end of shaft portion 18b.

A ring 32 is bolted to an internal shoulder 34 in shaft portion 18b and a collar 36 abuts an internal shoulder 18 on shaft portion 18a. Collar 36 has a spigot 39 which is a sliding fit in a bone in shaft portion 18a.

A tube 40 which is effectively a hollow bolt, is screwthreaded at its downstream end 42 and fits via the screwthread into the spigot 39 of collar 36.

The screwthread is of the kind described hereinbefore.

Tube 40 has a normal screwthread formed on its upstream end and is locked in position by the placing of a nut 44 and washer 46 thereon.

A fan retention shaft 48 has its downstream end 50, trapped between washer 46 and ring 32. Shaft portions land 18b are thus held together.

Rotary drive is transmitted from shaft portion 18a, through splines 18c, to shaft 1 8b and via flange 30 to fan stage 12. The drive also passes from shaft portion 18b, through splines 52 to fan retention shaft 48 and through flange 54, to torque tube 26.

Torque tube 26 is further connected via splines 56, collar 36.

If shaft portion 18b breaks at any point between bearings 51 and splines 52, fan stage 12 will slow and stop. A braking effect will thus be imparted via splines 52 to retaining shaft 48 and via flange 54 to torque tube 26 and from there via splines 56, to collar 36. Ring 32 however, will continue to rotate, by virtue of its bolted connection to shoulder 34 which is downstream of the break, and the splined connection 18c. Further, as ring 32 is connected via splines 58 to hollow bolt 40, the latter will also continue to rotate. A screwing action will thus be imparted to collar 36 which is not rotating and it will move downstream along aplines 56. Shaft portion 18a is then free to move downstream under the action of gas loads on turbine stage 16a.

Referring now to fig. 4 in which again, like parts have been given like numbers.

Hollow bolt 42 provided with a flange 60 which abuts shoulder 38 on shaft portion 18a. A screw threaded portion 62 is formed on the upstream end of bolt 42, and a nut 44 and washer 46 are fitted thereon, so as to trap retention shaft 48 against ring 32.

A torque tube 26 is fastened afore end to retention shaft 48 by rivets 64. The other end of torque tube 26 is connected via splines 66, to nut 44. The arrangemenu thus enables use of a much shorter and therefore lighter torque tube, than the arrangement of fig. 3.

Separation of shaft portions 18a, 18b in fig. 4, results in relative rotation between nut 44 and bolt 42, the latter being moved downstream and thus freeing shaft portion 18a.

An advantage is gained by the present invention in that, in those arrangements where turbines are located axially by the bearing structure which supports their associated shafts, the load imposed thereon via the torque tube screwthreads, will overcome the resistance offered by the bearing location features and push the turbine downstream, into the fixed structure.

Claims (8)

1. An assembly comprising a fan or compressor, a turbine stage and a shaft which connects said fan or compressor and turbine stage for co-rotation at a common speed and a torque tube, wherein one end of said torque tube is effectively locked to said shaft near its upstream end so as to prevent relative rotation therebetween and the other end is connected via means which engages said shaft at a position downstream of the upstream end thereof, such that on relative rotation occurring between said ends of said shaft, through breakage thereof at any point between said torque tube connections, said torque tube exerts a torque which reacts on said means to move it axially in a downstream direction and so enable the downstream end of the shaft to make a similar movement.

2. An assembly as claimed in claim 1 wherein said means comprises a screwthreaded land on the interior surface of the shaft, at a position downstream of said upstream end.

3. An assembly as claimed in claim 1 wherein said shaft comprises two stub shafts splined together in coaxial alignment and wherein said means comprises a collar which abuts a shoulder on the downstream stub shaft end and is in screwthreaded engagement with a fixed bolt and in splined engagement with said torque tube for achieving relative rotation with said bolt and downstream stub shaft and therefore axial movement in a downstream direction.

4. An assembly as claimed in claim 1 wherein said shaft comprises two stub shafts splined together in axial alignment and wherein said means comprises a hollow bolt which has a flanged downstream end by which said hollow bolt engages a shoulder on the downstream stub shaft and a screwthreaded upstream end, a nut on said screw threaded upstream end which engages said torque tube via splines for the purpose of achieving relatively rotation between the nut and hollow bolt, wherebyto move said hollow bolt downstream and to enable the downstream stub shaft to move in like manner.

5. An assembly as claimed in any previous claim, wherein the pitch angle of the screw thread is at least 45o

6. An assembly as claimed in any previous claim wherein the shaft is rotably supported in bearing structure at or near its mid length.

7. An assembly as claimed in claim 6 when dependant on any of claims 3 to 5 wherein said mid portion embraces saod mislapping- portions.

8. An assembly substantially as described in this specification, with reference to each of figs. 1 to 4 of the drawings.