GB2130340A - Gas turbine rotor assembly - Google Patents

Abstract

A gas turbine engine has a front fan (11) connected to its turbine shaft (19) through a coupling (22) which allows the fan to move bodily radially relative to the shaft (19) when the fan becomes unbalanced when a blade comes off, due for example to striking an ingested bird, and experiences a radial shift of its centre-of-mass. The coupling allows the fan to invert relative to the shaft whilst still transmitting torque through the coupling.The fan is constrained to rotate about the same axis as the shaft during normal balanced running by a shear pin (23) that does not transmit torque, and a resilient restoring means (30) is provided for urging the unbalanced rotor to be moved to bring its new centre-of-mass back on to the axis of rotation of the shaft (19). <IMAGE>

Description

SPECIFICATION Rotor assembly drive system This invention relates to the suporting and driving of rotors of gas turbine engines, and is particularly concerned with the problem of supporting and driving such rotors when the mass of the rotor becomes unbalanced and inversion ofthe rotor is permitted.

Imbalance of rotors, such as large compressorfans of gas turbine engines, can occurwhen part, or whole, of a fan blade becomes detached from the fan disc whilst the engine is running.

When a blade is lostthe rotor experiences a large out-of-balance load which causes the rotorto orbit bodily about its original axis of rotation. After a limited number of revolutions in this essentially unstable condition the rotor inverts, that is to say, it alters its mode of rotation so that its new rotational axis passes through its new centre of gravity and consequently assumes a stabilised rotation.

Inversion only occurs when the rotor is running well above its natural frequency. During normal running the natural frequency of the rotor is designed to be well above the maximum engine speed (typically 30% higher). When a blade comes offforexample due to striking an ingested bird, it is necessaryto lowerthe natural frequency of the rotor in some way to allowthe rotor to invert.

A number of prior proposals for catering for unbalanced running and inversion offan rotors have been suggested in the past. The first category of such proposals comprises the concept of providing a bearing assembly nearto the rotorthat is capable of floating, or being permanently deformed, only when unbalance and inversion occurs. Examples of such solutionsareto be found in our British Patents 1,421,377,1,418,907 and 1,421,540 (Agent's reference cases 21 7C, 230C and 273C respectively). With engines having the drive shaftforthe rotor mounted in three bearings one can arrangeforthe bearing nearest to the rotorto float or become permanently deformed whilst also allowing the second nearest bearing to move to accommodate whirling of the drive shaft.

However, in some engines, and in particular, in those designs in which the rotor drive shaft is supported in onlytwo bearings (as opposed to the usual three), it is impossibleto allowthe bearing nearestto the rotor to float or become permanently deformed.

A second category of prior proposals recognises the need to mount the rotor drive shaft in bearings that are relatively immovable in a radial direction and which are designed only to cope with the usual loads experienced during normal balanced engine running.

This is particularly desirable where the rotor or fan drive shaft is only supported in two bearings. This latter category of proposals usually disconnect the drive from the drive shaft to the rotor when imbalance occurs by breaking a frangible coupling in the drive path to the rotor. The rotor is then supported by structurethatis relatively moreflexible in transverse directions than the drive shaft. Usually a secondary drive path is provided to drive the rotor when the primary drive path is broken. British PatentApplication No.49445/73 (Agent's reference case 347C) relates to such a proposal where the primary torsional drive path is through a frangible coupling and a secondary drive path which by-passes the coupling is provided.

Afurther proposal falling in the latter category is described in ourco-pending British Patent Application No.52406/76 (Agent's reference case 630C). Here again in this design, the primary drive is taken through a frangible coupling and the rotor is supported on flexible structure as before. In this case the secondary drive path is th rough a torsionally stiff auxiliary shaft.

In both proposals described in British Patent Applications Nos. 49445/73 and 52406/76 the primary drive to the rotor is through a frangible coupling that must be robust enough to withstand torsional stresses during normal balanced running but be weak enough to breakwhen subjected to a predetermined value of transverse load when the rotor becomes unbalanced.

It is difficult to design a coupling to meet both criteria.

The present invention as claimed provides a rotor assembly which is connected to a drive shaft through a coupling that allows the rotorto move bodily radially relative to the shaft when it becomes unbalanced and to run inverted whilst still transmitting torque through the coupling.

Embodiments ofthe invention will now be described with reference to the accompanying drawings in which: Figure lisa schematic representation of a gas turbine aero engine ofthe by-pass type incorporating the present invention.

Figures 2 to 4 are cross-sectional views of part of the LP compressor drive of the engine of Figure 1 showing in greater detail different embodiments of the invention.

Figure5 is an end view of the coupling shown in Figure 4.

Referring to Figure 1 there is shown a by-pass type gas turbine aero engine 10 comprising in flow series a low pressure fan 11, an intermediate pressure axial flow compressor 12, a high pressure axial flow compressor 13, a combustion chamber 14, an HP turbine 15, an IPturbine 16, an LPturbine 17, and an exhaust jet pipe 18.

The details of construction ofthe engine are omitted for clarity.

The LP turbine 17 drives the fan rotor assembly 11 through a drive shaft 19which is mounted at the rear of the engine in a thruSt bearing 20 and atthefrontof the engine inaballracejournal bearing 21.

The fan rotor assembly 11 is connected to the shaft 19through an Oldham coupling 22through which torque is transmitted. The Oldham coupling 22 is prevented from operating as such by means of a shear pin 23. The shear pin 23 is located on the centre-line of the shaft 19so that it does nottransmittorque. The shear pin 23 is provided to ensure that the fan 11 rotates about the same axis of rotation as the shaft 19 during normal balanced running. The coupling 22 is shown in greater detail in Figure 2.

Referring to Figure 2 the Oldham coupling 22 comprises two clamping members 24,25, one of which (24) is bolted to the shaft 19 and the other of which carries the shear pin 23. Located between the clamping members 24,25 is a flange member26to which the hub of the fan 11 is bolted. The flange member 26 is also connected to the shear pin 23. The clamping member 24 is provided with a set of parallel splines which mate with complementary splines on one side of an annular intermediate member 27. The intermediate member 27 is provided on its other side with a second set of parallel splines which lie at right angles to the first set of splines. The second set of splines mate with complementarysplines on the flange member 26.The clamping members 24,25 are clamped together by bolts 28 which pass through oversize holes in the intermediate member 27 and flange member 26.

The shear pin 23 has a central circumferentially extending line of weakness 29, and the end ofthe shear pin 23 is fixed relative to the centreline axis of shaft 19 byvirtue of its connection through the clamping members 24 and 25. The other end ofthe shearpin 23 is connected bytheflangemember26to thefan hubsothat ifthe hub becomes unbalanced, dueto for example, to a blade breaking off,thehub and flange member 26 move in a radical direction. The flange member 27 moves along the splines between it and the intermediate member 27 and the intermediate member 27 moves along the splines between it and the clamping member 24. This movementsnapsthe shear pin 23 at its line of weakness 29.

The initial orbiting ofthe unbalanced rotor is resisted by a restoring means 30 in the form of a shaft 31 connected at one end to the shaft 19 and at its otherwise free end to the shear pin 23 (at the end of the shear pin that is connected to the hub). This shaft 31 provides a spring rate which imparts a restoring momentto the hub that urges the hub to move that its new centre of mass (resulting from the loss of a blade) is brought back on to the centre line ofthe shaft 31.

Otherforms of restoring means 30 such as leaf springs extending axially along the bore of shaft 19, or resilient blocks or annular members, or coil springs, between the shaft 19 and the shear pin 23 may be employed instead of the shaft 31.

Itwill beseen from abovethat atall times the universal coupling 22 transmits the torque from the shaft 19 to the fan but does not operate as a universal coupling until the rare occasion thatthe hub becomes sufficiently unbalanced to snap the shear pin 23. The shear pin 23 does not have to be strong enough to transmit torque, therefore, its strength can be optimised to withstand the normally experienced radial loads during the normal designed balanced running butweak enough to fail if the radial loads exceed a predetermined value commensurate with unbalanced running.

The Oldham coupling 22 described above is intended to use the frictional engagement between the intermediate member 27 and the other mating parts to provide some frictional damping ofthe initial radial movement of the unbalanced rotor as it orbits before running inverted. However, such damping as there may be must not preventthe restoring means 30 urging the new centre of mass of the unbalanced rotor back on to the centre line of shaft 19.

There are two schools ofthought; one believes that frictional damping between relatively movable components constituting a rotor (as opposed to frictional damping between the rotor as a whole and fixed structure) may be tolerated providing it is not excessive; the other school ofthought believes that any form offrictional damping between relatively movable components constituting a rotor is de-stabilizing.

Further research into these matters mayshed more light on the subject. However, the present invention contemplates the use of any form of coupling 23 (whether or not it provides frictional damping between the components ofthe coupling), that is capable oftransmitting torque from the shaft 19 to the hub, and which is also capable of allowing the fan to move bodily radially relativetothetheshaft 19 without the rotor tilting relative to the axis of the shaft 19.

Alternative forms of coupling are shown in Figs. 3to 5.

Referring to Figure 3 the coupling 22, instead of comprising an intermediate member 27 with splines which mate with complementary splines on the clamping members 24,25, may dispense with the intermediate member 27 and have planarfrictional surfaces similarto mating clutch plates on the confronting surfaces 33, 34 the flange member 26 and the clamping members 24,25. If desired auxiliary shear pins 32 (shown dotted) may be provided to restrict relative rotation between the clamping members 24,25 and theflange member 26.

The coupling 22 of Figure 2 may be replaced by one which employs drag links as shown in Figures 4 and 5.

Referring to Figures4 and 5, the coupling 22 incorporates a plurality of pairs of drag links 35 each of which is pivotally attached to one end of a flange 36 on the shaft 19 (the flange 36 effectively replaces the members 24 and 25 and the intermediate member 27 ofthe coupling shown in Fig. 2),forthe sake of clarity only one pair of drag links is shown. The diametrically opposied otherwise free ends ofthe drag links 35 are interconnected bya balance link37.Torqueappliedto shatt 19 istransmittedtothe hub 11 through links38 pivotally attached to theflange 36 at one end and pivotally mounted on the drag links 35 at a region intermediate the ends ofthe link 35. The links 35,37 and 38 are constructed to be strong enough to transmitthetorque applied to the coupling. The hub 11 is constrained to rotate aboutthe centre line of the shaft 19 by means of shear pins 23 that are designed to fracture in the eventthatthe hub becomes unba lanced.Arestoring means 30 in the form of an annular rubber block is located between the hub 11 and flange member 36.

Claims (10)

1. A rotor assembly comprising a rotor, a drive shaft mounted for rotation in bearings, a torsional drive coupling connecting the rotor to the shaft, the coupling being designed to allowthe rotorto move bodily radially relative to the shaftwithoutdisconnect- ingthetorsional drivethrough the coupling, constraining meansoperabletoconstraintherotorto rotate about the axis of rotation of the shaft but designed to release its constraint on the rotor in the event that the rotor becomes dynamically unbalanced and causes its centre of mass to shift radially relative to the axis of rotation of the shaft, and restoring means operable on the rotor when it becomes unbalanced to move the rotor bodily radially relative to the axis of rotation of the shaft to re-align the centre-of-mass of the rotoron the axis of rotation of the shaft.

2. A rotor assembly according to Claim 1 wherein the coupling is an Oldham type coupling comprising a driving member connected to, or forming part of, the shaft, a driven member connected to, offorming part ofthe rotor, and an intermediate member which co-operates with the driving and driven members to transmittorque from the shaftto the rotor, the intermediate member engaging the driving member through an engagement means which constrains the intermediate memberto move relative to the shaft in a first direction transverse to the shaft and engaging the driven memberthrough an engagement means which constrainsthe rotorto move relativeto the intermediate member in a second transverse direction which is normaltothefirsttransverse direction.

3. A rotor assembly according to Claim 2 wherein the intermediate member isfrictionally engaged with the driving and driven members to provide damping of radial movements of the rotor relative to the shaft when the rotor becomes unbalanced and moves bodily radially relative to the shaft.

4. A rotor assembly according to Claim 2 wherein the engagement means comprises a plurality of parallel mating splines.

5. A rotor assembly according to Claim 1 wherein the constraining means comprises one or more frangible components designed to breakwhen radial forces on the rotor due to the rotor becoming unbalanced exceed a predetermined value.

6. A rotor assembly according to Claim wherein the frangible component comprises a shear pin located on the axis of rotation of the shaft.

7. A rotor assembly according to Claim wherein the constraining means is constituted by the frictional engagement between two components.

8. A rotor assembly according to Claim 1 wherein the coupling comprises mutually contacting radial co-planarfrictionally engaged surfaces.

9. A rotor assembly according to Claim 1 comprising a driving member connected to the shaft, a driven member connected to the rotor and a plurality of pivotal links interconnecting the driving and driven members through which the torque is transmitted, the links being arranged relative to the driving and driven members to allow bodily relative movement in radial directions between the driving and driven members.

10. A rotor assembly substantially as herein de- scribed with reference to any one of the accompanying drawings.