GB2156908A - Bladed rotor assembly for gas turbine engine - Google Patents

Abstract

The rotor assembly has blades whose roots 20 engage with a circumferential, annular undercut groove 19 on the rotor. To enable the blades to be engaged with the rotor without requiring excessive cutting away of the area adjacent to the groove 19, the roots 20 are arranged to have dimensions which enable each blade to be assembled into the groove 19 while turned through 90 DEG about its longitudinal axis, clearance 22 being provided to enable the blade to move radially inwards while it is turned through 90 DEG to return to its normal position and engage its root with the groove 19. Special arrangements provide for radial positioning of the blades and enable the last blade to be inserted without fouling the adjacent blade platforms. <IMAGE>

Description

SPECIFICATION A Bladed Rotor Assembly for a Gas Turbine Engine This invention relates to a bladed rotor assembly for a gas turbine engine.

Such assemblies normally comprise rotor discs or other supporting rotors from which are carried pluralities of blades. It is necessary to provide a mechanical interlock between each of the blades and the rotor so as to restrain the blades against the high centrifugal loads exerted on these in operation.

Almost all bladed rotor assemblies use an undercut slot of some kind in the rotor with which a correspondingly shaped root on the blade engages.

The slot may be axially or circumferentially extending, the axially extending slots providing easy assembly of the blades to the rotor while the circumferentially extending slot provides greater rim strength but requires some kind of special arrangement to enable the blades to be assembled into the circumferential groove.

A common expedient is to provide a loading slot enabling each blade in turn to be moved radially into position then moved circumferentially to allow the next blade to be assembled into the loading slot.

This has the disadvantage that the loading slot has a serious weakening effect on the rotor rim being particularly deleterious to low cycle fatigue life.

The present invention provides a rotor disc construction enabling the blades to be assembled to the rotor without the need for a complete loading slot.

According to the present invention a bladed rotor for a gas turbine engine comprises a rotor having a circumferential annular undercut groove therein, and a plurality of rotor blades each having a root which engages in the undercut groove to retain the blade to the rotor, the groove and roots being dimensioned to enable each blade to be assembled to the rotor by inserting it into the groove when rotated by 90" about its longitudinal axis from its normal angular position, clearance being provided to allow each blade to be moved radially inwards from its normal radial location to allow it to be turned through 900 to its normal angular position without fouling the groove.The blades may have platforms which abut together to provide a substantially continuous surface on the completed assembly in which case the rotor may be provided with a cut-away portion of its outer surface adjacent to said groove so as to allow said rotation of each blade to take place without its platform fouling the rotor surface.

There may be radial location means with which the blades may be arranged to engage once they have been assembled to the rotor.

In order that the last blade of the assembly may be inserted it is preferred if its platform is cut-away to enable itto be rotated into position without fouling the platform of adjacent blades.

The invention will now be particularly described, merely by way of example, with reference to the accompanying drawings in which: Figure 1 is a partly broken-away view of a gas turbine engine having a bladed rotor assembly in accordance with the invention, Figure 2 is an enlarged, developed view of part of the bladed rotor assembly of Figure 1 in accordance with the invention, Figure 3 is a section on the line 3-3 of Figure 2, and Figure 4 is a section on the line 4-4 of Figure 2.

In Figure 1 there is shown a gas turbine engine which in the present case is illustrated as a front fan two shaft engine. It comprises a single stage fan 10 driven via a low pressure shaft 11 from a low pressure turbine 12, and a high pressure axial flow compressor 13 driven via a high pressure shaft 14 from a high pressure turbine 15. A combustion system 16 burns fuel mixed with the compressed air from the high pressure compressor 13 and provides the driving gases which power the turbines 15 and 12 and which provide some of the propulsive thrust of the engine.

As described so far the engine will be recognised as an entirely conventional two shaft front fan engine. Because the overall layout is entirely conventional this aspect is not described further in this specification. However, the detailed construction of part of the high pressure compressor 14 is unconventional, and it is here that the invention of the present application lies.

Thus the high pressure compressor 14 comprises a compressor drum carrying a plurality of rows of rotor blades 17. As in the conventional practice, rows of rotor blades 17 are interdigitated with rows of stator blades 18 to give compression. Each row of rotor blades 17 is carried from the drum byway of an annular circumferential undercut groove 19 in the drum, each of the blades 17 having a correspondingly shaped root 20 which engages beneath the undercut of the groove 19 to retain the blade to the rotor. This construction provides what is potentially a very strong drum, since the circumferential groove 19 does not in itself break the circumferential continuity of the drum.However, it is necessary to provide some means for loading the blades into the retaining groove; commonly this comprises a local enlargement of the groove which enables the blade roots to engage with the groove by radially inward motion followed by circumferential movement. Unfortunately this enlargement destroys the circumferential continuity of a fairly large cross-sectional area of the drum and weakens its resistance, particularly to the kind of stress known as low cycle fatigue.

In the present embodiment a different and novel structure is used which enables the groove itself to be completely annular without any local enlargement. As can best be seen from Figure 2 and 4 the groove 19 and roots 18 of the blades 17 are so dimensioned that each blade can be offered radially inwardly and enter the groove 19 while turned 90" about its longitudinal axis from its operative position. Once inserted in the groove the blade may be turned through 90" to engage its root 18 with the undercut of the groove 19. It may then be moved circumferentially to its required circumferential location.

In order that the blade 17may be turned through 90" in this way to engage its root with the groove, it is clearthat it must simultaneously be enabled to move radially inwards to allow the corners of the root to clear the groove. If, as in the present case, each blade 17 is provided with a platform 21, it is necessary to provide clearance to allow the platform to sweep through 90". Therefore, a shallow cut-away portion 22 of the outer surface of the rotor is provided in one location about the periphery of the rotor.This cut-away portion comprises arcuate recesses 22 on either side of the groove 19, and is made sufficiently large in plan view to enable the sweep of the platforms 20 through 90 , and sufficiently deep to allow the radially inward movement necessary simultaneously to give clearance between the root 20 and the groove 19.

As mentioned above, it is necessary to allow the blades to move radially inward to enable the rotation which engages them with the groove 19.

This radial motion is largely provided by the clearance between the under-surface 24 of the platform 20 and the underlying ring of outer surface 25 of the rotor (see Figure 4). The recesses 22 only extend the plan area of this part of the surface and do not in fact provide any additional radially inward motion. It is desirable to provide means for maintaining the correctly positioned blades in a radially outward location with their roots fully engaged with the groove 19 and to provide sealing between the platforms 20 and the rotor rim. In the present embodiment this function is carried out by a sealing wire 26 which extends circumferentially round the rotor on the surface 25 except in the vicinity of the recesses 22.The wire 26 does not, therefore, interfere with the radial displacement necessary to allow the blades to be engaged with the groove 19, but once engaged the blades are moved circumferentially so that one edge of each platform overlaps the wire 26. (This is best seen in Figure 4). For the majority of the blades, therefore, the sealing wire 26 provides a degree of radial location and forms a seal between platform and rotor which reduces leakage of air under the platforms.

For the majority of the blades, therefore, assembly to the rotor is carried out simply by insertion while rotated through 90 , rotation to the normal position, and circumferential motion to engage the sealing wire and take up the desired circumferential positioning. However, the lastfew blades cannot be assembled in this simple manner, because the sealing wire does not extend fully round the circumference and an alternative sealing! location means must be used, and because the last blade to be inserted must be able to be rotated without fouling its neighbours.

Therefore, the last five blades to be assembled are treated in a non-standard manner, and it is these five blades which are illustrated in Figure 2. Taking these blades in turn, the blade 28 at the extreme left is affected by the termination of the wire 26 at 27, and the whole width of its platform is not, therefore, sealed. A sealing plate 29 is, therefore, provided, which lies under the right hand portion of the platform of this blade where the wire 26 does not extend. The plate 29 is dimensioned to extend across the full width of the surface 25 and fills the entire circumferential width of the gap between the roots of the adjacent blades. It is assembled into position making use of the cut-away 21 as described below.

The next adjacent blade 30 is again affected by the local absence of the sealing wire, and sealing of one side of the platform and radial location is provided by the sealing plate 29. The other side of the platform of the blade 30 is cut-away at 31 with a semi-circular cut-out which allows a locking member 32 to be positioned in the groove 19, described below. Because of this cut-out and the positioning of the locking member it is not easy to provide a seal for the right-hand part of the platform of the blade 30, and hence a small leakage in this region is tolerated.

The next adjacent blade 33 is similar to the blade 30, but in this case the platform is cut-away at 34 on its left hand side to complete the circular aperture which allows placement of and access to the locking member 32. A second sealing plate 35 of similar dimensions to the plate 29 underlies the right-hand portion of the platform of the blade 33 to provide sealing of this part of the platform and radial location for the blade as a whole.

The locking member 32 mentioned above is a two-part construction comprising an outer, body portion 36 having a threaded central bore within which engages a screwed plug 37. The body portion 36 has an external shape similar to that of the roots 18 of the blade, but of somewhat smaller radial depth. The plug 37 may be screwed from the position shown in chain-dotted lines in Figure 3 to the solid line position, in which it projects from the bottom of the body portion 36 to engage with the bottom of the groove 19.

The member may, therefore, be assembled into the groove 19 between the blades 30 and 33 in exactly the same manner as the blades themselves, with the plug 37 screwed out. The plug 37 may then be partly screwed in to prevent substantial radially inward movement of the body 36 and consequent disengagement of the body from the groove; when the total assembly is completed further screwing in of the plug will lock the member 32 in place and prevent circumferential movement of the blade assembly relative to the rotor.

The next adjacent blade 38 to the blade 33 is in fact intended to be the last blade to be assembled to the rotor. In order that it may be assembled into the single blade space left without its platform fouling the platforms of adjacent blades when it is rotated into position, it is provided with a platform 39 which is cut-away at three of its corners. This platform, together with a cut-out 40 from the edge of the platform of the next adjacent blade 41, allows the necessary rotation of the blade. However, the cut away portions do raise a sealing problem, and this is dealt with as far as the left-hand side of the platform 39 is concerned by the plate 35.The sealing wire 26 extends to the point 42 in line with the left-hand side of the root of the blade 38, but in order to seal the gap left by the cut-away rear right corner and the cut-out 40 a further sealing plate 43 is provided. This underlays the right hand part of the platform of the blade 38 and the left hand part of the platform of the blade 41, and is of similar width to the plates 39 and 35 but is shorter to allow space for the wire 26 (see Figure 4).

These five blades, three sealing plates and one locking member comprise the special arrangements necessary to complete the assembly. Assembly is carried out in the following sequence: (a) All the blades are assembled to the rotor using the recesses 22 to gain access to the groove 19.

(b) All but these last five blades are assembled to the rotor.

(c) Blade 28 is then loaded and moved to the left to permit loading of the next blade.

(d) Blade 30 is loaded and moved to the left.

(e) Locking member 32 is loaded and moved to the left.

(f) Blade 33 is loaded and moved to the left.

(g) Blade 41 is loaded and moved to the right.

(h) Blade 38 is loaded using the recess 22 and the whole row of blades moved 2 blade to the left to allow loading of the sealing plate 43.

(i) Sealing plate 43 is assembled using the clearance provided by the recess 22.

(j) The blades are moved to the right by one blade pitch and sealing plate 35 is assembled through the recess 22.

(k) The blades are moved to the right by one blade pitch and sealing plate 29 is assembled.

(I) The blades are finally moved to the left by one blade pitch (to the position shown in Figure 2) and the plug 37 screwed in to lock the whole assembly circumferentially.

It will be seen that this construction enables the blades to be assembled to the rotor without requiring any cut-away of the retaining groove itself, the only local modification necessary being the recesses 22 which are only as deep as the surface 25. Clearly the embodiment described above has a number of desirable factors which are not necessarily essential to the invention, and it will be appreciated that, for instance, the locking member 32 and sealing wire 26 could readily be replaced by alternatives.

Claims (9)

1. A bladed rotor for a gas turbine engine comprising a rotor having a circumferential annular undercut groove therein, and a plurality of rotor blades each having a root which engages in the undercut groove to retain the blade to the rotor, the groove and roots being dimensioned to enable each blade to be assembled to the rotor by inserting it into the groove when rotated about its longitudinal axis by 90 from its normal angular position, clearance being provided to allow each blade to be moved radially inwards from its normal radial location to allow it to be turned through 90 to its normal angular position without fouling the groove.

2. A bladed rotor as claimed in Claim 1 and in which at least the majority of said blades have platforms which abut together to provide a substantially continuous surface on the completed assembly the rotor being provided with a cut-away portion of its outer surface adjacent said groove so as to allow said rotation of each blade to take place without its platform fouling the rotor surface.

3. A bladed rotor as claimed in Claim 1 and comprising radial location means with which the blades are arranged to engage once they have been assembled to the rotor.

4. A bladed rotor as claimed in Claim 3 in which said radial location means comprises a circumferentially extending sealing wire trapped between one edge of each of at least the majority of said platforms and said rotor.

5. A bladed rotor as claimed in Claim 4 and in which said radial location means also includes sealing plates extending below the platforms of a minority of said blades.

6. A bladed rotor as claimed in any one of the preceding claims and in which one said blade has at least one corner of its platform cut-away to allow it to be assembled as the last blade and rotated into position without fouling adjacent blades.

7. A bladed rotor as claimed in any one of the preceding claims and comprising locking means adapted to prevent circumferential motion of said blades relative to said rotor.

8. A bladed rotor substantially as hereinafter particularly described with reference to the accompanying drawings.

9. A gas turbine engine having a bladed rotor as claimed in any one of the preceding claims.