GB2105790A - Balanced bladed rotor - Google Patents

Abstract

A bladed rotor assembly, such as a multi-stage compressor of an axial flow turbofan aeroengine, has at least one stage of rotor blades 23 whose root portions 43 have holes 55 extending through them. Selected blades are provided with means for balancing the rotor assembly comprising plug weights 58 which are adhesively retained in the holes 55. <IMAGE>

Description

SPECIFICATION Bladed rotor assembly and rotor blade The present invention relates to a bladed rotor assembly and to a rotor blade for incorporation in the assembly, the rotor assembly being of the type used in axial fluid flow machinery such as gas turbine engines.

Gas turbine and compressor rotors tend to operate at high rotational speeds, and because of this any out-of-balance in the main rotating assembly of a gas turbine engine is capable of producing vibration and stresses which increase as the square of the rotational speed. Very accurate balancing, both static and dynamic, is therefore necessary.

The three basis methods of correcting unbalance of a rotor are redistribution of weight, addition of weight, and removal of weight. Redistribution of weight is possible for turbine and compressor rotors by interchanging blades which are of slightly different weights due to manufacturing tolerances.

However, this technique may not be sufficient in itself to correct all imbalances and involves timeconsuming extraction and refitting of the blades in a different order. Removal of weight is normally achieved by filing metal from balancing lands, but again this may not be sufficient to restore balance and is normally only employed when balancing individual components such as turbine or compressor shafts which are going to be incorporated into larger rotating assemblies.

The addition of weight is probably the most commonly used method, involving the use of small balance weights secured at appropriate points around the rotor, the most common practices being firstly the emplacement of metal plates or wire between the blade root and its associated retaining groove in the periphery of the rotor disc or drum, and secondly the retaining of balance weights by means of bolts on a bolting flange around the rotor disc. Both these methods may prove inconvenient due to difficulty in readily removing and replacing the balance weights, and in the latter case the extra machining involved in producing the bolting flange and associated components.

In the case of blades which are retained in the periphery of the disc or drum by means of fixing pins which pass through holes in the blade roots, it is known to use pins of different weights in order to balance the rotor, but such blade roots are passing out of use in the gas turbine engine industry because of their inferior performance compared with the more generally used "dovetail" and "firtree" types of roots, which do not employ retaining pins.

According to the present invention a bladed rotor assembly for an axial fluid flow machine has a plurality of rotor blades with root portions and a rotor member in whose periphery the blade root portions are retained, each blade having a hole in its root portion and at least one selected blade being provided with means for balancing the rotor assembly comprising plug means adhesively retained in the hole in the root portion. The root portions are advantageously of the "dovetail" or "firtree" (serated) type and preferably the holes extend through the root portions transversely of the directions of rotation of the rotor. It is convenient if the holes extend substantially parallel to the axis of rotation of the rotor assembly, and it is also convenient if they extend completely through the root portion from one of its end faces to the other.

The rotor assembly may comprise merely a single stage compressor or turbine, or it may comprise a plurality of stages, the aforesaid blades with holes in their roots being provided in at least one stage of the rotor assembly.

The plug means for balancing the rotor assembly may be of solid or tubular cross-section, and further may be of metallic or polymeric material. The means by which each plug means is adhesively retained in its hole may comprise a cureable polymer or a non-setting viscous adhesive. Alternatively, the plug means may itself comprise a cureable polymer injected into the hole.

The invention also includes a blade having a hole in its root portion and suitable for incorporation in the blade rotor assembly as one of the blades mentioned in the first consistory paragraph above.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic side elevation of a turbofan aeroengine with part of the outer casing and nacelle "broken-away" to reveal one likely location of a rotor assembly and rotor blade according to the invention; Figure 2 is an enlarged sectional view of the engine structure within the area A in Figure 1, showing the rotor assembly in detail; Figure 3 is an enlarged view of the part of the rotor assembly within the area B in Figure 2, showing a blade having a blade root modified in accordance with the invention; Figure 4 is a view on arrow C in Figure 3, but with the blade aerofoil portion deleted for clarity; and Figure 5 is a view on arrow D in Figure 3.

The drawings are not to scale.

Referring first to Figure 1, a turbofan aeroengine 1 has a core engine 3, a bypass duct 5 defined by casing and nacelle 7 surrounding the core 3, air intake 9, front fan 11, a ring of fan outlet guide vanes 13, and propulsion nozzles 15 and 17 forthe fan air and core engines streams respectively. Bypass duct 5 receives air from fan 11, which also supplies core engine 3 through core engine air intake 19, part of the flow path through the core compressor 21 being indicated by dashed lines. Fan 11 is driven from a low pressure turbine (not shown) in the core 3, compressor 21 being driven from an intermediate pressure turbine (not shown) in the core.

Turning to Figure 2, which shows structure to one side of the engine centre-line only and excludes the shafts and bearings on which the assembly is mounted, the first stage 23 of compressor 21 receives air for compression after it has passed through the ring of inlet guide vanes 35. Compressor 21 is the intermediate pressure compressor of the turbofan engine and consists of six stages of rotor blades 23 to 28 with corresponding rings of stator blades 29 to 34 after each stage. Each stage of rotor blades 23 to 28 is mounted on the periphery of a corresponding rotor disc 36 to 41, the rotor discs being provided with flanges extending between the stages so that they can be bolted to each other to produce the complete intermediate pressure compressor rotor drum assembly into which the blades are fixed by means of their root portions.

The compressor rotor drum and blade assembly 21 must be balanced as a complete unit before incorporation into the engine as a module. Because of the length of the compressor assembly 21, unbalance may be present at any radial and axial position and, when the assembly is rotating, this will produce dynamic imbalance (due to uneven centrifugal forces which act on the bearings (not shown) on which the assembly is mounted. To counteract these forces and dynamically balance the assembly, corrections are usually applied at both ends of the assembly, namely in the planes of the first and last stages of the compressor, 23 and 28 respectively. In order to determine what corrections are needed, the assembly 21 is tested on a dynamic balancing machine, which indicates both the magnitude and the angular position of the imbalance in each plane.

Having determined what correction is needed in the plane of the first stage 23, the means of applying the correction in accordance with the invention will now be explained.

Referring to Figures 3 to 5 we see that the radially inner aerofoil portion 24 of blade 23, its root portion 43, and the radially outer portion of disc 36. The root portion 43 of blade 23 is a rounded dovetail shape as seen in Figure 5 and during assembly is slid into a correspondingly shaped groove 45 whose dimensions are such that locking plate 47, when inserted into grooves 49,51 in the underside of blade platforms 53 and in the side of disc 36 respectively, holds the bottom of blade root 43 clear of the bottom of groove 45 and holds the shoulders of root 43 in intimate contact with the shoulders of groove 45.

Each blade root 43 has a stepped hole 55 extending through it in an axial direction (i.e. transversely of the direction of rotation of the rotor and parallel to the axis of rotation) from the forward face of the blade root to the rearward face. As can be seen from Figure 5, the major axis 56 of hole 55 lies in a radial plane 59 which bisects the blade root 43. The major portion of the hole has the larger bore, the small bore portion 57 occupying only a small proportion of the length of the hole at the forward face of the blade root 43.

According to the indication given by the balance machine, one or more selected blades in the first stage 23 are removed and reinserted in the same positions after a cylindrical plug of material 58 (shown cross-hatched) has been inserted into each hole 55, the plug or plugs 58 of material being of such a weight as to give the required correction in balance. The material of the plugs 58 is conveniently the same as that of the blade root 43, or is a material which is compatible with that of the root. For example, for a titanium blade root, 18/8 stainless steel would be a suitable plug material if titanium plugs could not be used due to the light weight.

As an alternative to the use of solid plugs of material 58 to insert in holes 55, tubular plugs can be used provided the corrections in dynamic balance required are of such a magnitude as not to require the heavier solid plugs.

The plugs 58 are retained in holes 55 primarily by an adhesive, the lock plate 47 and small bore portion 57 of hole 55 acting as secondary retaining features for safety. The adhesive may be a non-setting viscous adhesive such as one of the "Loctite" range (Registered Trade Mark), manufactured by the Kane Douglas Group Limited of Atherstone, Warwicks.

The use of such an adhesive requires that the plug be sized to the hole diameter quite accurately to give a tight but non-interfering fit. A preferred cheaper alternative is therefore to have an undersize plug with slacker tolerances, this being held in place by a curable elastomeric adhesive/filler such as "Silcoset 152", which is a silicone rubber which cures at room temperature by exposure, to atmospheric moisture.

This is supplied by Ambersil Limited, Daneshill, Basingstoke, Hampshire.

If the corrections required for dynamic balance are suitably small, the plug can be of a polymeric or elastomeric material, and in fact the curable silicone rubber adhesive/filler mentioned above can be used as a plug material simply by injecting it into selected holes 55 through a syringe. In this case there is no need to remove the blades 23 from the rotor assembly, the lock plate 47 being left in position and the uncured silicone rubber being injected through the small bore portion 57 of hole 55.

The above description is concerned with the use of the invention in the first stage of an intermediate pressure compressor, where the temperatures of the blade roots are such as to allow the use of the above-mentioned adhesives and fillers. In high temperature locations such as turbine rotors, metallic plugs (either solid or tubular) would be used and would be held in place by a suitable high temperature adhesive. For instance, high heat resisting aluminium filled enamel paint could be used as an adhesive. Such paints can be supplied by International Paint Limited (Aviation and Freight Container Coatings Division), P.O. Box 359, Rotten Park Street, Ladywood, Birmingham.

After the insertion of plugs 58 in the roots of the selected blades 23 at the required angular positions around the rotor and the consequent acheivement of dynamic balance for the complete compressor rotor assembly, the assembly is incorporated into the engine as a unit. If at some future stage the rotor assembly is dismantled for repair or overhaul and rebuilt with some new blades or with the blades replaced in a different relationship to each other from that required for acceptable balance, the plugs 58 can be pushed out of the blade roots by inserting a pusher tool through the small bore portion 57 of hole 55, and the rotor assembly re-balanced. If the removed plugs are suitable, they can be re-used where required.

Although in the above-described embodiment, the invention has been applied to balancing at the plane of the first stage of rotor blades 23 only, the invention is equally applicable to the balancing operation which is required in the plane of the last stage of rotor blades 28.

It is not essential that holes 55 extend completely through the blade root from one face to the other as shown, or even that they be in the orientation shown; for example, they could be blind-ended holes and they could extend radially upwards from the bottom face of the blade root. However, if blind-ended holes are to be utilised, it is of course necesary to form metallic plugs with some features such as an internal screw thread by which their ends can be gripped in order to extract them from the holes. This is not necessary for blind holes plugged with a polymer or elastomer composition, since these can easily be augured out.

Although the invention has been described as applied to blades with dovetail-shaped roots, it is applicable to blade roots having other shapes, such as the well-known "fir-tree" serrated type.

Claims (17)

1. A bladed rotor assembly for an axial fluid flow machine, said bladed rotor assembly having a plurality of rotor blades with root portions and a rotor member in whose periphery the blade root portions are retained, each blade having a hole in its root portion and at least one selected blade being provided with means for balancing the rotor assembly comprising plug means adhesively retained in the hole in the root portion.

2. A bladed rotor assembly according to claim 1, in which the root portions of the blades are of the "dovetail" type.

3. A bladed rotor assembly according to claim 1, in which the root portions of the blades are of the "firtree" serated type.

4. A bladed rotor assembly according to any of claims 1 to 3, in which the holes in the root portions of the blades extend transversely of the direction of rotation of the rotor.

5. A bladed rotor assembly according to claim 4, in which the holes in the root portions extend substantially parallel to the axis of rotation of the rotor assembly.

6. A bladed rotor assembly according to claim 5, in which the holes in the root portions extend completely therethrough from one end face of each root portion to the other end face.

7. A bladed rotor assembly according to any one of claims 1 to 6, said rotor assembly comprising a plurality of stages and the blades with holes in their root portions being provided in at least one stage of the rotor assembly.

8. A bladed rotor assembly according to any one of claims 1 to 7 having plug means of solid cross-section.

9. A bladed rotor assembly according to any one of claims 1 to 7 having plug means of tubular cross-section.

10. A bladed rotor assembly according to any one of claims 1 to 9 having plug means of a metallic material.

11. A bladed rotor assembly according to any one of claims 1 to 9 having plug means of a polymeric material.

12. A bladed rotor assembly according to any one of claims 1 to 11 in which each plug means is adhesively retained in its hole by means of a cureable polymer.

13. A bladed rotor assembly according to any one of claims 1 to 11 in which each plug means is adhesively retained in its hole by means of a non-setting viscous adhesive.

14. A bladed rotor assembly according to claim 8 having plug means comprising a cureable polymer injected into its hole.

15. A blade having a hole in its root portion in accordance with any one of claims 1 to 7.

16. A bladed rotor assembly substantially as described in this specification with reference to and as illustrated by Figures 3 to 5 of the accompanying drawings.

17. A blade for an axial fluid flow machine substantially as described in this specification with reference to and as illustrated by Figures 3 to 5 of the accompanying drawings.