Patent Publication Number: US-8974185-B2

Title: Balancing of rotatable components

Description:
FIELD OF THE INVENTION 
     The present invention relates to the balancing of rotors in a turbine engine and particularly the balancing of compressor or turbine rotors and balance weights and methods therefore. 
     BACKGROUND OF THE INVENTION 
     Referring to  FIG. 1 , a turbofan gas turbine engine  10  comprises in flow series an inlet  32 , a fan  2  and a core engine  4  comprising an intermediate pressure compressor  12 , a high pressure compressor  14 , a combustor  16 , a high pressure turbine  18 , an intermediate pressure turbine  20 , a low pressure turbine  22  and an exhaust. The fan  2 , compressors  12 ,  14 , and turbines  18 ,  20 ,  22  are all arranged to rotate about a central common engine axis  1 . Air is drawn into the engine  10 , as shown by arrow B, through the annular inlet  32  and into the fan  2 . The fan  2  compresses the air and a portion flows, in a downstream direction, into the core engine  4  where it is further compressed, mixed with fuel and burnt in the combustor  16 . 
     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 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 caused by manufacturing tolerance. However, this technique may not be sufficient in itself to correct all imbalances and involves time consuming 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 the most commonly used method, involving the use of small balance weights secured at appropriate points around the rotor. The present invention seeks to provide an improved method of balancing a rotor and an improved apparatus for balancing a rotor. 
     According to a first aspect of the invention there is provided a balanced rotor component for a gas turbine engine, the balanced component rotatable about an axis in use and having at least one slot having a profile holding a complementary profile of a root portion of a blade member, wherein the slot holds a balance weight which is in contact with the base of the slot at rest, the balance weight being further located within a cavity in the root portion. 
     The slot may be circumferential or axial. The blade member may be a compressor blade or a turbine blade. The turbine blade may be located in a turbine of a gas turbine engine. The compressor blade may be located in a compressor of a gas turbine engine. The rotor component may be a disc, ring or drum. The slot may be of the dovetail form, or in a fir-tree form or any other appropriate slot shape as known in the art. 
     Preferably the slot is axial and the cavity has at least an axially forward or an axially rearward wall defining the axially forward or axially rearward extent of the cavity. Preferably the cavity has both an axially forward and an axially rearward wall. 
     The axially forward and axially rearward walls may diverge and converge with respect to a centre line defined between the two walls to provide a cavity or recess of reflected trapezoid form. 
     Preferably the lateral sides of the cavity are open. 
     The radially inner surface of the balance weight may have a form which matches the base of the slot. 
     The slot may be axial and the balance weight has fore, aft sides and lateral sides, wherein at least one of the lateral sides has two portions joined at an obtuse angle. Preferably at least one of the lateral portions is spaced from a side wall of the slot. 
     The slot may be axial and the balance weight has fore, aft sides and lateral sides, wherein at least one of the lateral sides is spaced from a side wall of the slot. 
     The balance weight may have a radially extending protrusion engaging a hole in the root portion. 
    
    
     
       The invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  depicts a gas turbine engine incorporating an apparatus for balancing the rotor in accordance with the invention; 
         FIG. 2  depicts a portion of a compressor disc having an axial slot; 
         FIG. 3  depicts a balance weight located on a rotor blade; 
         FIG. 4  is a further view of a balance weight; 
         FIG. 5  is a view of a balance weight within an axial disc slot; 
         FIG. 6  is a further view of the balance weight within the axial disc slot; 
         FIG. 7  depicts a balance weight located on a rotor blade; 
         FIG. 8  depicts a balance weight within an axial disc slot; 
     
    
    
     Referring to  FIG. 1 , the fan stage  32 , the compressors  12 ,  14 , and the turbine  18 ,  20  and  22  can comprise a series of blades mounted in axial or circumferential slots. For the rest of the application the invention will be described with regard to axial slots but it will be appreciated that the invention may also be used in circumferential slots. It will be appreciated that the term axial and radial circumferential to the general direction in which the slot and the slots may be angled from the true axis or circumference of the component. Typically an axial slot will deviate less than 45° from the true axis and may also curve. 
     An axial slot in an intermediate pressure compressor is shown in  FIG. 2 . The slot is of the “dovetail” type with a portion  30  which receives a blade root and has a neck portion  34  which retains the blade and prevents it moving radially. It will be appreciated that the slot shown is only one in an array of slots which extends circumferentially around the disc and accordingly each land bisects adjacent slots. Similarly it will be appreciated that only one row of slots is shown and where the disc is part of an intermediate or high pressure compressor for example there may be a further row of slots either axially forward or axially rearward, or both axially forward and axially rearward of the slot depicted. 
     The disc is subject to significant stresses, one point of higher stress being indicated by the contour lines  38  though there are others regions subject to similarly high stress which are not shown. A region of higher stress can influence the allowed maximum working life of the disc which should not be permitted to fail in use. 
     A balance weight  40  which may be used to balance the disc is depicted in  FIGS. 3 and 4 . The weight is located in a cavity or recess in the root portion of the blade the cavity being open and defined between an axially forward wall  50  and an axially rearward wall  52  which prevent axial movement of the weight in use. The weight is permitted to slide circumferentially as induced by movement in use with circumferential stops being provided by the side walls of the slot. 
     The weight has a forward wall  42  and a rearward wall  44  and two lateral flanks  46 ,  48  which connect the forward wall and the rearward wall. The weight has a central cavity  54  the size of which may be varied to alter the mass of the weight whilst the outside “foot print” or form of the weight remains unchanged despite the mass of the weight selected to be used. 
     The lateral sides may be spaced from the wall of the slot or, more preferably, be formed as two or more portions joined at an obtuse angle. The lateral sides are arranged such that in the event of circumferential movement of the weight within the slot the lateral does not contact the walls of the slot at the point of its highest stress. Beneficially, this has been found to improve the working life of the disc by preventing additional stress load at a position which is already subject to high stress in use. 
     For the balance weight shown the extended length “D” of the forward and rearward walls,  42 ,  44  is around 27 mm with the lateral walls  46 ,  48  being spaced apart at a slightly shorter distance “B” of around 17 mm. It will be appreciated that these dimensions are exemplary and will depend on the size of the slot and the mass to be added by the balance weight, amongst other things. Also in the exemplary weight the length of portion  46 ″ should preferably be no more than 80% of the total distance between the fore and aft walls  42 , 44 . The obtuse angle  47  between the first portion  46 ′ and the second portion  46 ″ is selected such that the length “E” is no less than 20% of the extended length “D” of the forward wall  42 . These proportions are typical for a number of sizes of balance weights but may be varied provided a reasonable contact area over lengths  44  and  42  is provided. 
     The angle between the second portion of the lateral wall 46″ and rearward wall  44  may be 90° to produce a generally rectangular balance weight or, as shown, it may be acute to provide a weight of a parallelogram form, or it may be obtuse to provide a reflected trapezoid as shown in  FIG. 7 . 
     As shown in  FIGS. 5 and 6 , the weight is sized such that it may be located in the slot and had a radially inner face which closely matches the shape of the base of the slot. The shape of the weight on its inner surface provides an even contact with the disc when the engine is not running and also assists in locating the weight in its correct circumferential location. 
     In use the loading of the weight is through the flanks of the blade root to the disc. 
     In an alternative weight construction the recess in the blade root and the weight are of an inverted trapezoid construction as shown in  FIG. 7 . The weight shown has no cavity and is solid. A location feature  60  may be provided on the weight as shown in  FIG. 8  to facilitate positioning on the blade root. The location feature may be a dowel or projection on the weight or root and an complimentary receptacle on the opposing component. One benefit of the reflected trapezoid is that the lateral movement of the weight is inhibited. To avoid contact of the weight with the slot wall the length of the weight  62  is chosen to be less than the length of the recess  64  in the blade root. 
     It will be appreciated that embodiments of the invention provide a simple and elegant assembly where the weight is enclosed to reduce the risk of the weight being unintentionally released in use. The weight is simple to replace and the standard footprint enables cheap and simple replacement should a heavier or lighter weight be required. 
     The person of skill in the art would also realise that features described with respect to one embodiment should be considered appropriate for use in other embodiments where the modification is possible. The applicant has considered all possible combinations of the embodiments and each is individually disclosed in this application.