Abstract:
A method of balancing a rotor ( 10 ), the said method comprising the steps of determining the position of the center of mass of a rotor ( 10 ) to be balanced prior to the rotor ( 10 ) being machined to its final dimensions, and machining the rotor ( 10 ) to its final dimensions by removing material substantially along the length of the rotor ( 10 ). The material is removed in the direction of its axis such that the center of mass of the rotor is adjusted to be substantially co-incident with its axis of rotation ( 22 ).

Description:
This invention relates to rotor balancing and in particular the permanent balancing of rotors by the removal of material from the rotor to correct unbalance. 
   BACKGROUND 
   Rotating components such as wheels, crankshafts, drive shafts, gas turbine engine rotors etc, are balanced using various balancing methods which can be considered as either permanent or non permanent. In permanent balancing material is removed from the component to be balanced while in non-permanent methods balanced weights are added to the rotary component. 
   Rotating components can be balanced by the removal of material from specific balancing lands provided on the component to be balanced, or by the method of mass centering where location features, for example bearing surfaces, are offset machined to compensate for unbalance. In the method of mass centering the location features of the component are machined to their final dimensions such that the position of the axis of rotation, as determined by the location features, is made to be coincident with the center of mass of the component. 
   There are a number of disadvantages associated with mass centering. One disadvantage is that the component to be balanced is heavier than it would otherwise have to be because of the provision of relatively large balancing lands. Another disadvantage is that slave location features are required to hold the component while the normal location features are machined to their final dimensions. The requirement for relatively large lands and slave location features can add significantly to the weight of the component and this is a significant consideration in gas turbine aero engine applications. 
   Gas turbine engine rotors have traditionally been balanced using the weight variation present in a set of aerofoil blades to correct not only the disc unbalance but also that of the blades. This has been possible because the rotor blades have traditionally been attachably/detachably mounted in slots provided in the rim of the rotor disc so that the blades can be distributed around the circumference of the disc in such a way that the variations in their weight can be used to compensate for disc and blade unbalance. The net effect of this approach is a balanced bladed disc assembly having zero unbalance. 
   SUMMARY 
   Recent improvements in manufacturing technologies have enabled bladed discs, also referring to as blisks, or bliscs, to be manufactured with the rotor disc integrally formed with the rotor blades. Blisks may be machined from solid, but more usually the blades are friction welded to the rim of the disc. Blisks have a number of advantages when compared with more traditional blade and disc assemblies since the weight associated with the mounting features of the blade and disc can be eliminated from the rotor, and therefore blisks are increasingly used in modern gas turbine engines. The use of blisks, however, presents a new problem to the gas turbine engine designer in terms of rotor balancing since the method of using the mass variation present in a set of blades and balancing the rotor assembly accordingly is no longer available. The use of mass centering is not desirable for the aforementioned reasons, and therefore there is a requirement for a more appropriate balancing method for balancing rotors, and blisks in particular. 
   According to an aspect of the invention there is provided a method of balancing a rotor; the said method comprising the steps of: 
   determining the position of the center of mass of a rotor to be balanced prior to the rotor being machined to its final dimensions; 
   machining the rotor to its final dimensions by removing material substantially along the length of the rotor in the direction of its axis such that the centre of mass of the rotor is adjusted to be substantially co-incident with its axis of rotation. 
   One of the main advantages of using the method of the above aspects of the invention is that the weight of the rotor can be significantly reduced by significantly reducing the size of the balance correction lands present on the rotor. It is preferable that the above method is used to balance the rotor within predetermined limits as determined by the manufacturing tolerances for the rotor, and then correct relatively minor levels of unbalance by offset machining the balance correction lands to more finely balance the rotor. In this way the method of the present invention can be used to correct the major part of the unbalance in the rotor such that the size of the balance lands required for fine correction can be significantly reduced. 
   The balancing method of the present invention is different to that of mass centring since in the present method the rotor is machined to its final dimensions such that the centre of mass of the rotor is moved to be coincident with the axis of rotation, while in mass centring it is the axis of rotation of the rotor that is moved to be coincident with the centre of mass by machining the rotor location features. 
   The method comprises the step of determining an offset representative of the eccentricity of the centre of mass of the unbalanced rotor with respect to the said axis of rotation, and applying the offset to machine the rotor to its final dimensions such that the geometric centre of at least part of the rotor is offset with respect to the axis of rotation to balance the rotor. The offset is preferably 1 mm or less and therefore the unbalanced rotor is first pre-machined to allow an offset of 1 mm or less to be applied to the rotor during final machining. 
   Preferably, the rotor comprises a hub having a bore and the bore is machined such that its geometric centre is offset with respect to the said axis of rotation by an amount equal to the offset dimension and in the direction of the said eccentricity. Thus, the diameter of the bore may initially be 1 mm or so undersize to enable the bore to be machined with an offset of that amount or less when machined to its final dimensions during the balancing thereof. 
   Conveniently, the bore is machined to its final dimension by a machine tool cutter centred on the centre of mass of the unbalanced rotor. In this way the bore can be machined with the correct offset applied to the bore so that substantially more material is removed from the side of the disc on which the centre of mass of the unbalanced rotor is located. 
   The rotor comprises an outer periphery, and the outer periphery may additionally or alternatively be machined such that its geometric centre is offset with respect to the said axis of rotation by an amount equal to the said offset dimension and in a direction opposite to that of the said eccentricity. In this way rotor unbalance may be corrected by machining either the bore or the outer radially annular part of the rotor or both. 
   Conveniently, the offset dimension or eccentricity is determined by the static unbalance of the unbalanced rotor, the average diameter of the material to be removed and the length of the rotor over which material is to be removed. The offset dimension (or eccentricity) is determined by the formula: e=(4×U)/(π×D 2 ×L×ρ), where U is the static unbalance of the unbalanced rotor, D is the average diameter of the material to be removed from the unbalanced rotor, L is the axial length of the rotor over which material is to be removed and ρ is the density of the rotor material. The offset required to correct the unbalance of the rotor is calculated using the above formula. Thus, the method of the present invention is particularly suitable where the offset dimension required to correct unbalance in the rotor is relatively large in comparison with the manufacturing tolerance of the part of the component to be machined. 
   The rotor may comprise a turbine disc or compressor, preferably a gas turbine engine disc, more preferably a turbine or compressor blisk, that is to say a disc having integrally formed blades instead of attachably/detachably mounted blades. With conventional gas turbine engine disc and blade assemblies the level of unbalance is relatively low and offset dimensions calculated in accordance with the above mentioned formula are typically of the same order as the manufacturing tolerance and therefore it may not be economically practical to reduce initial unbalance in such assemblies. However the high level of unbalance commonly found in blisk results in offset dimensions much greater than the manufacturing tolerances required and therefore the method of the present invention provides a practical and cost effective way of reducing initial blisk unbalance. The method of the present invention therefore has particular advantages where the initial unbalance of the rotor is relatively high and/or in applications where manufacturing tolerances are relatively low. 
   The present invention also contemplates a rotor balanced according to the above method and in particular a gas turbine engine disc or blisk balanced according to the method of the invention. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     Various embodiments of the invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a partial cross section axi-symmetric view of a blisk rotor stage in a gas turbine engine; 
       FIG. 2  is a view similar to that of  FIG. 1  which shows material to be removed from the blisk in a final machining step in a method of balancing the blisk in accordance with an embodiment of the present invention; 
       FIG. 3  is a schematic representation of a rotatable component having an irregular shaped central bore for the purpose of illustrating the calculation of the offset to be applied in the method of the present invention; 
       FIG. 4   a  is a schematic representation of an offset vector in the plane of a rotor; 
       FIG. 4   b  is a schematic view of a rotor in a plane perpendicular to the rotor&#39;s axis of rotation, illustrating the application of the offset to the final machining of the external circumference of the rotatable component; and 
       FIG. 5  is a view similar to that of  FIG. 1  of a blisk rotor stage having a different configuration to that of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Referring to  FIG. 1 , a rotor stage of a gas turbine engine comprises a blisk  10  which comprises a disc part  12  and a plurality of circumferentially spaced aerofoil blades  14  which extend radially outwards from the disc rim  16 . The blisk is provided with forward and rearward mounting flanges,  18  and  20  respectively, for mounting the blisk to a shaft (not shown) of the gas turbine engine for rotation about the engine axis  22 . 
   In the drawing of  FIG. 1  the adjacent engine components are omitted for clarity and only part of the blade  14  is shown, ie that nearest the disc. The rotor stage  10  may be a compressor rotor stage or a turbine rotor stage since the method of the present invention is applicable to balancing both turbine and compressor rotors. 
   In the method of the invention, prior to finish machining the blisk of  FIG. 1  to its final dimensions shown in  FIG. 1 , the blisk is machined so that the average diameter  26  of the blisk&#39;s radially inner surface  24 , within the central bore of the disk, is slightly smaller than the final diameter of the finished disc. This is more readily seen in the drawing of  FIG. 2  where the undersize bore diameter is indicated by the line  28  which represents the bore surface prior to finish machining the blisk to its final dimensions. In a typical example the diameter of the bore of the blisk is about 1 mm undersize prior to finish machining. This additional material enables the bore to be finish machined with its geometric centre offset from the axis of rotation  22  to compensate for eccentricity of the centre of mass of the blisk with respect to the axis  22 . 
   The static unbalance of a rotor may be determined in accordance with the following formula;
 
 U=π×D   2   ×L×e×ρ/ 4
 
where, U represents the static unbalance, D represents the average diameter of the surface to be finish machined, L is the axial length of the part of the component being offset machined, e represents the offset or eccentricity to be applied, and ρ is the density of the component material.
 
   It follows therefore that the eccentricity (e) required to correct static unbalance is
 
 e =(4× U )/(π× D   2   ×L×ρ )
 
   The above referenced parameters are illustrated in the drawing of  FIG. 3  which shows the internal circumference of a rotor having a bore of irregular shape. The view of  FIG. 3  is similar to that of  FIGS. 1 and 2  in the sense that it shows an axi-symmetric cross section view of the rotor. As can be seen in the drawing of  FIG. 3  the offset (e) is shown applied to the outline of the rotary components such that the solid line  30  represents the outline of the inner diameter before finish machining and the chained line  32  represents the outline of the internal diameter after the component has been finish machined to its final dimensions. It is to be understood that the offset (e) results in different amounts of material being removed from different parts of the inner diameter around its circumference, since the offset is applied in the direction (angular position) of the centre of mass of the rotary component prior to finish machining. 
   The principle of offset machining to compensate for static unbalance in a rotor is illustrated with reference to  FIGS. 4   a  and  4   b . In this example balancing is achieved by removing material from the outer circumference of the component to be balanced rather than the radially inner circumference or radially inner bore surface in the examples of  FIGS. 1 to 3 . 
     FIG. 4   a  is a vector diagram showing the direction, or angular offset, of the centre of mass of the rotor to be balanced prior to finish machining. The angular offset vector  34  is shown with respect to a coordinate reference frame passing through the axis of rotation of the rotor represented by the point  36  in the drawing of  FIG. 4   a.  In the drawing of  FIG. 4   b  the diameter  38  of the outer circumference  40  of the rotary component is greater than is required in the finish component to allow material to be removed for balancing. In the drawing of  FIG. 4   b  the geometric centre of the outer circumference  40  and the axis of rotation are coincident and represented by the point  42  at the centre of the cross hairs  44 . The outer circumference  46  of the finish machined rotary component has a geometric centre represented by point  48  in the drawing which is offset from the geometric centre  42  by a distance  50  as determined by the above mentioned equation. Thus the distance between the geometric centres  42  and  48  is equivalent to the eccentricity of the mass centre of the unbalanced rotary component having the external circumference  40 . In this example the component is to be balanced by removing material from the outer circumference and therefore the offset  50  is applied in a direction diametrically opposite that of the unbalance vector  34 , that is to say with an offset angle equal to that of the vector  34 +180°. In the example of  FIGS. 4   a  and  4   b  the material in the region  52  between the outer circumference  40  of the unbalanced rotary component and the circumference  46  of the finish machined component is removed in the balancing process. In applications where the rotary component is machined using a machine tool cutter, the cutter may simply be centred on the geometric centre  48  to remove the unwanted material in the region  52  bounded by the circumferences  40  and  46 . Preferably the cutter is controlled by a numerically controlled machine tool. 
   It is to be understood that in balancing methods of the aforementioned type the offset is applied in the direction of the vector  34  where material is to be removed from the surface of the inner circumference of an internal bore, such as that defined by surface  24  in the blisk  10  of  FIGS. 1 and 2 . 
   In the method of the present invention material is removed along substantially the whole length of the component to be balanced and in the rotor of  FIGS. 1 and 2  material may be removed in the region between the flanges  18  and  20  and on the other sides of the flanges as indicated by lines  28  in  FIG. 2 . Material may also be removed additionally or alternatively from the gas washed surface at the rim  16  of the disc between the aerofoil blades  14 . 
   Referring now to  FIG. 5  which shows alternative blisk arrangement where the disc part  12  of the blisk extends radially and axially and may be offset machined along the downstream  56  side of the blisk, as indicated to the right of the drawing in  FIG. 5 , or on the upstream side  54  as indicated to the left of the drawing in  FIG. 5 . 
   Although aspects of the invention have been described with reference to the embodiments shown in the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications may be effected without further inventive skill and effort. For example, the invention may be used for balancing components other than blisk rotor stages in gas turbine engines, and may be used for balancing any type of rotational component where the initial unbalance is relatively high and generally too high for correcting by known methods of machining balance correction lands.