Abstract:
A method of balancing a rotor, where the rotor comprises a plurality of holes, comprises the steps of measuring an imbalance of the rotor, identifying at least one characteristic dimension associated with at least one hole, identifying a change to the or each characteristic dimension to reduce the imbalance, and changing the or each characteristic dimension.

Description:
FIELD OF THE INVENTION 
   This invention relates to the balancing of rotating components. More specifically, although not exclusively, it relates to the balancing of rotor discs or drums for gas turbine engines. 
   BACKGROUND OF THE INVENTION 
   The main rotating assemblies of gas turbine engines comprise compressors and turbines, rotating on shafts. As with any rotating assembly, any imbalance in these assemblies will give rise to vibration and stresses. The magnitude of these stresses increases as the square of the rotational speed. 
   Rotational speeds in gas turbine engines vary with the type of engine and the conditions of operation, but speeds of several thousand revolutions per minute are common. Because of these high rotational speeds, even slight unbalance in the main rotating assemblies can quickly give rise to damaging stresses, and so very accurate balancing of the assemblies is essential. 
   Two common ways of balancing a rotor disc or drum of a gas turbine engine are 
   i) by providing additional material (a “balance land”) on the rotor drum. This is then selectively machined away during manufacture to achieve the desired balance adjustment. The balance land is essentially parasitic mass. It cannot be assumed to carry any loads, as there is no guarantee it will be there in the final part. Conversely, for stress purposes it must be assumed that all the mass is present, in case none of it has to be machined away. The balance land is therefore an undesirable feature; 
   ii) by adding additional mass selectively to a component or assembly, in the form of, for example, screwed or riveted plugs, heavy wire, balancing plates or nuts. This method also adds parasitic mass to the component, and has the further disadvantage that the balancing masses may become detached from the component and cause damage elsewhere in the engine. 
   Conventionally, rotor discs and drums are connected to neighbouring components by means of bolted flanges. As well as the bolt holes, such rotors typically have stress defender holes, which help to reduce the increased stresses found at stress concentration features around the bolt holes. 
   SUMMARY OF THE INVENTION 
   This invention provides a means of balancing a rotor without adding parasitic mass to it, by adjusting the dimensions or positions of holes in the rotor. Typically these will be bolt holes or stress defender holes. 
   According to one aspect of the present invention, a method of balancing a rotor having a plurality of holes comprises the steps of measuring an imbalance of the rotor, identifying at least one characteristic dimension associated with at least one hole, identifying a change to the or each characteristic dimension to reduce the imbalance, and changing the or each characteristic dimension. 
   The characteristic dimension may be hole diameter, hole radial position, hole circumferential position, counterbore diameter or counterbore depth. 
   The hole may be a stress defender hole, or may be a bolt hole. 
   According to another aspect of the present invention, a rotor is provided, balanced according to the method described in any of the three preceding paragraphs. 
   The rotor may be part of a compressor for a gas turbine engine. 
   The rotor may be part of a gas turbine engine. 
   According to another aspect of the present invention, a rotor for a gas turbine engine has a plurality of bolt holes spaced around its circumference and a plurality of stress defender holes interposed between the bolt holes, and the rotor is balanced by a method comprising the steps of measuring an imbalance of the rotor, identifying a change to the diameter of at least one stress defender hole to reduce the imbalance, and changing the or each diameter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described more fully, by way of example, with reference to the following drawings in which: 
       FIG. 1  is a sectional side view of the upper half of a gas turbine engine of known type; 
       FIG. 2  shows part of a bolted flange of a compressor rotor drum balanced in known manner; 
       FIG. 3  shows part of a bolted flange of a compressor rotor balanced according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to  FIG. 1 , a gas turbine engine  10  comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate compressor  13 , a high pressure compressor  14 , a combustor  15 , a turbine arrangement comprising a high pressure turbine  16 , an intermediate pressure turbine  17  and a low pressure turbine  18 , and an exhaust nozzle  19 . 
   The gas turbine engine  10  operates in known manner so that air entering the intake  11  is first accelerated and compressed by the fan  12 . It is subsequently split into two airflows—a first airflow into the intermediate pressure compressor  13  and a second airflow which provides propulsive thrust. The intermediate pressure compressor  13  compresses the air directed into it before delivering it to the high pressure compressor  14  where further compression takes place. 
   The compressed air delivered from the high pressure compressor  14  is directed into the combustor  15 , where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through the high, intermediate and low pressure turbines  16 ,  17  and  18 , thereby driving them, before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low pressure turbines  16 ,  17  and  18  respectively drive the high and intermediate compressors  14  and  13  and the fan  12  by suitable connecting shafts. 
   The fan, compressors and turbines of the engine  10  each comprise one or more arrays of blades. The blades in each array are mounted so that they extend radially outwardly from a rotor. The rotors are mounted so that they can rotate about an axis X-X. It is common for several rotors in a given compressor or turbine to be connected together, for example by welding or by bolted flanges, to form a unit 
     FIG. 2  shows part of a rotor of known type. The rotor  20  has a plurality of bolt holes  22  spaced around its circumference. Bolts pass through these holes to secure the rotor to an axially adjacent rotor (not shown). Interposed between the bolt holes  22  are stress defender holes  24 . These help to reduce the increased stresses found at stress concentration features around the bolt holes. 
   The rotor is also provided with a balance land  26 . When the rotor is balanced, material is selectively machined away from the land  26 , as shown at  28 , to reduce the imbalance in the rotor. 
     FIG. 3  shows a part of a rotor balanced according to the invention. The rotor  30  has a plurality of bolt holes  32  spaced around its circumference, and interposed between these are stress defender holes  34 . One stress defender hole  36  has been made larger than the others; this alters the mass distribution of the rotor and thereby adjusts its balance. In this way the balance of the rotor can be adjusted without the need for the large parasitic mass of the balance land of  FIG. 2 . 
   It will be appreciated that the same effect of altering the mass distribution of the rotor could be achieved by means other than increasing the diameter of the stress defender holes. For example, their radial or circumferential position could be altered, or a counterbore of the required size could be added to a stress defender hole. 
   Alternatively or additionally, similar changes could be made to the positions or dimensions of one or more bolt holes. 
   Alternatively or additionally, extra holes (being neither bolt holes nor stress defender holes) could be provided in a rotor, specifically for balancing purposes, and the changes made to their positions or dimensions. 
   Furthermore, it will be appreciated that although the invention has been described in the context of a gas turbine engine, the principles set out in this specification could equally well be applied to any sort of rotor whose balance is to be corrected. 
   There is thus provided a method of balancing a rotor that avoids the difficulties inherent in the prior art, and allows the construction of rotors whose dimensions and weights are optimised.