Patent Publication Number: US-7912587-B2

Title: Method of balancing a gas turbine engine rotor

Description:
TECHNICAL FIELD 
     The invention relates generally to a method of balancing an assembly of rotary components of a gas turbine engine. 
     BACKGROUND OF THE ART 
     It is routine for gas turbine engines to have to pass stringent vibration acceptance tests following production. If an engine does not pass the vibration acceptance limit, it typically must be disassembled, re-balanced, and reassembled, which wastes time and resources. 
     Accordingly, there is a need to provide improved methods of balancing an assembly of rotary components. 
     SUMMARY 
     In one aspect, there is provided a method of balancing a rotor assembly comprising first and second rotors adapted to be coupled together, and a stack of intermediate components clamped between the first and second rotors, the method comprising: determining a relative angular position of the first and second rotors, the so angularly positioned first and second rotors respectively providing first and second reference faces defining a space therebetween for receiving the stack of intermediate components, and determining a stacking angular position of each of said intermediate components using geometrical data on said intermediate components and said first and second reference faces. 
     In a second aspect, there is provided a method of balancing a first rotor pack comprising a plurality of assembled rotor components and a coupling interface for connection to a second rotor pack, the method comprising: measuring said coupling interface to establish a reference axis line, and referencing said rotor components back to said reference axis line in order to establish individual angular stacking positions of said rotor components. 
     In a third aspect, there is provided a method of balancing a rotor assembly comprising first and second rotor packs, the first and second rotor packs being coupled to each other at a coupling interface, the method comprising separately balancing the first and second rotor packs, and determining the relative angular positioning of the first and second packs considering a measured geometry of the coupling interface. 
     In a fourth aspect, there is provided a method of balancing an assembly of rotary components including first and second main components and intermediate components adapted to be positioned in-between, each rotary component having at least one mating face, a respective reference and a plurality of stacking positions, the method comprising the steps of: 
     measuring the concentricity of the first and second main components; 
     measuring the parallelism of the mating faces of the first and second main components relative to the respective references; 
     generating an assembly unbalance for each combination of first and second main component stacking positions, determining the lowest assembly unbalance and defining the first and second main component stacking positions of the lowest assembly unbalance as optimal first and second main component stacking positions; 
     measuring the parallelism of the mating faces of each intermediate component relative to the respective references; 
     generating an assembly unbalance for each combination of intermediate component stacking positions relative to the optimal first and second main component stacking positions, determining the lowest assembly unbalance and defining the intermediate component stacking positions of the lowest assembly imbalance as optimal intermediate component stacking positions. 
     Further details of these and other aspects will be apparent from the detailed description and figures included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG. 1  is a schematic view of a gas turbine engine including an exemplary rotor assembly including a high pressure compressor (HPC) impeller and a high pressure turbine (HPT) first disk; 
         FIG. 2  is a sectional view of the rotor assembly of the gas turbine engine of  FIG. 1 , shown in cross-section along an axial centerline of the gas turbine engine; 
         FIG. 2   a  is an enlarged view of a connection between the HPC and the HPT shown in  FIG. 2 ; 
         FIG. 3  is a cross-sectional view showing the detail of a two-stepped spigot connection between the HPC impeller and the first turbine disk of the HPT pack shown in  FIG. 2 ; 
         FIG. 3   a  is an enlarged view of the spigot connection shown  FIG. 3 ; 
         FIG. 4  is a schematic cross-sectional view of the HPC impeller of  FIG. 3  mounted on a turntable for obtaining geometric parameters by means of a measuring system; 
         FIG. 5  is a schematic cross-sectional view of the first, turbine disk of  FIG. 3  mounted on a turntable for obtaining geometric parameters by means of the measuring system; 
         FIG. 6  is a schematic view of a series of points representing two different faces on the HPC impeller recorded in a 3-dimensional XYZ plane by the measuring system of  FIG. 4 ; 
         FIG. 7  is a flow chart showing a method of balancing an assembly or rotary components including first and second main components and intermediate components; 
         FIG. 8  shows a generic example of a possible spigot configuration; 
         FIGS. 9   a - 9   c  show examples of possible stacking arrangement of adjacent shaft-mounted components; 
         FIG. 10  is a schematic cross-sectional view of a turbine cover plate mounted on a turntable for obtaining geometric parameters by means of the measuring system; and 
         FIG. 11  is a schematic cross-sectional view of the HPT pack-turbine shroud housing assembly mounted on a turntable for obtaining geometric parameters by means of a measuring system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a compressor section  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel, and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. 
     Generally, the gas turbine engine  10  comprises a plurality of assemblies having rotary components mounted for rotation about a centerline axis  11  of the engine  10 . For instance, the compressor  14  section may include a high pressure compressor (HPC) pack  22  having multiple stages. The turbine section  18  downstream of the combustor  16  includes a high pressure turbine (HPT) pack  24  that drives the HPC  22  and a low pressure turbine (LPT)  26  that drives the fan  12 . 
       FIG. 2  shows an exemplary rotor assembly between the HPC pack  22  and the HPT pack  24  of the gas turbine engine  10 . The HPT pack  24  includes first and second turbine disks  27  and  28  carrying respective circumferential arrays of radially extending blades  30   a  and  30   b  (however, it is understood that the HPT  24  may have any number of stages, including only one stage, i.e. only one disk). The HPT pack  24  further comprises a front cover plate  23  and a rear cover plate  25 . As shown in  FIGS. 2 and 3 , the HPC pack  22  comprises, among other things, an impeller  32  (the exducer portion of which is shown in  FIGS. 3 and 4 ) adapted to be assembled to other HPC rotor stages  20   a ,  20   b ,  20   c  (schematically shown in  FIG. 1 ) to form the HPC pack or module. The impeller  32  is the last or downstream rotor component of the HPC pack  22 , and provided on an aft side of the impeller  32  is a hollow spigot projection  34  adapted to tightly receive in mating engagement a corresponding spigot projection  36  of the first turbine disc  27 . As best shown in  FIGS. 3 and 3   a , the spigot projection  34  of the impeller  32  in this embodiment has two axially-extending circumferential spigot, contact faces  38  and  40  respectively provided at first and second inside diameters of the impeller spigot projection  34 . The spigot projection  36  of the HPT first disk  27  has two corresponding mating axially-extending circumferential spigot contact faces  42  and  44  respectively provided at first and second outside diameters of the spigot  36 . The respective pairs of spigot contact faces  38 ,  42  and  40 ,  44  are adapted to telescopically engage by way of tight fit diameters. Mating in this way, the spigots dictate the relative alignment between the HPC pack  22  and HPT pack  24 . In other words, the HPT pack  24  radial positioning (i.e. relative to the centreline) is based on the spigot alignment with the HPC pack  22 . Deviations in spigot alignment result in deviations in alignment between the HPC and HPT packs. 
     As shown best in  FIG. 2   a , a plurality of intermediate components, sometimes referred to as a “clamp stack”, is mounted (by clamping between the rotors, in this example) between the impeller  32  and the first turbine disc  27 . More particularly, in the example of  FIGS. 2 and 2   a  a front runner seal  46 , a bearing  48 , a rear runner seal  50  and a spacer  52  are axially positioned one next to the other between the impeller  32  and the first turbine disc  27 . A tie shaft  54  extends axially centrally through the first and second turbine discs  27 ,  28 , through the spigot joint and into the impeller  32  to apply a compressive clamping load to the rotor assembly. The tie shaft  54  is securely engaged at a forward end to the impeller  32 . A nut  56  is threadably engaged on the aft end of the tie shaft  54  for axially clamping the clamp stack (i.e. front runner seal  46 , the bearing  48 , the rear runner seal  50  and the spacer  52 ) between a radially-extending circumferential rear abutment face  53  of the impeller  32  and a radially-extending circumferential front abutment face  55  of the first turbine disc  27 . It is understood that any suitable tightening means could be used to axially press the intermediate components, the impeller  32  and the HPT pack  24  together. 
     Referring still to  FIG. 2   a , the front runner seal  46 , the bearing  48 , the rear runner seal  50  and the spacer  52  are each provided with respective mating radially-extending circumferential front-and rear abutment faces  46   a ,  46   b ;  48   a ,  48   b ;  50   a ,  50   b  and  52   a ,  52   b . When clamped as described above, the front abutment face  46   a  of the front runner seal  46  is axially pressed against the rear abutment face  53  of the impeller  32 . The front abutment face  48   a  of the bearing  48  is axially pressed against the rear abutment face  46   b  of the front runner seal  46 . The front abutment face  50   a  of the rear runner seal  50  is axially pressed against the rear abutment face  48   b  of the bearing  48 . The front abutment face  52   a  of the spacer  52  is axially pressed against the rear abutment face  50   b  of the rear runner seal  50 . Finally, the front abutment face  55  of the first turbine disc  27  is axially pressed against the rear abutment face  52   b  of the spacer  52 . 
     The rotor assembly shown in  FIG. 2  is mounted within the engine coaxially with the engine centerline  11 , defined by bearings  47  and  48  (see  FIG. 1 ). It is desirable to minimize radial eccentricity of the assembly from the engine centerline  11 , to reduce rotor imbalance and, thus, vibration during engine operation. Although each rotary component of a gas turbine engine is manufactured with precision, it remains that tolerance effects will result in components which, among other things, are slightly off-center relative to (i.e. lack concentricity with) the axis of rotation and which have less than perfectly parallel mating faces (i.e. faces are not square). The effect of such eccentricities relative to the nominal engine centreline which, if ignored, may cause radial rotor deflection (i.e. vibration) in use. Consequently, these imperfections increase the vibration amplitude of an assembly and can result in considerable unbalance in the gas turbine engine. 
     As mentioned, there are at least two types of geometric deviations due to tolerancing which are considered in gas turbine rotor balancing, namely (1) lack of concentricity of axially-extending surfaces with a datum axis, or the existence of an eccentricity between a geometric centre of the surface of interest and a selected datum (such as a shaft centreline), and (2) lack of parallelism of a radially-extending faces, or a deviation from parallel between a face and a selected datum face. Lack of concentricity is sometimes referred in the art (and herein) to as radial deviation, radial run-out, centerline deviation or perpendicular plane deviation. Lack of parallelism is sometimes referred to in the art (and herein) as plane deviation, bi-plane deviation or face squareness deviation. 
     Tolerance effects in individual components can be addressed during assembly to provide a more balanced assembly, such as by adding counterbalance weights, and or by adjusting the relative angular alignment of components (known as stacking) to offset the unbalances of individual components against each other, to provide a cancellation effect with respect to the overall assembly. For example, two components having radial deviations can be angularly aligned with the radial deviations positioned 180 degrees from one another, to minimize their cumulative effect. In multi-piece assemblies, balancing optimization becomes more complex. 
     One approach to stacking rotor components to minimize deviations is to build a rotor serially, component by component, positioning each relative component to an arbitrary datum defined by a first bearing centreline (it being understood that rotors assemblies are typically supported by at least two bearings, and thus the bearings may be used to establish a reference for the axis of rotation). The bearing centreline is typically established by a bearing centre and a bearing face, the centreline passing through the centre and extending perpendicular to the face. For example, the concentricity for each component is determined relative to the bearing centreline. A first component is then placed in position (in fact, or virtually), and its radial deviation from the desired datum is noted. A second component is then mounted to the first, and stacked relative to the first such that overall radial deviation of the assembly is reduced (i.e. one attempts always to build back towards the datum line, so to speak, ideally to yield a rotor assembly with a net-zero concentricity deviation once all rotor components are assembled). Unfortunately, this method does not work well in all situations, such as where rotor systems having a connection between two rotor assemblies, such as a spigotted or curvic coupling between an HPC pack and an HPT pack. 
     For instance, a lack of concentricity or radial deviation of the axially-extending spigot contact faces  38 ,  40 ,  42  and  44  between the impeller  32  and the first turbine disk  27  may lead to an assembly unbalance if not taken into account when assembling the first turbine disk  27  to the impeller  32 . For example, referring to  FIG. 8 , shown is a simplified single spigot connection Sp-Sp between two rotors R 1 , R 2 . Although the individual components R 1  and R 2  may have been individually optimized to as that they do not have significant radial eccentricities, if the spigots lack concentricity, there will be a resulting eccentricity in the final rotor assembly R 1 -R 2 . 
     Furthermore, if the radially-extending abutment faces of a component are not parallel to one another, the interaction between the component and adjacent rotor components creates a mismatch between mating faces, which tends to cause unbalance. Referring to  FIG. 9   a  and  9   b , central shaft S has a plurality of components A, B, C and D with respective radially-extending mating faces a 1 , b 1 , b 2 , etc. which lack parallelism. Referring to  FIG. 9   b , when such components are clamped together under load, the shaft tends to deflect (δ) from the centreline in order to allow the mating faces a 1 , b 1 , b 2 , etc, to meet. Thus, the interaction between adjacent components is affected such that the center of mass of the assembly of  FIG. 9   b  is offset or displaced from the axis of rotation or centreline. 
     Either of the examples of the preceding two paragraphs could result in a rotor having a displaced center of mass. A displaced center of mass in the turbine pack of the engine of  FIG. 1 , where the turbine overhangs the bearings, will perform an orbital trajectory around the desired axis of rotation during operation thus creating vibration. Typically, the greater the displacement, the greater the vibration. 
     As mentioned, rotor assembly unbalance can be minimized by adjusting the stacking angle of each component in relation to the other rotor components, so as to cumulatively minimize the unbalancing effect of the lack of concentricity and the non-parallelism of the mounting ends (also referred to herein as radial abutment faces) of the rotor components. The stacking angle of each component is adjusted by rotating the component relative to adjoining components) about the centerline axis in the rotor stack. By optimizing the relative stacking angles for each component, the overall balance of the rotor can be optimized, by aligning the individual components so that unbalances are subtractive, rather than additive, tending to cancel one another out. This can result in an overall assembly with a minimal possible imbalance for a given set of components. 
     Referring again to  FIGS. 9   a - 9   b , it has been found that shaft deflection is proportional to the cumulative tolerance error in a clamp stack between two rotor assemblies (or any other reference faces). It has also been found that stacking the components clamped between two rotor assemblies significantly improves the geometry and hence measured out of balance of the overall rotor assembly. Referring to  FIG. 9   c , if one considers the relative lack of parallelism of the various mating faces a 1 , b 1 , b 2 , etc., an optimal arrangement of the faces may be found to minimize the net deflection (δ) of the assembly, once a clamping load is applied. To do so, conceptually speaking, the faces a 1  and d 3  of the outside components A, D (in this example) can be thought of as defining a space of certain shape and the remaining components (B, C in this example) are then arranged relative to one another and relative to components A, D, to fill the space as neatly as possible, so to speak. In other words, the components A-D are preferably stacked (i.e. angularly aligned) so that the mating faces (a 1 -b 1 , b 2 -c 2 , etc.) are as parallel as possible to one another within the given selection of components, all with the goal of providing a “best fit” of components within the space/shape defined by the outer or boundary surfaces a 1  and d 3 . It will be understood that the selection of components may also be altered, for example by substituting a component D with an unfavourable face characteristic for another component D “off the shelf”, to arrive at a more optimum face alignment, Although the above example, for illustration purposes assumes that the components A, D will define a pre-selected space within which the remaining components will be aligned to “fill”, it will be understood that the relative alignment of components A, D will also be considered an optimized, to provide the best possible shape to which the remaining components are best suited. Thus, as can be seen from  FIG. 9   c , an alignment of components is possible wherein face squareness error is minimized for the assembly, thereby reducing imbalance. 
     A rotor balancing example will now be considered for the gas turbine engine described above. As will be seen hereinbelow, numerous geometric parameters from the above described components of the high pressure rotor assembly are considered in the present technique in order to obtain the optimized component stacking angles that would provide the minimum rotor assembly unbalance, resulting in less vibration. Accordingly, different geometric inputs are required, such as 1) the parallelism of the radially-extending faces of the HPC and HPT components and of the intermediate parts (i.e. front inner seal  46 , bearing  48 , rear runner seal  50  and spacer  52 ) located between the HPC and HPT packs, 2) the concentricity of the HPC and HPT components, and 3) HPC impeller two spigot alignment geometry when the HPC pack is in an assembled state (as will be discussed further below with reference to  FIG. 6 ). The calculations and optimizations discussed further below are preferably processed by a computer, which employs various computer programs to compile the collected component geometric data and execute iterative processes to generate the best stacking optimization possible (i.e. the optimal stacking angles of the components) of the high pressure rotor assembly. 
     Now referring to  FIGS. 4 to 7 , we will see in details how the HPC stack  22 , the HPT stack  24  and the HPC-HPT assembly are balanced.  FIG. 7  depicts a method according to the present teachings. 
     Referring more particularly to  FIG. 4 , there is shown a measuring system  100  having a rotary table T and a plurality of probes P 1 -P 4  operatively connected to a programmable control system (not shown) which measures and processes the individual displacement readings from probes P 1 -P 4 . Probes P 1 -P 3 , in this set-up, are used to measure the concentricity, whereas probe P 4  is used to measure the parallelism of a front face  41  of the exducer of impeller  32 . A datum or imaginary axis of rotation is determine using data collected by probes P 1  and P 2 , and the output of the machine is the concentricity and parallelism provided by probe P 3  and P 4  respectively relative to the datum created by P 1  and P 2 . The same approach applies to other rotor components. The approach will now be discussed in detail. 
     Balancing of this rotor preferably begins with the impeller  32 . The exducer of the HPC impeller  32  is mounted front face down on the rotary table T and the probes P 1 -P 4  are positioned on predetermined surface points on the HPC impeller  32 . Particularly, as indicated in step  300  of  FIG. 7 , probes P 1  and P 2  are respectively used, to obtain geometric data on the concentricity of the HPC impeller  32  at the spigot contact surfaces  38  and  40  (it being understood that, at least initially, concentricity is measured relative to an axis of rotation of rotary table T). The probes P 3  and P 4  are used to obtain geometric data on the front side of the impeller  32 . Probe P 3  provides geometric data on the concentricity of the front inner diameter surface  39  of the exducer of impeller  32 , whereas probe P 4  provides geometric data on the parallelism of the front face  41  of the exducer of HPC Impeller  32 . Surface  39  and face  41  matingly engage the upstream adjacent HPC component, in this case the inducer of impeller  32  (not shown) and, thus, need to be taken into consideration in the determination of the HPC component stacking angles. 
     More specifically, measurement is done as follows. The measuring system  100  rotates the rotary table T, causing the exducer of HPC impeller  32  to rotate about the axis of rotation Z. The probes P 1 -P 4  remain stationary and in contact with the surfaces/faces of the exducer of HPC impeller  32  as the latter rotates. The probes P 1  and P 2  in contact with the inside spigot contact faces  38  and  40  record geometric data on the surface concentricity variations. More particularly, the probes P 1  and P 2  record the distance of each spigot contact face  38  and  40  from the axis of rotation Z at a series of points (i.e. angular locations). The measured points are preferably provided almost continuously around the circumference, to provide a multiple data points and thus improve the accuracy of measurement around the entire circumference. In a 3-dimensional coordinate system where the Z-axis is defined along the axis of rotation Z as shown in  FIG. 6 , each probe P 1 -P 3  records a series of data points in an X-Y plane around the circumference for a given Z value. 
     The data points representing spigot concentricity, recorded by probes P 1  and P 2 , are used to define a primary datum axis for the rotor assembly, as set forth by method step  300  of  FIG. 7 . More specifically, the data points recorded by each probe P 1 , P 2  may be connected to form respective circular formations  192  and  194  in the X-Y planes, as shown in  FIG. 6 . Theoretically, for a perfectly concentric component, the circular formations  192  and  194  would be perfectly centered about the Z-axis. However, in practice even the most precisely manufactured components have a slight eccentricity. Therefore, the primary datum axis is determined by connecting the center points  196  and  198  of the two circular formations  192  and  194  to provide a primary datum or reference axis  200 . The reference axis  200  defines the primary datum for the HPC components stacking (i.e. the stacking of the remaining HPC stages  20   a ,  20   b ,  20   c  and the inducer (not shown) of impeller  32  to the exducer of impeller  32 ), Spigot, contact surfaces  38  and  40  are thus used to define a primary datum or reference axis  200  for balancing of the HPC pack  22 . The selection of this primary datum will ultimately result in a better assembly stacking with the HPT stack, as will be seen below. 
     Once the HPC primary datum or reference axis  200  has been determined, the respective surfaces and faces of each other HPC components (e.g. the inducer and stages  20   a ,  20   b  and  20   c ) of the HPC pack  22  are preferably measured in a similar manner, in terms of concentricity and/or parallelism as described above, to acquire the relevant measured data as defined by method step  302  of  FIG. 7 . The measured data are then referenced back to the primary datum/reference axis  200  to determine the best HPC component stacking angles, considering the whole HPC assembly (method step  304  in  FIG. 7 ). This determination can be made in any suitable manner, however, in the preferred embodiment a computer, supplied with the measured concentricity and parallelism data, makes the determination in the following manner. Each geometric parameter, namely the parallelism and the concentricity of each component are used to produce a resultant vector representative of an eccentricity of the component. The eccentricity vectors of the rotating HPC components are added together to provide a final resultant vector that expresses the (lack of) concentricity of the HPC stack front journal end  47  in relation to the two impeller spigots (in this case) that are located at the back (downstream) end of the HPC stack. A numerical iteration process is then preferably used to converge toward a final solution of component angular positions which minimizes the magnitude of the vector. The solution creates the final eccentricity vector result that minimizes the HPC end-to-end eccentricity. Commercially available software can be used to process the iterative calculation. 
     The components of the HPC pack  22 , including the impeller  32 , are then physically assembled according to the calculated stacking angles, as set forth in method step  306  of the flowchart shown in  FIG. 7 . Depending on joint geometry, where a finite number of positions are available between adjacent components, the stacking angles may require to be rounded off to the nearest bolt hole location. The HPC pack  22 , that is the assembled components  20   a ,  20   b ,  20   c  and  32 , is then installed front end down on the rotary table T for verifying the actual concentricity deviation of the assembly (i.e. by measuring the concentricity deviation of the two spigot contact faces  38  and  40  of the impeller  32  relative to the rotary table axis), and the proper alignment and seating of the HPC rotor components assembled together, as indicated in step  308  of the flow chart sown in  FIG. 7 . Probes P 1  and P 2  are positioned in contact with the two spigot contact faces  38  and  40 , whereas probes P 3  and P 4  are respectively used to measure the parallelism and the concentricity at the front journal end of the HPC stack  22 , the front journal end being the interface between the front most HPC component  20   a  and the front end bearing  47 . The parallelism and concentricity measurements obtained by P 1 -P 4  are then compared with the predicted values to ensure that they correlate. As will be seen herein below, the measured deviations and concentricity angles (i.e. vectors indicating the magnitude and angle of the concentricity deviation in reference to the reference center line described by the front and rear bearings center line of the HPC stack) of the assembled HPC pack  22  will also be considered during the balancing optimization process of the HPT pack  24  and the clamp stack (front runner seal  46 , bearing  48  and rear runnel seal  50 ). The center line created by the back end impeller&#39;s spigots  38 ,  40  is compared to the center line described by the front and rear bearings of the HPC stack. The difference in the two center lines determines the concentricity off-set of the impeller spigots  38 ,  40  in the engine running position (step  308 ). This concentricity off-set vector information is used to position the HPT pack in order to minimize the overall HPT pack unbalance in reference to the centerline defined by the front and rear bearings of the HPC stack. In other words, the HPT components will be positioned in such a manner that they will counteract the concentricity offset created by the HPC impeller spigots. 
     Balancing of the HPT pack will now be described. As shown in  FIG. 5 , the HPT first disk  27  is installed rear face down on the rotary table T and is measured, in a similar manner as described above with reference to the exducer of impeller  32 , to acquire concentricity and parallelism data, as follows. Just as for the HPC pack  22 , the measurement of the concentricity deviation of the spigot contact surfaces  42  and  44  is used to establish a primary datum (e.g. see a reference axis  200  of  FIG. 6 ) for the HPT components stacking. This corresponds to step  310  of  FIG. 7 . Particularly, probes P 1  and P 2  obtain geometric data on the concentricity of the high pressure turbine first disk  27  at the spigot contact faces  42  and  44 . Probe P 3  obtains data on the concentricity of an annular aft flange  29  of the first disk  27  on which the second turbine disk  28  is fitted, as shown in  FIGS. 2 and 2   a . Probe P 4  provides geometric data on the parallelism of a rear abutting face  31  of the first disk  27  and against which the second turbine disk  28  is axially mated. 
     In a second probe set-up configuration, as shown in dotted outline in  FIG. 5 , further measurements are taken. In particular, probes P 2  is removed and probe P 1 ′ is repositioned to obtain geometric data on the parallelism of front face  33 . The first disk  27  is then rotated by the rotary table to obtain a second set of geometry data on the first disk  27  from the measurements of probes P 1 ′, P 3  and P 4 . In this configuration, probes P 1 ′ and P 4  permit to measure parallelism deviation between front face  33  and rear face  31 . Rear face  31  is used as the reference for measuring the deviation of front face  33 . 
     Still referring to  FIG. 5 , the probes are then set in a third configuration, wherein probes P 1  and P 2  are used to obtain geometric data on the concentricity of the high pressure turbine first disk  27  at the spigot contact faces  42  and  44  (like in the first probe configuration), P 3  is removed while probe P 4 ″ is used to obtain geometric data on the parallelism of the front abutment face  55  (which will be placed in mating engagement with spacer  52  (see FIGS.  2 / 2   a ) in the final assembly). Probe P 3  is not used in this third probe set-up. 
     After having so measured the turbine disk  27 , the concentricity and parallelism of the other components of the HPT pack are measured as indicated in step  312  of  FIG. 7 . For instance, as shown in  FIG. 10 , the front cover plate  23  is installed on the rotary table T to obtain geometric data on the parallelism of the axially front and rear mating faces  23   a  and  23   b  relative to the first turbine disk  27  (see FIGS.  2 / 2   a ). Rear face  23   b  is used as the reference or datum surface to evaluate the face axial run out (i.e. parallelism). The collected data on the axial face parallelism deviation between the front and rear mounting ends of the first disk  27  and the front cover plate  23  (i.e. between face  23   a  and face  33 ) are then preferably used to calculate (e.g. by computer) the optimal angular stacking position of the front cover plate  23  relative to the first disk  27 . 
     Though not depicted in the Figures, geometric data are also collected on the second turbine disk  28 , in a manner similar to that described above with reference to  FIG. 5 . More particularly, the second turbine disk  28  is installed front face down on the rotary table T and probes are appropriately positioned to measure the parallelism of front and rear mating faces  28   a  and  28   b , and the concentricity of faces  28   c  and  28   d  (see  FIG. 2 ). Faces  28   a  and  28   c  are respectively used as the datum face and datum inside diameter to evaluate the face perpendicular plane deviation and the centerline deviation. 
     Likewise, as discussed above with reference to  FIG. 10 , the rear cover plate  25  is installed on the rotary table to obtain geometric data on mating faces/surfaces  25   a ,  25   b ,  25   c  and  25   d  (see  FIG. 2 ) in order to determine the parallelism and concentricity of these surfaces/faces, as described hereinbefore. Face  25   a  and surface  25   c  are respectively used as the datum face and datum inside diameter to determine the parallelism and the concentricity of the coverplate. 
     The deviations in concentricity and parallelism measured for the rear cover plate  25 , the second turbine disk  28  and the previously-stacked front cover plate-first turbine disk assembly are used, together with the previously measured deviations and concentricity angles (i.e. vectors indicating the magnitude and angle of the concentricity deviation) of the assembled HPC pack  22  to calculate the optimized angular stacking angles between the previously-stacked front cover plate-first turbine disk assembly, the second turbine disk  28  and the rear cover plate  25  (step  316  in  FIG. 7 ). As described before, preferably this is done by iterative computer process, in which eccentricity vectors are optimized to a minimal size. 
     This process of stacking discs and coverplates recognizes that the disc and coverplate are simply another “stack” which are to be considered in the rotor assembly, since eccentricities between the coverplate and the disc can tend to bend the assembly. Hence, this “stack” is also preferably considered in a comprehensive stacking analysis of the rotor assembly. 
     The computer also preferably predicts the total radial (concentricity) deviation of the HPT stack (i.e. between HPT spigot and rear coverplate) for the computed optimized stacking, angles, which will be used later. The additional input of the actual deviations of the HPC pack  22  (measured earlier at step  308 ) allows the computer to consider the effect of the alignment of the two impeller spigot faces  38  and  40  relative to the centerline axis  11  defined by bearings  47  and  48 . As mentioned hereinbefore, the concentricity off-set of the impeller spigots  38 ,  40  relative to the center line defined by bearings  47  and  48  is used to position the HPT pack in order to counteract the concentricity offset created by the HPC impeller spigots. 
     The HPT stack  24  is then assembled (step  318  in  FIG. 7 ) according to the calculated optimized stacking angles and the assembly is mounted in the turbine shroud housing  66 . Thereafter, as shown in  FIG. 11 , the HPT stack  24  and the turbine shroud housing  66  are installed front end down to the rotary table T. A pair of probes P 1 , P 2 , is provided to measure the centerline deviation of the spigot surfaces  42  and  44  at the front mounting end of the first turbine disk  27 , in a manner similar to as described above. A third probe P 3  is provided for measuring the concentricity deviation of surface  25   d  of the rear cover plate  25 . These geometric data obtained are compared and validated with the concentricity values predicted for the HPT pack, as discussed above in the preceding step. 
     In the next step corresponding to step  314  in  FIG. 7 , each of the intermediate components or clamp stack (i.e. the front runner seat  46 , the bearing  48 , the rear runner seal  50  and the spacer  52 ) between the HPC pack  22  and the HPT pack  24  is also individually measured (not shown) to obtain data on the parallelism between their respective front and rear abutment faces. In this way, the face axial run out (i.e. deviation from parallel) of each intermediate component is individually ascertained. 
     Then, to establish the stacking angle of the HPT pack  24  relative to the HPC pack  22  as set forth in step  320  in  FIG. 7 , the measured face axial run out of the spacer  52 , the output of the turbine pack optimization computer program (i.e. the angular indexation of the component) and the measured deviations of the assembled HPC pack  22  are used (e.g. by the computer) to establish the stacking angle of the overall HPC-HPT assembly. The spacer is installed first for ease of assembly only and could, thus, be not considered in the determination of the angular position of the HPT pack vs. the HPC pack. Referring again to  FIG. 3   a , when the overall HPC-HPT assembly is assembled and stacked according to the predicted stacking angle, it will be appreciated that the shoulder  53 , of HPC spigot  34  and the shoulder  55  of the HPT spigot  36  define an envelope in which the clamp stack will ultimately be assembled. 
     The next step corresponds to step  322  in  FIG. 7  and relates to the stacking of the clamp stack. As discussed above with reference to  FIG. 9   c , preferably the parallelism of faces is considered and arranged so as to provide a “best fit” (i.e. minimize face error) to the envelope defined between spigot shoulders  53  and  55 . However, in this gas turbine embodiment, since the spacer  52  effectively forms a part of the HPC-HPT assembly, the clamp stack envelope is in fact defined by HPC spigot shoulder  34   b  and front face  52   a  of spacer  52 , since the stacking angle of the spacer  52  has already been selected with reference to the stacking of the HPT pack to the HPC pack. The measured parallelism deviations of the front runner seal  46 , the bearing  48  and the rear runner seal  50  are therefore used (e.g. by the computer), together with the measured deviations of the assembled HPC pack  22 , the output of the turbine pack optimization program and data “simulating” the effect of the high pressure turbine first disk  27  front face  55  squareness (i.e. perpendicularity) relative to the spigot surfaces  38 ,  40 ,  42  and  44 . In other words, The computer provides the HPT stack assembly indexing position relative to the HPC stack and therefore predicts the envelope defined between the HPC spigot shoulder  53  and front face  52   a  of spacer  52 . The computer program determines (e.g. by an iterative process of the type described above) the best stacking angles of the front runner seal  46 , the bearing  48  and the rear runner seal  50  to minimize face error within the envelope defined between HPC spigot shoulder  53  and front face  52   a  of spacer  52 . The next and final step in balancing is to stack each component of the assembly in the determined stacking angles. Using the calculated data, the clamp stack components (front runner seal  46 , the bearing  48  and the rear runner seal  50  and spacer  52 ) are assembled to the HPC pack (step  324  in  FIG. 7 ), and the HPT pack is installed on the HPC (step  326  in  FIG. 7 ) to provide an overall HPC-HPT assembly. Measurements are made to verify that the predicted deviations and run-outs have been obtained in fact. 
     The method of balancing an assembly of rotary components exemplified herein advantageously helps improve gas turbine engine vibration acceptance. As a result, re-test costs are reduced. As seen herein above, the geometric data obtained by measuring each component of the high pressure rotor assembly are considered using spigot interfaces as primary datum for both the HPC pack  22  and the HPT pack  24 . Although the use of a spigot connection is discussed, the approach applies as well to a rotor assembly having a curvic coupling between HPC and HPT—the skilled reader will appreciate that, rather than using two concentricity measurements to establish the primary datum (i.e. see  FIG. 6 ), a concentricity and squareness (parallelism) measurement of the curvic coupling could be used instead to establish the primary datum. Concentricity and squareness of the curvic coupling can be measured in any suitable fashion, including using known techniques for doing so. 
     The method of balancing an assembly of rotary components described herein considers all possible component stacking positions, within each rotor stack and within the overall assembly, to achieve optimum unbalance of the assembly as a whole. Thus, the optimized stacking position does not necessarily position the component in its most balanced (i.e. concentric and parallel) position when considered only in context of its closest neighbours, but rather represents the optimized position to provide the most balanced (i.e. concentric and parallel) position of the entire assembly. Rather, when all the components of a given assembly are considered as a whole, the result is optimal. 
     As can be seen from the above description, preferably the balancing of the HPC and HPT packs is optimized separately for each pack, and the assembly of the two is also optimized to ensure the overall rotor assembly is also optimized. Relative to a rotor where the entire assembly is balance/optimized at once as a whole, this technique permits, for example, better interchangeability of HPT packs should it be desirable to remove an HPT pack from an engine and replace it with another. By analyzing the HPC and HPT separately, and then together as an assembly, this type of interchangeability is facilitated without compromising rotor balance. 
     The above description is exemplary only, and changes may be made. For example, instead of using an iterative process based on all the components characteristics to find the optimum stacking optimization angles, other techniques may be used. For example, a less rigorous optimization method may look at finding the best stacking angles by optimizing one part at a time and not considering the whole assembly. It is also understood that the methodology can be used for any other suitable rotor constructions, such as other turbine rotors, and is not limited to the specific rotor or coupling embodiments discussed here. 
     The present stacking optimization method, could be applied to two rotor components (e.g. an HPC and an HPT) having a single spigot interface, and is not limited to the double spigot interlaces as described above. As mentioned above, a curvic or other type of coupling may also be used. According to the present teachings, the rotor-rotor connection simply dictates a certain alignment of the two rotors which should be considered in balancing such a rotor. For instance, the stacking position between the first and second rotors could instead by optimized by angularly positioning the second rotor (e.g. HPT) so as to off-set the eccentricity of the first rotor (e.g. HPC) resulting in the lowest possible unbalance between the two. Thus, the primary datum established by the first rotor is the basis for the optimization. In short, the reference point could be the turbine stack as opposed to the HPC stack. Once the optimal stacking positions of the first and second main components have been established, the parallelism of the mating faces of the first and second main components and all the intermediate components can be considered to determine the combination of stacking positions that yields the lowest assembly unbalance. 
     Therefore, the above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Still further examples are: the method of balancing an assembly of rotary components may be applied to any suitable rotor assembly; and although it is preferred to use both the concentricity and parallelism data in determining optimal stacking as described above, the two need not be used together, and may be used individually or in combination with other rotor measurements. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.