Abstract:
A method of stacking a plurality of wheels and spacers forming a rotor for a turbine, the wheels and spacers having a plurality of circumferentially spaced, axially extending openings spaced radially from axes of the wheels and spacers including providing a support fixture having an axis and disposing a plurality of alignment rods about the fixture in circumferentially spaced relation to one another about and generally parallel to the axis. Each of the rods has a cross-section with a radial dimension less than a circumferential dimension. The method includes disposing the wheels and spacers on the support fixture with the alignment rods extending through the openings with clearances between the rods and margins of the openings being greater in a radial direction than in a circumferential direction.

Description:
This is a divisional of application Ser. No. 09/275,633, filed Mar. 24, 1999, U.S. Pat. No. 6158,102, the entire content of which is hereby incorporated by reference in this application. 
    
    
     TECHNICAL FIELD 
     The present invention relates to apparatus and methods for aligning openings through wheels and spacers during assembly to form a turbine rotor stack enabling subsequent insertion of steam tubes into the aligned openings with the tightest possible clearances between the openings and tubes and particularly relates to a fixture having alignment pins shaped to enable tangential alignment of the elements of the rotor stack without constricting radial alignment. 
     BACKGROUND OF THE INVENTION 
     Gas turbine rotors are typically formed by stacking the rotor wheels and spacers axially one against the other. Bolt holes are provided through the wheels and spacers and receive bolts which are used to finally secure the wheels and spacers to one another to form the rotor. The wheels and spacers in final assembly also have rabbeted joints. That is, axially projecting flanges formed on the spacers underlie and fit tightly against axially oppositely extending flanges formed on the wheels. To form the rabbeted joints, the wheels are typically heated in an oven prior to assembly in the stack to expand the flanges of the wheels so that, after stacking and upon cool-down, the flanges of adjacent wheels and spacers fit tightly relative to one another. 
     During stack-up of the wheels and spacers, the bolt holes of the wheels and spacers are fitted over bolts projecting from a fixture. The bolts remain in the rotor assembly and maintain the wheels and spacers stacked relative to one another. Consequently, to enable the stack-up of the wheels and spacers on the bolts, substantial clearances between the bolt holes through the wheels and spacers and the bolts are necessary in the radial direction and corresponding clearances are therefore also provided in the circumferential direction. A need has developed, however, for a much tighter alignment of the stacked wheels and spacers which cannot be provided by the alignment of the bolt holes and bolts during the assembly stack-up consistent with the need to accommodate radial expansion and contraction of the heated wheels during the stack-up. 
     This need has arisen as a result of a new advanced steam-cooled gas turbine design of the assignee of the present invention wherein certain parts of the rotor are steam-cooled. In this advanced steam-cooled turbine design, a plurality of openings, in addition to the bolt holes, are provided through the wheels and spacers of the rotor to accommodate a plurality of circumferentially spaced tubes extending generally axially through the rotor for supplying steam to the steam-cooled parts, i.e., first and second stage rotor buckets, and returning the spent cooling steam to the rotor bore assembly. The supply and return tubes are thin-walled structures which extend through openings in bushings provided in circumferentially spaced apertures of the stacked wheels and spacers. Tight clearances between the tubes and bushing openings are highly desirable. The steam-carrying tubes desirably have as large a diameter as possible to maximize steam flow, as well as have very tight clearances with the openings to preclude high stresses on the tubes. Thus, there is a need to tightly tolerance the openings through the wheels and spacers which carry the steam-cooling tubes while concurrently enabling radial contraction of the wheels to form tightly rabbeted joints. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a plurality of alignment pins are employed in a fixture for reception in the openings of the wheels and spacers. The alignment pins are specifically configured to allow for radial misalignment of the openings due to thermal expansion and contraction, tolerance stack-up and wheel-to-spacer mismatch due to rabbet mechanical growth. The alignment pins, however, enable closely toleranced circumferential alignment of the wheels, spacers and aft shaft bushing openings sufficiently to install the steam-carrying tubes into the aligned openings after the rotor has been constructed with a tight clearance and minimal stress in use. The configuration of the alignment pins allows tight tangential alignment of the wheels and spacers without constricting radial alignment thereof and enables the pins to find the average position of all the openings so that the most each opening can be off in a circumferential direction is the opening&#39;s tolerance relative to the average true circumferential position of each pin. To accomplish this, the alignment pins have a radial dimension less than their dimension in the circumferential direction. Preferably, the pins are generally hexagonal in cross-sectional configuration with major and minor axes extending in circumferential and radial directions. 
     To accomplish the foregoing, the rotor fixture comprises a stand having a plurality of precision-located alignment openings which receive alignment pins circumferentially spaced from one another. The pins upstand from the fixture. Each pin has a cross-section with a radial dimension substantially less than its circumferential dimension to accommodate radial expansion and contraction during assembly of the rotor, while at the same time affording alignment of the wheels and spacers so that the average position of all the aligned openings is equal to or less than the tolerance of the openings relative to the average true position of each pin. 
     To form the stack in accordance with the present invention, the bolts are also provided on the fixture and upstand the full length of the stack. The height of the alignment pins above the fixture is adjustable so that the pins can be periodically raised as the wheels and spacers are stacked one on top of the other. (The following description proceeds with stacking four wheels and three spacers on an aft shaft to form a four-stage rotor, with the aft wheel being designated the fourth wheel and the forward wheel the first wheel, it being appreciated that the stacking method hereof can be applied to rotors having different numbers of wheels and spacers and hence a different number of stages.) To begin the rotor assembly, the aft shaft including the integral aft shaft disk is disposed on the fixture with the bolts being received through bolt apertures on the aft shaft disk and the alignment pins being received through slave bushings on the aft shaft disk. With the aft shaft on the fixture, an initial wheel, e.g., the fourth wheel, is heated in an oven. Once the fourth wheel is heated, it is placed on the aft shaft disk with the bolts and alignment pins being received through its bolt holes and openings, respectively. By initially heating the fourth wheel, the forwardly directed flange of the aft shaft disk lies radially inwardly of the now radially expanded, axially directed, aft flange of the fourth wheel. While the fourth wheel remains heated, the 3-4 spacer is then applied to the fixture with the bolts being received in the bolt holes of the spacer and the alignment pins being received in the spacer openings. The aft-directed flange of the 3-4 spacer is received radially within the radially expanded forwardly directed flange of the fourth wheel. The fourth wheel is then allowed to cool. Consequently, the aft flange of the fourth wheel tightly engages the forward flange of the aft shaft disk and the forward flange of the fourth wheel engages the aft flange of the 3-4 spacer to form tight rabbeted joints. It will be appreciated that the fourth wheel contracts radially as it is allowed to cool down and engage the corresponding flanges. 
     The alignment pins are then elevated in the fixture to receive the next wheel/spacer set, i.e., the third wheel and the 2-3 spacer. The third wheel is first heated and applied over the bolts and alignment pins similarly as the fourth wheel and the 34 spacer were applied over the bolts and alignment pins. It will be appreciated that the radial contraction of the third wheel during cool-down enables a tight fit between the flanges of the third wheel and the 2-3 and 3-4 spacers on axially opposite sides of the third wheel to form the rabbeted joints. After cool-down, the alignment pins are again raised relative to the fixture to receive the second wheel and 1-2 spacer. The second wheel is thus heated and the heated second wheel and 1-2 spacer are similarly applied to the bolts and alignment pins. After cool-down of the second wheel forming the tightly engaged rabbeted joints between the second wheel and the 1-2 and 2-3 spacers on axially opposite sides of the second wheel, the first or final wheel is heated, similarly applied to the bolts and alignment pins and allowed to cool down to form the rabbeted joint with the 1-2 spacer. 
     It will be appreciated that upon cooling of the wheels, the flanges radially contract to form portions of the rabbeted joints. That radial contraction is accommodated by the large clearance between the reduced radial dimension of the alignment pins and the openings through the wheels and spacers. However, because the alignment pins have a circumferential dimension corresponding to the tolerance of each spacer or wheel opening relative to the average true position of each pin, a tight alignment of the wheel and spacer openings in a circumferential direction is achieved. 
     In a preferred embodiment according to the present invention, there is provided a fixture for forming a turbine rotor having stacked axially aligned wheels and spacers, each of the wheels and spacers having a plurality of circumferentially spaced openings thereabout for alignment with one another, comprising a support having an axis for registration with axes of the aligned wheels and spacers, at least a pair of alignment rods spaced radially from the axis of the support and circumferentially from one another, the rods being located about the support for reception in the openings through the wheels and spacers, each of the rods having a radial dimension less than a circumferential dimension such that spacing between the rods and margins of the openings in a radial direction is greater than spacing between the rods and margins of the openings in a circumferential direction. 
     In a further preferred embodiment according to the present invention, there is provided a method of stacking a plurality of wheels and spacers forming a rotor for a turbine, the wheels and spacers having a plurality of circumferentially spaced, axially extending openings spaced radially from axes of the wheels and spacers, comprising the steps of providing a support fixture having an axis, disposing a plurality of alignment rods about the fixture in circumferentially spaced relation to one another about and generally parallel to the axis, each rod having a cross-section with a radial dimension less than a horizontal dimension and disposing the wheels and spacers on the support fixture with the alignment rods extending through the openings with clearances between the rods and margins of the openings being greater in a radial direction than in a circumferential direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary perspective view of a gas turbine rotor with parts broken out and in cross-section illustrating a stacked rotor wheel and spacer construction with steam tubes applied in accordance with the present invention; 
     FIG. 2 is a schematic perspective illustration of a rotor fixture employing the alignment pins of the present invention; 
     FIG. 3 is a view similar to FIG. 2 illustrating the aft shaft of a rotor stackup mounted on the alignment fixture; 
     FIGS. 4-7 are fragmentary cross-sectional views of the fixture illustrating the step-by-step stacking of the rotor wheels and spacers; and 
     FIG. 8 is an enlarged plan view of an alignment pin. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is illustrated a portion of a gas turbine rotor, generally designated  10 , assembled in accordance with the present invention. Rotor  10  includes an aft bore tube assembly  12 , an aft shaft  14  having a forward aft shaft disk  16  and rotor wheels  18 ,  20 ,  22  and  24  axially spaced one from the other by spacers  26 ,  28  and  30 . In the illustrated preferred embodiment, the rotor turbine comprises four stages, each including a wheel and a spacer, the first stage being only partially shown. The outer rims of the wheels mount turbine buckets, not shown, while the outer rims of the spacers lie in radial opposition to nozzles, also not shown. The advanced gas turbine design of assignee, part of which is illustrated in FIG. 1, comprises a steam-cooled, four-stage turbine having steam supply and return tubes  31  and  32 , respectively. Tubes  31  and  32  are circumferentially spaced about and extend axially of the rotor  10  and lie in communication with radial steam supply and return tubes  34  and  36 , respectively. Steam is supplied through the bore tube assembly  12  to the radial tubes  34  and returning spent cooling steam is supplied to the bore tube assembly  12  from radial tubes  36 . The stack of wheels, spacers and aft shaft disk are bolted one to the other as in conventional rotor construction, a bolt B being illustrated. Thus, the bolt holes B.H. pass axially through each of the wheels and spacers and lie in axial registry with one another at circumferentially spaced-apart positions at locations radially inwardly of the steam tubes  31  and  32 . As noted previously, the steam tubes  31  and  32  are thin-walled structures inserted into the rotor after assembly and which require close tolerance fit-ups with the openings  33  through the wheels, spacers and aft shaft disk. Consequently, the present invention provides a fixture, generally designated  40 , in FIGS. 2 and 3, for stacking the wheels, spacers and aft shaft disk to form the rotor assembly  10  with the steam tube openings aligned and tightly toleranced. 
     Referring to the fixture  40  illustrated in FIG. 2, a pair of fixture plates  42  and  44  are spaced vertically one from the other and are aligned about an axis A. Each plate includes a plurality of bolt holes  46  which receive elongated bolts B which extend the length of the rotor for securing the wheels, spacers and aft shaft to one another. The circle of bolt openings  46  lies radially inwardly relative to a circle of steam tube openings in each of the fixture plates  42  and  44 . The plates  42  and  44  are accurately precision-mounted relative to one another and have alignment rods  48  extending through the registering aligned openings  47 . The rods  48  are slidable vertically within the openings to selected adjusted positions and are selectively retained in those positions by pins extending through lateral holes, not shown, in the rods  48  bearing against the stops maintaining the rods at a selected elevation. The rods terminate at their upper ends in eyehooks  49  such that they can be grasped and raised to a higher elevation, as will become clear from the ensuing description. 
     The purpose of the alignment pins is to circumferentially align the wheels, spacers and aft shaft disk openings  33  sufficiently to install the steam tubes at a later time. The pins are also designed to allow for radial opening misalignment due to thermal expansion, tolerance stack-up and wheel-to-spacer mismatch due to rabbet mechanical growth. As best illustrated in FIG. 8, each of the alignment rods  48  has a radial dimension a, the direction of which is indicated by the arrow in FIG. 8 substantially less than its dimension b in a circumferential direction. The cross-sectional shape of the alignment rods  48  is preferably generally hexagonal with a minimum dimension in a radial direction and a larger maximum dimension in a circumferential direction at the circumferential edge  50  of the alignment rods. This particular dimensional configuration of the alignment rods enables tangential alignment of the wheels, spacers and aft shaft disk without constricting radial alignment thereof and enables the rods to find the average position of all the openings so that the most each opening can be off in a circumferential direction is an opening&#39;s tolerance relative to the average true circumferential position of each pin. 
     Referring back to FIG. 3, the first step in the assembly of the rotor is to dispose the aft shaft  14  on the fixture with the aft shaft disk  16  resting on the upper fixture plate  42 . The aft shaft disk  16  has radially opening slots spaced circumferentially about its periphery for receiving the alignment rods  48 , as well as the bolts (the bolts and bolt holes not being shown). Referring to FIG. 4, and with the aft shaft disk on the upper fixture plate  42 , the fourth wheel and the 3-4 spacer  26  are next applied to the aft shaft. Before placing the fourth wheel  18  on the fixture, fourth wheel  18  is heated in an oven O.V. such that the forward and aft axially extending flanges  54  and  56 , respectively, are thermally radially expanded. After wheel  18  has been heated, wheel  18  is aligned with the alignment rods  48  and the bolts and lowered onto the forward face of the aft shaft disk  16 . With the radial expansion of the aft flange  56  of wheel  18 , the forward flange  58  of the aft shaft disk  60  is received within the expanded flange  56 . On the forward axial face of wheel  18 , the flange  54  likewise has expanded radially outwardly. The 3-4 spacer, at ambient temperature, is then applied to the fixture with its aft flange  60  lying radially inwardly of the wheel flange  54 . It will be appreciated, upon cool-down, that the wheel  18  contracts in a radial direction to form a tight-fitting rabbeted joint between the forward and aft flanges of the wheel and the aft and forward flanges of the spacer and aft shaft disk, respectively. Radial contraction is accommodated by the cross-sectional configuration of the alignment rods (see FIG. 8) while the circumferential dimension of the alignment rods are closely toleranced to the wheel and spacer openings maintaining tight tolerances in the circumferential direction. 
     Referring now to FIG. 5, the next wheel and spacer combination, i.e., the third wheel  20  and the 2-3 spacer  28  are applied to the stack. The alignment rods are first elevated to accommodate the wheel  20  and spacer  28 . Similarly as with the fourth wheel  18 , the third wheel  20  is likewise initially heated in the oven O.V. Wheel  20  is then lowered onto the alignment rods and bolts for engagement against the forward face of the 3-4 spacer  26 . The 2-3 spacer  28  is next lowered, at ambient temperature, onto the alignment rods and bolts for disposition on the forward face of the third wheel  20 . It will be appreciated that the respective forward and aft flanges  62  and  64 , respectively, of the third wheel  20  radially contract for tight fitting engagements with the aft flange  66  and forward flange  68  of the 2-3 spacer  28  and the 34 spacer  26 , respectively. Thus, upon contraction of the heated wheel  20 , forward and aft rabbeted joints are formed with the adjoining spacers  28  and  26 , respectively. The alignment rods accommodate the radial contracting movement while maintaining the circumferential alignment of the stacked wheels and spacers in a closely toleranced fit relative to the circumferential dimension of the alignment rods. 
     Referring now to FIG. 6, the alignment rods are once again elevated to a height to accommodate the next wheel and spacer combination disposed on the stack. Prior to lowering the second wheel  22  onto the stack, the second wheel is placed in oven O.V. to heat the wheel and radially expand the axial flanges for forming the rabbet joint. After heating, the third wheel is lowered onto the bolts and alignment rods and the 1-2 spacer  30 , at ambient temperature, is then lowered on top of the heated second wheel  22 . As in the prior spacer/wheel combinations, the heat expanded flanges of the third wheel contract in a radial inward direction to engage and form rabbeted joints with the axially directed flanges of the 1-2 spacer  30  and 2-3 spacer  28 . Finally, and referring to FIG. 7, the first-stage wheel  18  is heated in oven O.V. and lowered onto the bolts and alignment rods. Its aft-facing flange contracts radially inwardly upon cooling to engage the forward flange of the 1-2 spacer  30  to form the rabbeted joint therewith. 
     At this stage, the wheels, spacers and aft shaft disk are aligned with very close and tight tolerances in the circumferential direction. The rabbeted joints maintain the openings in radial alignment with one another. Thus, the steam tubes may be disposed axially through the axially aligned registering openings of the spacers, wheels and aft disk with very tight tolerances therebetween. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.