Abstract:
A method is provided for forming a circular array of rotatable blades from blades having an integral shroud with a leading planar surface and a trailing planar surface, the leading planar surface being generally parallel to an axial radial plane passing through the center of the root of the blade and the trailing surface, if extended, intersecting the axial radial plane passing through the center of the root of the blade to form an angle generally equal in degrees to 360 divided by the number of blades forming the circular array. Strict radial alignment is provided by assembling the complete blade row such that all blade center lines coincide with the groove center lines, with the shroud mating surfaces in complete contact and in tangential compression.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to turbine blades, and more particularly to rotating blades for a turbine. 
     In turbines, e.g., steam turbines, a plurality of rotating arrays of foils or blades are arranged circumferentially about a rotor. Reaction of steam or gases against the blades produces rotation of the rotor and associated blade arrays. The forces acting on these rotating blades, including centrifugal forces caused by rotation, tend to throw the blades radially outward and generate large forces on the blade attachment structure. In many turbines, the attachment structure comprises a Christmas tree shape root structure which slides into a mating slot or groove in the rotor. When the blade is properly positioned, lugs extending from opposing sides of the root structure share equally in retaining a blade to the rotor, however, if a blade is aligned off of a radial line of the rotor through the center of the rotor groove, lugs on one side of the root structure may support more force than those on the opposite side leading to potential overstress conditions. Accordingly, it is desirable to assure that turbine blades are aligned with their respective center lines oriented along radial lines of the turbine rotor. 
     Such arrays of rotating blades are often joined together at their tip by a shroud ring which are normally riveted to the blade via a tenon made integral with the blade. The tenon being an abrupt change in cross-section of the blade is subject to higher stresses due to bending moments imposed by the shroud ring and provides crevices wherein corrosion products are accumulated; provide a steam seal over the top of the blade and also however, the shroud rings greatly reduce blade vibration. One prior art turbine blade which eliminates the problems of tenon stress and corrosion is disclosed in U.S. Pat. No. 4,533,298, issued to Albert J. Partington et al on Aug. 6, 1985, assigned to the assignee of the present invention, and incorporated herein by reference. 
     Partington et al. teaches a plurality of rotatable blades disposed in a circular array, each blade comprising a root portion, which fastens the blades to the rotor, an airfoil shaped blade portion having a leading edge and a trailing edge, and a shroud portion made integral with the blade portion and disposed on the radially outer end of the blade portion. The shroud portion has a leading planar surface and trailing surface, one of the planar surfaces being disposed generally parallel to an axial radial plane passing through the central portion of the root portion, and the other planar surface if extended forming an angle with the radial axial plane passing through the center of the root portion generally equal in degrees to 360 divided by the number of blades forming the circular array. Such turbine blades are disclosed in U.S. Pat. No. 4,602,412, also issued to Partington et al on July 29, 1986, assigned to the assignee of the present invention, and incorporated herein by reference. 
     Requirements of strict radial alignment imposed to prevent unequal loading on the sides of a turbine blade root portion, and tight contact between the respective shroud to minimize vibration during full-speed operation and wear of the root portions during turning gear operation have necessitated a change in the approach to assembling rows of turbine blades having integral shrouds. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a method of assembling a row of turbine blades upon a rotor, each of the blades having an integral shroud portion. More specifically, it is an object of the present invention to provide a row assembly process for integral shroud blades which prevents unequal loading on the sides of the turbine blade root portions. 
     Another object of the present invention is to provide a row assembly process for integral shroud blade which minimizes vibration during full-speed operation, as well as minimizes wear of the root portions during turning gear operation. 
     Still another object of the present invention is to provide a row assembly process for integral shroud blades in which the complete blade row is assembled such that all blade center lines coincide with the groove center lines, and with the shroud mating surfaces in complete contact and in tangential compression. 
     Briefly, these and other objects of the present invention are accomplished by a row assembly process for integral shroud blades generally comprising the steps of providing an anchored blade to start the row, monitoring any subsequent deflection of the anchored blade to determine its movement from a predetermined radial position, inserting a closing blade next to the anchored blade without locking the closing blade within its groove, sequentially inserting each of the remaining blades within their grooves, locking them in place, and prestressing each blade in a direction towards the anchored blade, positioning the blade radially with respect to the their grooves, relieving the force exerted upon the anchored blade by the prestressing steps, removing the anchored blade in order to complete the circular array, removing the closing blade to determine the size of the gap between the shroud portions of the blades installed on either side thereof, modifying the shroud portion of the closing blade to provide an interference fit within the gap, and thereafter installing the modified blade within its groove locking it in place. 
     These and other objects, advantages and novel features of the present invention will become more apparent from the following detailed description of a preferred embodiment when considered in conjunction with the accompanying drawings wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial sectional view of a circular array of a rotatable blades disposed in a rotor; 
     FIG. 2 is an axial view of an integral shroud blade; 
     FIG. 3 is a radial view of the integral shroud blade shown in FIG. 2; 
     FIG. 4 is a tangential view of the integral shroud blade; 
     FIG. 5 is an axial view of a radial position gauge utilized in accordance with the present invention; 
     FIG. 6 is an tangential view of another form of the radial position gauge in operative position with a turbine blade; 
     FIG. 7 is a radial view of the gauge shown in FIG. 6; 
     FIG. 8 is an elevational view illustrating one method of calibration of an illustrative form of the radial position gauge; 
     FIG. 9 is a partial sectional view of a circular array of rotatable blades disposed in a rotor, illustrating details of the method according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, wherein like characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1 a portion of a circumferential array of exemplary turbine blades 2 disposed in a portion of a turbine rotor 4. 
     As shown best in FIGS. 2, 3 and 4, each turbine blade 2 comprises a Christmas tree shaped root portion 6 which registers with a similarly shaped slot or groove 8 in the rotor 4 to fasten the blades 2 to the rotor 4. A locking device is used in order to prevent axial movement of the blades 2. The root portion 6 extends from a blade platform 16. 
     Extending radially outwardly from the blade platform portion 16 is an airfoil shaped blade portion 18 having leading and trailing edges 20 and 22, respectively. Disposed radially outwardly of the blade portion 18 and made integral therewith, in accordance with the above referenced U.S. Pat. No. 4,533,298, is a shroud portion 24. The shroud portion 24 has a leading planar surface 26 and a trailing planar surface 28. 
     The leading planar surface 26 extends beyond the leading edge 20 of the blade portion 18, while the trailing edge 22 of the blade portion 18 extends substantially beyond the trailing planar surface 28 of the shroud portion 24. As is conventional, the outer periphery of the shroud portion 24 is machined to form a cylindrical ring which cooperates with labyrinth seals to form a rotating seal. 
     Each of the blades 2 is desirably aligned on a radial line of the rotor 4, or more specifically the blade center line 32 desirably coincides with a radial line of the rotor 4. In such an alignment, each lug portion 36 on the root portion 6 shares the centrifugal forces on blades 2 during turbine operation. If one of the blades 2 is misaligned, its root portion 6 may be stressed off-center causing a force increase on one or more of the lug portions 36 on only one side of the root portion 6. Such force increase may exceed the stress capability of the root structure resulting in blade root damage or may damage the rotor groove 8 in which the blade root portions 6 are retained. Since blade separation, whether from root failure or rotor groove failure, can be disastrous in a turbine, it is desirable to assure proper blade alignment at the time of assembly. 
     Referring generally now to FIGS. 5-8, a radial position gauge 38, as disclosed in U.S. patent application Ser. No. 913,881, filed Sept. 30, 1986, in the names of Marshall J. Rouse and Albert J. Partington, assigned to the assignee of the present invention and incorporated herein by reference, is illustrated. As shown in FIG. 5, a simplified view looking axially along the surface of the turbine rotor 4 shows a blade 2 in phantom lines with a left-hand form of the radial position gauge 38. The gauge 38 includes an elongated base member 40 and a standard 42 extending substantially perpendicular from the base member 40. First and second elevated contact points in the form of tooling balls 44 and 46 are attached in spaced apart relationship to a lower surface of the member 40. The tooling balls 44 and 46, well known in the machinist&#39;s art, provide contact points between the gauge 38 and the surface of the rotor 4 and support the member 40 adjacent to the rotor 4 in a plane substantially parallel to a tangent line to the rotor surface. The balls 44 and 46 may be attached to the member 40 in a manner well known in the art such as, for example, by being drilled and tapped and connected by screw-type studs. The standard 42 has one end fixedly attached to member 40 by bolts or welds (not shown). For simplicity of construction end use, both member 40 and standard 42 are preferably formed or machined as elongated bars having substantially parallel opposing surfaces 48, 50 and 52, 54, respectively. 
     Standard 42 supports an arm member 56 extending from a lower portion thereof and carrying a tooling ball 58. The arm member 56 is positioned such that the tooling ball 58 creates a third contact point for contacting a surface 60 of the blade platform portion 16 when the gauge 38 is in a measuring position. It should be noted at this juncture that platform 16 is a precision machined portion of the blade 2 and is carefully situated with respect to the root portion 6, surface 60 being machined parallel to the blade center line 32. 
     At a distal end of the standard 42 from member 40, there is located a second arm 62 which supports a measuring apparatus 64 including a dial indicator 66 and a reciprocating plunger 68. The arm 62 supports the apparatus 64 such that the plunger 68 is brought into contact with the trailing planar surface 28 of the shroud portion 24 of blade 2 when the gauge 38 is in its measuring position. The apparatus 64 is calibrated with respect to the third contact point so that any deviation of the alignment of blade 2 off a rotor radial line through the center of the blade root will result in a non-zero reading. Shroud portion 24, like surface 60, is a precision machined portion of the blade 2. 
     Turning briefly to FIG. 8, there is illustrated one method and apparatus for calibrating the gauge 38. A gauge calibration stand 74 comprising a base 76 and a perpendicular member 78 is used to establish zero calibration of the dial indicator 66. The base 76 has a precision machined flat top surface 80, member 78 being attached to base 76 to form a perpendicular reference. The gauge 38 is placed on surface 80 and positioned with tooling ball 58 in contact with member 78. Depending upon the particular design or configuration of the turbine blade 2 to be aligned, various thickness shims 82 are positioned between member 78 and plunger 68 of the measuring apparatus 64. With the gauge 38 in position against member 78, a bezel lock 84 is released to allow bezel 86 on the dial indicator 66 to be turned for setting a zero reading, lock 84 thereafter being tightened. This completes the calibration of gauge 38 and prepares it for use in determining alignment of blades 2 on the rotor 4. 
     The method of determining blade alignment can be appreciated by reference to FIGS. 5-6. The gauge 38 is placed on the rotor 4 adjacent to one of the blades 2, with the tooling balls 44 and 46 resting on the surface of the rotor 4 assuring alignment of the gauge 38 with a rotor radius. As is well known, the surface of the rotor 4 includes a plurality of alternating lands and valleys (shown in FIG. 6) in the axial direction, the lands having the slots or grooves 8 formed therein for receiving the root portions 6 of the blades 2. The gauge 38 is positioned for use on the surface of the rotor 4 in a valley adjacent a blade 2 to be measured, and is moved circumferentially on the surface of the rotor 4 until the tooling ball 58 contacts the blade platform portion 16. In order to avoid having to hold the gauge 38 in place for checking alignment of each blade 2, an elastic cord 88, e.g., a bungy cord, is wrapped around the rotor 4 and over base member 40. For that purpose, there is provided a retaining member 90 attached to the base member 40 having a double-folded lip for engaging and slidably holding cord 88. With ball 58 in position against platform 16, any non-zero reading of the dial indicator 66 represents a deviation of the blade centerline 32 from a rotor radius line. The blade 2 may be replaced if the deviation exceeds allowable limits. Further details relating to the radial position gauge 38 may be found in the above referenced U.S. patent application Ser. No. 913,881, now U.S. Pat. No. 4,718,172. 
     Referring now to FIG. 9, the row assembly process according to the present invention will now be explained. In preparation for such process, the radial position gauge 38 is calibrated, and installed upon the rotor 4 by the elastic cord 88. An anchor blade 92, comprised generally of a standard blade 2 having a support plate 94 attached thereto (such as by welding) and a means for adjusting 96 the radial position of the blade 92 is inserted within a selected groove 8 (G2 as shown). The adjusting means 96 includes a threaded portion 98 attached to the support plate 94, an adjusting screw 100 threadedly coupled within the threaded portion 98, and a swivel foot portion 102 attached to the adjusting screw 100 at its end. 
     In a rotor 4 having N number of grooves 8 (G1-GN as shown in FIG. 9), the row assembly process according to the present invention is started by inserting the anchor blade 92 within groove G2 and shimmed medium tight to provide a preload on the adjusting screw 100. The anchor blade 92, as installed, is held in place from rocking primarily by the adjusting means 96, not the shims or root-groove fit. An anchor blade deflection monitor 104, such as a bar 106 inserted within a notch 108 formed in the support plate 94, is rigidly coupled to the support plate 94 such as by clamping means (not shown). Thereafter, a monitor blade 110 is inserted within the next vacant groove in a clockwise direction (G6 as shown in FIG. 8) in order to provide a measurable gap 112 between the bar 106 and the monitor blade 110. The monitor blade 110 is shimmed tightly within its groove by shim 116. 
     After the anchor blade 92, with its deflection monitor 104, and the monitor blade 110 are installed, the remaining blades, starting with a closing blade 114, are installed from the inlet side of the rotor 4 at a 10 o&#39;clock or 2 o&#39;clock position thereof. With the anchor blade 92, the anchor blade deflection monitor 104 and the closing blade 114 in place, the blades 2 are installed in descending numerical order in a counterclockwise fashion around the rotor. That is, the first blade 2 to be installed after installation of the closing blade 114 is the blade which is installed within the groove 8 shown in FIG. 9 as GN, followed by GN-1, GN-2, . . . 9,8, and 7, where N is the highest numbered blade in the row equal to the number of blades per row. 
     A flat shim 116, having a thickness which will allow the shroud portion 24 of the installed blade to fall against the shroud portion 24 of an adjacent blade 2, may be inserted under the root portion 6 within the groove 8. The shims 116 should be installed within the grooves 8 so that they do not interfere with the radial gauge 38, and shim clearance should be less than 0.003 inches. A blade locking device is conventionally installed in order to lock the blade 2 in place. At least one tapered wedge 118 is then inserted under the concave side of the platform portion 16 to provide a tight metal-to-metal contact between adjacent shroud portions 24. The radial position of the installed blade 2 is then measured in accordance with the above referenced U.S. patent application Ser. No. 913,881, and recorded. Ten blades 2 on both sides the closing blade 114 are leaned towards the closing blade 114. That is, blades 2 in grooves number GN to GN-9 are leaned towards the closing blade 114, while blades 2 in grooves number G11 to G2 are leaned towards the closing blade 114. The blades 2 installed within grooves number GN-10 to G12 preferably have no lean at all. 
     Periodically, the gap 112 between the another deflection monitor 104 and the monitor blade 110 is measured, recorded, and adjusted until deflection changes are negligible. The anchor blade deflection monitor 104 may suitably be removed after the blade 2 in groove number G10 is installed. Blade assembly is continued in a descending sequence of blade numbers until the blade 2 in groove number G7 has been assembled. 
     At the start of the row closing process, the anchor blade deflection monitor 104 has been removed, but the anchor blade 92 is still in place, with bldaes 2 in grooves number G1, GN, GN-1, . . . G-9,8, and G7 having been installed. All blades 2 have a locking device, such as a pin as disclosed in the above referenced U.S. Pat. No. 4,533,298 and 4,602,412, installed with the exception of the closing blade 114 in groove number G1. The closing blade 114 is locked by any convenient means, one means being shown in copending U.S. patent application Ser. No. 844,496, filed Mar. 26, 1986 now U.S. Pat. No. 4,676,723 of William E. Kiger and Jurek Ferleger, assigned to the assignee of the present invention, and incorporated herein by reference. Furthermore, all blades 2 will have one or more tapered wedges 118 on the concave side of the blade 2 between the underside of the blade platform 16 and the top of the steeple, tending to push each of the shroud portions 24 towards the shroud portion 24 of the anchor blade 92. 
     Referring again to FIG. 9, the radial positions of blades 2 installed within grooves number GN and G7 must be verified and recorded before starting the closing process. The gap between the shroud portion 24 of the blades 2 installed within grooves number G7 and G1 is thereafter measured with any suitable means. Starting with the blade 2 installed within groove number GN/2 (i.e., that groove 8 having the number equal to half of the total number of grooves in a row), and working towards the anchor blade 92, each of the tapered wedges 118 are moved one at a time from the concave to the convex side of the platform portion 16 so that in both halves of the row the blades 2 are being forced towards the blade 2 in groove number GN/2 and away from the closing blade 114. This relieves the force on the anchor blade 92 and holds the shroud portions 24 tight together so that the anchor blade 92 can be removed. After the tapered wedges 118 have been reversed on half of the row from the blade 2 within groove number GN/2 to the blade 2 within groove number GN, as well as on the blade 2 within groove number G1. The anchor blade 92 is then removed. 
     Any change in the radial position of the blades 2 contained within grooves number GN and G7, as a result of reversing the tapered wedges 118 on half of the row and removing the anchor blade 92, is determined. Next,the blades 2 necessary for installation within grooves number G6,G5,G4 and G3 are installed as described herein above. 
     A closing blade interference will cause the blades 2 on the both sides thereof to move away from the closing blade 114. It can be assumed that the blades 2 in grooves number GN to GN-9 will change the radial position values by I/2 in the positive direction, where I is the interference, and the blades 2 in grooves number G2 to G11 will change their values by I/2 in the negative direction. Thereafter, the maximum interference I that will not cause any of the ten blades on either side of the closing blade 114 to be out of an allowable radial position is determined. The blade 2 contained within groove number G1 is then removed in order that the blade 2 for groove number G2 may be temporarily installed. However, before installation of the blade 2 within groove number G2, a total shroud length to be removed from the blades 2 for installation within grooves number G1 and G2 must be determined. That is, by measuring the shroud length of the closing blade 114, and the opening for the closing blade 114, the total shroud length to be removed from the blades 2 for installation within grooves number G1 and G2 is equal to the closing blade shroud length minus the opening for the closing blade and the maximum interference. The shroud length to be removed from the blade 2 for installation within groove number G2 should be substantially equal to half of the total shroud length to be removed. Thereafter, the blade 2 installation within the groove number G2 is sized and installed within the groove number G2. The opening for the closing blade is then remeasured, and appropriate amount of shroud length removed therefrom, the remaining gap between the blade 2 installed within groove number G2 and GN spread by means such as a bolt spreader (not shown), and the modified closing blade 114 installed therein. After the closing blade 114 is locked in place, all of the tapered wedges 118 must be removed. The outer peripheral surface of the shroud portions 16 of the circular array of blades 2 may be suitably machined thereafter to form a cylindrical surface which cooperates with a labyrinth seal to form a rotating seal. 
     The complete assembly process described herein above includes preparation, starting the row assembly, typical blade assembly, closing the row, and locking the closing blade. Further details may be found in Process Specification (Fed.Code 8857) No. 80308 LV (revision 1), entitled &#34;Assembly of Integral Shroud Blades&#34;, of Westinghouse Electric Corporation. Ideally, after all blades are assembled, all blade radial lines coincide with the groove center lines, with the shroud mating surfaces in complete contact and in tangential compression. Thus, the blade root is allowed to seat in an acceptable position during turbine operation while minimizing blade motion during rotor turning gear operation. Obviously many modifications and variations of the present invention are possible in light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.