Abstract:
The gas-turbine rotor is composed of a plurality of rotor disks which are placed one next to another in the axial direction thereof and fastened by spindle bolts piercing through all the rotor disks in the axial direction. Each of the rotor disks has annular protrusions on both sides and are joined to the adjoining rotor disks by abutting the top faces of the annular protrusions against the annular protrusions of the adjoining rotor disks. Grooves having semicircular cross sections are formed on the top face of each of the annular protrusions. When the rotor is assembled, the grooves of the adjoining rotor disks join and form circular holes. Cylindrical torque pins having flanges at both ends are fitted into the circular holes formed by the grooves. The surfaces of the inner side walls of the annular protrusions are formed as spherical surfaces. The faces of the flanges of the torque pins which contact the inner side walls are formed as spherical shapes matching the curvature of the spherical surfaces of the inner side walls.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gas-turbine rotor. More specifically, the present invention relates to a gas-turbine rotor constructed by assembling a plurality of rotor discs. 
     2. Description of the Related Art 
     In a gas turbine, high pressure, high temperature combustion gas generated by burning fuel in high pressure combustion air is used for driving a turbine through which the energy of the combustion gas is converted into a mechanical output. Usually, combustion air is supplied by an axial-flow compressor driven by the turbine. Thus, usually the rotors of the axial-flow compressor and the output turbine are combined together to form an integral gas-turbine rotor. Further, the axial-flow compressor portion of the gas-turbine rotor is usually constructed by placing a number of rotor disks one next to another and by fastening the rotors in the axial direction using through bolts. Rotor blades of the axial-flow compressor are embedded on the outer peripheries of the respective rotor disks. In other words, an assembled gas-turbine rotor is used in the gas turbine. 
     FIG. 7 illustrates a general construction of an assembled rotor of a gas turbine. In FIG. 7, a gas-turbine rotor  1  is composed of a turbine rotor assembly  20  which generates rotary mechanical power from the flow of combustion gas and a compressor rotor assembly  10  connected to, and driven by, the turbine rotor assembly  20  through an intermediate shaft  25 . The gas-turbine rotor  1  in FIG. 7 is a “cold end drive type” in which rotary mechanical power for driving an external load is taken from the compressor rotor assembly side end  15 . 
     As can be seen from FIG. 7, both of the turbine rotor  20  and compressor rotor  10  are composed of rotor disks. The rotor disks are placed one next to another in the axial direction and fastened together by through bolts. For example, in the compressor rotor  10 , rotor disks  50  having compressor rotor blades embedded on the outer peripheries thereof are placed one next to another in the axial direction, and all the rotor disks  50  are fixed together by spindle bolts  51  piercing through the rotor disks  50  in the axial direction. 
     As explained later, on the side faces of the respective rotor disks  50 , at the portions where the rotor disk abuts to adjoining rotor disk, grooves  33  having semicircular cross sections are formed in the radial direction. When the adjoining rotor disks are assembled together in such a manner that the grooves  33  of both rotor disks align, the pairs of semicircular grooves form circular holes  35  extending in radial directions. As explained later, torque pins  40  are inserted into the circular holes  35 . 
     FIG. 8 is a perspective view illustrating the grooves  33  and torque pins  40  of the compressor rotor disk  50  in detail. As can be seen from FIG. 8, an annular protrusion  53  concentric with the center of the rotor disk is formed on each of the side faces  50   a  of the rotor disc  50 . The annular protrusion  53  has a generally rectangular cross section. The inner periphery (i.e., the inner side wall)  53   a  and the outer periphery (i.e., the outer side wall)  53   b  of the annular protrusion  53  are formed as cylindrical surfaces concentric with the center of the rotor disk  50 . The top face  53   c  of the annular protrusion is formed a flat plane perpendicular to the center axis of the rotor disk  50 . 
     When the rotor disks  50  are assembled, the top face  53   c  of the annular protrusion  53  of the rotor disk  50  abuts the top face  53   c  of the annular protrusion of the adjoining rotor disk. Thus, the top faces  53   c  of the annular protrusions  53  of the adjoining rotor disks closely contact to each other when the spindle bolts  51  are fastened.  51   a  in FIG. 8 designates bolt holes piercing through the respective rotor disks  50  for receiving the spindle bolts  51 . 
     As shown in FIG. 8, grooves  33  having semicircular cross sections are formed on the respective top faces  53   a  and extending in the radial direction. The grooves  33 , together with the grooves  33  on the top face  53   a  of the annular protrusion  53  of the adjoining rotor disk, form circular torque pin holes  35  which penetrate the annular protrusions  53  in the radial direction when the annular protrusions of the adjoining rotor disks are joined. 
     When the rotor disks  50  are assembled, a cylindrical pin (a torque pin)  40  is fitted into each of the torque pin holes  35 . The torque pins  40  function as keys for transmitting rotation torque between the adjoining rotor disks  50  and prevents relative angular movement between the rotor disks. 
     As can be seen from FIG. 7, since the number of the rotor disks  50  (i.e., the compression stages) of the compressor rotor assembly  10  is larger than the number of the rotor disks of the turbine assembly  20 , the axial length of the compressor rotor assembly  10  is larger than that of the turbine rotor assembly  20 . Further, in the cold end drive type gas-turbine rotor in FIG. 7, rotary torque is transmitted to an external load (such as a generator) from the turbine rotor assembly  20  via the compressor rotor assembly  10 . Therefore, the torque required for driving the external load, in addition to the torque required for driving compressor assembly  10 , must be transmitted from one rotor disk to the adjoining rotor disk during the operation of the gas turbine. 
     Further, a relatively steep temperature gradient along the axial direction is generated in the compressor rotor assembly during the operation of gas turbine. This causes the radial clearances of the bolt holes  51   a  relative to the spindle bolts  51  to change in accordance with the temperature gradient. Therefore, the radial clearances of the bolt holes  51   a  relative to the spindle bolts are different in the respective rotor disks  50 . Thus, though the spindle bolts  51  fasten the rotor disks  50  to each other in the axial direction, they cannot transmit a large torque between the rotor disks. Therefore, torque pins  40  are required for transmitting torque between the rotor disks. 
     During the operation of the gas turbine, centrifugal force due to the rotation of the gas-turbine rotor is exerted on the torque pins  40 . In order to prevent the torque pins  40  from coming out from the torque pin holes  35  due to the centrifugal force, a stopper flange  40   a  is formed on each of the torque pins  40  at the inner end thereof (i.e., the end of the torque pin located nearer the center of the rotor disk). Further, in order to prevent the torque pins  40  from falling out from the torque pin holes  35  due to their own weight when the gas-turbine rotor is at rest, another flange  40   b  is formed on each of the torque pins  40  at the outer end thereof (i.e., the end of the torque pin located far from the center of the rotor disk). 
     Usually, the stopper flange  40   a  is formed as a disk plate having flat faces on both sides thereof while the inner side wall  53   a  is formed as a cylindrical surface. Therefore, in order to ensure uniform contact between the flanges  40   a  and the inner side wall  53   a  when the centrifugal force is exerted on the torque pins  40 , the inner face  53   a  of the annular protrusion  53  must be machined flat, i.e., spot facing must be formed on the surface of the inner side walls  53   a  at the portion around the torque pin holes  35  where the flanges  40   a  contact the inner face. 
     However, since the spot facings must be formed on the inner side walls  53   a  of both annular protrusions  53  of the adjoining rotor disks continuously, the machining of the spot facings requires elaborate machining work. 
     In order to ensure uniform contact between the stopper flange  40   a  and both inner side walls  53   a  of adjoining rotor disks, the accuracy of the machining of the spot facings must be high. In order to obtain a high accuracy of the machining, the inner side walls  53   a  of the adjoining rotor disks must be machined to form spot facings in the condition where two adjoining rotor disks are assembled. However, when two rotor disks are assembled, i.e., when the two rotors are joined by abutting the top faces  53   c  of the annular protrusions  53  against each other, the inner side walls  53   a  of the annular protrusions are located inside of the annular protrusions  53 . Therefore, the following steps are required for machining the inner side walls  53   a  in order to form spot facings around the torque pin holes  35 . 
     a) Placing an L-shaped bit used for machining the spot facing in the groove  33  of one of the rotor disks before the two rotor disks are joined. 
     b) Assembling the two rotor disks so that the grooves  33  of both rotor disks align and that the L-shaped bit penetrates the torque pin hole  35  formed by the grooves with the cutting bit portion of the L-shaped bit being placed inside of the annular protrusions of both rotor disks. 
     c) Turning the L-shaped bit while keeping the two rotor disks  50  are assembled in order to form a circular spot facing around the torque pin hole  35 . 
     d) Disassembling the rotor disks  50  in order to remove the L-shaped bit after completing the machining of the spot facing. 
     The above steps a) to d) must be repeated for all of the torque pin holes  35 . Usually, 10 to 20 torque pin holes  35  are required for one rotor disk and one compressor rotor assembly consists of 10 to 20 rotor disks. Therefore, the steps a) to d) must be repeated as many as 400 times before completing the machining of the spot facings of a whole compressor rotor. This increases the cost and the time required for manufacturing the gas-turbine rotor. 
     If the spot facing on the inner side wall  53   a  can be machined without assembling the rotor disks, theoretically, a large part of the above-mentioned problem may be solved. However, if the spot facing machining is carried out without joining two rotor disks, the L-shaped bit must be turned in the semicircular groove  33 . This causes intermittent cutting, i.e., the L-shaped bit cuts the metal only during its half turn and races during the remaining half turn. This causes the bit to hit the edge of the inner side wall  53   a  every time it turns. In this case, therefore, the cutting speed is restricted to a significantly low level in order to prevent damage of the L-shaped bit. Thus, the time required for machining is not largely reduced. 
     SUMMARY OF THE INVENTION 
     In view of the problems in the related art as set forth above, the object of the present invention is to provide a means for largely reducing the time and cost required for manufacturing the gas-turbine rotor while ensuring uniform contact between the flanges of the torque pins and the inner side walls of the annular protrusions of the rotor disks. 
     The object as set forth above is achieved by a gas-turbine rotor, according to one aspect of the present invention, which includes a rotor assembly fabricated of a plurality of rotor disks, placed one next to another in the axial direction thereof and a plurality of spindle bolts piercing through the rotor disks and fastening the rotor disks to each other in the axial direction, comprising annular protrusions formed on both sides of each of the rotor disks and concentric with the center thereof, the annular protrusions having inner side walls facing the center of the rotor disk and outer side walls facing opposite the inner side walls and flat top surfaces, the rotor disks abutting against each other by contacting the top faces of the annular protrusions to the top faces of the annular protrusions of adjoining rotor disks, a plurality of grooves having semicircular cross sections formed on the top faces and extending in the radial direction of the rotor disk, the semicircular grooves, together with the grooves on the adjoining rotor disks, forming circular holes piercing through the annular protrusions when the rotor disks are assembled, cylindrical pins one each fitted into the respective circular holes for transmitting rotational torque between the rotor disks, each of the cylindrical pins being provided with a flange at least on the end thereof located near the center of the rotor disk, wherein, the surface of the inner side wall of an annular protrusion is formed as a spherical surface having the center thereof on the central axis of the rotor disk, and the surface of the face of the flange of the cylindrical pin abutting the inner side wall of the annular protrusion is formed as a spherical surface matching the spherical surface of the inner side wall of the annular protrusion. 
     According to this aspect of the invention, the surfaces of the inner walls of the annular protrusions are formed as spherical surfaces instead of cylindrical surfaces. Further, the faces of the flanges of the cylindrical pins contacting the inner side walls are also formed as spherical surfaces matching the spherical surfaces of the inner side walls. Therefore, the faces of the flanges of the pin uniformly contact the inner side walls of the annular protrusions when the centrifugal force is exerted on the pins without the need for forming the spot facings on the inner side wall surfaces. 
     Further, since the spherical surfaces of the inner side walls of the annular protrusions can be machined easily and accurately without assembling the rotor disks, time and cost required for manufacturing the gas-turbine rotor can be greatly reduced. 
     According to another aspect of the present invention, there is provided a gas-turbine rotor including a rotor assembly fabricated of a plurality of rotor disks placed one next to another in the axial direction thereof and a plurality of spindle bolts piercing through the rotor disks and fastening the rotor disks to each other in the axial direction comprising, annular protrusions formed on both sides of each of the rotor disks and concentric with the center thereof, the annular protrusions having inner side walls facing the center of the rotor disk and outer side walls facing opposite the inner side walls and flat top surfaces, the rotor disks abutting against each other by contacting the top faces of the annular protrusions to the top faces of the annular protrusions of adjoining rotor disks, a plurality of grooves having semicircular cross sections formed on the top faces and extending in the radial direction of the rotor disk, the semicircular grooves, joined by the grooves on the adjoining rotor disks, forming circular holes piercing through the annular protrusions when the rotor disks are assembled, cylindrical pins each one fitted into the respective circular holes for transmitting rotational torque between the rotor disks, each of the cylindrical pins being provided with a flange at least on the end thereof located near the center of the rotor disk, wherein the surface of the inner side wall of the annular protrusions is formed as a cylindrical surface having the center thereof on the central axis of the rotor disk, and the surface of the face of the flange of the cylindrical pin abutting the inner side wall of the annular protrusion is formed as a cylindrical surface matching the cylindrical surface of the inner side wall of the annular protrusion. 
     According to this aspect of the invention, the surface of the inner side wall of the annular protrusion is formed as a cylindrical surface same as in the related art. However, in this embodiment, the face of the flange of the pin contacting the inner side wall is also formed as a cylindrical surface which matches the cylindrical surface of the inner side wall. Therefore, the faces of the flanges of the pin uniformly contact the inner side walls of the annular protrusions when the centrifugal force is exerted on the pins without the need for forming the spot facings on the inner side wall surfaces. 
     Further, according to yet another aspect of the present invention, there is provided a gas-turbine rotor including a rotor assembly fabricated of a plurality of rotor disks placed one next to another in the axial direction thereof and a plurality of spindle bolts piercing through the rotor disks and fastening the rotor disks to each other in the axial direction comprising, annular protrusions formed on both sides of each of the rotor disks and concentric with the center thereof, the annular protrusions having inner side walls facing the center of the rotor disk and outer side walls facing opposite the inner side walls and flat top surfaces, the rotor disks abutting against each other by contacting the top faces of the annular protrusions to the top faces of the annular protrusions of adjoining rotor disks, a plurality of grooves having semicircular cross sections formed on the top faces and extending in the radial direction of the rotor disk, the semicircular grooves, together with the grooves on the adjoining rotor disks, forming circular holes piercing through the annular protrusions when the rotor disks are assembled, cylindrical pins one each fitted into the respective circular holes for transmitting rotational torque between the rotor disks, each of said cylindrical pins being provided with a flange at least on the end thereof located near the center of the rotor disk, wherein the surface of the face of the flange facing the inner side walls of the annular protrusions contacts the inner side wall of only one of the rotor disks adjoining each other. 
     According to this aspect of the invention, the face of the flange contacts the inner side wall of the annular protrusions of only one of rotor disks adjoining each other. When the inner side walls of the annular protrusion is machined without assembling the adjoining rotor disks, the accuracy of the machining must be relatively high so that the inner side walls of the annular protrusions of the adjoining rotor disks form a continuous surface in order to ensure uniform contact of the face of the flange and the inner side walls of the annular protrusions of the adjoining rotor disks. 
     However, the centrifugal force exerted on the cylindrical pin is relatively small. Therefore, if the face of the flange of the cylindrical pin uniformly contacts the inner side walls of the annular protrusions of one of the adjoining rotor disks, problems do not occur. In other words, the face of the flange does not need to contact both of the inner side walls of the annular protrusions of the adjoining rotor disks. Therefore, in this aspect of the invention, the inner side walls of the annular protrusion abutting each other have different dimensions so that only one of them contacts the face of the flange of the cylindrical pin. For example, if the surface of the inner side walls of the annular protrusions are formed as spherical or cylindrical surfaces having the center on the central axis of the rotor disks, the surface of the inner side walls of the annular protrusions which abut against each other are machined in such a manner that the diameter of one of the inner side walls becomes smaller than the diameter of the other inner side wall so that only the inner side wall having a smaller radius contacts the face of the flange of the cylindrical pin. By doing so, the accuracy of the machining of the inner side walls can be lowered even if the inner side walls are machined without joining the adjoining rotor disks and, thereby, the time and the cost required for manufacturing a rotor are further reduced. 
     Further, if the face of the flange contacts only one of the inner side walls, a flange having flat face may be used. In this case, the spot facings must be machined around the semicircular grooves in order to ensure uniform contact of the flat faces of the flanges and the curved inner side wall surface. However, in this case, the spot facing is required for only one of the surfaces of the inner side walls of the adjoining rotor disks. Therefore, the amount of machining work required for forming the spot facing becomes half that of the case where inner side walls of both of the adjoining rotor disks are machined to form spot facings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood from the description, as set forth hereinafter, with reference to the accompanying drawings in which: 
     FIG. 1 is a partial sectional view of a rotor disk used in the gas-turbine rotor according to a first embodiment of the present invention; 
     FIG. 2 is a longitudinal sectional view of a torque pin used in the first embodiment; 
     FIG. 3 is a partial sectional view of a rotor disk used in the gas-turbine rotor according to a second embodiment of the present invention; 
     FIG. 4A is a side view of a torque pin used in the second embodiment; 
     FIG. 4B is a view taken from the direction B—B in FIG. 4A; 
     FIG. 5 is a partial schematic sectional view of rotor disks, according to a third embodiment of the present invention, adjoining each other when the rotor disks are assembled; 
     FIG. 6 is a partial schematic sectional view of rotor disks, according to a fourth embodiment of the present invention, adjoining each other when the rotor disks are assembled; 
     FIG. 7 is a longitudinal sectional view illustrating the construction of a conventional gas-turbine rotor; and 
     FIG. 8 is a partial perspective view showing the shape of an annular protrusion and a torque pin of the rotor disks in FIG.  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, embodiments of the gas-turbine rotor according to the present invention will be explained with reference to FIGS. 1 through 6. 
     (1) First Embodiment 
     FIGS. 1 and 2 are sectional views of a rotor disk  50  and torque pin  40 , respectively, taken along a plane including the central axis of the disk according to the first embodiment of the present invention. In FIGS. 1 and 2, reference numerals the same as those in FIGS. 7 and 8 represent similar elements. 
     In this embodiment, annular protrusions  53  are provided on both side faces  50   a  of rotor disk  50 . The top faces  53   c  of the annular protrusions  53  are formed as flat planes perpendicular to the center axis CL of the rotor disk. When the rotor disks are assembled, the top faces  53   c  of the adjoining rotor disks abut each other. Further, semicircular grooves  33  extending in the radial direction are formed on the top face  53   c  of the annular protrusion  53 . 
     In the conventional rotor disk in FIG. 8, the surface of the inner side wall  53   a  of the annular protrusion is formed as a cylindrical surface having a central axis coinciding with the central axis (FIG. 8, CL) of the rotor disk  50 . The portions of the surface of the inner side wall  53   a  around the grooves  33  are machined to form flat planes, i.e., spot facings are formed on the surface of the inner side wall  53   a  around the groove  33 . 
     In contrast to the conventional rotor disk, the surface of the inner side wall  53   a  of the annular protrusion  53  is formed as a spherical surface having its center on the central axis CL of the disk  50  in this embodiment. Further, no spot facings are formed on the surface of the inner side wall  53   a  around the grooves  33 . 
     In this embodiment, the center of the spherical surface of the inner side wall  53   a  coincides with the point where the plane of the flat top face  53   c  of the annular protrusion  53  intersects the central axis CL of the rotor disk  50 . The radius R of the inner side wall  53   a  is appropriately determined in accordance with the location, thickness and height of the annular protrusion. 
     Further, in this embodiment, the torque pin  40  is cylindrical shape having flanges on both ends. The surface of the face  401   a  of the flange  40   a  of the torque pin which contacts the inner side wall  53   a  is formed as a spherical surface having a radius the same as the radius of the inner side wall  53   a . The spherical surface of the face  401   a  has its center O on the central axis of the cylindrical pin  40 . Therefore, when the torque pin  40  is fitted into the radial semicircular groove  33 , the curvature of the face  401   a  of the flange  40   a  matches the curvature of the inner side wall  53   a  and, thereby, the face  401   a  uniformly contacts the inner side wall  53   a.    
     Since the surface of the inner side wall  53   a  of the annular protrusion  53  is formed as a spherical surface having its center on the central axis of the rotor disk  50 , the inner side wall  53   a  can be easily machined with high accuracy without assembling the adjoining rotors. Namely, the inner side wall  53   a  can be machined to a desired spherical shape by turning the rotor disk  50  around the central axis thereof while abutting the cutting bit against the inner side wall  53   a  in the radial direction. The face  401   a  of the flange  40   a  of the torque pin  40  also can be machined in a desired spherical shape easily and accurately in the manner similar to the inner side wall  53   a.    
     Therefore, according to the present embodiment, uniform contact between the flange face  401   a  and the inner side wall  53   a  is possible without forming the spot facing on the inner side wall  53   a . Thus, the time and the cost required for machining a large number of spot facings on the inner side wall  53   a  are not required in this embodiment and, thereby the time and the cost required for manufacturing the gas-turbine rotor can be greatly reduced. 
     In this embodiment, the surface of the face  401   a  of the flange  40   a  is formed as a spherical surface so that the whole area of the face  401   a  uniformly contacts the inner side wall  53   a . However, since the centrifugal force exerted on the torque pin  40  during the operation of the gas turbine is relatively small, the face  401   a  may be machined in a conical shape instead of a spherical shape so that the flange face  401   a  contacts the inner side wall  53   a  only at its outer periphery. 
     Further, though the flange face  401   a  preferably uniformly contacts the inner side walls  53   a  of the annular protrusions  53  of both of the adjoining rotor disks  50  when the rotor disks are assembled, practically, problems do not occur even if the flange face  401   a  contacts the inner side wall  53   a  of one of the annular protrusions  53  as long as the contact is uniform. Therefore, the inner side walls  53   a  of the adjoining rotor disks  50  do not necessarily form one continuous spherical surface when the annular protrusions  53  of adjoining rotors are joined. In other words, even if the two inner side walls  53   a  are not flush when the two annular protrusions  53  are joined, practically, no problems occur as long as the flange face  401   a  contacts one of the inner side walls  53   a  uniformly. 
     (2) Second Embodiment 
     Next, a second embodiment of the present invention is explained with reference to FIGS. 3 and 4. 
     FIG. 3 is a partial sectional view of the rotor disk  50  of the present embodiment, similar to FIG.  1 . In this embodiment, similarly to the conventional rotor disk in FIG. 8, the inner side walls  53   a  of the annular protrusions  53  of the rotor disk  50  are formed as cylindrical surfaces having a radius R and concentric with the rotor disk  50 . However, spot facings are not formed on the inner side walls  53   a  around the radial semicircular grooves  33 . 
     FIG. 4A is a side view of the cylindrical torque pin  40  used in this embodiment and FIG. 4B is view of the same seeing from the direction B—B in FIG.  4 A. As can be seen from FIGS. 4A and 4B, the surface of the flange face  401   a  of the flange  40   a  of the torque pin  40  is formed as a cylindrical surface having a radius R, the same as the radius of the inner side wall  53   a . Therefore, in this embodiment, since the flange face  401   a  matches the inner side walls  53   a  of the annular protrusions  53  when the adjoining rotor disks  50  are joined, the flange face  401   a  uniformly contacts with the inner side walls  53   a  of the adjoining rotor disks without the need for forming the spot facings on the inner side walls  53   a  around the grooves  33 . 
     In this embodiment, the inner side wall  53   a  of the annular protrusion  53  can be machined in a manner the same as that of the conventional rotor disk in FIG.  8 . However, since the machining of the spot facings on the inner side wall  53  is not required, elaborate machining works in the condition where the two adjoining rotor disks are assembled is not required. It is true that relatively complicated machining work is required for forming cylindrical surfaces on the flange faces  401   a  of the torque pins  40 . However, since the torque pin  40  is much smaller than the rotor disk  50 , the time and the cost required for machining the flange faces  401   a  is much smaller than time and cost required for forming spot facings on the inner side walls  53   a  of the rotor disk. Therefore, according to the present embodiment, the time and the cost required for manufacturing the gas-turbine rotor is greatly reduced as a whole. 
     (3) Third Embodiment 
     Next, a third embodiment of the present invention will be explained with reference to FIG.  5 . FIG. 5 is a schematic sectional view of the rotor disks  50  in the assembled condition. In FIG. 5, two adjoining rotor disks  50  are joined by abutting the top faces  53   c  (not shown) of the annular protrusions  53  to each other. In this condition, the radial semicircular grooves ( 33  in FIG. 1) on both annular protrusions  53  join each other and form a circular torque pin holes  35 . In FIG. 5, the surfaces of the inner side walls ( 53   a  in FIG. 1) of the annular protrusions  53  are formed as spherical surfaces having their centers on the central axis of the rotor disks  50 . 
     However, though the inner side walls of both adjoining rotor disks have spherical surfaces, the radius R 1  of the spherical surface of the inner side wall  53   a   1  of one of the adjoining rotor disks is smaller than the radius R 2  of the spherical surface of the inner side wall  53   a   2  of the other of the adjoining rotor disks in this embodiment. The surface of the flange face  401   a  of the torque pin  40  is formed as a spherical surface having a radius R 1 , the same as the smaller radius R 1  of the inner side wall  53   a   1 . Therefore, as shown in FIG. 5, the flange face  401   a  of the torque pin  40  only contacts the inner side wall  53   a   1  having a radius R 1  when the rotor disks are assembled. In other words, the centrifugal force exerted on the torque pin  40  during the operation of the gas turbine is received by the contact only between the flange face  401   a  and the inner side wall  53   a   1 . 
     As explained before, since the centrifugal force exerting on the torque pin  40  is relatively small, practically no problems occur even if the flange face  401   a  of the flange  40  contacts only one of the inner side wall ( 53   a   1 ) as long as the flange face  401   a  uniformly contacts the inner side wall  53   a   1 . 
     Therefore, in this embodiment, the radius of the inner side wall of one of the adjoining rotor disk is intentionally set at a value smaller than the radius of the inner side wall of the other of the adjoining rotor disk so that only one of the inner side wall contacts the flange face  401   a  when the adjoining rotor disks are assembled. When the inner side walls of the rotor disks are machined without assembling the adjoining rotor disks, a relatively high accuracy of machining is required if the surfaces of the inner side walls of the rotors adjoining each other must be joined continuously (i.e., without forming a step at the seam of two surfaces). Therefore, in this embodiment, by intentionally forming a step at the seam of inner side walls of the adjoining rotors, the accuracy of the machining of the inner side walls can be lowered to some extent while maintaining the uniform contact between the flange face  401   a  and one of the inner side walls. Thus, according to this embodiment, the time and the cost required for manufacturing the gas turbine can be further reduced. 
     Though the present embodiment is explained with reference to the example in which the surfaces of the inner side walls  53   a  are formed as spherical surfaces, the surface of the inner side walls  53   a  may be formed as cylindrical surfaces. In this case, if the radius of the surface of the inner side wall of one of the adjoining rotor disks is set at a smaller value than the inner side wall of the other of the adjoining rotor disks, the same advantageous effect can be obtained. 
     (4) Fourth Embodiment 
     FIG. 6 is a partial section view similar to FIG. 5 which illustrates a fourth embodiment of the present invention. 
     In this embodiment, the surface of the inner side walls  53   a  of the annular protrusions  53  of the rotor disks  50  are formed as cylindrical surfaces concentric with the rotor disks. Further, similarly to the third embodiment, the radius R 1  of the inner side wall  53   a   1  of one of the adjoining rotor disks is smaller than the radius R 2  of the inner side wall  53   a   2  of the other of the adjoining rotor disks. Therefore, also in this embodiment, the flange face  401   a  of the torque pin  40  only contacts the inner side wall  53   a   1  having a smaller radius R 1 . 
     However, the surface of the flange face  401   a  of the torque pin  40  is formed as a flat plane in this embodiment. Therefore, a spot facing  54  (a flat surface) is formed on the surface of the inner side wall  53   a   1  in order to obtain uniform contact between the flange face  401   a  and the inner side wall  53   a   1 . However, the spot facing  54  is formed only on the surface of the inner side wall  53   a   1  having a smaller radius R 1 . Further, the depth of the spot facing  54  is determined in such a manner that the flange face  401   a  does not contact the inner side wall  53   a   2  having a larger radius R 2  when the flange face  401   a  contacts the bottom plane of the spot facing  54 . In this embodiment, a torque pin  40  having a flat flange face  401   a  can be used by forming a spot facing  54  on the inner side wall  53   a   1  which contacts the flange face  401   a . Therefore, according to this embodiment, a uniform contact between the flange face  401   a  and the inner side wall  53   a   1  can be obtained although a torque pin  40  having a flat flange face  401   a  is used. 
     It is true that the machining of the inner side wall  53   a   1  is required for forming the spot facings  54  in this embodiment. However, the machining of the spot facings are required for only one of the inner side walls adjoining each other and the machining can be carried out without assembling the adjoining rotor disks. Further, since the number of the spot facings to be machined is a half of that in the conventional rotor disk in FIG. 8, the time and the cost required for the machining of the spot facings are largely reduced although the cutting speed of the bit must be lowered when the spot facings are machined without assembling the adjoining rotor disks. Thus, the time and the cost required for manufacturing the gas-turbine rotor can be greatly reduced.