Abstract:
A turbomolecular pump comprises a casing, a rotor having a rotor blades arranged in multiple stages in the casing axially thereof, and a stator having stator blades arranged in multiple stages and alternately located between the rotor blades. Each of the stator blades has an inner ring portion, an outer ring portion spaced-apart from the inner ring portion, and blades connected between the inner and outer ring portions along a circumferential direction thereof. Each of the blades has a first end connected to an outer peripheral edge of the inner ring portion and a second end opposite the first end connected to an inner peripheral edge of the outer ring portion. A reinforcement member is disposed on the inner ring portion of each of the stator blades along the entire circumference of the inner ring portion for reinforcing the stator blade.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a turbomolecular pump, and more particularly to a turbomolecular pump having improved stator blades. 
     2. Description of the Related Art 
     A turbomolecular pump is widely used as a vacuum apparatus for a semiconductor manufacturing equipment. 
     The turbomolecular pump has stator blades and rotor blades which are disposed on a stator portion and a rotor portion, respectively, in a multistage arrangement in an axial direction, and the rotor portion is rotated with a motor at high speed so that a vacuum (exhaust) action is performed. 
     FIGS.  11 ( a ) to  11 ( c ) show the structures of the rotor blade and the stator blade of the turbomolecular pump described above. FIG.  11 ( a ) shows an arrangement between the rotor blade and the stator blade, FIG.  11 ( b ) is a sectional perspective view showing a rotor that is cut along upper and lower planes of the rotor blade, and FIG.  11 ( c ) is a perspective view showing a part of the stator blade. 
     As shown in FIG.  11 ( a ), the turbomolecular pump is composed of a rotor  60  and a stator  70  that are fixedly disposed to rotor axes rotating at high speed. 
     The rotor  60  is composed of a rotor body  61  that accommodates a motor and magnetic bearings inside thereof, a rotor ring portion  64  arranged at an outer circumference of the rotor body  61 , and a plurality of blades  63  provided to the rotor ring portion  64  radially in a radial direction and tilted at a predetermined angle with respect to the rotational axis. 
     On the other hand, the stator  70  is composed of a spacer  71  and a stator blade  72  that are arranged between rotor blades  62  at the respective stages, while being supported its outer circumferential side between the spacers  71  and  71 . 
     The spacer  71  is a cylindrical shape having stepped portions, and the length of each stepped portion in an axial direction, located inside thereof, is varied in accordance with the intervals between the respective stages of the rotor blades  62 . 
     The stator blade  72  is composed of an outer ring portion  73 , part of outer circumferential portion of which is sandwiched by the spacers  71  in circumference direction, an inner ring portion  74 , and a plurality of blades  75  both ends of which are supported radially with a predetermined angle by the outer ring portion  73  and the inner ring portion  74 . The inner diameter of the inner ring portion  74  is formed to have a larger size than the outer diameter of the rotor body  61  so that an inner circumferential surface  77  of the inner ring portion  74  and an outer circumferential surface  65  of the rotor body  61  do not contact with each other. 
     In order to arrange the stator blade  72  between the rotor blades  62  at the respective stages, each stator blade  72  is divided into two parts in circumference. The stator blade  72  is made from a thin plate such as a stainless or aluminum thin plate that is divided into two. An outer portion having a semi-ring profile and portions for blades  75  of the stator blade  72  are cut out by means of etching from the thin plate, and the portions for blades  75  are folded by means of press machining to have a predetermined angle. Thus, the shape shown in FIG.  11 ( c ) is obtained. 
     In the thus formed turbomolecular pump, the rotor  60  is designed to be rotated with a motor at several tens of thousands r.p.m., so that an exhaust action is effected from the upstream side to the downstream side of FIG.  11 ( a ) 
     In such conventional turbomolecular pump, since the support of the stator blades  72  by a spacer  71  is carried out with a cantilever configuration and the stator blades  72  are divided into two parts in circumference, large deflection would occur in the case that excess loads were applied to the stator blades  72 . In particular, in the stator blades  72  formed by means of press machining, since the thickness of the plate is thin, there have been cases where the open end on the center side was largely deflected at the portion divided into two parts. 
     For that reason, in the case where a large fluctuation occurred in a load of gas due to malfunction, etc. of valves attached to a vacuum chamber, the stator blades were caused to largely deflect with the result that, in the worst case, blades  75  of the stator blades were brought into contact with blades  63  of the rotor blades were damaged. 
     Further, in the case where such a structure was employed that a magnetic bearing was used for the rotor axis, there also occurred the case in which the stator blades  72  and the rotor blades  62  broken when brought into contact with each other due to vibration generated at the time of a trouble with the magnetic bearing device or of a touch-down of a touch down bearing upon a power failure. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above-mentioned problems inherent in the conventional turbomolecular pump, and therefore has a primary object of the present invention to provide a turbomolecular pump with stator blades having a structure in which deflections are not relatively occurred. 
     Further, a secondary object of the present invention is to provide a turbomolecular pump with stator blades having a structure in which deflections are not relatively occurred. 
     Further, a secondary object of the present invention is to provide a turbomolecular pump with stator blades having a structure in which breakage of the stator blades are hardly occurred even if the stator blades are brought into contact with rotor blades due to deflections. 
     In order to attain the primary object of the present invention, according to the present invention, a reinforcement portion is arranged to the inner ring portion of each stator blade. 
     Further, according to the present invention, the reinforcement portion is constructed of a rib structure formed in the inner ring portion of the stator blade. 
     Still further, according to the present invention, said reinforcement portion is constructed of engagement means formed at end portions of the divided inner ring portion of the stator blade for engaging one end portion of the divided inner ring portion of said stator blade and the other end portion of the divided inner ring portion facing thereto. 
     In order to attain the secondary object of the present invention, according to the present invention, the blades of the stator blades at the respective stages comprise a multi-layer of plural pairs of blades overlapped with each other, and the phases of the divided positions at the respective layers are shifted with each other. 
     Further, according to the present invention, the blades of the rotor blades at the respective stages are provided to the rotor ring portion that is disposed to the rotor corresponding with the stage; and an outer diameter of the inner ring portion of the blades of the stator blades is smaller than an outer diameter of the rotor ring portion. 
     Further, according to the present invention, steps are formed at the outer ring portion so that the blades of the stator blades are allowed to contact with the outer ring portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a cross-sectional view showing the entire structure of a turbomolecular pump according to an embodiment of the present invention; 
     FIG. 2 shows the structure of a stator blade according to a first embodiment of the present invention, in which FIG.  2 ( a ) is a partially perspective view of the stator blade, and FIG.  2 ( b ) to  2 ( e ) show rib structure portions of the stator blade in which various shapes are employed; 
     FIG. 3 shows the structure of a stator blade according to a second embodiment of the present invention, in which FIG.  3 ( a ) is a perspective view showing both end portions of inner ring portions facing to each other; and FIG.  3 ( b ) is a cross-sectional view showing the engagement state of FIG.  3 ( a ); 
     FIG. 4 shows the structure of the stator blade according to a first modification example of the second embodiment of the present invention, in which FIG.  4 ( a ) is a perspective view showing both end portions of the inner ring portions facing to each other; and FIG.  4 ( b ) is a cross-sectional view showing the engagement state of FIG.  4 ( a ); 
     FIG. 5 is a perspective view showing both end portions of the inner ring portions of the stator blade facing to each other according to a second modification example of the second embodiment of the present invention; 
     FIG. 6 are conceptual views showing arrangements of a stator blade according to a third embodiment of the present invention, in which FIG.  6 ( a ) shows two pairs of the stator blades each consisting of two-divided stator blades to be overlapped so that coupling positions are shifted by 90° to each other, and FIGS.  6 ( b ) to  6 ( d ) show examples of the overlapping state of the pairs of stator blades; 
     FIG. 7 is a cross-sectional view showing a stator blade and a rotor blade of a turbomolecular pump according to a fourth embodiment of the present invention; 
     FIG. 8 is a cross-sectional view showing a stator blade and a rotor blade of a turbomolecular pump according to a first modification example of the fourth embodiment of the present invention; 
     FIG. 9 is a cross-sectional view showing a stator blade and a rotor blade of a turbomolecular pump according to a second modification example of the fourth embodiment of the present invention; 
     FIG. 10 is a cross-sectional view showing a stator blade and a rotor blade of a turbomolecular pump according to a third modification example of the fourth embodiment of the present invention; and 
     FIG. 11 shows the structures of a rotor blade and a stator blade of the conventional turbomolecular pump, in which FIG.  11 ( a ) shows an arrangement between the rotor blade and the stator blade, FIG.  11 ( b ) is a sectional perspective view showing a rotor that is cut along upper and lower planes of the rotor blade, and FIG.  11 ( c ) is a perspective view showing a part of the stator blade. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, detailed descriptions will be made of the preferred embodiments of the present invention with reference to FIG. 1 to FIG.  10 . 
     (1) Outline of Embodiments 
     According to a first embodiment of the present invention, a rib structure is employed to an inner ring portion  74  of a stator blade  72 . As a specific rib structure, a variety of shapes such as a semicircular-shape, a semiellipse-shape, a U-shape, or reversed V-shape in cross-section in a radial direction may be employed. Those shapes may be formed by means of a press machining, or by attaching with welding, etc. 
     According to a second embodiment of the present invention, claws are formed by means of folding or welding, etc., at the connecting portions of two-divided stator blades  72 . As a result, rigidity is enhanced at the two-divided portion where the stator blades  72  are faced to each other, which hardly causes deflection of the blades. 
     According to a third embodiment of the present invention, the pair of the stator blades  72  each consisting of two-divided stator blades are overlapped to form a two-layer structure. Further, phases of the two-divided positions of the stator blades in the respective layers are shifted by 90° to each other, 
     According to a fourth embodiment of the present invention, there is employed a structure in which even in the case where the stator blades  72  and the rotor blades  62  are brought into contact with each other, blades  75  of the stator blades (hereinafter simply referred to as “blades S”) and blades  63  of the rotor blades (hereinafter simply referred to as “blades R”), which form planes of discontinuity and are the weakest portions in the structure, are prevented from contacting with each other. Specifically, the length of the blades S  75  in a radial direction is lengthened (extend) inwardly, so that in the case where the stator blades  72  are deflected, the top end portions  76  of the blades S  75  on the center side is brought into contact with a rotor ring portion  64  which is a plane of continuity of the rotor blades  62 , thereby preventing the blades S  75  from contacting with the blades R  63 . Further, an abutting portion (leg portion) is provided to the inner ring portion  74  of each stator blade  72 , with the result that even in the case where the stator blades  72  are largely deflected, the leg portion is allowed to contact with the rotor ring portion  64 . With taking a structure described above, it can be prevented both planes of discontinuity (blades S  75  and blades R  63 ) from directly contacting with each other, with the result that the stator blades  72  and the rotor blades  62  are hardly damaged. 
     (2) Details of Embodiments 
     Detailed descriptions of preferred embodiments of the present invention will be made hereinafter with reference to FIG. 1 to FIG.  10 . It is to be noted that, in the present embodiments, the same reference numerals are used to explain the same components in the conventional turbomolecular pump shown in FIG. 11, and the descriptions thereof are appropriately omitted. Only different portions between the conventional structure and the present embodiments are described. 
     FIG. 1 is a cross-sectional view showing the entire structure of a turbomolecular pump according to an embodiment of the present invention. 
     A turbomolecular pump  1  above is installed, for example, in semiconductor manufacturing equipment for exhausting a process gas from a chamber, etc. In this example, a flange  11  is formed at the top end portion of a casing  10 , and is allowed to join the semiconductor manufacturing equipment, etc., with bolts. 
     As shown in FIG. 1, the turbomolecular pump  1  is provided with a rotor shaft  18  that is substantially cylindrical and is arranged at the center portion of the casing  10 . Arranged to the outer periphery of the rotor shaft  18  is a rotor body  61  having a substantially inverted U-shaped in cross-section to be attached to the top portion of the rotor shaft  18  with bolts  19 . Around the outer periphery of the rotor body  61 , the rotor ring portions  64  are arranged in multistage manner, and the rotor blades  62  are arranged to the respective rotor ring portions  64 . The rotor blades  62  at the respective stages include a plurality of the blades  63  with an open end. 
     Further, the turbomolecular pump  1  is provided with the rotor  60  and the stator  70 . 
     The stator  70  is constructed of the plurality of stator blades  72  and the cylindrical spacers  71  having stepped portions. The stator blades at the respective stages are divided into two as described later, and are inserted between the rotor blades  62  at the respective stages outwardly to be assembled. The stator blades  72  at the respective stages are sandwiched in a circumferential direction at the outer ring portion  73  between the spacers  71  and  71 , respectively, thereby being retained between the rotor blades  62 . 
     The stator  70  is fixedly disposed to the inner periphery of the casing  10 . 
     The rotor blades  62  and the stator blades  72  according to the present embodiment serve as an exhaust stage, an intermediate stage, and a compression stage from the upstream thereof. It should be noted that the present invention is not limited to a three-stage structure consisting of the exhaust, intermediate, and compression stages, and a variety of structures may be employed such as a two-stage structure consisting of the exhaust stage and the compression stage, a two-stage structure in which each stage plays another function, and a structure with no limitation in the function of each stage. 
     The turbomolecular pump  1  further includes a magnetic bearing  20  for supporting the rotor shaft  18  with magnetic force, and a motor  30  for generating torque to the rotor shaft  18 . 
     The magnetic bearing  20  includes radial electromagnets  21  and  24  for generating a magnetic force in a radial direction to the rotor shaft  18 , radial sensors  22  and  26  for detecting the position of the rotor shaft  18  in a radial direction, axial electromagnets  32  and  34  for generating a magnetic force in an axial direction to the rotor shaft  18 , a metal disk  31  to which force generated by the axial electromagnets  32  and  34  is acted, and an axial sensor  36  for detecting the position of the rotor shaft  18  in an axial direction. 
     The radial electromagnet  21  is composed of two pairs of electromagnets that are disposed so as to be orthogonal with each other. The respective pairs of electromagnets are disposed at an upper position than the motor  30  of the rotor shaft  18 , while sandwiching the rotor shaft  18  therebetween. 
     Provided between the radial electromagnet  21  and the motor  30  are two pairs of radial sensors  22  facing with each other and sandwiching the rotor shaft  18  therebetween, and being adjacent to the radial electromagnet  21  side. The two pairs of radial sensors  22  are disposed so as to cross at right angles with each other in correspondent with two pairs of radial electromagnets  21 . 
     Furthermore, two pairs of electromagnets  24  are similarly disposed at a lower position than the motor  30  of the rotor shaft  18  so as to be orthogonal with each other. 
     Between the radial electromagnet  24  and the motor  30 , two pairs of radial sensors  26  are similarly provided so as to be adjacent to the radial electromagnet  24 . 
     By supplying excitation current to these radial electromagnets  21  and  24 , the rotor shaft  18  is magnetically levitated. This excitation current is controlled bin correspondence with the position detection signals from the radial sensors  22  and  26  upon the magnetic levitation. As a result, the rotor shaft  18  is secured at the prescribed position in the radial direction. 
     Onto the lower portion of the rotor shaft  18 , a discoid metal disk  31  formed of the magnetic substance is fixed. Each one pair of axial electromagnets  32  and  34  facing with each other are disposed while sandwiching this metal disk  31  therebetween. Further, the axial sensors  36  are disposed facing with each other at the lower end portion of the rotor shaft  18 . 
     The excitation current of the axial electromagnets  32  and  34  is controlled in correspondent with the position detection signal from the axial sensor  36 . As a result, the rotor shaft  18  is secured at the prescribed position in the axial direction. 
     The magnetic bearing  20  includes a magnetic bearing controlling section disposed within a controller  45  for magnetically levitating the rotor shaft  18  by feedback controlling the excitation current of the radial electromagnets  21  and  24  and the axial electromagnets  32  and  34 , respectively, on the basis of the detection signals of these radial sensors  22  and  26  and the axial sensor  36 . 
     The touch down bearings  38  and  39  are disposed at the upper and lower sides of the rotor shaft  18 . 
     In general, the rotor portion consisting of the rotor shaft  18  and respective portions attached thereto is axially supported in a non-contact state by the magnetic bearing  20  during its rotation with the motor  30 . The touch down bearings  38  and  39  play a part for protecting the entire device by axially supporting the rotor portion in place of the magnetic bearing  20  when the touch down occurs. 
     Therefore, the touch down bearings  38  and  39  are arranged so that the inner race of the bearings  38  and  39  are in the non-contact state against the rotor shaft  18 . 
     The motor  30  is disposed between the radial sensor  22  and the radial sensor  26  inside the casing  10  substantially at the center position of the rotor shaft  18  in the axial direction. The rotor shaft  18 , the rotor  60  and the rotor blades  62  fixed thereto are allowed to rotate by applying a current to the motor  30 . 
     An exhaust port  52  for exhausting the processed gas or the like from the semiconductor manufacturing equipment is disposed at the lower portion of the casing  10  of the turbomolecular pump  1 . 
     Also, the turbomolecular pump is connected to the controller  45  through the connector  44  and the cable. 
     FIGS.  2 ( a ) to  2 ( e ) show the structure of the stator blade  72  according to the first embodiment of the present invention. 
     As shown in FIG.  2 ( a ), the stator blade  72  is constructed of an outer ring portion  73  part of the outer circumference side of which is sandwiched in the circumferential direction by the spacers  71 , the inner ring portion  74 , and a plurality of blades  75  both ends of which are radially supported with a predetermined angle by the outer ring portion  73  and the inner ring portion  74 . The inner diameter of the inner ring portion  74  is formed larger than the outer diameter of the rotor body  61 , so that the inner circumferential plane  77  of the inner ring portion  74 , and the outer circumferential plane  65  of the rotor body  61  do not contact with each other (refer to FIG.  11 ( a )). 
     A rib structure portion  80  that functions as the reinforcement member is formed at the inner ring portion  74 . This rib structure portion  80  is formed in the circumferential direction from an end face  78  of the two-divided inner ring portion  74  to the end face  78  on the other side. The rigidity with respect to the deflection of the inner ring portion  74  can be enhanced by the provision of the rib structure portion  80 . 
     The stator blade  72  is made from a thin plate such as a stainless or aluminum thin plate. An outer portion having a semi-ring profile and portions for blades  75  of the stator blade  72  are cut out by means of etching from the thin plate, and the portions for blades  75  are folded by means of press machining to have a predetermined angle. Then, the rib structure portion  80  is press-machined, and to thereby form the stator blades  72  shown in FIG.  2 ( a ) is obtained. 
     As the specific shape (sectional shape in a radial direction) of the rib structure portion  80 , though it is optional, it is possible to employ a variety of shapes such as a semicircular-shape with a radius R (FIG.  2 ( b )), a semiellipse-shape having a plane portion with the length of b in the radial direction and being chamfered with the radius R (FIG.  2 ( c )), a U-shape with the length of b and the height of h in the radial direction (FIG.  2 ( d )), or a reversed V-shape with the height of h and the width of b (FIG.  2 ( e )) and the like. 
     Further, in FIGS.  2 ( b ) to  2 ( e ), as the rib structure portion  80 , the shapes that are press-machined so as to protrude in the upward direction of the drawings are shown. However, the press machining may be conducted so that the rib structure portion protrudes in the downward direction of the drawings. 
     In the stator blades  72  of the first embodiment of the present invention, the description was made of the case in which in order to decrease the amount of deflection of the blades in an axial direction, the rib structure portion  80  was formed by press machining as reinforcement portion in the inner ring portion  74 . However, other structure may be employed as the reinforcement portion. 
     For example, the reinforcement member may be a separate structure which can be fixed to the inner ring portion  74  in a circumferential direction by welding or the like from one end face  78  of the two-divided inner ring portion  74  to the end face  78  on the other side. As the shape of the reinforcement member in cross-section in a direction that is orthogonal to the longitudinal direction of the reinforcement member, a variety of shapes such as square, triangle, semicircular, or semiellipse may be employed. 
     FIGS.  3 ( a ) and  3 ( b ) shows the structure of the stator blade  72  according to a second embodiment of the present invention, in which both end portions of inner ring portions facing to each other are shown. 
     In FIGS.  3 ( a ) and  3 ( b ), one end out of a pair of two-divided inner ring portions  74  is denoted by reference symbol  74   a,  and the other end is denoted by reference symbol  74   b.  Further, if a right side end portion of the both inner ring portions  74   a  and  74   b  viewed from the rotor axis  18  side is assumed as one end portion, and a left side end portion is assumed as the other end portion, the shapes of the one end portion and the other end portion of the inner ring portion  74   a  are formed identical to that of the inner ring portion  74   b.  FIG.  3 ( a ) shows the one end portion of the inner ring portion  74   a  and the other end portion of the inner ring portion  74   b.    
     Note that the relationship between the one end portion and the other end portion of the inner ring portions  74   a  and  74   b  is the same as in modification examples shown in FIGS.  4 ( a ) and  4 ( b ) and FIG.  5 . 
     As shown in FIGS.  3 ( a ) and  3 ( b ), in order to enhance the rigidity of the two-divided stator blades  72 , reinforcing means comprised of engagement claws  81   a  and  81   b  are provided as engagement members to one of the two-divided end faces  78   a  of the inner ring portion  74 . Also, provided to the two-divided end face  78   b  on the other side of the inner ring portion  74  is another engagement member in the form of an engagement claw  81   c.    
     Although the dimensions of widths b 1 , b 2 , and b 3  of the respective engagement claws  81   a,    81   b,  and  81   c  in a radial direction are optional, the total value of b 1 +b 2 +b 3  must be equal to or smaller than the width of the inner ring portion  74  in the radial direction. Also, the distance between the engagement claw  81   a  and the engagement claw  81   b  is required to be equal or larger than the width b 2  of the engagement claw  81   c.  Furthermore, the lengths  11  of the engagement claws  81   a  and  81   b  and the length  12  of the engagement claw  81   c  are also optional. The above-mentioned relationships are the same as in the respective modification examples shown in FIGS.  4 ( a ) and  4 ( b ) and FIG.  5 . 
     As shown in FIG.  3 ( a ), in the engagement claws  81   a  and  81   b,  the joining portion to the inner ring portion  74   a  are curved upwardly as much of the thickness of the inner ring portion  74 . Similarly, in the engagement claw  81   c,  the joining portion to the inner ring portion  74   b  is curved upwardly as much of the thickness of the inner ring portion  74 . 
     The engagement claws  81   a ,  81   b,  and  81   c  in accordance with the present embodiment are machined by folding the engagement claw portions integrally formed. However, the curved engagement claws  81  may be separate members which are fixed to the inner ring portion  74  by welding. It is to be noted that in the case of welding or the like of the engagement claws, it may take a structure in which the engagement claws  81  are overlapped on the inner ring portion  74  and then welded. The methods of provision of the engagement claws described above are the same as in the modification examples shown in FIGS.  4 ( a ) and  4 ( b ) and FIG.  5 . 
     FIG.  3 ( b ) shows an engagement state of the inner ring portions  74   a  and  74   b  when a pair of the two-divided stator blades  72  is coupled. 
     As shown in the figure, the pair of the two-divided inner ring portions  74   a  and  74   b  are engaged with the engagement claws  81 , and the rigidity against the deflection from the upper side to the down side of the drawing is improved. 
     It is to be noted that if it is designed such that the engagement claws are disposed to the lower side of the inner ring portion  74 , the rigidity against the deflection from the lower side to the upper side of the drawing may also be improved. 
     FIG. 4 shows the abutting portion of the inner ring portions according to a first modification example of the second embodiment of the present invention. 
     In this modification example, as shown in FIG.  4 ( a ), engagement claws  82   a  and  82   c  curved downwardly at the joining portion as much of the thickness of the inner ring portion  74  are provided to one end portion of the inner ring portion  74   a,  and an engagement claw  82   b  curved upwardly at the joining portion as much of the thickness of the inner ring portion  74  is provided therebetween. 
     Further, the other end of the inner ring portion  74   b  is a flat plate with no engagement claw, and as shown in FIG.  4 ( b ), the engagement claws  82   a  and  82   c  are engaged with the lower side of the inner ring portion  74   b  and the engagement claw  82   b  is engaged with the upper side thereof. 
     According to the first modification example, one end portion of the pair of inner ring portions  74   a  and  74   b  are engaged with both upper and lower surfaces of the other end portion thereof through the engagement claws  82   a,    82   b,  and  82   c.  As a result, the rigidity against the deflections from both the upper side and the lower side of the drawing may be improved. 
     FIG. 5 shows the abutting portion of the inner ring portions according to a second modification example of the second embodiment of the present invention. 
     In this modification example, as shown in FIG. 5, a sandwiching claw  83   a  is provided to one end portion of the inner ring portion  74   a,  and an engagement claw  83   b  curved upwardly at the joining portion as much of the thickness of the inner ring portion  74  is provided to the outer end of the inner ring portion  74   b.    
     The sandwiching claw  83   a  is formed of a member having an L-shape in cross-section, and is configured such that an open end side of a lower horizontal bar portion of the L is attached to a face  77   a  facing to the rotor of inner ring portion  74   a,  and the lower horizontal bar portion is extended upwardly in an axial direction as much of the thickness of the inner ring portion  74 , and in addition, a vertical bar portion of the L is extended in a radial direction. 
     In this modification example, the engagement claw  83   b  is sandwiched by the inner ring portion  74   a  and the sandwiching claw  83   a.  As a result, the rigidity against the deflections from both the upper side and the lower side of the drawing may be improved. 
     The sandwiching claw  83   a  is formed by cutting out a rectangular portion integrally with the inner ring portion  74   a,  and folding this rectangular portion by means of press machining in an axial direction and in an opposite direction to the axial center direction. 
     It should be noted that the shape of the sandwiching claw  83   a  in cross-section is not limited to L-shape, and a U-shape in cross-section may be employed. The sandwiching claw in this case has such a profile that the length  11  of the L-shape vertical bar portion of the sandwiching claw  83   a,  shown in FIG. 5, is longer than the width b 2  of the engagement claw  83   b,  and the tip end thereof is further folded toward the inner ring portion  74   a.    
     In addition, the sandwiching claw  83   a  may be formed separately from the inner ring portion  74   a  to fixed to the inner ring portion  74  by welding. In the case where the sandwiching claw is to be welded to the inner ring portion  74   a  by welding, the sandwiching claw may be welded not only on the face  77   a  facing to the rotor but also on the surface facing to the rotor blades  62 . In this case, depending upon the welding position of the sandwiching claw  82 , the position at which the engagement claw  83   b  is arranged, may be adjusted. In this way, provision of the sandwiching claw  83  on the surface facing to the rotor blades may prevent the interval between the outer periphery of the rotor body  61  from narrowing. 
     FIG. 6 is a conceptual view showing arrangements of a stator blade  72  according to a third embodiment of the present invention. 
     In the third embodiment, two-divided stator blades  72   a  and  72   b  and two-divided stator blades  72   c  and  72   d  are overlapped to constituted the stator blades  72  at the respective stages. 
     As shown in FIG.  6 ( a ), the phase of the two-divided position of a pair of the stator blades  72   a  and  72   b  and that of the other pair of the stator blades  72   c  and  72   d  are shifted by 90° to each other to be then overlapped. It should be noted that if the phases of the divided positions of the respective pairs do not coincide with each other, the shift thereof is not limited to 90°, for example, arbitrary angle such as 30°, 45°, or 60° may be shifted. 
     FIGS.  6 ( b ) to  6 ( d ) show examples of the overlapping methods of the two pairs of the stator blades  72  according to the third embodiment of the present invention. 
     As a first method, as shown in FIG.  6 ( b ), there is employed a case in which the upper side stator blades  72   a  and  72   b  and the lower side stator blades  72   c and  72   d  are abutted at the outer ring portions  73  and the inner ring portions  74  to each other. As a result, the blades  75   a  and  75  and the blades  75   c  and  75   d  are disposed at the upper and lower sides, respectively. 
     As a second method, as shown in FIG.  6 ( c ), it is configured such that the upper stator blades  72   a  and  72   b  and the lower stator blades  75   a  and  75   b  are disposed with predetermined intervals so that the blades  75   a  and  75   b  and the blades  75   c  and  75   d  are opposedly disposed to each other. It is to be noted that the given interval between the upper and lower stator blades  72  is set on the basis of a spacer disposed between the outer ring portion  73  and the inner ring portion  74  of the stator blades  72 . 
     As a third method, as shown in FIG.  6 ( d ), the conventional stator blades shown in FIG.  11 ( a ) are used for the upper side stator blades  72   a  and  72   b.  Note that the lower stator blades  72   c  and  72   d  have no blades  75 , and ventilation holes are formed by punching out the portions for the blades  75 . 
     It should be noted that, in the first and second methods shown in FIGS.  6 ( b ) and  6 ( c ), the blades  75   a,    75   b,    75   c  and  75   d  having a length of a half of the conventional ones are used so that the length covering the upper and lower layers is identical with that of the conventional ones. 
     As described above, according to the first to third embodiments and the modification examples thereof, the rigidity of the stator blades can be improved. For that reason, the distance between the stator blades  72  and the rotor blades  62  can be made shorter than the conventional ones, thereby being capable of enhancing the down-sizing of the apparatus and the exhaust performances. 
     FIG. 7 is a cross-sectional view showing the stator blades and the rotor blades of a turbomolecular pump according to a fourth embodiment of the present invention. 
     In this embodiment, there is employed a structure in which the blades S  75  and the blades  63  that are planes of discontinuity and are the weakest portions in structure, are prevented from contacting with each other, thereby preventing damage to the the stator blades  72  and the rotor blades  62 . 
     Specifically, the length of the blades S  75  in a radial direction extends in an axial center direction so that the top end portions  76  of the blades S  75  on the center side are arranged between the rotor ring portions  64  and  64 . 
     As described above, with the top end portions  76  of the blades S  75  on the center side being arranged between the rotor ring portions  64  and  64 , even if the stator blades  72  are largely deflected, the top end portions  76  of the blades S  75  on the center side are brought into contact with a rotor ring portion  64  which is a plane of continuity of the rotor blades  62 , thereby preventing damage to the blades S  75 . 
     FIG. 8 is a cross-sectional view showing a stator blade and a rotor blade of a turbomolecular pump according to a first modification example of the fourth embodiment of the present invention. 
     In this modification example, an abutting portion  85  is provided to the inner ring portion  74  of each stator blade  72 , thereby preventing the blade S  75  and blades R  63  from contacting with each other. 
     As shown in FIG. 8, the abutting portion  85  has a substantially U-shape in cross-section, and has such a structure that the abutting portion is folded back in an opposite direction to the axial center. 
     Further, the abutting portion  85  is configured to satisfy the relation of δ1≦δ2≦x, where the distance between the upper most top end face  85   a  in the axial direction of the abutting portion  85  and its upper facing rotor ring portion  64  is prescribed as “δ2,” and the distance between the top end face of the blades S  75  and the lower end face of the blade R  63  is prescribed as “δ1.” 
     In this case, “X” means the distance between the upper most top end face  85   a  in the axial direction of the abutting portion  85  and it supper facing rotor ring portion  64  in the case where the upper most top end face  85   a  in the axial direction of the abutting portion  85  and the top end face  76  on the center side of the blades S  75  are simultaneously brought into contact with the rotor blades  62 , when the stator blades were deflected. 
     In the first modification example of the fourth embodiment of the present invention, as shown in FIG. 8, the abutting portion  85  is folded back against the axial direction to have a U-shape in cross-section. As a result, the abutting portion  85  functions as a spring, thereby being capable of absorbing the impact at the time when the upper most top end face  85  is brought into contact with the rotor ring portion  64 . 
     FIG. 9 is a cross-sectional view showing a stator blade and a rotor blade of a turbomolecular pump according to a second modification example of the fourth embodiment of the present invention. 
     In this modification example, similar to the first modification example, the abutting portion that abuts against the rotor ring portion  64  is provided to the inner ring portion  74  of the stator blades  72 . However, in the second modification example, an abutting portion  86  having a rectangular shape is provided to the inner ring portion  74  of the stator blades  72 . 
     The condition relating to the distance δ2 between the top end face  86   a  of the abutting portion  86  and its upper facing rotor ring portion  64  is similar to that of the first modification example of the fourth embodiment of the present invention. 
     FIG. 10 is a cross-sectional view showing a stator blade and a rotor blade of a turbomolecular pump according to a third modification example of the fourth embodiment of the present invention. 
     In this third modification example, the abutting portion  86  according to the second modification example of the fourth embodiment of the present invention is disposed to the inner ring portion  74 . Also, the distance between the end portion of the blades S  75  and the spacers  71  side in a radial direction and the spacers  71  is configured to be wider than the distance between the distal end of the blades R  63   a  and  63   b  and the spacers  71  so that the length of blades S  75  in a radial direction become shorter than the length of the blades R  63   a  and  63   b.    
     Further, the outer ring portion  73  of the stator blade  72  is configured to have a stepped portion between a first ring portion  87   a  on the side being sandwiched by the spacers  71  and a second ring portion  87   b  for supporting blades S  75 . The stepped portion is provided to the whole outer ring portion  73  of the stator blade  72  in a circumferential direction thereof. The length of the first ring portion  87   a  in the axial direction is set to the length so that the top face thereof is positioned between the blades R  63   a  and  63   b.    
     It should be noted that since the position of the first ring portion  87   a  held by the spacers  71  in the axial direction is moved to the upper than the conventional ones, the length of the spacers  71  is adjusted based on the shape of the outer ring portion  73  of the blade S  75 . 
     The outer ring portion  73  is configured so as to satisfy the relation of 0&lt;δ3&lt;P, where the distance between the top face of the first ring portion  87   a  and the blades R  63   a  is prescribed as s 3 . In this case, value “P” means the distance between the first ring portion  87   a  and the blades R  63   a  in the case where blades R  63   a  are brought into contact with the first ring portion  87   a  and the blades S  75 , simultaneously, when the rotor blades  62  were deflected downwardly. 
     According to the thus configured third modification example of the fourth embodiment of the present invention, when the stator blades  72  were deflected, the abutting portion  86  is brought into contact with the rotor ring portion  64 , thereby preventing the damage of the blades S  75 . 
     On the other hand, in the case where the rotor blades  62  are largely downwardly deflected, the blades R  63   a  positioned at the upper portion of the outer ring portion  73  of stator blades  72  are brought into contact with the first ring portion  87   a  of the outer ring portion  73 . In the case where the blades R  63   a  and  63   b  are largely upwardly deflected, the blades R  63   b  are brought into contact with the second ring portion  87   b.  Since the first and second ring portions  87   a  and  87   b  form the plane of continuity in a circumferential direction, even if the blades R  63  are brought into contact therewith, the damage of the blades R  63   a  and  63   b  can be prevented. 
     It should be noted that the abutting portions  85  and  86 , according to the first, second, and third modification examples of the fourth embodiment of the present invention, are provided to the whole inner ring portion  74  in a circumferential direction from one end portion to the other end portion. 
     Alternatively, the abutting portion  85  or  86  may be provided to both one end portion and the other end portion of the inner ring portion  74 . In the latter case, at least one abutting portion  85  or  86  may further be provided between one end portion and the other end portion. 
     As described above, descriptions have been made of the respective embodiments and their modification examples. However, the present invention is not limited thereto, various modifications may be adopted if such modifications fall within the scope of claim described in each claim. 
     For example, among the respective stator blades  72  described in the first, second and third embodiments of the present invention, the stator blades may be configured by a combination at least two structures. For example, the combination of the first and second embodiments allow the provisions of the rib structure (enhancement portion (reinforcement member) as well as the engagement claws for engaging one end with the other end, to the inner ring portion  74  of the stator blades  72 . 
     Further, as another modification example of the second embodiment of the present invention, such a configuration may be employed in which a concave portion in a radial direction is formed at one end face  78   a  of the two-divided inner ring portion  74   a,  and a convex portion to be fitted to the concave portion is provide to the other end face  78   b  of the two divided inner ring portion  74   b.    
     As described above, according to the first to third embodiments of the present invention, even if a large fluctuation occurred in a load of gas, since the rigidity of the stator blades  72  is improved, the deflection of the stator blades  72  is restrained. As a result, the stator blades  72  are hardly brought into contact with the rotor blades  62 . 
     Furthermore, according to the fourth embodiment of the present invention, even if the stator blades  72  and the rotor blades  62  are brought into contact with each other, before the portions that are weak in structure (the blades S  75  and the blades  63 ) are brought into contact with each other, other portions are allowed to contact with each other, thereby being capable of preventing the fatal damages of the stator blades  72  and the rotor blades  62 . 
     Even in the case where the magnetic bearing is subjected to touch down against the touch down bearing, if the structures according to the first to third embodiments of the present invention is employed, the deflection the stator blades can be prevented. If the structure according to the fourth embodiment of the present invention is used, contact between the blades can also be prevented. 
     As described above, according to the present invention, it is possible to provide the turbomolecular pump with stator blades having a structure in which deflections are not relatively occurred. The stator blades are hardly deflected, thereby being capable of narrowing the distance between the stator blades and the rotor blade. As a result, the downsizing of the turbomolecular pump may be realized and exhaust performances may be improved. 
     Further, according to the present invention, it is possible to provide a turbomolecular pump with stator blades having a structure in which even if the deflection of the stator blades occurred, since the contacts between the blades S and the blades R can be prevented, breakage of the stator blades hardly occurrs.