Abstract:
A turbo-molecular pump evacuates gas with a rotor that rotates at a high speed. The turbo-molecular pump comprises a casing, a stator fixedly mounted in the casing and having stator blades, a rotor rotatably provided in the casing and having rotor blades alternating with the stator blades, and a radial turbine blade pumping section having a spiral ridge-groove section provided on at least one of surfaces, facing each other, of the stator blade and the rotor blade. At least one of the stator blade and the rotor blade which are located at a first stage of the radial turbine blade pumping section has such a shape that at least one of the stator blade and the rotor blade is smaller in thickness in a direction of gas flow.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a turbo-molecular pump for evacuating gas with a rotor that rotates at a high speed, and more particularly to a turbo-molecular pump having a radial turbine blade pumping section in a casing.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 12 of the accompanying drawings shows a conventional turbo-molecular pump having a radial turbine blade pumping section in a casing. As shown in FIG. 12, the conventional turbo-molecular pump comprises a rotor R and a stator S which are housed in a casing  10 . The rotor R and the stator S jointly make up an axial turbine blade pumping section L 1  and a radial turbine blade pumping section L 2 . The stator S comprises a base  14 , a stationary cylindrical sleeve  16  vertically mounted centrally on the base  14 , and stationary components of the axial turbine blade pumping section L 1  and the radial turbine blade pumping section L 2 . The rotor R comprises a main shaft  18  inserted in the stationary cylindrical sleeve  16 , and a rotor body  20  fixed to the main shaft  18 .  
           [0005]    Between the main shaft  18  and the stationary cylindrical sleeve  16 , there are provided a drive motor  22 , and upper and lower radial bearings  24  and  26  provided above and below the drive motor  22 . An axial bearing  28  is disposed at a lower portion of the main shaft  10 , and comprises a target disk  28   a  mounted on the lower end of the main shaft  18 , and upper and lower electromagnets  28   b  provided on the stator side. Further, touchdown bearings  29   a  and  29   b  are provided at upper and lower portions of the stationary cylindrical sleeve  16 .  
           [0006]    With this arrangement, the rotor R can be rotated at a high speed under 5-axis active control. The rotor body  20  in the axial turbine blade pumping section L 1  has disk-like rotor blades  30  integrally provided on an upper outer circumferential portion thereof. In the casing  10 , there are provided stator blades  32  disposed axially alternately with the rotor blades  30 . Each of the stator blades  32  has an outer edge clamped by stator blade spacers  34  and is thus fixed. Each of the rotor blades  30  has a wheel-like configuration which has a hub at an inner circumferential portion thereof, a frame at an outer circumferential portion thereof, and inclined blades (not shown) provided between the hub and the frame and extending in a radial direction. Thus, the turbine blades  30  are rotated at a high speed to make an impact on gas molecules in an axial direction for thereby evacuating gas.  
           [0007]    The radial turbine blade pumping section L 2  is provided downstream of, i.e. below the axial turbine blade pumping section L 1 . In the radial turbine blade pumping section L 2 , the rotor body  20  has disk-like rotor blades  36  integrally provided on an outer circumferential portion thereof in the same manner as the axial turbine blade pumping section L 1 . In the casing  10 , there are provided stator blades  38  disposed axially alternately with the rotor blades  36 . Each of the stator blades  38  has an outer edge clamped by stator blade spacers  40  and is thus fixed.  
           [0008]    Each of the stator blades  38  is in the form of a follow disk, and as shown in FIGS. 13A and 13B, each of the stator blades  38  has spiral ridges  46  which are formed in the front and backside surfaces thereof and extend between a central hole  42  and an outer circumferential portion  44 , and spiral grooves  48  whose widths are gradually broader radially outwardly and which are formed between the adjacent ridges  46 . The spiral ridges  46  on the front surface, i.e. upper surface of the stator blade  38  are configured such that when the rotor blade  36  is rotated in a direction shown by an arrow A in FIG. 13A, gas molecules flow inwardly as shown by a solid line arrow B. On the other hand, the spiral ridges  46  on the backside surface, i.e. lower surface of the stator blade  38  are configured such that when the rotor blade  36  is rotated in a direction shown by the arrow A in FIG. 13A, gas molecules flow outwardly as shown by a dotted line arrow C. Each of the stator blade  38  is usually composed of two half segments, or three or more divided segments. The stator blades  38  are assembled by interposing the stator blade spacers  40  so that the stator blades  38  alternate with the rotor blades  36 , and then the completed assembly is inserted into the casing  10 .  
           [0009]    With the above configuration, in the radial turbine blade pumping section L 2 , a long evacuation passage extending in zigzag from top to bottom between the stator blades  38  and the rotor blades  36  is constructed within a short span in the axial direction, thus achieving high evacuation and compression performance without making the radial turbine blade pumping section L 2  long in the axial direction.  
           [0010]    In the radial turbine blade pumping section L 2 , the outer diameter D 1  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  is set to the same dimension in all stages, and the inner diameter D 2  of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  is set to the same dimension in all stages.  
           [0011]    However, in the case of the conventional turbo-molecular pump having the radial turbine blade pumping section L 2 , as shown in FIG. 14, the gap G 1  between the stator blade  38  located at the first stage in the radial turbine blade pumping section L 2  and the rotor blade  30  located immediately above this first-stage stator blade  38  and at the lowermost stage in the axial turbine blade pumping section L 1  is constant. Therefore, the cross-sectional area of the flow passage extending along the upper surface of the stator blade  38  toward the inner circumferential side of the stator blade  38 , i.e. the inner circumferential side of the radial turbine blade pumping section L 2  decreases drastically in proportion to the radius of the stator blade  38 . Consequently, the gas is prevented from flowing smoothly to the inner circumferential side of the radial turbine blade pumping section L 2  to cause stagnation of the gas. Further, when the gas turns its flow direction from the axial direction to the radial direction, the gas cannot be smoothly flowed to be stagnated, thus lowering the evacuation performance of the pump.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention has been made in view of the above drawbacks in the conventional turbo-molecular pump. It is therefore an object of the present invention to provide a turbo-molecular pump which can create smooth gas flow therein and prevent the evacuation performance from lowering.  
           [0013]    According to a first aspect of the present invention, there is provided a turbo-molecular pump comprising: a casing; a stator fixedly mounted in the casing and having stator blades; a rotor rotatably provided in the casing and having rotor blades, the rotor blades alternating with the stator blades; and a radial turbine blade pumping section having a spiral ridge-groove section provided on at least one of surfaces, facing each other, of the stator blade and the rotor blade; wherein at least one of the stator blade and the rotor blade which are located at a first stage of the radial turbine blade pumping section has such a shape that the at least one of the stator blade and the rotor blade is smaller in thickness in a direction of gas flow.  
           [0014]    With the above arrangement, at least one of the cross-sectional area of the flow passage defined between the stator blade at the first stage in the radial turbine blade pumping section and the rotor blade located immediately above this first-stage stator blade and at the lowermost stage in the axial turbine blade pumping section and the cross-sectional area of the flow passage defined between the rotor blade at the first stage in the radial turbine blade pumping section and the stator blade located immediately above this first-stage rotor blade and at the lowermost stage in the axial turbine blade pumping section is prevented from being drastically smaller in the direction of gas flow. Thus, the gas flowing from an upstream side into the radial turbine blade pumping section can be guided smoothly toward the inner circumferential side of the radial turbine blade pumping section.  
           [0015]    According to a second aspect of the present invention, there is provided a turbo-molecular pump comprising: a casing; a stator fixedly mounted in the casing and having stator blades; a rotor rotatably provided in the casing and having rotor blades, the rotor blades alternating with the stator blades; and a radial turbine blade pumping section having a spiral ridge-groove section provided on at least one of surfaces, facing each other, of the stator blade and the rotor blade; wherein an outer diameter of the rotor at its portion facing an inner circumferential surface of a stator blade at a first stage in the radial turbine blade pumping section is smaller than an outer diameter of the rotor at its portion facing an inner circumferential surface of a stator blade at any one of stages subsequent to the first stage.  
           [0016]    With this arrangement, the cross-sectional area of the flow passage in an axial direction defined between the inner circumferential surface of the stator blade at the first stage and the outer circumferential surface of the rotor at its portion facing the inner circumferential surface of this first-stage stator blade is enlarged for thereby guiding the gas toward a radial direction in flow passages upstream and downstream of the flow passage in the axial direction.  
           [0017]    According to a third aspect of the present invention, there is provided a turbo-molecular pump comprising: a casing; a stator fixedly mounted in the casing and having stator blades; a rotor rotatably provided in the casing and having rotor blades, the rotor blades alternating with the stator blades; and a radial turbine blade pumping section having a spiral ridge-groove section provided on at least one of surfaces, facing each other, of the stator blade and the rotor blade; wherein one of an inner diameter of the stator and an outer diameter of the spiral ridge-groove section at its portion facing an outer circumferential surface of a rotor blade at a first stage in the radial turbine blade pumping section is larger than an inner diameter of the stator and an outer diameter of the spiral ridge-groove section at its portion facing an outer circumferential surface of a rotor blade at any one of stages subsequent to the first stage.  
           [0018]    With this arrangement, the cross-sectional area of the flow passage in an axial direction defined between the outer circumferential surface of the rotor blade at the first stage and the inner circumferential surface of the stator at its portion facing the outer circumferential surface of this first-stage rotor blade or the outer diameter of the spiral ridge-groove section is enlarged for thereby guiding the gas toward a radial direction in flow passages upstream and downstream of the flow passage in the axial direction. Generally, the inner circumferential surface of the stator at its portion facing the outer circumferential surface of this first-stage rotor blade and the outer diameter of the spiral ridge-groove section have the same dimension.  
           [0019]    According to a fourth aspect of the present invention, there is provided a turbo-molecular pump comprising: a casing; a stator fixedly mounted in the casing and having stator blades; a rotor rotatably provided in the casing and having rotor blades, the rotor blades alternating with the stator blades; and a radial turbine blade pumping section having a spiral ridge-groove section provided on at least one of surfaces, facing each other, of the stator blade and the rotor blade; wherein an outer diameter of the rotor at its portion facing an inner circumferential surface of a stator blade at a first stage in the radial turbine blade pumping section is smaller than an outer diameter of the rotor at its portion facing an inner circumferential surface of a stator blade at any one of stages subsequent to the first stage; one of an inner diameter of the stator and an outer diameter of the spiral ridge-groove section at its portion facing an outer circumferential surface of a rotor blade at a first stage in the radial turbine blade pumping section is larger than an inner diameter of the stator and an outer diameter of the spiral ridge-groove section at its portion facing an outer circumferential surface of a rotor blade at any one of stages subsequent to the first stage.  
           [0020]    The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a cross-sectional view of a turbo-molecular pump according to a first embodiment of the present invention;  
         [0022]    [0022]FIG. 2 is an essential part of the turbo-molecular pump shown in FIG. 1;  
         [0023]    [0023]FIG. 3 is a cross-sectional view of a turbo-molecular pump according to a second embodiment of the present invention;  
         [0024]    [0024]FIG. 4 is an essential part of the turbo-molecular pump shown in FIG. 3;  
         [0025]    [0025]FIG. 5A is a horizontal cross-sectional view showing the cross-sectional area of flow passage in a portion around a stator blade and a rotor blade at a first stage of the turbo-molecular pump shown in FIG. 3;  
         [0026]    [0026]FIG. 5B is a perspective view showing a part of the flow passage shown in FIG. 5A;  
         [0027]    [0027]FIG. 6 is an enlarged view showing an essential part of a turbo-molecular pump according to a third embodiment of the present invention;  
         [0028]    [0028]FIG. 7 is an enlarged view showing an essential part of a turbo-molecular pump according to a fourth embodiment of the present invention;  
         [0029]    [0029]FIG. 8 is an enlarged view showing an essential part of a turbo-molecular pump according to a fifth embodiment of the present invention;  
         [0030]    [0030]FIG. 9 is a cross-sectional view of a turbo-molecular pump according to a sixth embodiment of the present invention;  
         [0031]    [0031]FIG. 10 is a cross-sectional view of a turbo-molecular pump according to a seventh embodiment of the present invention;  
         [0032]    [0032]FIG. 11 is a cross-sectional view of a turbo-molecular pump according to an eighth embodiment of the present invention;  
         [0033]    [0033]FIG. 12 is a cross-sectional view of a conventional turbo-molecular pump;  
         [0034]    [0034]FIG. 13A is a plan view of a stator blade shown in FIG. 12;  
         [0035]    [0035]FIG. 13B is a cross-sectional view of the stator blade shown in FIG. 13A; and  
         [0036]    [0036]FIG. 14 is an enlarged view showing a part of the turbo-molecular pump shown in FIG. 12. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    Next, turbo-molecular pumps according to embodiments of the present invention will be described below with reference to FIGS. 1 through 11. Like or corresponding parts are denoted by like or corresponding reference numerals throughout views. Those parts of turbo-molecular pumps according to the present invention which are identical to or correspond to those of the conventional turbo-molecular pump shown in FIGS. 12 through 14 are denoted by identical reference numerals, and will not be described in detail below.  
         [0038]    [0038]FIGS. 1 and 2 show a turbo-molecular pump according to a first embodiment of the present invention. In this embodiment, a turbo-molecular pump has an axial turbine blade pumping section L 1  and a radial turbine blade pumping section L 2  which comprise a turbine blade section, respectively, shown in FIGS. 12 through 14. As shown in FIGS. 1 and 2, the stator blade  38  at the first stage in the radial turbine blade pumping section L 2  has a tapered surface  38   a  which is gradually inclined downwardly in a radially inward direction to make the stator blade  38  gradually smaller in thickness so that the gap G between this first-stage stator blade  38  and the rotor blade  30  located immediately above the first-stage stator blade  38  and at the lowermost stage in the axial turbine blade pumping section L 1  is gradually larger toward the inner circumferential side of the stator blade  38 , i.e. the inner circumferential side of the radial turbine blade pumping section L 2 . Other details of the turbo-molecular pump according to the present embodiment are identical to those of the conventional turbo-molecular pump shown in FIGS. 12 through 14.  
         [0039]    According to the present embodiment, the cross-sectional area of the flow passage defined between the stator blade  38  at the first stage in the radial turbine blade pumping section L 2  and the rotor blade  30  located immediately above this first-stage stator blade  38  and at the lowermost stage in the axial turbine blade pumping section L 1  is prevented from being gradually smaller in the direction of gas flow. Thus, the gas flowing from the axial turbine blade pumping section L 1  to the radial turbine blade pumping section L 2  can be guided smoothly toward the inner circumferential side of the radial turbine blade pumping section L 2 .  
         [0040]    In this embodiment, the stator blade  38  at the first stage has a thickness which is smaller toward a radially inward direction. However, the stator blade  38  at the first stage has such a shape as to be thinner in a step-like manner so that the gap G between this first-stage stator blade  38  and the rotor blade  30  located at the lowermost stage in the axial turbine blade pumping section L 1  is larger in the step-like manner. It is important that the cross-sectional area of the flow passage per unit length in the direction of gas flow is substantially the same.  
         [0041]    [0041]FIGS. 3 and 4 show a turbo-molecular pump according to a second embodiment of the present invention. In the present embodiment, in the radial turbine blade pumping section L 2 , the outer diameter Dr 1  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at the first stage, the outer diameter Dr 2  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at the second stage, and the outer diameter Dr n  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at other stages have the relationship of Dr 1 &lt;Dr 2 &lt;Dr n . Further, the inner diameter Ds 1  of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  at the first stage, the inner diameter Ds 2  of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  at the second stage, and the inner diameter Ds n  of the stator (outer diameter of the spiral ridge-groove portion) at its portion facing the outer circumferential surface of the rotor blade  36  at other stages have the relationship of Ds 1 &gt;Ds 2 &gt;Ds n . Other details of the turbo-molecular pump according to the second embodiment are identical to those of the conventional turbo-molecular pump shown in FIGS. 12 through 14.  
         [0042]    According to the present embodiment, the cross-sectional area S 1  (see FIG. 5A) of the flow passage F 1  in an axial direction defined between the inner circumferential surface of the stator blade  38  at the first stage in the radial turbine blade pumping section L 2  and the outer circumferential surface of the rotor, and the cross-sectional area S 2  (see FIG. 5A) of the flow passage F 2  in an axial direction defined between the outer circumferential surface of the rotor blade  36  at the first stage in the radial turbine blade pumping section L 2  and the inner circumferential surface of the stator are enlarged for thereby guiding the gas smoothly toward a radial direction in flow passages upstream and downstream of the flow passage F 1  and the flow passage F 2 .  
         [0043]    Specifically, as shown in FIGS. 4, 5A and  5 B, if the stator blade  38  has the inner diameter of Dr 0  and the rotor blade  36  has the outer diameter of Ds 0 , then the above cross-sectional areas S 1  and S 2  are expressed by the following formulas:  
           S   1 ={( Dr   0 /2) 2 −( Dr   1 /2) 2 }·π 
           S   2 ={( Ds   1 /2) 2 −( Ds   0 /2) 2 }·π 
         [0044]    On the other hand, in the case where the width of the flow passage defined by the spiral groove at the inner circumferential edge is W i , the width of the flow passage defined by the spiral groove at the outer circumferential edge W 0 , the hight of the flow passage defined by the spiral groove at the inner circumferential edge H i , the hight of the flow passage defined by the spiral groove at the outer circumferential edge H 0 , and the number of ridges J, the cross-sectional area S i  of the flow passage at the inner circumferential edge and the cross-sectional area S 0  of the flow passage at the outer circumferential edge are expressed by the following formulas:  
         
       S 
       i 
       =W 
       i 
       ×H 
       i 
       ×J  
     
         
       S 
       0 
       =W 
       0 
       ×H 
       0 
       ×J  
     
         [0045]    Therefore, the outer diameter Dr 1  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at the first stage and the inner diameter Ds 1  of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  at the first stage are set to such dimensions that the cross-sectional area S 1  of the flow passage F 1  is equal to or larger than the cross-sectional area S i  of the flow passage at the inner circumferential side, and the cross-sectional area S 2  of the flow passage F 2  is equal to or larger than the cross-sectional area S 0  of the flow passage at the outer circumferential side. Thus, the stagnation of gas flow in the radial turbine blade pumping section L 2  can be avoided.  
         [0046]    If the shape of the spiral ridge-groove section on the front surface of the stator blade  38  is different from that on the backside surface of the stator blade  38 , then the cross-sectional area S 1  of the flow passage F 1  is equal to or larger than the larger of the two cross-sectional areas S 1  at the inner circumferential side. If the shape of the spiral ridge-groove section on the backside surface of the stator blade  38  is different from that on the front surface of the stator blade  38  at the next stage, then the stagnation of the gas flow in the radial turbine blade pumping section L 2  can be avoided by allowing the cross-sectional area S 2  of the flow passage F 2  to be equal to or larger than the larger of the two cross-sectional areas S 0  at the outer circumferential side.  
         [0047]    According to this embodiment, the outer diameters Dr 1 , Dr 2  and Dr n  of the rotor at their portions facing the inner circumferential surfaces of the stator blades  38  in the radial turbine blade pumping section L 2  have the relationship of Dr 1 &lt;Dr 2 &lt;Dr n . However, if the number of stages is n, the following formula should hold:  
         Dr 1 ≦Dr 2 ≦ . . . ≦Dr n  (on condition that Dr 1 =Dr 2 = . . . =Dr n  is excepted therefrom)  
         [0048]    Further, according to this embodiment, the inner diameters Ds 1 , Ds 2  and Ds n  of the stator at their portions facing the outer circumferential surfaces of the rotor blades  36  have the relationship of Ds 1 &gt;Ds 2 &gt;Ds n . However, if the number of stages is n, the following formula should hold:  
         Ds 1 ≧Ds 2 ≧ . . . ≧Ds n  (on condition that Ds 1 =Ds 2 = . . . =Ds n  is excepted therefrom)  
         [0049]    This relationship holds true for other embodiments of the present invention.  
         [0050]    [0050]FIG. 6 shows a turbo-molecular pump according to a third embodiment of the present invention. According to the third embodiment, in the radial turbine blade pumping section L 2 . the outer diameter Dr 1  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at the first stage, the outer diameter Dr 2  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at the second stage, and the outer diameter Dr n  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at other stages have the relationship of Dr 1 &lt;Dr 2 &lt;Dr n . Further, the inner diameter Ds of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  at the first stage in the radial turbine blade pumping section L 2  is set to be equal in all stages.  
         [0051]    With this arrangement, the cross-sectional area S 1  (see FIG. 5A) of the flow passage F 1  in an axial direction defined between the inner circumferential surface of the stator blade  38  at the first stage in the radial turbine blade pumping section L 2  and the outer circumferential surface of the rotor is enlarged for thereby guiding the gas smoothly toward a radial direction in flow passages upstream and downstream of the flow passage F 1 .  
         [0052]    [0052]FIG. 7 shows a turbo-molecular pump according to a fourth embodiment of the present invention. According to the fourth embodiment, in the radial turbine blade pumping section L 2 , the inner diameter Ds 1  of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  at the first stage, the inner diameter Ds 2  of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  at the second stage, and the inner diameter Ds 2  of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  at other stages have the relationship of Ds 1 &gt;Ds 2 &gt;Ds n . Further, the outer diameter Dr of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at the first stage in the radial turbine blade pumping section L 2  is set to be equal in all stages.  
         [0053]    With this arrangement, the cross-sectional area S 2  of the flow passage F 2  (see FIG. 5A) in an axial direction defined between the outer circumferential surface of the rotor blade  36  at the first stage in the radial turbine blade pumping section L 2  and the inner circumferential surface of the stator is enlarged for thereby guiding the gas smoothly toward a radial direction in flow passages upstream and downstream of the flow passage F 2 .  
         [0054]    [0054]FIG. 8 shows a turbo-molecular pump according to a fifth embodiment of the present invention. The turbo-molecular pump according to the fifth embodiment incorporates the features of the turbo-molecular pump according to the first embodiment and the features of the turbo-molecular pump according to the second embodiment. More specifically, the stator blade  38  at the first stage in the radial turbine blade pumping section L 2  has a tapered surface  38   a  which is gradually inclined downwardly in a radially inward direction to make the stator blade  38  gradually smaller in thickness so that the gap G between this first-stage stator blade  38  and the rotor blade  30  located immediately above the first-stage stator blade  38  and at the lowermost stage in the axial turbine blade pumping section L 1  is gradually larger toward the inner circumferential side of the stator blade  38 . Further, in the radial turbine blade pumping section L 2 , the outer diameter Dr 1  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at the first stage, the outer diameter Dr 2  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at the second stage, and the outer diameter Dr n  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at other stages have the relationship of Dr 1 &lt;Dr 2 &lt;Dr n . Further, the inner diameter Ds 1  of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  at the first stage, the inner diameter Ds 2  of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  at the second stage, and the inner diameter Ds n  of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade  36  at other stages have the relationship of Ds 1 &gt;Ds 2 &gt;Ds n . With this arrangement, the turbo-molecular pump according to the fifth embodiment can obtain the synergistic effect of the turbo-molecular pumps according to the first and the second embodiments.  
         [0055]    [0055]FIG. 9 shows a turbo-molecular pump according to a sixth embodiment of the present invention. In this embodiment, a turbo-molecular pump has an axial thread groove pumping section L 3  comprising cylindrical thread grooves and a radial turbine blade pumping section L 2  at the upper and lower sides thereof. Specifically, in this turbo-molecular pump, the rotor body  20  has a cylindrical thread groove section  54  having thread grooves  54   a,  and the thread groove section  54  and the casing  10  jointly make up the axial thread groove pumping section L 3  for evacuating gas by way of a dragging action of the thread grooves in the rotor R which rotates at a high speed. In the radial turbine blade pumping section L 2 , the stator blade  38  at the first stage has a tapered surface  38   a  which is gradually inclined downwardly in a radially inward direction to make the stator blade  38  gradually smaller in thickness.  
         [0056]    According to this embodiment, the axial thread groove pumping section L 3  comprising the cylindrical thread grooves functions effectively in the pressure range of 1 to 1000 Pa, and hence this turbo-molecular pump can be operated in the viscous flow range close to the atmosphere although the ultimate vacuum is low.  
         [0057]    [0057]FIG. 10 shows a turbo-molecular pump according to a seventh embodiment of the present invention. In the seventh embodiment, a turbo-molecular pump has an axial thread groove pumping section L 3  comprising cylindrical thread grooves between the axial turbine blade pumping section L 1  and the radial turbine blade pumping section L 2  which comprise a turbine blade section. Specifically, the rotor body  20  has a thread groove section  54  having thread grooves  54   a  formed in an outer circumferential surface thereof at its intermediate portion, and the thread groove section  54  is surrounded by a thread groove pumping section spacer  56 , thereby constituting the axial thread groove pumping section L 3  for evacuating gas molecules by way of a dragging action of the thread grooves in the rotor R which rotates at a high speed. In the radial turbine blade pumping section L 2 , the outer diameter Dr 1  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at the first stage, the outer diameter Dr 2  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at the second stage, and the outer diameter Dr n  of the rotor at its portion facing the inner circumferential surface of the stator blade  38  at other stages have the relationship of Dr 1 &lt;Dr 2 &lt;Dr n . Further, the inner diameter Ds 1  of the stator at its portion facing the outer circumferential surface of the rotor blade  36  at the first stage in the radial turbine blade pumping section L 2 , and the inner diameter Ds n  of the stator at its portion facing the outer circumferential surface of the rotor blade  36  at other stages have the relationship of Ds 1 &gt;Ds n . According to this embodiment, three-stage pumping structure is constructed to thus improve pumping speed of the turbo-molecular pump.  
         [0058]    [0058]FIG. 11 shows a turbo-molecular pump according to an eighth embodiment of the present invention. According to the eighth embodiment, a turbo-molecular pump has an axial turbine blade pumping section L 1  and a radial turbine blade pumping section L 2  which comprise a turbine blade section shown in FIGS. 12 through 14. As shown in FIG. 11, the rotor blade  36  at the first stage in the radial turbine blade pumping section L 2  has a tapered surface  36   a  which is gradually inclined downwardly in a radially outward direction to make the rotor blade  36  gradually smaller in thickness so that the gap between the first-stage rotor blade  36  and the stator blade  32  located immediately above the first-stage rotor blade  36  and at the lowermost stage in the axial turbine blade pumping section L 1  is gradually larger toward the outer circumferential side of the rotor blade  36 , i.e. the outer circumferential side of the radial turbine blade pumping section L 2 . Other details of the turbo-molecular pump according to the present embodiment are identical to those of the conventional turbo-molecular pump shown in FIGS. 12 through 14.  
         [0059]    According to the present embodiment, the gas flowing from the axial turbine blade pumping section L 1  to the radial turbine blade pumping section L 2  can be guided smoothly toward the outer circumferential side of the radial turbine blade pumping section L 2 .  
         [0060]    As described above, according to the above embodiments, the turbo-molecular pumps have the radial turbine blade pumping section, and the axial pumping section comprising turbine blades or thread grooves. However, the principles of the present invention are also applicable to a turbo-molecular pump having only the radial turbine blade pumping section. Further, the combination of the radial turbine blade pumping section and the axial pumping section is not limited to the above embodiments. Furthermore, although the spiral ridge-groove sections are formed in the stator blades of the stator in the embodiments, the spiral ridge-groove sections may be provided on the rotor blades of the rotor, or both of the stator blades of the stator and the rotor blades of the rotor.  
         [0061]    As described above, according to the present invention, the gas flowing from an axial direction to a radial direction can be smoothly guided, and the stagnation of the gas flow in the radial turbine blade pumping section can be avoided for thereby allowing the gas to flow smoothly and preventing evacuation performance from being lowered.  
         [0062]    Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.