Abstract:
A molecular pump has a casing having a gas inlet port and a gas discharge port, a stator disposed within the casing, a shaft disposed in the casing concentrically with the stator, a bearing rotatably supporting the shaft for undergoing rotation relative to the stator in a preselected direction of rotation, a rotor mounted on the shaft for rotation therewith in the preselected direction of rotation, and a motor for rotationally driving the shaft. A flange is connected to the casing at the gas inlet port. The flange has at least one mounting hole for receiving a fastener, a first elongated through-hole disposed in communication with the mounting hole, and a second elongated through-hole spaced apart from the first elongated through-hole in a direction toward a direction of rotation opposite to the preselected direction of rotation to define between the first and second elongated through-holes a deformable thin-wall portion for undergoing deformation due to a shock resulting from a torque acting on the casing in the preselected direction of rotation.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a molecular pump and, more particularly, to a turbo-molecular pump which is used for evacuating a vacuum vessel, for example. 
   2. Description of the Related Art 
   Molecular pumps such as turbo-molecular pumps and screw groove pumps are frequently used to evacuate vacuum vessels such as a semiconductor manufacturing system and an electron microscope which require high vacuum. 
   These molecular pumps have inlet ports provided with flanges adapted to be fixed to evacuating ports of vacuum vessels such as by bolts, respectively. An O-ring or gasket is interposed between the flange and the evacuating port of the vacuum vessel so as to keep air tightness between the molecular pump and the evacuating port. 
   Inside the molecular pump, there are provided a rotor section which is pivotally supported so as to be rotatable and which can be rotated at a high speed by a motor section, and a stator section fixed to a casing of the molecular pump. 
   In the molecular pump, the rotor section rotates at a high speed so that the rotor section and the stator section exhibit an evacuating effect. By this evacuating effect, a gas is sucked from the gas inlet port of the molecular pump and exhausted from a gas discharge port. 
   Usually, the molecular pump exhausts a gas in a molecular flow range (a range in which a vacuum degree is high so that the frequency of collision among molecules is low). To exhibit an evacuating ability in the molecular flow range, the rotor section is required to rotate at a high speed such as on the order of 30,000 revolutions per minute. 
   Meanwhile, in a case where some trouble has occurred during operation of the molecular pump so that the rotor section has collided with the stator section and the other fixed members within the molecular pump, the angular momentum of the rotor section is transmitted to the stator section, fixed members and the like so that a larger torque is instantly generated which rotates the entire molecular pump in a rotating direction of the rotor section. This torque also exerts a large stress to the vacuum vessel through the flange. 
   Thereupon, the following techniques have been proposed to mitigate such a shock. 
   [Patent-Related Reference 1] JP-A-1998-274189 
   [Patent-Related Reference 2] JP-A-1996-114196 
   Both of the techniques proposed in the patent-related reference 1 and the patent-related reference 2 are to provide a buffering mechanism at a flange disposed at a gas inlet port of a turbo-molecular pump. 
     FIG. 23  a flange having the buffering mechanism as proposed in the patent-related reference 1. 
   In  FIG. 23 , a flange  201  is provided at a gas inlet port of the turbo-molecular pump. The flange  201  is provided with a plurality of bolt holes  203  in elongated hole shapes on the same circle along an arc of the flange  201  and concentrically therewith. Contrary, the flange at the vacuum vessel side has the same outer diameter and inner diameter as the flange  201 , and is provided with bolt holes in normal shapes (having cylindrical inner peripheral surfaces) arranged on the same circle concentrically with the flange itself at the vacuum vessel side. 
   The flange  201  and the flange at the vacuum vessel side are concentrically aligned with each other, the bolts  202  are then inserted through the bolt holes of them, respectively, and nuts are threadedly fitted over these bolts and then tightened, so that the turbo-molecular pump is fixed to the vacuum vessel. 
   Upon mounting the turbo-molecular pump onto the vacuum vessel, the bolts  202  are to be fixed at the ends of the bolt holes  203  in the rotation direction of the rotor. Then, in the case of a torque being generated which rotates the turbo-molecular pump in the rotation direction of the rotor when the rotor section is broken and touches the stator section and the like, the flange  201  slides (slips) in the rotation direction of the rotor so that the shock caused by the torque in the turbo-molecular pump can be buffered. 
   Further, the patent-related reference  1  also discloses a technique where each bolt hole (of circular cross section) of the flange  201  is formed to be sufficiently larger than the outer diameter of the bolt  202 , and a buffering material is interposed between the bolt  202  and bolt hole  203 . 
   The patent-related reference 2 describes a technique for absorbing the torque caused in the turbo-molecular pump by breakage of the rotor section and the like, by plastically deforming the bolts for joining the turbo-molecular pump to the vacuum vessel into an elbowed shape. 
   To plastically deform the bolts in the above manner, the bolt holes of the flange at the turbo-molecular pump side are formed into elongated hole shapes in the rotation direction of the rotor, and a thin plate portion in a pawl shape for deforming the bolt into the elbowed shape is formed near a bottom of each elongated hole. 
   When the structure for absorbing a shock by the flange portion of the turbo-molecular pump is used identically to the techniques disclosed in the patent-related references 1, 2, the safety of the turbo-molecular pump is enhanced. Further, the mounting strength between the flange portion of the turbo-molecular pump and the flange portion of the vacuum vessel side can be then reduced as compared with a case of absence of such a buffering mechanism (i.e., when the absorbing mechanism is absent, it is required to enhance the mechanical strength of the mounting portions so as to withstand an occurring torque, and required to enhance the mounting strength), and the manufacturing cost, working cost and the like can be reduced. 
   However, the patent-related reference 1 describing the bolt holes  203  formed into the elongated hole shapes presents a problem of complicated positioning (phasing) of the bolts on the installing job site. Also, there is a disadvantage that the shock-absorbing properties are changed depending on the tightening state of the bolts. Further, there is a problem of an increased cost, in case of using a buffering material. 
   Further, in the technique described in the patent-related reference 2, the shock-absorbing properties are changed depending on the natures (material, rigidity, property relative to shearing stress, and the like) of bolts to be used. It is thus desirable to specify a bolt for mounting, in case of guaranteeing a predetermined shock-absorbing property. Unfortunately, many kinds of bolts having the same shapes and different natures are distributed, so that the distribution, mounting and the like of turbo-molecular pumps are complicated in case of specifying the combination of turbo-molecular pump and bolts which are members different from each other. Also, when bolts of types different from specified ones are used, the used bolts are likely to rupture so that the turbo-molecular pump is dropped away from the vacuum vessel. Moreover, there is another problem of an increased machining cost, due to the thin plate portion in the pawl shape machined in the elongated hole. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a molecular pump having an inexpensive and stable buffering mechanism that achieves shock-absorbing properties. 
   To achieve the above object, the present invention of a first aspect provides a molecular pump including a cylindrical casing which is provided with a gas inlet port and a gas discharge port; a stator which is formed within the casing; a shaft which is disposed concentrically with the stator; a bearing which rotatably supports the shaft so as to be rotatable relative to the stator; a rotor which is mounted on the shaft and rotates integrally with the shaft; a motor which drives and rotates the shaft; and a flange portion which is provided at the gas inlet port side of the casing and is provided with a buffering portion to be deformed by a shock due to a torque acting on the casing in the rotation direction of the rotor. 
   To achieve the above object, in the invention of a second aspect, the flange portion is provided with a plurality of bolt holes for fixing the flange portion; and the buffering portion is provided with a thin-wall portion provided adjacently to the bolt hole in a direction opposite to the rotation direction of the rotor. 
   To achieve the above object, in the invention of a third aspect, the thin-wall portion comprises a cutout section formed in an axial direction of the bolt hole. 
   To achieve the above object, in the invention of a fourth aspect, the buffering portion is constituted by an elongated hole section having a width which is directed in the radial direction of the rotor and is changed along the rotation direction of the rotor. 
   To achieve the above object, in the invention of a fifth aspect, the elongated hole section is provided with a positioning portion for positioning a bolt. 
   To achieve the above object, the invention of a sixth aspect provides a flange for connecting a gas inlet port of a molecular pump to an evacuating port of a vacuum vessel, wherein the flange includes a plurality of bolt holes for fixing the flange, and a thin-wall portion provided adjacently to the bolt hole in the rotation direction of a rotor. 
   To achieve the above object, in the invention of a seventh aspect, the flange portion is provided with a plurality of bolt holes for fixing the flange portion, and the buffering portion includes a thin-wall portion in a flat plate shape provided adjacently to the bolt hole in a direction opposite to the rotation direction of the rotor, and a through hole formed apart from the bolt hole in the direction opposite to the rotation direction of the rotor and via the thin-wall portion. 
   To achieve the above object, in the invention of an eighth aspect, the bolt hole is provided with a guiding portion for guiding a bolt inserted through the bolt hole toward a center of the thin-wall portion. 
   To achieve the above object, in the invention of a ninth aspect, the thin-wall portion has a plastic deformation strength lower than a rupture strength of a bolt inserted through the bolt hole. 
   To achieve the above object, in the invention of a tenth aspect, the thin-wall portion has a plastic deformation strength lower than a rupture strength of a bolt inserted through the bolt hole. It is enough for the plastic deformation strength that the plastic deformation strength in the direction opposite to the rotation direction of a rotor is lower than the rupture strength of the bolt. 
   To achieve the above object, in the invention of an eleventh aspect, the molecular pump further includes a washer interposed between a bolt-head of a bolt inserted through the bolt hole and the flange portion, and a portion at least touching the flange portion is existent in a region of the washer between the center of the bolt and a washer end in the rotation direction of the rotor, at a position where the bolt has been moved in the direction of the thin-wall portion by a shock caused by collision of the rotor. 
   To achieve the above object, in the invention of a twelfth aspect, the molecular pump further includes a washer interposed between a bolt-head of a bolt inserted through the bolt hole and the flange portion, and a portion at least touching the flange portion is existent in a region of the washer between the center of the bolt and a washer end in the rotation direction of the rotor, at a position where the bolt has been moved in the direction of the thin-wall portion by a shock caused by collision of the rotor. 
   To achieve the above object, in the invention of a thirteenth aspect, the molecular pump further includes a washer interposed between a bolt-head of a bolt inserted through the bolt hole and the flange portion, and a portion at least touching the flange portion is existent in a region of the washer between the center of the bolt and a washer end in the rotation direction of the rotor, at a position where the bolt has been moved in the direction of the thin-wall portion by a shock caused by collision of the rotor. 
   To achieve the above object, in the invention of a fourteenth aspect, the molecular pump further includes a washer interposed between a bolt-head of a bolt inserted through the bolt hole and the flange portion, and a portion at least touching the flange portion is existent in a region of the washer between the center of the bolt and a washer end in the rotation direction of the rotor, at a position where the bolt has been moved in the direction of the thin-wall portion by a shock caused by collision of the rotor. 
   According to the present invention, a molecular pump having an inexpensive and stable buffering mechanism that achieves shock-absorbing properties can be provided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view showing an example of configuration for mounting a molecular pump of an embodiment in accordance with the present invention to a vacuum vessel; 
       FIG. 2  is a sectional view of the molecular pump of the embodiment of the present invention, showing a cross section in the axial direction; 
       FIG. 3  is a view showing a flange of the molecular pump viewed from a gas inlet port side; 
       FIG. 4  is a view for explaining a flange in accordance with another embodiment; 
       FIG. 5  is a view for explaining a flange in accordance with a further embodiment; 
       FIG. 6  is a view for explaining a flange in accordance with still another embodiment; 
       FIG. 7  is a view for explaining a flange in accordance with a still further embodiment; 
       FIG. 8  is a view for explaining a flange in accordance with yet another embodiment; 
       FIG. 9  is a view for explaining a flange in accordance with a yet further embodiment; 
       FIG. 10  is a view for explaining a flange in accordance with another embodiment; 
       FIG. 11  is a view for explaining a flange in accordance with a further embodiment; 
       FIG. 12  is a view for explaining a flange in accordance with still another embodiment; 
       FIG. 13  is a view for explaining a flange in accordance with a still further embodiment; 
       FIG. 14  is a view for explaining a flange in accordance with yet another embodiment; 
       FIG. 15  is a view for explaining a flange in accordance with a yet further embodiment; 
       FIG. 16  is a view for explaining a flange in accordance with another embodiment; 
       FIG. 17  is a view for explaining a flange in accordance with a further embodiment; 
       FIG. 18  is a view for explaining a flange in accordance with still another embodiment; 
       FIG. 19  is a view for explaining a relationship between a plastic deformation strength of a thin-wall portion and a rupture strength of a bolt; 
       FIG. 20  is a view for explaining parameters for determining a plastic deformation strength of a thin-wall portion; 
       FIG. 21  is a view for explaining a conventional washer; 
       FIG. 22  is a view for explaining a washer of an embodiment in accordance with the present invention; and 
       FIG. 23  is a view for explaining a flange having a buffering mechanism proposed in the patent-related reference 1. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will now be described in detail with reference to  FIG. 1  through  FIG. 16 . 
   (1) Summary of Embodiments 
   In the embodiments of the present invention, a thin-wall portion is provided at a position confronting with each bolt mounting hole of a flange in a direction opposite to the rotation direction of a rotor. In a case where a shock to the whole of the molecular pump is caused by a torque due to touching a rotor section with a stator section or the like, the thin-wall portion is plastically deformed so that the energy for rotating the molecular pump is absorbed. 
   The forming patterns of the thin-wall portion are variously conceivable, and it is possible to provide a cavity portion  72  adjacent to each bolt hole  14  in a flange  61  of  FIG. 3 , for example. The cavity portion  72  is a through hole penetrating the flange  61 . Thereby, a thin-wall portion  71  is formed between the bolt hole  14  and the cavity portion  72 . 
   If a shock in the rotation direction of a rotor section is caused by rotor section breakage, etc., the flange  61  slides in the rotation direction of rotor section together with the molecular pump. Then a bolt that fixes the flange  61  to a flange of a vacuum vessel hits the thin-wall portion  71 , so that the thin-wall portion  71  is plastically deformed in the direction of arrow B. Thus, by the plastic deformation of the thin-wall portion  71 , energy for rotating the molecular pump is consumed as energy for plastically deforming the thin-wall portion  71 , so that the shock caused by the molecular pump is buffered. 
   (2) Details of Embodiments 
     FIG. 1  is a view showing an example of a configuration for mounting a molecular pump  1  of an embodiment in accordance with the present invention to a vacuum vessel  205 . 
   The molecular pump  1  is a vacuum pump which exhibits an evacuating effect by a rotor section rotating at a high speed and a fixed stator section, and which is a turbo-molecular pump, screw groove pump, or one having both structures of them. 
   The molecular pump  1  has a gas inlet port provided with a flange  61 , and is provided with a gas discharge port  19  at an exhausting side. 
   The vacuum vessel  205  constitutes a vacuum system such as a semiconductor manufacturing system or a mirror barrel of electron microscope, and is provided with a flange  62  at an evacuating port. 
   The flanges  61 ,  62  are provided with pluralities of bolt holes formed on the same positions on the same circle, respectively, concentrically with these flanges. Then bolts  65  are inserted through these bolt holes and nuts  66  are threadedly fitted over these bolts  65  and tightened, so that the molecular pump  1  is mounted and fixed to a lower portion of the vacuum vessel  205 . The gas within the vacuum vessel  205  is sucked from the gas inlet port of the molecular pump  1 , and exhausted from the gas discharge port  19 . Thereby, reaction gas or other gases for manufacturing semiconductors can be evacuated from the vacuum vessel  205 . 
   Although the molecular pump  1  is mounted to the lower portion of the vacuum vessel  205  in such a shape for hanging the molecular pump from the vacuum vessel  205  in the illustrated embodiment, the mounting position of the molecular pump  1  is not limited thereto, and it is possible to horizontally lay the molecular pump  1  and mount the same to the side portion of the vacuum vessel  205 , or to make the molecular pump  1  upside-down and to mount the gas inlet port thereof to the upper portion of the vacuum vessel  205 . 
   Further, a valve for regulating a flow rate of an evacuated gas may be provided between the evacuating port of the vacuum vessel  205  and the gas inlet port of the molecular pump  1 . 
   Generally, the gas discharge port  19  is connected to a roughing vacuum pump such as a rotary pump. 
     FIG. 2  is a sectional view of the molecular pump  1  of the embodiment of the present invention, showing a cross section in the axial direction. 
   In this embodiment of the present invention, a molecular pump of a so-called hybrid vane type will be explained as an example, which comprises a turbo-molecular pump section and a screw groove pump section. 
   A casing  16  constituting an armoring body of the molecular pump  1  is in a cylindrical shape, and constitutes a frame of the molecular pump  1  together with a disk-shaped base  27  provided at a bottom of the casing  16 . Structures for causing the molecular pump  1  to exhibit an evacuating function are housed within the casing  16 . 
   The structures exhibiting the evacuating function are generally constituted by a rotor section  24  pivotally supported so as to be rotatable and a stator section fixed to the casing  16 . From a standpoint of a pump type, a gas inlet port  6  side is constituted by a turbo-molecular pump section, and the gas discharge port  19  side is constituted by a screw groove pump section. 
   The rotor section  24  is constituted by rotor vanes  21  provided at the gas inlet port  6  (turbo-molecular pump section) side, a cylindrical member  29  provided at the gas discharge port  19  (screw groove pump section) side, and a shaft  11  and the like. Each rotor vane  21  is constituted by blades installed to radially extend from the shaft  11  so as to be inclined through a predetermined angle from a plane perpendicular to the axis of the shaft  11 , and these rotor vanes  21  are formed in a plurality of stages in the axial direction of the turbo-molecular pump section. 
   The cylindrical member  29  is a member having an outer peripheral surface in a cylindrical shape, and constitutes the rotor section  24  of the screw groove pump section. 
   The shaft  11  is a columnar member constituting an axis of the rotor section  24 , and a component comprising the rotor vanes  21  and cylindrical member  29  is screwed to an upper end of the shaft  11  by bolts  25 . 
   A permanent magnet is fixed to an outer peripheral surface of the shaft  11  at a substantially central portion in the axial direction, and constitute a rotor of a motor section  10 . The magnetic poles around an outer periphery of the shaft  11  formed by this permanent magnet are an N pole over a half circumference of the outer peripheral surface and an S pole over the remaining half circumference. 
   Further, those portions of magnetic bearing portions  8 ,  12  at the rotor section  24  side which pivotally support the shaft  11  in the radial direction are formed at the gas inlet port  6  side and gas discharge port  19  side relative to the motor section  10  of the shaft  11 , and a portion of a magnetic bearing portion  20  at the rotor section  24  side which pivotally supports the shaft  11  in the axial direction (thrust direction) is formed at a lower end of the shaft  11 . 
   Those portions at the rotor side of displacement sensors  9 ,  13  are formed near the magnetic bearing portions  8 ,  12 , respectively, so as to detect a displacement of the shaft  11  in the radial direction. 
   Those portions of the magnetic bearing portions  8 ,  12  and displacement sensors  9 ,  13  at the rotor side are constituted by steel plates laminated in the rotational axial direction of the rotor section  24 . This is to prevent occurrence of eddy current in the shaft  11  due to magnetic fields generated by coils constituting those portions of the magnetic bearing portions  8 ,  12  and displacement sensors  9 ,  13  at the stator side. 
   The rotor section  24  as described above is formed of a metal such as stainless steel or aluminum alloy. 
   The stator section is formed at an inner periphery side of the casing  16 . This stator section is constituted by stator vanes  22  provided at the gas inlet port  6  (turbo-molecular pump section) side, a screw groove spacer  5  provided at the gas discharge port  19  (screw groove pump) side, and the like. 
   Each stator vane  22  is constituted by blades extending from the inner peripheral surface of the casing  16  toward the shaft  11  so as to be inclined through a predetermined angle from a plane perpendicular to the axis of the shaft  11 , and these stator vanes  22  are formed in a plurality of stages in the axial direction of the turbo-molecular pump section alternately with the rotor vanes  21 . The stator vanes  22  at the stages are separated from one another by spacers  23  in cylindrical shapes. 
   The screw groove spacer  5  is a columnar member having an inner surface provided with spiral grooves  7 . The inner peripheral surface of the screw groove spacer is opposed to an outer peripheral surface of the cylindrical member  29  with a predetermined clearance (gap). The direction of the spiral groove  7  formed on the screw groove spacer  5  is directed toward the gas discharge port  19  when a gas is transferred within the spiral groove  7  in the rotation direction of the rotor section  24 . The depth of the spiral groove  7  becomes shallower toward the gas discharge port  19 , so that the gas transferred within the spiral groove  7  is more compressed as the gas approaches the gas discharge port  19 . 
   The stator section is formed of a metal such as stainless steel or aluminum alloy. 
   The base  27  is a disk-shaped member, and a stator column  18  in a cylindrical shape concentrical with the rotational axis of the rotor is mounted at a radial center of the base  27  in the direction of the gas inlet port  6 . 
   The stator column  18  supports those portions of the motor section  10 , magnetic bearing portions  8 ,  12 , and displacement sensors  9 ,  13  at the stator side. 
   In the motor section  10 , stator coils of a predetermined number of poles are equidistantly disposed on the inner periphery side of the stator, so that a rotating magnetic field can be generated around the magnetic poles formed at the shaft  11 . Further, a collar  49  which is a cylindrical member formed of a metal such as stainless steel is disposed at the outer periphery of the stator coils, so as to protect the motor section  10 . 
   The magnetic bearing portions  8 ,  12  are constituted by coils arranged at 90° intervals around the rotation axis. Further, the magnetic bearing portions  8 ,  12  attract the shaft  11  by the magnetic fields generated by these coils, so as to magnetically levitate the shaft  11  in the radial direction. 
   The magnetic bearing portion  20  is formed at the bottom of the stator column  18 . The magnetic bearing portion  20  is constituted by a disk protruded from the shaft  11  and coils disposed above and under the disk, respectively. The magnetic fields generated by these coils attract the disk so that the shaft  11  is magnetically levitated in the radial direction. 
   The gas inlet port  6  of the casing  16  is provided with the flange  61  protruded toward the outer periphery side of the casing  16 . The flange  61  is provided with the bolt holes  14  for inserting the bolts  65  therethrough, respectively, and a groove  15  for fitting therein an O-ring for holding air tightness relative to the flange  62  at the vacuum vessel  205  side. The flange  61  is provided with a mechanism for buffering a shock to be caused by the molecular pump  1  in the rotation direction of the rotor section  24 . This mechanism will be described later in detail. 
   The molecular pump  1  constituted in the above manner operates as follows, so as to evacuate a gas from the vacuum vessel  205 . 
   Firstly, the magnetic bearing portions  8 ,  12 ,  20  magnetically levitate the shaft  11 , so that the rotor section  24  is pivotally supported in a space in a non-touching manner. 
   Next, the motor section  10  operates so as to rotate the rotor in a predetermined direction. The rotational speed is on the order of 30,000 revolutions per minute, for example. In the embodiment of the present invention, the rotation direction of the rotor section  24  is a clockwise direction when viewed in a direction of arrow A in  FIG. 2 . It is also possible to constitute the molecular pump  1  so as to rotate in the counterclockwise direction. 
   When the rotor section  24  rotates, the gas is sucked from the gas inlet port  6  by the action of the rotor vanes  21  and stator vanes  22 , and is compressed as the gas advances to the lower stages. 
   The gas compressed at the turbo-molecular pump section is further compressed at the screw groove pump section, and is exhausted from the gas discharge port  19 . 
     FIG. 3  is a view showing the flange  61  viewed from the direction of arrow A in  FIG. 2 . To simplify the view, the groove  15  of the O-ring and the internal structure of the molecular pump  1  are not shown. 
   As shown, the flange  61  is provided with the plurality of bolt holes  14  at predetermined intervals on the same circle concentrically with the flange  61  itself. 
   Each bolt hole  14  is in an elongated hole shape in the rotation direction of the rotor section  24  and in a substantially wedge shape such that the width of the hole at the end in the rotation direction of the rotor section  24  is wider and is conversely narrowed toward the other end in the opposite direction. 
   The end of each bolt hole  14  in the rotation direction of the rotor section  24  is in an arcuate shape analogous to the bolt  65  such that the bolt  65  can be inserted thereinto with a predetermined clearance, and the bolt  65  is inserted into this end. 
   Since the width of the bolt hole  14  becomes narrower toward the other end of this hole, the outer diameter of the bolt  65  hits an inner wall of the bolt hole  14  and the bolt  65  is inhibited from sliding into the other end direction even when the bolt  65  is intended to be slid into the other end direction. Thereby, the bolt  65  is positioned at the end of the bolt hole  14 . 
   Each cavity portion  72  penetrating the flange  61  along the elongated direction is provided at the outer periphery side of the bolt hole  14 , and thereby the thin-wall portion  71  is formed between the bolt hole  14  and cavity portion  72 . 
   The thickness of the thin-wall portion  71  is on the order of 0.5 millimeters to several millimeters, depending on the material, thickness and the like of the flange  61 . 
   Next, the buffering function of the flange  61  as constituted above will be explained below. 
   When the rotor section  24  collides with the stator section by rupture of the rotor section  24  or the like in the molecular pump  1  during a high speed rotation of the rotor section, a shock due to torque is caused that intends to rotate the whole of the molecular pump  1  in the rotation direction of the rotor section  24 . 
   Then, the flange  61  tends to slide and rotate in the rotation direction of the rotor section  24  with respect to the flange  62  of the vacuum vessel  205 . 
   Contrary, the position of each bolt  65  is fixed with respect to the flange  62  (the bolt hole of the flange  62  is assumed to be a normal circular one), each bolt  65  relatively moves in the other end direction within the bolt hole  14  as the flange  61  rotates into the rotation direction of the rotor section  24 . 
   Since the width of the bolt hole  14  becomes narrower toward the other end direction, the side wall of the inner periphery of the bolt hole  14  hits the bolt  65  so that the thin-wall portion  71  is pushed in the direction of arrow B (i.e., a direction oriented to the outer radial direction from a tangential direction opposite to the rotation direction of the rotor section  24 ) and plastically deformed. 
   The energy for rotating the molecular pump  1  is consumed as energy for plastically deforming the thin-wall portion  71  during the plastic deformation of the thin-wall portion  71 , so that the shock is mitigated. 
   In the embodiment of the present invention as described above, the flange  61  is provided with the buffering mechanism constituted to be plastically deformed by a torque for rotating the molecular pump  1 , so that the safety is enhanced, even if the rotor section  24  were ruptured, and even when trouble has occurred such that deposits accumulated at the rotor section  24 , stator section and the like upon evacuating the reaction gas from the semiconductor manufacturing system collides with each other within the molecular pump  1 . 
   It is also possible to fill a rubber or other elastic member in the bolt hole  14  and cavity portion  72 , as a buffering member. 
   Further, it is constitutionally possible to form the bolt hole  14  of the flange  61  into a normal screw hole of a circular cross section while providing the bolt hole of the flange  62  at the vacuum vessel  205  side with a thin-wall portion, or to provide thin-wall portions at the bolt holes of both of the flanges  61 ,  62 , respectively. 
   When a thin-wall portion is provided at the flange  62  at the vacuum vessel  205  side, the thin-wall portion is provided at a position confronting with each bolt hole of the flange  62  in the rotation direction of the rotor. 
     FIG. 4  is a view for explaining a flange  61   a  in accordance with another embodiment of the flange  61 . 
   The flange  61   a  has a cutout section  73 , instead of the cavity portion  72  of the flange  61 . 
   When a large torque in the rotation direction of the rotor section  24  in the molecular pump  1  is caused and the molecular pump rotates due to breakage of the rotor section  24 , for example, the bolt  65  hits the thin-wall portion  71  so that the thin-wall portion  71  is plastically deformed in the direction of arrow B. Thereby, energy for rotating the molecular pump  1  is absorbed, so that the shock caused in the molecular pump  1  is mitigated. 
   The machining of the cutout section  73  is easier than the cavity portion  72 , so that the manufacturing cost can be reduced. 
     FIG. 5  is a view for explaining a flange  61   b  in accordance with a further embodiment of the flange  61 . 
   In the flange  61   b , each bolt hole  14  is a normal one having a circular cross section, and adapted to position the bolt  65 . Further, a cavity portion  77  is formed at a predetermined distance from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24 . The cavity portion  77  is a through hole having a circular cross section of an inner diameter smaller than that of the bolt hole  14 . The portion  76  between the bolt hole  14  and cavity portion  77  constitutes a thin-wall portion  76 . 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   b  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  76  and cavity portion  77  are pressurized and plastically deformed in a direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
     FIG. 6  is a view for explaining a flange  61   c  in accordance with still another embodiment of the flange  61 . 
   In the flange  61   c,  the bolt hole  14  is a normal bolt hole having a circular cross section. Further, a cavity portion  79  is formed at a predetermined distance from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24 . The cavity portion  79  is a through hole having a circular cross section of an inner diameter smaller than that of the bolt hole  14 . Moreover, a cavity portion  80  is formed at a predetermined distance from the cavity portion  79  in the direction opposite to the rotation direction of the rotor section  24 . The cavity portion  80  is a through hole having a circular cross section of an inner diameter smaller than that of the cavity portion  79 . 
   The portions between the bolt hole  14  and cavity portion  79  and between the cavity portion  79  and cavity portion  80  constitute thin-wall portions, respectively. 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   c  constituted as described above so that the molecular pump  1  is rotated, these thin-wall portions and the cavity portions  79 ,  80  are pressurized and plastically deformed in a direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
     FIG. 7  is a view for explaining a flange  61   d  in accordance with a still further embodiment of the flange  61 . 
   In the flange  61   d,  each bolt hole  14  is a normal one having a circular cross section. Further, a cavity portion  83  is formed at a predetermined distance from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24 . The cavity portion  83  is a through hole having a circular cross section of the same inner diameter as that of the bolt hole  14 . The portion between the bolt hole  14  and cavity portion  83  constitutes a thin-wall portion  82 . 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   d  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  82  and cavity portion  83  are pressurized and plastically deformed in a direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
   The inner diameter of the cavity portion  83  can be constituted to be larger than that of the bolt hole  14 . 
     FIG. 8  is a view for explaining a flange  61   e  in accordance with yet another embodiment of the flange  61 . 
   In the flange  61   e,  each bolt hole  14  is a normal one having a circular cross section. Further, a cavity portion  86  is formed at a predetermined distance from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24 . The cavity portion  86  is a through hole having a circular cross section of the same inner diameter as that of the bolt hole  14 . In this embodiment, the distance between centers of the bolt hole  14  and cavity portion  86  is set to be shorter than the sum of the radii of the bolt hole  14  and cavity portion  86 , and the bolt hole  14  and cavity portion  86  are interconnected with each other. 
   Further, the constricted region between the bolt hole  14  and cavity portion  86  forms a thin-wall portion  85 . 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   e  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  85  is pressurized and plastically deformed in a direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
     FIG. 9  is a view for explaining a flange  61   f  in accordance with a yet further embodiment of the flange  61 . 
   In the flange  61   f,  each bolt hole  14  is a normal one having a circular cross section. Further, a cavity portion  89  constituted by a through hole having a crescent cross section is formed at a predetermined distance from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24 . The crescent cross section is arranged so that its concave portion is confronted with the bolt hole  14  via thin-wall portion  88 . Further, the R-shape of the concave portion is set so that the thickness of the thin-wall portion  88  becomes substantially uniform. 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   f  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  88  is pressurized and plastically deformed in a direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
     FIG. 10  is a view for explaining a flange  61   g  in accordance with another embodiment of the flange  61 . 
   In the flange  61   g,  each bolt hole  14  is a normal one having a circular cross section. Further, a cavity portion  92  is formed at a predetermined distance from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24 . 
   The cavity portion  92  is constituted by three through holes each having a circular cross section. Two of these through holes have the same inner diameters, and are formed to be separated from the bolt hole  14  via thin-wall portion  91  and aligned in the radial direction. Thereby, a point intermediate between these two through holes is set to be positioned on a circle passing through the center of the bolt hole  14  and concentric with the flange  61   g.  Further, the remaining one through hole is formed at an opposite side to the rotation direction of the rotor section  24  and beyond the former two through holes, and the center of this remaining through hole is positioned on the circle passing through the center of the bolt hole  14  and concentric with the flange  61   g.    
   In such a cavity portion  92 , the thin-wall portion  91  is formed between the cavity portion  92  and bolt hole  14 , and thin-wall portions are further formed between the three through holes constituting the cavity portion  92 . 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   g  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  91  as well as the thin-wall portions between the three through holes constituting the cavity portion  92  are pressurized and plastically deformed in a direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
     FIG. 11  is a view for explaining a flange  61   h  in accordance with a further embodiment of the flange  61 . 
   In the flange  61   h,  each bolt hole  14  is a normal one having a circular cross section. Further, a cutout section  95  is formed apart from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24  and via thin-wall portion  94 . 
   The cutout section  95  is formed in a direction (direction of arrow D in  FIG. 11 ) oriented from the thin-wall portion  94  to an outer radial direction from a tangential direction of a circumference of the flange  61   h.    
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   h  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  94  is pressurized and plastically deformed in the direction of arrow D by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
     FIG. 12  is a view for explaining a flange  61   i  in accordance with still another embodiment of the flange  61 . 
   In the flange  61   i,  each bolt hole  14  is a normal one having a circular cross section. Further, a cutout section  98  is formed apart from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24  and via thin-wall portion  97 . 
   The cutout section  98  is formed to hollow out an outer periphery of the flange  61   i  in the radial direction, via thin-wall portion  97 . 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   i  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  97  is pressurized and plastically deformed in a direction of arrow C by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
     FIG. 13  is a view for explaining a flange  61   j  in accordance with a still further embodiment of the flange  61 . 
   In the flange  61   j,  the bolt hole  14  is a normal bolt hole having a circular cross section. Further, a cavity portion  101  is formed apart from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24  and via thin-wall portion  100 . 
   The cavity portion  101  is formed of two arcuate through holes. These two through holes are circumferentially juxtaposed with each other and arranged at predetermined distances from the bolt hole  14 , respectively, such that the concave portions of these through holes confront with the bolt hole  14 . Thereby, a thin-wall portion  102  is also formed between the two through holes. 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   j  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  100  and thin-wall portion  102  are pressurized and plastically deformed in a direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
     FIG. 14  is a view for explaining a flange  61   k  in accordance with yet another embodiment of the flange  61 . 
   In the flange  61   k,  the bolt hole  14  is a normal bolt hole having a circular cross section. Further, a cavity portion  104  is formed apart from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24  and via thin-wall portion  103 . 
   The cavity portion  104  is formed of two through holes in elongated hole shapes. These two through holes are circumferentially juxtaposed with each other and arranged at predetermined distances from the bolt hole  14 , respectively, such that those sides having the larger curvatures of the elongated holes confront with the bolt hole  14 . Thereby, a thin-wall portion  105  is also formed between the two through holes. 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   k  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  103  and the thin-wall portion  105  formed between the two through holes are pressurized and plastically deformed in a direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
     FIG. 15  is a view for explaining a flange  61   l  in accordance with a yet further embodiment of the flange  61 . 
   In the flange  61   l,  the bolt hole  14  is a normal bolt hole having a circular cross section. Further, a cavity portion  109  is formed apart from the bolt hole  14  in the direction opposite to the rotation direction of the rotor section  24  and via thin-wall portion  113 . 
   The cavity portion  109  is constituted by through holes  110 ,  111 ,  112  having circular cross sections, respectively. The through hole  111  and through hole  110  are formed at inner and outer peripheral sides, respectively, and the through hole  112  is formed between the through holes  110 ,  111 . 
   The distance between centers of the through hole  110  and through hole  112  is set to be smaller than the sum of the radii of the through holes  110 ,  112  so that the through holes  110 ,  112  are continuous with each other. 
   Similarly, the distance between centers of the through hole  111  and through hole  112  is set to be smaller than the sum of the radii of the through holes  111 ,  112  so that the through holes  111 ,  112  are continuous with each other. Although the inner diameter of the through hole  112  is set to be larger than those of the through holes  111 ,  110  concerning the cavity portion  109 , all of these through holes may have the same inner diameters, and the inner diameter of the through hole  112  may be smaller than those of the through holes  110 ,  111 . 
   Further, the center of the through hole  112  is positioned apart from the centers of the through holes  110 ,  111  in a direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ). Thus, a thin-wall portion  113  formed between the cavity portion  109  and bolt hole  14  is made convex in the direction of arrow C. 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   l  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  113  is pressurized and plastically deformed in the direction of arrow C (the direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 . Thereby, the shock is absorbed. 
   The cavity portion  109  can be formed by simply forming through holes at three positions by a milling machine, for example, so that machining thereof is easy. 
     FIG. 16(   a ) is a view for explaining a flange  61   m  in accordance with another embodiment of the flange  61 .  FIG. 16(   b ) is an enlarged view near the bolt hole  14 . 
   In the flange  61   m,  the bolt hole  14  is a normal bolt hole having a circular cross section. 
   Further, an elongated hole  119  which is a through hole in an elongated hole shape in the radial direction is formed in a direction of arrow C of the bolt hole  14  (a direction opposite to the rotation direction of the rotor section  24 ) and at a position having a distance from the center of the bolt hole  14  which distance is shorter than the inner diameter of the bolt hole  14 . Thereby, the bolt hole  14  is continuous with the elongated hole  119  in the direction of arrow C. The inner diameter of the elongated hole  119  in the elongated direction is set to be larger than the inner diameter of the bolt hole  14 . 
   Further, the position of the elongated hole  119  in the direction C is set so that an arc drawn by extending the inner diameter of the bolt hole  14  within the elongated hole  119  is tangent to an inner peripheral surface of the elongated hole  119 . Then, elongated holes  115 ,  116  in the shapes analogous to the elongated hole  119  are formed in the direction C of the elongated hole  119 , and a thin-wall portion  117  is formed between the elongated hole  119  and elongated hole  115 . Further, a thin-wall portion  118  is formed between the elongated hole  115  and elongated hole  116 . 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   m  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  117  is pressurized and plastically deformed in the direction of arrow C (the direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 . Then, the plastically deformed thin-wall portion  117  further presses and plastically deforms the thin-wall portion  118 . Thereby, the thin-wall portions  117 ,  118  are plastically deformed so that the shock is absorbed. 
   Although the buffering mechanism has been constituted by providing a plastically deformable thin-wall portion(s) near the bolt hole  14  of the flange  61  as described above, the shapes of the thin-wall portions are not limited to the embodiments described above and various configurations are additionally conceivable. 
   Further, although the molecular pump  1  is of the hybrid vane type constituted by the turbo-molecular pump section and the screw groove pump section, the type of the molecular pump  1  is not limited thereto, and may be of a full vane type of turbo-molecular pump in which the pump is wholly constituted by stator vanes and rotor vanes from the gas inlet port  6  side up to the gas discharge port  19  side. 
   According to the embodiments of the present invention as explained above, the following effects can be obtained. 
   (1) The shock in the rotation direction of the rotor section  24  can be effectively absorbed, by such a simple structure that cavity portions or cutout sections are provided near the bolt hole  14  so as to form the thin-wall portion(s). 
   (2) The structure is simple, so that the manufacturing is inexpensive. 
   (3) Since the buffering mechanism is constituted at the flange  61 , use can be made irrespectively of the internal structure of the molecular pump  1 . 
   (4) Since the flange  61  is provided with the buffering mechanism, the joining portions between the molecular pump  1  and vacuum vessel  205  can withstand the practical use even when the strength of these joining portions are weaker than the conventional one. Thus, it is possible to reduce the number of bolts  65  or to use a bolt  65  of strength lower than the conventional one, for example, and additionally, it becomes unnecessary to provide a shell-like safety cover (safety cover for covering the whole of the molecular pump  1 ) so that a total cost can be reduced. 
   (5) The position of the bolt  65  within the bolt hole  14  can be readily determined, so that the workability is improved. 
   Next, an example of a buffering mechanism, which can be readily analyzed by a computer, will be explained. 
   By a recent remarkable advancement in an analysis technique utilizing a computer, it has become possible to previously calculate a buffering effect by a buffering mechanism. 
   Since a molecular pump is an expensive product, it becomes possible to restrict the number of experiments using real molecular pumps by conducting such experiments after previously conducting simulations by a computer so as to select out candidates of shapes for buffering mechanism. 
   Particularly, since a molecular pump is an expensive product, the developing cost can be reduced by conducting such simulations. 
   In the simulation, a model as a calculation target is created by setting the data such as shape, dimensions and material as parameters of the buffering mechanism, and thereafter a magnitude of shock to be caused within the molecular pump is inputted and the way in which the buffering mechanism absorbs this shock is numerically calculated. To the numerical calculation, a known theory such as a finite element method is applied. 
   After selecting out those candidates of buffering mechanism which exhibit desired effects while changing the parameters, fracture experiments of molecular pumps are actually performed and compared with the simulation results. 
   Based on the comparison results, the buffering mechanism to be actually practiced is determined. 
   In case of designing a buffering mechanism by performing simulations in the above manner, it is important to select a shape which can be easily calculated and easily machined. 
   As shapes satisfying such requirements, there are thin-wall portions formed into flat plate shapes. 
   When the thin-wall portions are in the flat plate shapes, the thickness of portions to be plastically deformed becomes uniform so that the calculation is very easy. Further, machining is also easy, and the experimental results fit well. 
   Examples of cases where thin-wall portions are formed into flat plate shapes will be described below, with reference to  FIG. 17 ,  FIG. 18 . 
     FIG. 17(   a ) is a view for explaining a flange  61   n  in accordance with a further embodiment of the flange  61 , and  FIG. 17(   b ) is an enlarged view near the bolt hole  14 . 
   In the flange  61   n,  the bolt hole  14  is a normal bolt hole having a circular cross section. 
   Further, an elongated hole  124  which is a through hole in an elongated hole shape in the radial direction is formed in a direction of arrow C of the bolt hole  14  (a direction opposite to the rotation direction of the rotor section  24 ) and at a position having a distance from the center of the bolt hole  14  which distance is shorter than the inner diameter of the bolt hole  14 . Thereby, the elongated hole  124  is continuous with the bolt hole  14  in the direction of arrow C. The inner diameter of the elongated hole  124  in the elongated direction is set to be larger than the inner diameter of the bolt hole  14 . 
   Further, the position of the elongated hole  124  in the direction C is set so that an arc drawn by extending the inner diameter of the bolt hole  14  within the elongated hole  124  is tangent to an inner peripheral surface of the elongated hole  124 . Then, an elongated hole  120  in the shape analogous to the elongated hole  124  is formed in the direction C of the elongated hole  124 , and a thin-wall portion  122  is formed between the elongated hole  124  and elongated hole  120 . 
   Since the elongated hole  124  and elongated hole  120  are parallel to each other, the thin-wall portion  122  is in a flat plate shape having a constant thickness. 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   n  constituted as described above so that the molecular pump  1  is rotated, the thin-wall portion  122  is pressurized and plastically deformed in the direction of arrow C (the direction opposite to the rotation direction of the rotor section  24 ) by the bolt  65  inserted through the bolt hole  14 , so that the shock is absorbed. 
     FIG. 18(   a ) is a view for explaining a flange  61   p  in accordance with still another embodiment of the flange  61 , and  FIG. 18(   b ) is an enlarged view near the bolt hole  14 . 
   The flange  61   p  has its buffering portion constituted by the bolt hole  14  having a circular cross section, a guiding portion  136  for guiding the bolt  65  toward a thin-wall portion  132 , and elongated holes  134 ,  130  (first and second through-holes) acting as through holes for forming the thin-wall portion  132 . 
   The bolt hole  14  is a through hole through which the bolt  65  is inserted. The inner diameter of the bolt hole  14  is set to be larger than the outer diameter of the bolt  65  by a predetermined value, and a predetermined clearance is set between an inner wall surface of the bolt hole  14  and an outer surface portion of the bolt  65 . 
   The elongated hole  134  is connected to the bolt hole  14  in the direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) via guiding portion  136 . 
   The guiding portion  136  is a gap formed in the radial direction, and this gap has a width which is set to be substantially equal to or larger than the outer diameter of the bolt  65  and smaller than the inner diameter of the bolt hole  14 . 
   When a larger torque is caused in the rotation direction of the rotor section  24  and the flange  61   p  is rotated, the bolt  65  is to be guided through the guiding portion  136  toward the center of the thin-wall portion  132 . 
   The simulation is performed on the assumption that the bolt  65  hits the center of the thin-wall portion  132 , and the bolt  65  can be guided to the position assumed by the simulation by forming the guiding portion  136 . 
   The elongated hole  130  is formed parallelly to the elongated hole  134 , in the direction of arrow C of the elongated hole  134 . The length of the elongated hole  134  in the longitudinal direction is set to be the same as the elongated hole  130 , and the thin-wall portion  132  is formed between the elongated hole  134  and elongated hole  130 . 
   The thin-wall portion  132  is formed of inner wall surfaces of the elongated hole  134  and elongated hole  130 , and constitutes a flat plate shape having a constant thickness. 
   The thickness of the thin-wall portion  132  is set by performing a simulation and experiments. 
   The length of the thin-wall portion  132  in the radial direction of the flange  61   p  is such that the side surface of the bolt  65  touches the thin-wall portion  132  at least upon plastic deformation of the guiding portion  136 . 
   Further, if the plastic deformation to be caused in the thin-wall portion  132  spreads to regions beyond the portion to be touched the bolt  65 , such regions to which the plastic deformation spreads may be formed into flat plate shapes. 
   When a large torque in the rotation direction of the rotor section  24  is caused in the molecular pump  1  using the flange  61   p  constituted as described above so that the molecular pump  1  is rotated, the bolt  65  inserted through the bolt hole  14  moves in the direction C relative to the flange  61   p.    
   At this time, the bolt  65  is guided by the guiding portion  136  and collides with the central portion of the thin-wall portion  132 . The thin-wall portion  132  is plastically deformed by this collision, and buffers the shock. 
   Thereby, the bolt  65  can be collided with the intended position (central portion) of the thin-wall portion  132  by providing the guiding portion  136  for guiding the bolt  65 , so that the thin-wall portion  132  can be plastically deformed in the same manner as calculated by the simulation. 
     FIG. 19  is a view for explaining a relationship between a plastic deformation strength of the thin-wall portion  132  and a rupture strength of the bolt  65 . 
     FIG. 19(   a ) shows the bolt hole  14  in  FIG. 18(   a ). Here, the center of the bolt  65  is assumed to be an origin O, and an x-axis is set from the origin O in a direction opposite to the rotation direction of the rotor section  24 . The thin-wall portion  132  after deformation is shown by a dotted line. 
   When the molecular pump  1  is rotated, the bolt  65  touches the thin-wall portion  132  at x=a and thereafter reaches x=b while deforming the thin-wall portion  132 . 
     FIG. 19(   b ) is a graph having an abscissa representing a moved amount of the bolt  65  and an ordinate representing a load P(x) acting on the bolt  65  as the bolt  65  moves. 
   As shown in  FIG. 19(   b ), the load P(x) starts to act on the bolt  65  at x=a, and the load gradually increases up to x=b. During this interval, mainly the thin-wall portion  132  is deformed. 
   When the bolt  65  reaches x=b, the thin-wall portion  132  hits the side surface of the elongated hole  130  and does not deform any more and thereafter the bolt  65  is deformed. During the interval where the bolt  65  moves in a +x direction while being deformed, the load P(x) increases steeply, and the bolt  65  ruptures upon reaching a fracture point. 
   In this embodiment of the present invention, since the plastic deformation strength of the thin-wall portion  132  is set to be lower than the rupture strength of the bolt  65 , the load P(x) required for the rupture of the bolt  65  is larger than the load P (x) required for deformation of the thin-wall portion  132  as described above. Thereby, the thin-wall portion  132  can be deformed to the maximum extent before the bolt  65  ruptures. Thus, it becomes possible to prevent the bolt  65  from rupturing before the thin-wall portion  132  is fully deformed, so that the thin-wall portion  132  can sufficiently exhibit the buffering effect. 
     FIG. 20  is a view for explaining parameters which determine the plastic deformation strength of the thin-wall portion  132 . 
   The plastic deformation strength of the thin-wall portion  132  is determined by a thickness t of the thin-wall portion  132 , a length L of the thin-wall portion  132 , a thickness T of the flange  61 , material of the flange  61  and the like. 
   By inputting these parameter into a simulation software, the plastic deformation strength of the thin-wall portion  132  is automatically calculated. 
   Since the rupture strength of the bolt  65  is previously known and the material and thickness T of the flange  61  are previously determined, the shape of the thin-wall portion  132  is designed within a range satisfying the conditions while varying the thickness t of the thin-wall portion  132  and the length L of the thin-wall portion  132 . 
   Next, a washer to be mounted on the bolt  65  is explained. 
   Although a case where the washer is mounted between the flange  61   p  and the bolt  65  is explained below, this can be applied to other types of flanges  61 . 
     FIGS. 21(   a ),  21 ( b ) are views for explaining a conventional washer.  FIG. 21(   a ) is a plan view, and  FIG. 21(   b ) is a cross-sectional view. 
   A washer  141  is a ring-like disk member having an outer diameter larger than that of a bolt-head of the bolt  65  and an inner diameter larger than an outer diameter of a thread portion of the bolt  65 . 
   The washer  141  is mounted on the flange  61   p  by inserting the bolt  65  therethrough, and is urged by the bolt-head onto a surface of the flange  61   p  in the mounted state. 
   The washer  141  constituted thereby is moved in the direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) together with the bolt  65  upon deformation of the thin-wall portion  132 . 
   At this time, the bolt  65  receives a force from the thin-wall portion  132 , in a direction opposite to the direction of arrow C. Thus, a force F acts on a washer end  142  at a side opposite to the direction of arrow C of the bolt hole  14 , which force F drops the washer end into the bolt hole  14 . 
   However, the washer end  142  is positioned above the bolt hole  14 , and it is impossible to generate a force for supporting the bolt  65  against the force F. 
   Thus, the washer end  142  drops into the bolt hole  14  and the bolt  65  is inclined, so that it becomes difficult to plastically deform the thin-wall portion  132  equally. 
     FIGS. 22(   a ), ( b ) are views for explaining a washer for improving the above described defect.  FIG. 22(   a ) is a plan view and  FIG. 22(   b ) is a cross-sectional view. 
   A washer  145  has a rectangular shape and is elongated in the moving direction of the flange  61   p.    
   Thereby, it becomes possible to prevent the bolt-head from dropping into the bolt hole  14  even when the bolt  65  is moved in the direction of arrow C (a direction opposite to the rotation direction of the rotor section  24 ) and the thin-wall portion  132  is plastically deformed, because the washer  145  touches the surface of the flange  61   p  in any positions from the center of the bolt-head up to a washer end  146 . 
   Thereby, it is possible to prevent the bolt  65  from inclining upon plastic deformation of the thin-wall portion  132 , so that the thin-wall portion  132  can be plastically deformed equally. 
   Thus, the thin-wall portion  132  can be plastically deformed as correctly as simulated. 
   The shape of the washer  145  is not limited to the rectangular shape, and variously conceivable depending on the shape of the bolt hole  14 . 
   For example, since the washer is to be existent between the bolt-head of the bolt  65  inserted into the bolt hole  14  and the flange  61   p,  it is enough that a portion at least touching the flange  61   p  is existent in a region of the washer  145  between the center of the bolt-head and the washer end  146  in the rotation direction of the rotor section  24 , at a position where the bolt  65  has been moved in the direction of the thin-wall portion  132  by a shock due to the torque caused in the casing  16  by collision of the rotor section  24 . 
   Alternatively, it is enough that the distance from the center of the bolt  65  up to the washer end  146  in the rotation direction of the rotor section  24  is at least larger than a length which is the sum of a distance from the center of the bolt  65  up to the end of the bolt hole  14  in the rotation direction of the rotor section  24 , and a moved amount of the bolt  65  in the direction of the thin-wall portion  132  by a shock due to a torque caused in the casing  16  by collision of the rotor section  24 . 
   Further, it is alternatively enough that the washer  145  includes a portion having a width wider than that of the bolt hole  14  toward the rotation direction of the rotor section  24  after the bolt  65  is moved. 
   As described above, the thin-wall portion constituting the buffering mechanism is constituted into the flat plate shape, so that the simulation is facilitated and machining becomes easy. 
   Thereby, the developing cost and manufacturing cost of the molecular pump provided with the buffering mechanism can be reduced. 
   Further, the plastic deformation strength of the thin-wall portion is set to be lower than the rupture strength of the bolt, so that the buffering function of the buffering mechanism can be exhibited to the maximum extent. 
   Moreover, by using the washer having its longitudinal direction coincident with the moving direction of the flange, it becomes possible to restrict the inclination of the bolt upon plastic deformation of the thin-wall portion and to plastically deform the thin-wall portion uniformly. Thereby, the excellent buffering function obtained by the simulation can be realized. 
   Although the first embodiment of the present invention has been described above, the present invention is not limited to the described embodiments and can be variously modified within a scope recited in the appended claims.