Abstract:
A flange of a molecular pump is connected to a vacuum vessel by first bolts that extend through elongate holes in the flange and are bolted to the vacuum vessel. One or more buffering members are attached to the flange by the first bolts that extend through first openings in the buffering members and by second bolts that extend through second openings in the buffering members and are bolted to the flange. In the event of malfunction of the molecular pump that generates an abnormally high torque, the buffering members plastically deform and absorb the shock energy caused by the abnormal torque thereby preventing damage to the vacuum vessel.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a molecular pump and a connecting device for connecting a vessel incorporating a rotator rotating at a high speed to another vessel. 
   2. Description of the Related Art 
   For evacuation of a vacuum vessel requiring high vacuum used in, for example, semiconductor manufacturing equipment or electron microscope equipment, a molecular pump having high evacuating performance has been used. As such a molecular pump, for example, a turbo-molecular pump, a thread groove pump, and the like are available. 
   Evacuation of the vacuum vessel is accomplished in a state in which an inlet of molecular pump is attached to an outlet of vacuum vessel with bolts, etc. 
   In the molecular pump, a rotor section is pivotally supported so as to be rotated at a high speed by the action of a motor. Also, in the molecular pump, a stator section is provided in such a manner as to be fixed to a housing. 
   The molecular pump is configured so that the rotor section and the stator section perform evacuating operation by high-speed rotation of the rotor section. By this evacuating operation, gas is sucked through the inlet of molecular pump and is exhausted through an outlet thereof. 
   Usually, the molecular pump that accomplishes evacuation in a molecular flow region carries out evacuation by rotating the rotor section at a high speed (for example, 30,000 rpm). 
   If extreme disturbance or a trouble such as deformation of rotor section or stator section occurs during the operation of molecular pump in which the rotor section rotates at a high speed in this manner, and thus the rotor section comes into contact with a fixed member such as the stator section, angular momentum (moment of momentum) of rotor section is transmitted to the housing. 
   Thereby, there is produced torque that rotates the whole of molecular pump in the rotation direction of the rotor section. This torque also gives a great stress to the vacuum vessel to which the molecular pump is attached. 
   Conventionally, a technique for easing shock caused by torque generated at such emergency time has been proposed in Patent Documents listed below.
         [Patent Document 1] Unexamined Japanese Patent Publication No. 8-114196   [Patent Document 2] Unexamined Japanese Patent Publication No. 10-274189       

   Patent Document 1 has proposed a technique in which torque generated in a turbo-molecular pump by breakage of rotor section or by other causes is absorbed by plastically deforming bolts that join the turbo-molecular pump to a vacuum vessel into a chevron shape. 
   In order to deform the bolts in this manner, bolt holes in a flange on the turbo-molecular pump side are formed in an elongated shape in the rotation direction of the rotor, and a claw-shaped thin sheet portion is formed near the bottom of elongated hole to deform the bolts into a chevron shape. 
   Patent Document 2 has proposed a technique in which shock of torque generated in the turbo-molecular pump is eased by sliding the flange in the rotation direction of the rotor. 
   Specifically, an elongated hole shaped bolt hole is formed along the arc of flange, and the turbo-molecular pump is installed to the vacuum vessel via this bolt hole. By sliding the elongated hole shaped bolt hole, the turbo-molecular pump is rotated relative to the vacuum vessel, and thereby shock energy is consumed as its rotational energy. 
   SUMMARY OF THE INVENTION 
   In the turbo-molecular pump proposed in Patent Document 1, shock is absorbed by deforming the bolts. However, in such a configuration, it is difficult to ensure a sufficient stroke necessary for absorbing shock energy. 
   Also, in the turbo-molecular pump proposed in Patent Document 2, there is a fear of being incapable of sufficiently absorbing shock energy only by the rotational energy caused by the sliding of the elongated hole shaped bolt hole. 
   Accordingly, an object of the present invention is to provide a molecular pump and a connecting device capable of effectively absorbing shock energy due to abnormal torque generated at an emergency time during malfunction of the molecular pump by using a simple construction. 
   In an invention of a first aspect, to achieve the above object, there is provided a molecular pump which is attached to a vacuum vessel to exhaust gas in the vacuum vessel, including a casing provided with an inlet and an outlet; a rotor which is pivotally supported in the casing and is provided with a gas transfer mechanism for transferring gas from the inlet to the outlet; a motor for rotating the rotor; a flange-shaped attachment portion which is formed in an end portion of the casing or in a portion opposite thereto and is formed with a fastening hole penetrating in the thickness direction and an elongated hole extending from the fastening hole in the direction opposite to the rotation direction of the rotor; a buffering member having a buffering portion, a first fixing portion which is provided at one end of the buffering portion and has a fastening hole, and a second fixing portion formed at the other end of the buffering portion; fastening means for fastening the buffering member to the casing via the fastening hole in the attachment portion and the fastening hole in the buffering member; and fixing means for fixing the second fixing portion of the buffering member to the attachment portion. 
   In an invention of a second aspect, to achieve the above object, in the invention of the first aspect, the buffering member is formed by a plurality of members having a different stress-strain characteristic. 
   The buffering member described in the invention of the second aspect may be configured by combining members, for example, having different strain before the stress reaches the maximum value. 
   In an invention of a third aspect, to achieve the above object, in the invention of the first or second aspect, the fastening means has a higher strength than the fixing means. 
   In an invention of a fourth aspect, to achieve the above object, in the invention of the first, second, or third aspect, the fixing means is arranged on the side opposite to the side on which the elongated hole is formed with respect to the fastening means. 
   The fixing means described in the invention of the fourth aspect may be arranged on the side opposite to the rotation direction of the rotor with respect to the fastening means. 
   In an invention of a fifth aspect, to achieve the above object, in the invention of the first, second, third, or fourth aspect, at least one of the fastening means and the fixing means is formed by a bolt having a head at the terminal end thereof, and the fixing portion of the buffering member which is in contact with the head of the bolt is in contact with the head of the bolt over the whole circumference thereof. 
   In an invention of a sixth aspect, to achieve the above object, there is provided a connecting device for connecting a first vessel and a second vessel, at least one of which incorporates a rotator, to each other, including a flange formed with a fastening hole which is formed in a connecting portion between one vessel of the first vessel and the second vessel and the other vessel thereof and has a fastening hole penetrating in the thickness direction and an elongated hole extending from the fastening hole in the direction such that the one vessel moves relatively when being subjected to a shock due to torque in the rotation direction of the rotator; a buffering member having a buffering portion, a first fixing portion which is provided at one end of the buffering portion and has a fastening hole, and a second fixing portion formed at the other end of the buffering portion; fastening means for fastening the first vessel to the second vessel via the fastening hole in the flange and the fastening hole in the buffering member; and fixing means for fixing the second fixing portion of the buffering member to the flange. 
   The buffering member described in the invention of the sixth aspect may be formed by a plurality of members, for example, having a different stress-strain characteristic. The fastening means may be configured so as to, for example, have a higher strength than the fixing means. Further, the fixing means may be arranged, for example, on the side opposite to the side on which the elongated hole is formed with respect to the fastening means. Also, the configuration may be such that, for example, at least one of the fastening means and the fixing means is formed by a bolt having a head at the terminal end thereof, and the fixing portion of the buffering member which is in contact with the head of the bolt is in contact with the head of the bolt over the whole circumference thereof. 
   According to the present invention, by deforming the buffering member provided in the connecting portion between the first vessel and the second vessel, there can be provided a molecular pump and a connecting device capable of effectively consuming shock energy due to torque generated when the rotator operates abnormally. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1(   a )- 1 ( b ) are views showing one example of a connection mode of a molecular pump to a vacuum vessel in accordance with a first embodiment; 
       FIG. 2(   a ) is a perspective view and  FIG. 2(   b ) a front view showing a construction of a connecting portion indicated as portion A in  FIG. 1 ; 
       FIG. 3  is a graph showing a stress-strain characteristic of a first buffering member, a second buffering member, and a composite of the first and second buffering members; 
       FIG. 4(   a ) is a sectional view taken along the line X-X′ of a connecting portion indicated as portion A in  FIG. 1  and  FIG. 4(   b ) is a perspective view of the first and second buffering members, showing a state before a shock is generated at emergency time; 
       FIG. 5(   a ) is a sectional view taken along the line X-X′ of a connecting portion indicated as portion A in  FIG. 1  and  FIG. 5(   b ) is a perspective view of the first and second buffering members, showing a state after a shock has been generated at emergency time; 
       FIG. 6(   a )- 6 ( c ) are views showing modifications of shape of a first buffering member and a second buffering member; 
       FIGS. 7(   a )- 7 ( b ) are views showing one example of a connection mode of a molecular pump to a vacuum vessel in accordance with a second embodiment; 
       FIG. 8(   a ) is a perspective view and  FIG. 8(   b ) a front view showing a construction of a connecting portion indicated as portion B in  FIG. 7 ; 
       FIG. 9(   a ) is a sectional view taken along the line Y-Y′ of a connecting portion indicated as portion B in  FIG. 7  and  FIG. 9(   b ) is a perspective view of the first and second buffering members, showing a state before a shock is generated at emergency time; 
       FIG. 10(   a ) is a sectional view taken along the line Y-Y′ of a connecting portion indicated as portion B in  FIG. 7  and  FIG. 10(   b ) is a perspective view of the first and second buffering members, showing a state after a shock has been generated at emergency time; and 
       FIG. 11  is an explanatory view of a modification of a connecting device for connecting a molecular pump to a vacuum vessel shown in the first embodiment and the second embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of a connecting device in accordance with the present invention will now be described in detail with reference to  FIGS. 1 to 11 . 
   1. First Embodiment 
     FIG. 1  is a view showing one example of a connection mode of a molecular pump to a vacuum vessel in accordance with a first embodiment. 
   A molecular pump  1  is a vacuum pump that performs an evacuating function by the evacuating operation of a rotor section R rotating at a high speed and a fixed stator section S. As the molecular pump  1 , a turbo-molecular pump, a thread groove pump, or a composite pump having a construction of both of the pumps can be used. 
   The molecular pump  1  has a cylindrical casing  3  that forms a housing together with a base  10 , and the casing  3  is provided with an inlet  8  and an outlet  12 . 
   The casing  3  contains a structure for causing the molecular pump  1  to perform the evacuating function. 
   The structure for performing the evacuating function is broadly made up of the rotatably supported rotor section R and the stator section S fixed to the casing  3 . The rotor section is rotated at a high speed by the action of a motor M. 
   That is, the casing  3  forms a vessel incorporating a rotator. 
   Also, the base  10  forms a vessel incorporating a part of a gas transfer mechanism. 
   At the end of the inlet of the casing  3 , a flange  11  is formed so as to protrude to the outer periphery side from the casing  3 . The flange  11  is used as a connecting device for connecting the molecular pump  1  to a vacuum vessel  2 . 
   The vacuum vessel  2  forms a vacuum device such as semiconductor manufacturing equipment or a lens barrel for electron microscope, and is provided with an outlet. 
   The molecular pump  1  is connected to the outlet of the vacuum vessel  2  via the flange  11  by using fastening means such as bolts. 
     FIG. 2  is a perspective view and front view showing a construction of a connecting portion indicated as portion A in  FIG. 1 . 
   The molecular pump in accordance with the first embodiment has the connecting portions indicated as portion A in  FIG. 1  provided at a plurality of places. 
   The molecular pump  1  is connected to the vacuum vessel  2  by fastening the flange  11  to the vacuum vessel  2  with bolts  41  each having a head at the terminal end thereof. The bolt  41  is fastening means for fastening the casing  3  to the vacuum vessel  2 . 
   The vacuum vessel  2  is provided with bolt holes  21  for threadedly fixing the bolts  41 . At the inner periphery of the bolt hole  21 , a thread groove is provided to fix the bolt  41 . By fixing the bolts  41  in the bolt holes  21 , the flange  11  is connected to the vacuum vessel  2 . 
   Between the bearing surface of the bolt  41  and the flange  11 , a first buffering member  31  and a second buffering member  32  are interposed. 
   The first buffering member  31  and the second buffering member  32  are lapped on each other so that the first buffering member  31  is located on the flange  11  side. 
   One end of each of the first buffering member  31  and the second buffering member  32  is fixed to the vacuum vessel  2  with the bolt  41 , and the other end thereof is fixed to the flange  11  with a bolt  42  having a head at the terminal end thereof. 
   The bolt  42  forms fixing means for fixing the first buffering member  31  and the second buffering member  32  to the flange  11  in the connecting portion. 
   The first buffering member  31  is provided, in the central portion thereof, with an elongated hole  311  of an elongated shape extending in the lengthwise direction. 
   The elongated hole  311  is formed so that the bolt  42  penetrates an end portion on the side of rotation direction of the rotor section and the bolt  41  penetrates an end portion on the opposite side. That is, both end portions in the lengthwise direction of the elongated hole  311  function as bolt holes for inserting the bolts  41  and  42 . 
   Both end portions of the first buffering member  31  including both of the end portions in the lengthwise direction of the elongated hole  311  function as fixing portions for fixing the first buffering member  31  with the bolts  41  and  42 . The fixing portion on the side of the bolt  41  is called a first fixing portion, and the fixing portion on the side of the bolt  42  is called a second fixing portion. 
   Therefore, the length in the lengthwise direction of the elongated hole  311  is determined based on an interval at which the bolt  41  and the bolt  42  are arranged. For example, the length is determined by adding the radiuses of bolts  41  and  42  to the interval at which the bolt  41  and the bolt  42  are arranged, and further by adding a clearance formed at the time of bolt penetration. 
   The elongated hole  311  is formed by a through hole provided with no thread groove. 
   The first buffering member  31  generates a tensile stress in buffering portions  312   a  and  312   b  which are formed on both sides of the elongated hole  311  and extend in the lengthwise direction. 
   In accordance with a necessary tensile stress, the material of the first buffering member  31  and the cross-sectional areas of the buffering portions  312   a  and  312   b  are determined. 
   The second buffering member  32  is made up of a washer portion  321  and a washer portion  322  provided at both ends and a buffering portion  325  provided between the washer portions  321  and  322 . 
   The washer portion  321  functions as a first fixing portion for fixing the second buffering member  32  with the bolt  41 , and the washer portion  322  functions as a second fixing portion for fixing the second buffering member  32  with the bolt  42 . The washer portion  321  is in contact with the head of the bolt  41  over the whole circumference thereof, and the washer portion  322  is in contact with the head of the bolt  42  over the whole circumference thereof. 
   The washer portion  321  and the washer portion  322  are provided with a bolt hole  323  and a bolt hole  324  for inserting the bolt  41  and the bolt  42 , respectively. 
   Each of the bolt holes  323  and  324  is a circular hole, and the diameter thereof is determined by adding a clearance at the time of bolt penetration to the diameter of the bolt  41  or the bolt  42 . 
   Each of the bolt holes  323  and  324  is formed by a through hole provided with no thread groove. 
   The buffering portion  325  is a buffering mechanism for buffering a shock by generating a tensile stress, and is formed of a member having a distortion stress at least lower than that of the washer portions  321  and  322 . 
   Therefore, when an excessive shock is generated, the buffering portion  325  reaches a deformation limit at an earlier stage than the washer portions  321  and  322 . 
   All of the washer portions  321  and  322  and the buffering portion  325  can be formed of the same member. In this case, adjustment is made by changing the thickness, width, etc. of the buffering portion  325  so that a proper stress characteristic can be obtained. 
   Here, the stress characteristic that the first buffering member  31  and the second buffering member  32  have is explained. 
     FIG. 3  is a graph showing a stress-strain characteristic of the first buffering member  31 , the second buffering member  32 , and a composite of the first and second buffering members  31  and  32 . 
   As shown in  FIG. 3 , the first buffering member  31  and the second buffering member  32  are each formed of a member having a different stress-strain characteristic. 
   The stress shown in the graph of  FIG. 3  means a force resisting to an external force generated in a member subjected to torque due to external force, i.e., a shock. In the first embodiment, since a tensile action takes place on the first buffering member  31  and the second buffering member  32 , this stress is a tensile stress. 
   For the first buffering member  31 , the stress at the early stage of strain is lower than that of the second buffering member  32 , but the strain before the stress reaches the maximum value Pa is larger than the strain of the second buffering member  32  before the stress reaches the maximum value Pb. 
   By combining members having a different stress characteristic and by utilizing the superiority of these members, a stress-stain characteristic (broken line) of a composite of the first and second buffering members  31  and  32  can be obtained. 
   As seen from the stress-stain characteristic of the composite, a high stress can be generated at the early stage of strain. Even after the stress has reached the maximum value Pb and the second buffering member has been broken, a stress can be generated by the first buffering member. 
   Thus, a high stress can be generated over a wide range of strain. Therefore, at a limited strain, energy given by a large external force, that is, energy due to a shock can be consumed. 
   The stress characteristic differs depending on the material and shape of buffering member. In order to obtain a proper stress characteristic, the first buffering member  31  and the second buffering member  32  may be formed of a member made of a resin such as a reinforced plastic, not of a metal such as aluminum, iron, or copper. 
   As shown in  FIG. 2 , the flange  11  is formed with a bolt hole  111  for inserting the bolt  41 . This bolt hole  111  is a fastening hole for the bolt  41 , which is fastening means for fastening the casing  3  to the vacuum vessel  2 , to be inserted through. 
   Further, an elongated hole  113  is formed so as to extend from the bolt hole  111  in the direction opposite to the rotation direction of the rotor. 
   The elongated hole  113  formed in the flange  11  may be configured by an elongated hole extending in both directions from the bolt hole  111 , provided that the elongated hole is at least a hole including the elongated hole  113  extending from the bolt hole  111  in the direction opposite to the rotation direction of the rotor. 
   Also, the flange  11  is formed with a bolt hole  112  for threadedly fixing the bolt  42 . The bolt hole  112  is provided on the rotor rotation direction side of the bolt hole  111 . 
   At the inner periphery of the bolt hole  21 , a thread groove is provided to fix the bolt  41 . 
   All of the bolt hole  111 , the end portion on the side opposite to the rotor rotation direction of the elongated hole  311 , and the bolt hole  323  are provided concentrically with the bolt hole  21 . 
   Also, the end portion on the rotor rotation direction side of the elongated hole  311  and the bolt hole  324  are provided concentrically with the bolt hole  112 . 
   In the first embodiment, the first buffering member  31  and the second buffering member  32  are arranged so as to be held between the bearing surface of the bolt  41  and the flange  11 . The first buffering member  31  and the second buffering member  32  may be arranged between the vacuum vessel  2  and the flange  11 . In this case, a construction is used in which gastightness can be secured in a connecting portion between the vacuum vessel  2  and the casing  3 . 
   In the first embodiment, the first buffering member  31  and the second buffering member  32  are fixed to the flange  11  with the bolt  42 . However, as means for fixing the first buffering member  31  and the second buffering member  32  to the flange  11 , for example, staking or welding may be performed. 
   In the first embodiment, the flange  11  is fixed to the vacuum vessel  2  by threadedly fixing the bolt  41  in the bolt hole  21 . However, for example, in the case where a flange portion like the flange  11  is provided at the outlet of the vacuum vessel  2 , a through hole is formed in this flange portion of the vacuum vessel  2 , and the casing  3  may be fixed to the flange portion of the vacuum vessel  2  by using a bolt and a nut via this through hole and the bolt hole  111 . 
   In this case, the bolt and the nut function as fastening means for fastening the casing  3  to the vacuum vessel  2 . 
   Also, the casing  3  may be fastened to the vacuum vessel  2  by using nuts installed from both directions without the use of the bolt having a head. In this case, the nut in contact with the second buffering member serves as the head in contact with the buffering member. 
   Next, the buffering function in the connecting portion thus constructed is explained. 
   If during the operation of the molecular pump  1 , that is, at the time of high-speed rotation of the rotor section, extreme disturbance or a trouble such as deformation of rotor section or stator section or other malfunction occurs, and thus the rotor section comes into contact with a fixed member such as the stator section, a shock is generated due to torque that tends to rotate the whole of the molecular pump  1  in the rotation direction of the rotor section. 
   The shock due to this torque also gives a high stress to the vacuum vessel  2  connected to the molecular pump  1 . 
     FIG. 4  is a sectional view taken along the line X-X′ of a connecting portion indicated as portion A in  FIG. 1 , showing a state before a shock is generated at emergency time. 
   Also,  FIG. 5  is a sectional view taken along the line X-X′ of a connecting portion indicated as portion A in  FIG. 1 , showing a state after a shock has been generated at emergency time. 
   If a shock is generated due to abnormal torque that tends to rotate the whole of the molecular pump  1  in the rotation direction of the rotor section, the flange  11  slips and rotates in the rotation direction of the rotor section with respect to the vacuum vessel  2  due to this shock. 
   At this time, since the position of the bolt  41  is fixed by the bolt hole  21  of the vacuum vessel  2 , the flange  11  moves in the rotation direction of the rotor section along the elongated hole  113 . 
   That is, the vacuum vessel  2  moves relatively in the opposite direction with respect to the flange  11 . Therefore, the elongated hole  113  is formed so as to extend from the bolt hole  111  in the direction such that the vacuum vessel  2  moves relatively with respect to the flange  11 . 
   The bolt  42  fixed to the flange  11  moves in the rotation direction of the rotor section. 
   At this time, the bolt  42  moves in the rotation direction of the rotor section while applying a force to the end portion on the rotor rotation direction side of the elongated hole  311  and the bolt hole  324 . Also, a force equivalent to the force applied by the bolt  42  is applied to the end portion on the side opposite to the rotation direction of the rotor section of the elongated hole  311  and the bolt hole  323  in the direction opposite to the rotation of the rotor section by the bolt  41 . 
   The buffering portions that are formed at both sides of the elongated hole  311  and extend in the lengthwise direction and the buffering portion  325  are pulled by the force applied by the bolt  41  and the bolt  42 , so that a tensile stress acts (is generated). If the tensile stress acts, the first buffering member  31  and the second buffering member  32  are plastically deformed. 
   Specifically, for the first buffering member  31 , since the buffering portions that are formed on both sides of the elongated hole  311  and extend in the lengthwise direction transform and elongate, the length in the lengthwise direction of the elongated hole  311  increases, and accordingly the thickness decreases. 
   For the second buffering member  32 , the stress in the buffering portion  325  reaches the maximum value Pb, and hence the buffering portion  325  transforms and breaks at a position near the center thereof. 
   In the process in which the first buffering member  31  and the second buffering member  32  are transformed and plastically deformed, the energy that rotates the molecular pump  1  is consumed or absorbed by the first buffering member  31  and the second buffering member  32 , whereby a shock is eased. 
     FIGS. 6(   a ) to  6 ( c ) are views showing modifications of shape of the first buffering member  31  and the second buffering member  32 . 
   The first buffering member  31  and the second buffering member  32  can be formed of a member having the shape shown in  FIGS. 6(   a ) to  6 ( c ). 
   The buffering member shown in  FIG. 6(   a ) is formed by washer portions  51  and  52  provided at both ends and a buffering portion  53  provided between the washer portions  51  and  52 . 
   The washer portions  51  and  52  function as fixing portions for fixing the buffering member with the bolts  41  and  42 , respectively. Also, the washer portions  51  and  52  are in contact with the heads of the bolts over the whole circumference thereof. 
   The washer portions  51  and  52  are each provided with a bolt hole  511  and a bolt hole  521  for inserting the bolts  41  and  42 , respectively. 
   Each of the bolt holes  511  and  521  is a circular hole, and the diameter thereof is determined by adding a clearance at the time of bolt penetration to the diameter of the bolt  41  or the bolt  42 . 
   Each of the bolt holes  511  and  521  is formed by a through hole provided with no thread groove. 
   The buffering portion  53  is a buffering mechanism for buffering a shock by generating a tensile stress, and is formed into a shape having a distortion stress at least lower than that of the washer portions  51  and  52 . 
   The buffering portion  53  has a construction such that shock energy is absorbed easily. Specifically, the buffering portion  53  is formed in a band shape such that the width thereof is narrower than that of the washer portions  51  and  52 . 
   That is, the buffering member shown in  FIG. 6(   a ) has, at both ends thereof, the washer portions each formed with a bolt hole, and the band-shaped buffering portion the width of which is narrower than that of the washer portions is formed between the washer portions. 
   The stress characteristic of the buffering portion  53  can be adjusted to a proper value by changing the length, width, and thickness thereof. 
   The buffering member shown in  FIG. 6(   b ) is formed by washer portions  61  and  62  provided at both ends and a buffering portion  63  provided between the washer portions  61  and  62 . 
   The washer portions  61  and  62  function as fixing portions for fixing the buffering member with the bolts  41  and  42 , respectively. Also, the washer portions  61  and  62  are in contact with the heads of the bolts over the whole circumference thereof. 
   The washer portions  61  and  62  are each provided with a bolt hole  611  and a bolt hole  621  for inserting the bolts  41  and  42 , respectively. 
   Each of the bolt holes  611  and  621  is a circular hole, and the diameter thereof is determined by adding a clearance at the time of bolt penetration to the diameter of the bolt  41  or the bolt  42 . 
   Each of the bolt holes  611  and  621  is formed by a through hole provided with no thread groove. 
   The buffering portion  63  is a buffering mechanism for buffering a shock by generating a tensile stress, and is provided with slits formed alternately from one side in the lengthwise direction to the opposite side. 
   That is, the buffering member shown in  FIG. 6(   b ) has, at both ends thereof, the washer portions each formed with a bolt hole, and the buffering portion having slits formed alternately from one side in the lengthwise direction to the opposite side is formed between the washer portions. 
   The stress characteristic of the buffering portion  63  can be adjusted to a proper value by changing the thickness of member, the slit length, and the number of slits. 
   The buffering member shown in  FIG. 6(   c ) has washer portions  71  and  72  provided at both ends and an elongated hole  73  provided between the washer portions  71  and  72 . 
   The washer portions  71  and  72  function as fixing portions for fixing the buffering member with the bolts  41  and  42 , respectively. Also, the washer portions  71  and  72  are in contact with the heads of the bolts over the whole circumference thereof. 
   The washer portions  71  and  72  are each provided with a bolt hole  711  and a bolt hole  721  for inserting the bolts  41  and  42 , respectively. 
   Each of the bolt holes  711  and  721  is a circular hole, and the diameter thereof is determined by adding a clearance at the time of bolt penetration to the diameter of the bolt  41  or the bolt  42 . 
   Each of the bolt holes  711  and  721  is formed by a through hole provided with no thread groove. 
   The buffering member shown in  FIG. 6(   c ) buffers a shock by generating a tensile stress in buffering portions that are formed at both sides of the elongated hole  73  and extend in the lengthwise direction, like the first buffering member  31 . 
   That is, the buffering member shown in  FIG. 6(   c ) has, at both ends thereof, the washer portions each formed with a bolt hole, and the elongated hole extending in the lengthwise direction is formed between the washer portions. 
   The stress characteristic of the buffering portions that are formed at both sides of the elongated hole  73  and extend in the lengthwise direction can be adjusted to a proper value by changing the length or width of the elongated hole  73 . 
   The above-described first buffering member  31 , the second buffering member  32 , and modifications shown in  FIGS. 6(   a ) to  6 ( c ) may be used singly as a buffering member for buffering a shock generated at emergency time. 
   However, for the buffering member having a construction such that the bolts are installed penetratingly at both ends of the elongated hole like the first buffering member  31 , since the bearing surface of bolt laps on the elongated hole, the bolt head is easily tilted in the direction toward the center or elongated hole when the flange  11  is moved by a shock. 
   Therefore, when the buffering member having no washer portions for stabilizing the bearing surfaces of the bolts  41  and  42 , like the first buffering member  31 , is used singly, it is preferable that washers dedicated to the bolts  41  and  42  be individually provided separately from the buffering member. 
   When a plurality of buffering members like the first buffering member  31  are used in a lapped manner, too, it is preferable that washers dedicated to the bolts  41  and  42  be individually provided separately from the buffering member. 
   Also, a plurality of buffering members may be selected from the first buffering member  31 , the second buffering member  32 , and the modifications shown in  FIGS. 6(   a ) to  6 ( c ) and combinedly used, or a plurality of same buffering members may be used. 
   In such a case, it is preferable that the buffering member provided with the washer portions for stabilizing the bearing surfaces of the bolts  41  and  42  be arranged on the outside, i.e., at a position at which the buffering member is in contact with the bolt heads. 
   The molecular pump  1  in accordance with the first embodiment is configured so that the strength of the bolt  41  is at least higher than the strength of the bolt  42 . 
   Thereby, breakage of the bolt  41  occurring earlier than the breakage of the bolt  42  when the bolt is subjected to a shock can be restrained. Therefore, shock energy can be absorbed sufficiently before all of the installed bolts  41  are broken. 
   Also, when the bolt  42  is broken, i.e., is deformed, shock energy can be absorbed in the process of deformation of the bolt  42 . 
   In the first embodiment, the bolt hole  112  is provided on the rotor rotation direction (arrow-marked direction) side of the bolt hole  111 , by which shock energy generated at emergency time is consumed by a tensile stress acting on the first buffering member  31  and the second buffering member  32 . 
   The configuration may be such that the bolt hole  112  is provided on the side opposite to the rotor rotation direction of the bolt hole  111 , by which shock energy generated at emergency time is consumed by a compressive stress acting on the first buffering member  31  and the second buffering member  32 . 
   In this case as well, the first buffering member  31  and the second buffering member  32  are formed by a plurality of members having a different stress compression characteristic so that a high stress can be generated over a wide range of strain due to compression. 
   By causing the compressive stress to act on the first buffering member  31  and the second buffering member  32 , the energy that rotates the molecular pump  1  is consumed and a shock is eased in the process in which these members are plastically deformed. 
   According to the first embodiment, a shock of breaking torque can be consumed effectively by the use of a simple and inexpensive structure such as the first buffering member  31  and the second buffering member  32  regardless of the internal construction of the molecular pump  1 . 
   Also, according to the first embodiment, if a tensile stress or a compressive stress can be caused to act sufficiently in the range of rotational movement distance (stroke) of the molecular pump  1 , which depends on the lengthwise distance of the elongated hole  113  provided in the flange  11 , the first buffering member  31  and the second buffering member  32  can be formed of a plastic member. 
   2. Second Embodiment 
   Next, a second embodiment of the present invention will be described. In this embodiment, the same reference numerals are applied to the same elements as those in the first embodiment, and the detailed explanation of the same elements is omitted. 
     FIG. 7  is a view showing one example of a connection mode of a molecular pump  1  to a vacuum vessel  2  in accordance with the second embodiment. 
   The molecular pump  1  is the same vacuum pump as the molecular pump  1  explained in the first embodiment. 
   At the end of an inlet of a casing  3 , a flange  11 ′ is formed so as to protrude to the outer periphery side from the casing  3 . The flange  11 ′ is used as a connecting device for connecting the molecular pump  1  to the vacuum vessel  2 . 
   The vacuum vessel  2  forms a vacuum device such as semiconductor manufacturing equipment or a lens barrel for electron microscope, and is provided with an outlet. At the end of this outlet, a flange  22  is formed so as to protrude to the outer periphery side from the outlet like the molecular pump  1 . The flange  22  is used as a connecting device when the molecular pump  1  is connected to the vacuum vessel  2 . 
   The molecular pump  1  is connected to the outlet of the vacuum pump  2  via the flanges  11 ′ and  22  by using fastening means such as bolts. 
     FIG. 8  is a perspective view and front view showing a construction of a connecting portion indicated as portion B in  FIG. 7 . 
   The molecular pump in accordance with the second embodiment has the connecting portions indicated as portion B in  FIG. 1  provided at a plurality of places. 
   The molecular pump  1  is connected to the vacuum vessel  2  by fastening the flange  22  to the flange  11 ′ with bolts  41  each having a head at the terminal end thereof. The bolt  41  is fastening means for fastening the casing  3  to the vacuum vessel  2 . 
   The flange  11 ′ is provided with a bolt hole  114  for threadedly fixing the bolt  41 . At the inner periphery of the bolt hole  114 , a thread groove is provided to fix the bolt  41 . By fixing the bolts  41  in the bolt holes  114 , the flange  22  is connected to the casing  3  of the molecular pump  1 . 
   Between the bearing surface of the bolt  41  and the flange  22 , a first buffering member  31  and a second buffering member  32  are interposed. 
   The first buffering member  31  and the second buffering member  32  are lapped on each other so that the second buffering member  32  is located on the bearing surface side of the bolt  41  and the first buffering member  31  is located on the flange  22  side. 
   One end of each of the first buffering member  31  and the second buffering member  32  is fixed to the flange  11 ′ of the molecular pump  1  with the bolt  41 , and the other end thereof is fixed to the flange  22  of the vacuum vessel  2  with a bolt  42  having a head at the terminal end thereof. 
   The bolt  42  forms fixing means for fixing the first buffering member  31  and the second buffering member  32  to the flange  22  in the connecting portion. 
   Since the first buffering member  31  and the second buffering member  32  have the same configuration as that of the members explained in the first embodiment, detailed explanation thereof is omitted. 
   The flange  22  is formed with a bolt hole  221  for inserting the bolt  41 . This bolt hole  221  is a fastening hole for the bolt  41 , which is fastening means for fastening the casing  3  to the vacuum vessel  2 , to be inserted through. 
   Further, an elongated hole  223  is formed so as to extend from the bolt hole  221  in the rotation direction of the rotor. 
   The elongated hole formed in the flange  22  may be configured by an elongated hole extending in both directions from the bolt hole  221 , provided that the elongated hole is at least a hole including the elongated hole  223  extending from the bolt hole  221  in the rotation direction of the rotor. 
   Also, the flange  22  is formed with a bolt hole  222  for threadedly fixing the bolt  42 . The bolt hole  222  is provided on this side opposite to the rotor rotation direction of the bolt hole  221 . 
   At the inner periphery of the bolt hole  114 , a thread groove is provided to fix the bolt  41 . 
   All of the bolt hole  221 , the end portion on the rotor rotation direction side of an elongated hole  311 , and a bolt hole  323  are provided concentrically with the bolt hole  114 . 
   Also, the end portion on the opposite side of the rotor rotation direction of the elongated hole  311  and a bolt hole  324  are also provided concentrically with the bolt hole  222 . 
   In the second embodiment, the first buffering member  31  and the second buffering member  32  are arranged so as to be held between the bearing surface of the bolt  41  and the flange  22 . The first buffering member  31  and the second buffering member  32  may be arranged between the flange  22  and the flange  11 ′. In this case, a construction is used in which gastightness can be secured in a connecting portion between the vacuum vessel  2  and the casing  3 . 
   In the second embodiment, the first buffering member  31  and the second buffering member  32  are fixed to the flange  22  with the bolt  42 . However, as means for fixing the first buffering member  31  and the second buffering member  32  to the flange  22 , for example, staking or welding may be performed. 
   In the second embodiment, the flange  22  is fixed to the vacuum vessel  2  by threadedly fixing the bolt  41  in the bolt hole  114 . However, for example, a through hole is formed in the flange  11 ′, and the casing  3  may be fixed to the vacuum vessel  2  by using a bolt and a nut via this through hole and the bolt hole  221 . 
   In this case, the bolt and the nut function as fastening means for fastening the casing  3  to the vacuum vessel  2 . 
   Also, the casing  3  may be fastened to the vacuum vessel  2  by using nuts installed from both directions without the use of the bolt having a head. In this case, the nut in contact with the second buffering member serves as the head in contact with the buffering member. 
   Next, the buffering function in the connecting portion thus constructed is explained. 
     FIG. 9  is a sectional view taken along the line Y-Y′ of a connecting portion indicated as portion B in  FIG. 7 , showing a state before a shock is generated at emergency time. 
   Also,  FIG. 10  is a sectional view taken along the line Y-Y′ of a connecting portion indicated as portion B in  FIG. 7 , showing a state after a shock has been generated at emergency time. 
   If a shock is generated due to torque that tends to rotate the whole of the molecular pump  1  in the rotation direction of the rotor section, the flange  11 ′ slips and rotates in the rotation direction of the rotor section with respect to the flange  22  due to this shock. 
   That is, the flange  22  moves relatively in the opposite direction with respect to the flange  11 ′. Therefore, the elongated hole  223  is formed so as to extend from the bolt hole  221  in the direction such that the flange  11 ′ moves relatively with respect to the flange  22 . 
   The bolt  41 , which is fixed to the bolt hole  114  in the flange  11 ′, moves in the rotation direction of the rotor section along the elongated hole  223  due to the movement of the flange  11 ′. 
   The flange  22  moves relatively on the direction opposite to the flange  11 ′. Therefore, as viewed relatively with respect to the flange  11 ′, the bolt  42  moves while applying a force to the end portion on the side opposite to the rotor rotation direction of the elongated hole  311  and the bolt hole  324  in the direction opposite to the rotation direction of the rotor section. Also, a force equivalent to the force applied by the bolt  42  is applied to the end portion on the rotor rotation direction side of the elongated hole  311  and the bolt hole  323  in the rotation direction of the rotor section by the bolt  41 . 
   The buffering portions that are formed at both sides of the elongated hole  311  transform and extend in the lengthwise direction and the buffering portion  325  are pulled by the force applied by the bolt  41  and the bolt  42 , so that a tensile stress acts (is generated). If the tensile stress acts, the first buffering member  31  and the second buffering member  32  are plastically deformed. 
   In the process in which the first buffering member  31  and the second buffering member  32  are transformed and plastically deformed, the energy that rotates the molecular pump  1  is consumed and absorbed by the first buffering member  31  and the second buffering member  32 , whereby a shock is eased. 
   Also, in the first buffering member  31  and the second buffering member  32  shown in the second embodiment as well, the modifications shown in  FIGS. 6(   a ) to  6 ( c ) in accordance with the first embodiment can be applied. 
   As in the first embodiment, modifications shown in  FIGS. 6(   a ) to  6 ( c ) may be used singly as a buffering member for buffering a shock generated at emergency time. 
   However, for the buffering member having a construction such that the bolts are installed penetratingly at both ends of the elongated hole like the first buffering member  31 , since the bearing surface of bolt laps on the elongated hole, the bolt head is easily tilted in the direction toward the center or elongated hole when the flange  22  is moved relatively with respect to the flange  11 ′ by a shock. 
   Therefore, when the buffering member having no washer portions for stabilizing the bearing surfaces of the bolts  41  and  42 , like the first buffering member  31 , is used singly, it is preferable that washers dedicated to the bolts  41  and  42  be individually provided separately from the buffering member. 
   When a plurality of buffering members like the first buffering member  31  are used in a lapped manner, too, it is preferable that washers dedicated to the bolts  41  and  42  be individually provided separately from the buffering member. 
   Also, a plurality of buffering members may be selected from the first buffering member  31 , the second buffering member  32 , and the modifications shown in  FIGS. 6(   a ) to  6 ( c ) and combinedly used, or a plurality of same buffering members may be used. 
   In such a case, it is preferable that the buffering member provided with the washer portions for stabilizing the bearing surfaces of the bolts  41  and  42  be arranged on the outside, i.e., at a position at which the buffering member is in contact with the bolt heads. 
   In the second embodiment as well, the configuration is such that the strength of the bolt  41  is at least higher than the strength of the bolt  42 . 
   Thereby, breakage of the bolt  41  occurring earlier than the breakage of the bolt  42  when the bolt is subjected to a shock can be restrained. Therefore, shock energy can be absorbed sufficiently before all of the installed bolts  41  are broken. 
   Also, when the bolt  42  is broken, i.e., is deformed, shock energy can be absorbed in the process of deformation of the bolt  42 . 
   In the second embodiment as well, the configuration may be such that the bolt hole  222  is provided on the rotor rotation direction side of the bolt hole  114 , by which shock energy generated at emergency time is consumed by a compressive stress acting on the first buffering member  31  and the second buffering member  32 . 
   In this case as well, the first buffering member  31  and the second buffering member  32  are formed by a plurality of members having a different stress compression characteristic so that a high stress can be generated over a wide range of strain due to compression. 
   By causing the compressive stress to act on the first buffering member  31  and the second buffering member  32 , the energy that rotates the molecular pump  1  is consumed and a shock is eased in the process in which these members are plastically deformed. 
   According to the second embodiment, a shock of breaking torque can be consumed effectively by the use of a simple and inexpensive structure such as the first buffering member  31  and the second buffering member  32  regardless of the internal construction of the molecular pump  1 . 
   Also, according to the second embodiment, if a tensile stress or a compressive stress can be caused to act sufficiently in the range of rotational movement distance (stroke) of the molecular pump  1 , which depends on the lengthwise distance of the elongated hole  223  provided in the flange  22 , the first buffering member  31  and the second buffering member  32  can be formed of a plastic member. 
   Also, in combination with the configuration of connecting portion between the vacuum vessel  2  and the casing  3  shown in the second embodiment, the configuration of connecting portion between the vacuum vessel  2  and the casing  3  shown in the first embodiment may be used. 
   In this case, the connecting portion of the first embodiment is configured so that the flange  22  shown in the second embodiment corresponds to the vacuum vessel  2  shown in the first embodiment. 
   Also, the bolt  41  may be formed by a fastening member for fastening the casing  3  to the vacuum vessel  2  by using nuts installed from both directions without the use of the bolt having a head so that the bolt  41  can be used for both of the connecting portion shown in the first embodiment and the connecting portion shown in the second embodiment. 
   As shown in the second embodiment, there can be provided a vacuum vessel capable of effectively consuming a shock of breaking torque by using a simple and inexpensive construction, the vacuum vessel being connected with a molecular pump for exhausting gas, which has a casing provided with an inlet and an outlet; a rotor which is pivotally supported in the casing and is provided with a gas transfer mechanism for transferring gas from the inlet to the outlet; and a motor for rotating the rotor, and being provided with a flange-shaped connecting portion which is formed in a joining portion with the molecular pump and is formed with a fastening hole penetrating in the thickness direction and an elongated hole extending from the fastening hole in the rotation direction of the rotor; a buffering member having a buffering portion, a first fixing portion which is provided at one end of the buffering portion and has a fastening hole, and a second fixing portion formed at the other end of the buffering portion; fastening means for fastening the buffering member to the casing via the fastening hole in the connecting portion and the fastening hole in the buffering member; and fixing means for fixing the second fixing portion of the buffering member to the connecting portion. 
     FIG. 11  is an explanatory view of a modification of a connecting device for connecting the molecular pump  1  to the vacuum vessel  2  shown in the first embodiment and the second embodiment. 
   As shown in  FIG. 11 , the same configuration as that of the connecting methods shown in the first and second embodiments can also be used in a connecting portion (portion C) between a casing  8  and a casing  9 , which are formed by dividing the casing  3  of the molecular pump  1  into two in the axial direction of the rotor, and a connecting portion (portion D) between the casing  9  or the casing  3  and the base  10  forming the housing of the molecular pump  1 . 
   The connecting portion shown as portion C in  FIG. 11  is explained. 
   The casing  8  is formed with a flange  81  protruding to the outer periphery side at the end on the base  10  side. Also, the casing  9  is formed with a flange  91  protruding to the outer periphery side at the end on the inlet side. 
   In the casings  8  and  9 , there is provided a rotor section which is rotated at a high speed by the action of a motor. 
   That is, the casings  8  and  9  form a vessel incorporating a rotator. 
   Here, it is assumed that when a shock is generated at emergency time as described above, the casing  8  is subjected to a greater shock than the casing  9 . This assumes a case where, for example, the inlet side of rotor section is formed by a turbo-molecular pump, and a thread groove pump is formed on the base  10  side. 
   When the configuration of connecting method shown in the first embodiment is applied, the casing  8  is regarded as the casing  3  shown in the first embodiment, and the casing  9  is regarded as the vacuum vessel  2  shown in the first embodiment. That is, the flange  81  corresponds to the flange  11  shown in the first embodiment, and the flange  91  corresponds to the vacuum vessel  2 . 
   Therefore, the flange  81  and the flange  91  are made to correspond to the flange  11  and the vacuum pump  2  shown in the first embodiment, and in this state, the same configuration as that shown in the first embodiment is used. 
   Specifically, the flange  81  is provided with holes corresponding to the bolt hole  112 , the bolt hole  111 , and the elongated hole  113 , and the flange  91  is provided with a hole corresponding to the bolt hole  21 , each hole being formed at a position corresponding to the rotation direction of the rotor section. As in the first embodiment, the first buffering member  31 , the second buffering member  32 , the bolt  41 , and the bolt  42  are arranged. 
   By configuring the connecting portion (portion C) between the casing  8  and the casing  9  in this manner, a shock of breaking torque can be consumed effectively by using a simple and inexpensive construction. 
   When the configuration of connecting method shown in the second embodiment is applied, the casing  8  is regarded as the casing  3  shown in the second embodiment, and the casing  9  is regarded as the vacuum vessel  2  shown in the second embodiment. That is, the flange  81  corresponds to the flange  11 ′ shown in the second embodiment, and the flange  91  corresponds to the flange  22 . 
   Therefore, the flange  81  and the flange  91  are made to correspond to the flange  11 ′ and the flange  22  shown in the second embodiment, and in this state, the same configuration as that shown in the second embodiment is used. 
   Specifically, the flange  81  is provided with a hole corresponding to the bolt hole  114 , and the flange  91  is provided with holes corresponding to the bolt hole  222 , the bolt hole  221 , and the elongated hole  223 , each hole being formed at a position corresponding to the rotation direction of the rotor section. As in the second embodiment, the first buffering member  31 , the second buffering member  32 , the bolt  41 , and the bolt  42  are arranged. 
   By configuring the connecting portion (portion C) between the casing  8  and the casing  9  in this manner, a shock of breaking torque can be consumed effectively by using a simple and inexpensive construction. 
   The connecting portion shown as portion D in  FIG. 11  is explained. 
   The casing  9  is formed with a flange  92  protruding to the outer periphery side at the end on the base  10  side. Also, the base  10  is formed with a flange  101  protruding to the outer periphery side at the end on the inlet side. 
   In the casings  8  and  9 , there is provided a rotor section which is rotated at a high speed by the action of a motor. 
   The casing  9  may be a divided casing, or may be a non-divided casing  3  as shown in  FIG. 1 . 
   When the configuration of connecting method shown in the first embodiment is applied, the casing  9  is regarded as the casing  3  shown in the first embodiment, and the base  10  is regarded as the vacuum vessel  2  shown in the first embodiment. That is, the flange  92  corresponds to the flange  11  shown in the first embodiment, and the flange  101  corresponds to the vacuum vessel  2 . 
   Therefore, the flange  92  and the flange  101  are made to correspond to the flange  11  and the vacuum pump  2  shown in the first embodiment, and in this state, the same configuration as that shown in the first embodiment is used. 
   Specifically, the flange  92  is provided with holes corresponding to the bolt hole  112 , the bolt hole  111 , and the elongated hole  113 , and the flange  101  is provided with a hole corresponding to the bolt hole  21 , each hole being formed at a position corresponding to the rotation direction of the rotor section. As in the first embodiment, the first buffering member  31 , the second buffering member  32 , the bolt  41 , and the bolt  42  are arranged. 
   The flange  92  may be connected directly to the base  10  without providing the flange  101  on the base  10 . 
   By configuring the connecting portion (portion D) between the casing  9  and the base  10  in this manner, a shock of breaking torque can be consumed effectively by using a simple and inexpensive construction. 
   When the configuration of connecting method shown in the second embodiment is applied, the casing  9  is regarded as the casing  3  shown in the second embodiment, and the base  10  is regarded as the vacuum vessel  2  shown in the second embodiment. That is, the flange  92  corresponds to the flange  11 ′ shown in the second embodiment, and the flange  101  corresponds to the flange  22 . 
   Therefore, the flange  92  and the flange  101  are made to correspond to the flange  11 ′ and the flange  22  shown in the second embodiment, and in this state, the same configuration as that shown in the second embodiment is used. 
   Specifically, the flange  92  is provided with a hole corresponding to the bolt hole  114 , and the flange  101  is provided with holes corresponding to the bolt hole  222 , the bolt hole  221 , and the elongated hole  223 , each hole being formed at a position corresponding to the rotation direction of the rotor section. As in the second embodiment, the first buffering member  31 , the second buffering member  32 , the bolt  41 , and the bolt  42  are arranged. 
   By configuring the connecting portion (portion D) between the casing  9  and the base  10  in this manner, a shock of breaking torque can be consumed effectively by using a simple and inexpensive construction. 
   The connection configuration shown in the first or second embodiment can be used in the connecting portion between the vacuum vessel  2  and the casing  3 , the connecting portion between the divided casings  8  and  9 , and the connecting portion between the casing  3  or the casing  9  and the base  10 . 
   The connection configuration shown in the first or second embodiment may be used singly in any of these connecting portions, or may be used in all of these connecting portions.