Patent Publication Number: US-6659227-B2

Title: Oil leak prevention structure for vacuum pump

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an oil leak prevention structure of a vacuum pump that draws gas by rotating a rotary shaft to move a gas conveying body in a pump chamber. 
     Japanese Laid-Open Patent Publication No. 63-129829 and No. 3-11193 each disclose a vacuum pump. The pump of either publication introduces lubricant oil into the interior of the pump. Either pump prevents lubricant oil from entering regions where oil is not desirable. 
     The vacuum pump disclosed in Japanese Laid-Open Patent Publication No. 63-129829 includes a plate attached to a rotary shaft to prevent oil from entering a chamber for an electric generator. Specifically, when moving along the surface of the rotary shaft toward the generator chamber, oil reaches the plate. The centrifugal force of the plate spatters the oil to an annular groove formed about the plate. The oil flows to the lower portion of the annular groove and is then drained to the outside along an oil passage connected to the lower portion. 
     The vacuum pump disclosed in Japanese Laid-Open Patent Publication No. 3-11193 has an annular chamber for supplying oil to a bearing and a slinger provided in the annular chamber. When moving along the surface of a rotary shaft from the annular chamber to a vortex flow pump, oil is thrown away by the slinger. The thrown oil is then sent to a motor chamber through a drain hole connected to the annular chamber. 
     The plate (slinger) is a mechanism that integrally rotates with a rotary shaft to prevent oil from entering undesirable regions. The oil leak entry preventing operation utilizing centrifugal force of the plate (slinger) is influenced by the shape of the plate (slinger), and the shape of the walls surrounding the plate (slinger). 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide an oil leak prevention mechanism that effectively prevents oil from entering a pump chamber of a vacuum pump. 
     To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, the invention provides a vacuum pump. The vacuum pump draws gas by operating a gas conveying body in a pump chamber through rotation of a rotary shaft. The vacuum pump has an oil housing member, a stopper and an annular oil chamber. The oil housing member defines an oil zone adjacent to the pump chamber. The rotary shaft has a projecting section that projects from the pump chamber to the oil zone through the oil housing member. The stopper has a circumferential surface. The stopper is located on the rotary shaft to integrally rotate with the rotary shaft and prevents oil from entering the pump chamber. The oil chamber collects oil. The oil chamber is located about an axis of the rotary shaft to surround the circumferential surface of the stopper. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG.  1 ( a ) is a cross-sectional plan view illustrating a multiple-stage Roots pump according to a first embodiment of the present invention; 
     FIG.  1 ( b ) is an enlarged partial cross-sectional view of the pump shown in FIG.  1 ( a ); 
     FIG.  2 ( a ) is a cross-sectional view taken along line  2   a — 2   a  in FIG.  1 ( a ); 
     FIG.  2 ( b ) is a cross-sectional view taken along line  2   b — 2   b  in FIG.  1 ( a ); 
     FIG.  3 ( a ) is a cross-sectional view taken along line  3   a — 3   a  in FIG.  1 ( a ); 
     FIG.  3 ( b ) is a cross-sectional view taken along line  3   b — 3   b  in FIG.  1 ( a ); 
     FIG.  4 ( a ) is a cross-sectional view taken along line  4   a — 4   a  in FIG.  3 ( b ); 
     FIG.  4 ( b ) is an enlarged cross-sectional view of FIG.  4 ( a ); 
     FIG.  5 ( a ) is a cross-sectional view taken along line  5   a — 5   a  in FIG.  3 ( b ); 
     FIG.  5 ( b ) is an enlarged cross-sectional view of FIG.  5 ( a ); 
     FIG.  6 ( a ) is an enlarged cross-sectional view of the pump shown in FIG.  1 ( a ); 
     FIG.  6 ( b ) is an enlarged cross-sectional view of FIG.  6 ( a ); 
     FIG. 7 is an exploded perspective view illustrating part of the rear housing member, the first shaft seal, and a leak prevention ring of the pump shown in FIG.  1 ( a ); 
     FIG. 8 is an exploded perspective view illustrating part of the rear housing member, the second shaft seal, and a leak prevention ring of the pump shown in FIG.  1 ( a ); 
     FIG. 9 is an enlarged cross-sectional view illustrating a second embodiment of the present invention; and 
     FIG. 10 is an enlarged cross-sectional view illustrating a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A multiple-stage Roots pump  11  according to a first embodiment of the present invention will now be described with reference to FIGS.  1 ( a ) to  8 . 
     As shown in FIG.  1 ( a ), the pump  11 , which is a vacuum pump, includes a rotor housing member  12 , a front housing member  13 , and a rear housing member  14 . The front housing member  13  is coupled to the front end of the rotor housing member  12 . A lid  36  closes the front opening of the front housing member  13 . The rear housing member  14  is coupled to the rear end of the rotor housing member  12 . The rotor housing member  12  includes a cylinder block  15  and chamber defining walls  16 , the number of which is four in this embodiment. As shown in FIG.  2 ( b ), the cylinder block  15  includes a pair of blocks  17 ,  18 . Each chamber defining wall  16  includes a pair of wall sections  161 ,  162 . 
     As shown in FIG.  1 ( a ), a first pump chamber  39  is defined between the front housing member  13  and the leftmost chamber defining wall  16 . Second, third, and fourth pump chambers  40 ,  41 ,  42  are each defined between two adjacent chamber defining walls  16  in this order from the left to the right as viewed in the drawing. A fifth pump chamber  43  is defined between the rear housing member  14  and the rightmost chamber defining wall  16 . 
     A first rotary shaft  19  is rotatably supported by the front housing member  13  and the rear housing member  14  with a pair of radial bearings  21 ,  37 . Likewise, the second rotary shaft  20  is rotatably supported by the front housing member  13  and the rear housing member  14  with a pair of radial bearings  21 ,  37 . The first and second rotary shafts  19 ,  20  are parallel with each other and extend through the chamber defining walls  16 . The radial bearings  37  are supported by bearing holders  45  that are installed in the rear housing member  14 . The bearing holders  45  are fitted in first and second recesses  47 ,  48  that are formed in the rear side of the rear housing member  14 , respectively. 
     First, second, third, fourth, and fifth rotors  23 ,  24 ,  25 ,  26 ,  27  are formed integrally with the first rotary shaft  19 . Likewise, first, second, third, fourth, and fifth rotors  28 ,  29 ,  30 ,  31 ,  32  are formed integrally with the second rotary shaft  20 . As viewed in the direction along the axes  191 ,  201  of the rotary shafts  19 ,  20 , the shapes and the sizes of the rotors  23 - 32  are identical. However, the axial dimensions of the first to fifth rotors  23 - 27  of the first rotary shaft  19  become gradually smaller in this order. Likewise, the axial dimensions of the first to fifth rotors  28 - 32  of the second rotary shaft  20  become gradually smaller in this order. 
     The first rotors  23 ,  28  are accommodated in the first pump chamber  39  and are engaged with each other. The second rotors  24 ,  29  are accommodated in the second pump chamber  40  and are engaged with each other. The third rotors  25 ,  30  are accommodated in the third pump chamber  41  and are engaged with each other. The fourth rotors  26 ,  31  are accommodated in the fourth pump chamber  42  and are engaged with each other. The fifth rotors  27 ,  32  are accommodated in the fifth pump chamber  43  and are engaged with each other. The first to fifth pump chambers  39 - 43  are not lubricated. Thus, the rotors  23 - 32  are arranged not to contact any of the cylinder block  15 , the chamber defining walls  16 , the front housing member  13 , and the rear housing member  14 . Further, the rotors of each engaged pair do not slide against each other. 
     As shown in FIG.  2 ( a ), the first rotors  23 ,  28  define a suction zone  391  and a pressure zone  392  in the first pump chamber  39 . The pressure in the pressure zone  392  is higher than the pressure in the suction zone  391 . Likewise, the second to fourth rotors  24 - 26 ,  29 - 31  define suction zones and pressure zones in the associated pump chambers  40 - 42 . As shown in FIG.  3 ( a ), the fifth rotors  27 ,  32  define a suction zone  431  and a pressure zone  432 , which are similar to the suction zone  391  and the pressure zone  392 , in the fifth pump chamber  43 . 
     As shown in FIG.  1 ( a ), a gear housing member  33  is coupled to the rear housing member  14 . A pair of through holes  141 ,  142  is formed in the rear housing member  14 . The rotary shafts  19 ,  20  extend through the through holes  141 ,  142  and the first and second recesses  47 ,  48 , respectively. The rotary shafts  19 ,  20  thus project into the gear housing member  33  to form projecting portions  193 ,  203 , respectively. Gears  34 ,  35  are secured to the projecting portions  193 ,  203 , respectively, and are meshed together. An electric motor M is connected to the gear housing member  33 . A shaft coupling  44  transmits the drive force of the motor M to the first rotary shaft  19 . The motor M thus rotates the first rotary shaft  19  in the direction indicated by arrow R 1  of FIGS.  2 ( a ) to  3 ( b ). The gears  34 ,  35  transmit the rotation of the first rotary shaft  19  to the second rotary shaft  20 . The second rotary shaft  20  thus rotates in the direction indicated by arrow R 2  of FIGS.  2 ( a ) to  3 ( b ). Accordingly, the first and second rotary shafts  19 ,  20  rotate in opposite directions. The gears  34 ,  35  form a gear mechanism to rotate the rotary shafts  19 ,  20  integrally. 
     As shown in FIGS.  4 ( a ) and  5 ( a ), a gear accommodating chamber  331  is formed in the gear housing member  33  and retains lubricant oil Y for lubricating the gears  34 ,  35 . The gear accommodating chamber  331  and the first and second recesses  47 ,  48  form a sealed oil zone. The gear housing member  33  and the rear housing member  14  thus form an oil housing, or an oil zone adjacent to the fifth pump chamber  43 . The gears  34 ,  35  rotate to lift the lubricant oil Y in the gear accommodating chamber  331 . The lubricant oil Y thus lubricates the radial bearings  37 . 
     As shown in FIGS.  1 ( a ) and  2 ( b ), a hollow  163  is defined in each chamber defining wall  16 . Each chamber defining wall  16  has an inlet  164  and an outlet  165  that are connected to the hollow  163 . Each adjacent pair of the pump chambers  39 - 43  are connected to each other by the hollow  163  of the associated chamber defining wall  16 . 
     As shown in FIG.  2 ( a ), an inlet  181  is formed in the block  18  of the cylinder block  15  and is connected to the suction zone  391  of the first pump chamber  39 . As shown in FIG.  3 ( a ), an outlet  171  is formed in the block  17  of the cylinder block  15  and is connected to the pressure zone  432  of the fifth pump chamber  43 . When gas enters the suction zone  391  of the first pump chamber  39  from the inlet  181 , rotation of the first rotors  23 ,  28  moves the gas to the pressure zone  392 . The gas is compressed in the pressure zone  392  and enters the hollow  163  of the adjacent chamber defining wall  16  from the inlet  164 . The gas then reaches the suction zone of the second pump chamber  40  from the outlet  165  of the hollow  163 . Afterwards, the gas flows from the second pump chamber  40  to the third, fourth, and fifth pump chambers  41 ,  42 ,  43  in this order while repeatedly compressed. The volumes of the first to fifth pump chambers  39 - 43  become gradually smaller in this order. When the gas reaches the suction zone  431  of the fifth pump chamber  43 , rotation of the fifth rotors  27 ,  32  moves the gas to the pressure zone  432 . The gas is then discharged from the outlet  171  to the exterior of the vacuum pump  11 . That is, each rotor  23 - 32  functions as a gas conveying body for conveying gas. 
     The outlet  171  functions as a discharge passage for discharging gas to the exterior of the vacuum pump  11 . The fifth pump chamber  43  is a final-stage pump chamber that is connected to the outlet  171 . Among the pressure zones of the first to fifth pump chambers  39 - 43 , the pressure in the pressure zone  432  of the fifth pump chamber  43  is the highest, and the pressure zone  432  functions as a maximum pressure zone. 
     As shown in FIG.  1 ( a ), first and second annular shaft seals  49 ,  50  are securely fitted about the first and second rotary shafts  19 ,  20 , respectively, and are located in the first and second recesses  47 ,  48 , respectively. Each of the first and second shaft seals  49 ,  50  rotates with the corresponding rotary shaft  19 ,  20 . A seal ring  51  is located between the inner circumferential surface of each of the first and second shaft seals  49 ,  50  and the circumferential surface  192 ,  202  of the corresponding rotary shaft  19 ,  20 . Each seal ring  51  prevents the lubricant oil Y from leaking from the associated recess  47 ,  48  to the fifth pump chamber  43  along the circumferential surface  192 ,  202  of the associated rotary shaft  19 ,  20 . 
     As shown in FIG.  4 ( a ), the shaft seal  49  includes a small diameter portion  59  and a large diameter portion  60 . As shown in FIG.  4 ( b ), space exists between the outer circumferential surface  491  of the large diameter portion  60  and the circumferential surface  471  of the first recess  47 . Also, space exists between the end surface  492  of the first shaft seal  49  and the bottom  472  of the first recess  47 . As shown in FIG.  5 ( a ), the second shaft seal  50  includes a small diameter portion  81  and a large diameter portion  80 . As shown in FIG.  5 ( b ), space exists between the circumferential surface  501  of the large diameter portion  80  and the circumferential surface  481  of the second recess  48 . Also, space exists between the end surface  502  of the second shaft seal  50  and the bottom  482  of the second recess  48 . 
     Annular projections  53  coaxially project from the bottom  472  of the first recess  47 . In the same manner, annular projections  54  coaxially project from the bottom  482  of the second recess  48 . Further, annular grooves  55  are coaxially formed in the end surface  492  of the shaft seal  49 , which faces the bottom  472  of the first recess  47 . In the same manner, annular grooves  56  are coaxially formed in the front side  502  of the shaft seal  50 , which faces the bottom  482  of the second recess  48 . Each annular projection  53 ,  54  projects in the associated groove  55 ,  56  such that the distal end of the projection  53 ,  54  is located close to the bottom of the groove  55 ,  56 . Each projection  53  divides the interior of the associated groove  55  of the first shaft seal  49  to a pair of labyrinth chambers  551 ,  552 . Each projection  54  divides the interior of the associated groove  56  of the second shaft seal  50  to a pair of labyrinth chambers  561 ,  562 . 
     The projections  53  and the grooves  55  form a first labyrinth seal  57  corresponding to the first rotary shaft  19 . The projections  54  and the grooves  56  form a second labyrinth seal  58  corresponding to the second rotary shaft  20 . In this embodiment, the end surface  492  and the bottom  472  are formed along a plane perpendicular to the axis  191  of the first rotary shaft  19 . Likewise, the end surface  502  and the bottom  482  are formed along a plane perpendicular to the axis  201  of the rotary shaft  20 . In other words, the end surface  492  and the bottom  472  are seal forming surfaces that extend in a radial direction of the first shaft  19 . Likewise, the end surface  502  and the bottom  482  are seal forming surfaces that extend in a radial direction of the second shaft  50 . 
     As shown in FIGS.  4 ( b ) and  7 , a first helical groove  61  is formed in the outer circumferential surface  491  of the large diameter portion  60  of the first shaft seal  49 . As shown in FIGS.  5 ( b ) and  8 , a second helical groove  62  is formed in the outer circumferential surface  501  of the large diameter portion  80  of the second shaft seal  50 . Along the rotational direction R 1  of the first rotary shaft  19 , the first helical groove  61  forms a path that leads from a side corresponding to the gear accommodating chamber  331  toward the fifth pump chamber  43 . Along the rotational direction R 2  of the second rotary shaft  20 , the second helical groove  62  forms a path that leads from a side corresponding to the gear accommodating chamber  331  toward the fifth pump chamber  43 . Therefore, each helical groove  61 ,  62  exert a pumping effect and convey fluid from a side corresponding to the fifth pump chamber  43  toward the gear accommodating chamber  331  when the rotary shafts  19 ,  20  rotate. That is, each helical groove  61 ,  62  forms pumping means that urges the lubricant oil Y between the outer circumferential surface  491 ,  501  of the associated shaft seal  49 ,  50  and the circumferential surface  471 ,  481  of the associated recess  47 ,  48  to move from a side corresponding to the fifth pump chamber  43  toward the oil zone. The circumferential surface  471 ,  481  of each recess  47 ,  48  functions as a sealing surface. The outer circumferential surface  491 ,  501  of the large diameter portion  60 ,  80  of each shaft seal  49 ,  50  faces the corresponding circumferential surface  471 ,  481 . 
     As shown in FIG.  3 ( b ), first and second discharge pressure introducing channels  63 ,  64  are formed in a chamber defining surface  143  of the rear housing member  14 . The chamber defining surface  143  defines the fifth pump chamber  43 , which is at the final stage of compression. As shown in FIG.  4 ( a ), the first discharge pressure introducing channel  63  is connected to the maximum pressure zone  432 , the volume of which is varied by rotation of the fifth rotors  27 ,  32 . The first discharge pressure introducing channel  63  is connected also to the through hole  141 , through which the first rotary shaft  19  extends. As shown in FIG.  5 ( a ), the second discharge pressure introducing channel  64  is connected to the maximum pressure zone  432  and the through hole  142 , through which the second rotary shaft  20  extends. 
     As shown in FIGS.  1 ( a ),  4 ( a ), and  5 ( a ), a cooling loop chamber  65  is formed in the rear housing member  14 . The loop chamber  65  surrounds the shaft seals  49 ,  50 . Coolant water circulates in the loop chamber  65  to cool the lubricant oil Y in the recesses  47 ,  48 , which prevents the lubricant oil Y from being evaporated. 
     As shown in FIGS.  1 ( b ),  6 ( a ) and  6 ( b ), an annular leak prevention ring  66  is fitted about the small diameter portion  59  of the first shaft seal  49  to block flow of oil. The leak prevention ring  66  includes a first stopper  67  having a smaller diameter and a second stopper  68  having a larger diameter. The front end portion  69  of the bearing holder  45  defines an annular first oil chamber  70  and an annular second oil chamber  71  about the leak prevention ring  66 . The first oil chamber  70  surrounds the first stopper  67 , and the second oil chamber  71  surrounds the second stopper  68 . 
     A circumferential surface  671  is located in the first oil chamber  70 . A circumferential surface  681  of the second stopper  68  is located in the second oil chamber  71 . The circumferential surface  671  of the first stopper  67  faces a circumferential surface  702 , which defines the first oil chamber  70 . The circumferential surface  681  of the second stopper  68  faces a circumferential surface  712 , which defines the second oil chamber  71 . 
     An end surface  672  of the first stopper  67  faces a end surface  701 , which defines the first oil chamber  70 . A first end surface  682  of the second stopper  68  faces and is located in the vicinity of a end surface  711 , which defines the second oil chamber  71 . A second end surface  683  of the second stopper  68  faces and is widely separated from a first end surface  601  of a third stopper  72 . The third stopper  72  will be discussed below. 
     The third stopper  72  is integrally formed with the large diameter portion  60  of the first shaft seal  49 . An annular oil chamber  73  is defined in the first recess  47  to surround the third stopper  72 . A circumferential surface  721  of the third stopper  72  is defined on a portion that projects into the third oil chamber  73 . Also, the circumferential surface  721  of the third stopper  72  faces a circumferential surface  733  defining the third oil chamber  73 . The first end surface  601  of the third stopper  72  faces and is located in the vicinity of a first end surface  731  defining the third oil chamber  73 . A second end surface  722  of the third stopper  72  faces and is located in the vicinity of a second end surface  732  defining the third oil chamber  73 . 
     A drainage channel  74  is defined in the lowest portion of the first recess  47  and the end  144  of the rear housing  14  to return the oil Y to the gear accommodation chamber  331 . The drainage channel  74  has an axial portion  741 , which extends along the axis  191  of the first rotary shaft  19 , and a radial portion  742 , which extends perpendicular to the axis  191 . The axial portion  741  is communicated with the third oil chamber  73 , and the radial portion  742  is communicated with the gear accommodation chamber  331 . That is, the third oil chamber  73  is connected to the gear accommodating chamber  331  by the drainage channel  74 . The drainage channel  74  is axially formed in the first embodiment. However, the drainage channel  74  may be inclined downward toward the gear accommodating chamber  331 . 
     As shown in FIG.  5 ( a ), the leak prevention ring  66  is attached to the small diameter portion  81  of the second shaft seal  50 . The leak prevention ring  66  has the same structure as the leak prevention ring  66  attached to the first shaft seal  49 . Thus, detailed explanations are omitted. A third stopper  72  is formed on the large diameter portion  80  of the second shaft seal  50 . The third stopper  72  has the same structure as the third stopper  72  formed on the first shaft seal  49 . Thus, detailed explanations are omitted. As shown in FIG.  5 ( b ), the first and second oil chambers  70 ,  71  are defined radially inward of the bearing holder  45 , and the third oil chamber  73  is defined in the second recess  48 . The drainage channel  74  is formed in the lowest portion of the second recess  48 . The third oil chamber  73  is connected to the gear accommodating chamber  331  by the drainage channel  74 . The drainage channel  74  is axially formed in the first embodiment. However, the drainage channel  74  may be inclined downward toward the gear accommodating chamber  331 . 
     The lubricant oil Y stored in the gear accommodating chamber  331  lubricates the gears  34 ,  35  and the radial bearings  37 . After lubricating the radial bearings  37 , the oil Y enters a through hole  691  formed in the projection  69  of each bearing holder  45  through a space  371  in each radial bearing  37 . Then, the oil Y moves toward the corresponding first oil chamber  70  via a space g 1  between the end surface  672  of the corresponding first stopper  67  and the end surface  701  of the corresponding first oil chamber  70 . At this time, some of the oil Y that reaches the end surface  672  of the first stopper  67  is thrown to the circumferential surface  702  or the end surface  701  of the first oil chamber  70  by the centrifugal force generated by rotation of the first stopper  67 . At least part of the oil Y thrown to the circumferential surface  702  or the end surface  701  remains on the circumferential surface  702  or the end surface  701 . Then, the remaining oil Y falls along the surfaces  701 ,  702  by the self weight and reaches the lowest area of the first oil chamber  70 . After reaching the lowest area of the first oil chamber  70 , the oil Y moves to the lowest area of the second oil chamber  71 . 
     After entering the first oil chamber  70 , the oil Y moves toward the second oil chamber  71  through a space g 2  between the first end surface  682  of the second stopper  68  and the end surface  711  of the second oil chamber  71 . At this time, the oil Y on the first end surface  682  is thrown to the circumferential surface  712  or the end surface  711  of the second oil chamber  71  by the centrifugal force generated by rotation of the second stopper  68 . At least part of the oil Y thrown to the circumferential surface  712  or the end surface  711  remains on the circumferential surface  712  or the end surface  711 . The remaining oil Y falls along the surfaces  711 ,  712  by the self weight and reaches the lowest area of the second oil chamber  71 . After reaching the lowest area of the second oil chamber  71 , the oil Y moves to the lowest area of the third oil chamber  73 . 
     After entering the second oil chamber  71 , the oil Y moves toward the third oil chamber  73  through the space g 3  between the first end surface  601  of the third stopper  72  and the first end surface  731  of the third oil chamber  73 . At this time, the oil Y on the first end surface  601  is thrown to the circumferential surface  733  or the first end surface  731  of the third oil chamber  73  by the centrifugal force generated by rotation of the third stopper  72 . At least part of the oil thrown to the circumferential surface  733  or the first end surface  731  remains on the circumferential surface  733  or the first end surface  731 . Then, the remaining oil falls along the surfaces  731 ,  733  by the self-weight and reaches the lowest area of the third oil chamber  73 . 
     After reaching the lowest area of the third oil chamber  73 , the oil Y is returned to the gear accommodating chamber  331  by the corresponding drainage channel  74 . 
     The first, second, and third oil chambers  70 ,  71 , and  73  and the spaces g 1 , g 2 , and g 3  form a bent path, which extends from the fifth pump chamber  43  to the gear accommodating chamber  331 . Likewise, another bent path is formed around the second shaft seal  50 . 
     The above illustrated embodiment has the following advantages. 
     (1-1) While the vacuum pump is operating, the pressures in the five pump chambers  39 ,  40 ,  41 ,  42 ,  43  are lower than the pressure in the gear accommodating chamber  331 , which is a zone exposed to the atmospheric pressure. Thus, the atomized lubricant oil Y moves along the surface of the leak prevention rings  66  and the surface of the shaft seals  49 ,  50  toward the fifth pump chamber  43 . The atomized lubricant oil Y is more easily liquefied in a bent path than in a straight path. That is, when the atomized lubricant oil Y collides with the wall forming a bent path, the atomized lubricant oil Y is easily liquefied. The path along which the atomized lubricant oil Y in the first oil chamber  70  moves is bent by the first stopper  67  located in the first oil chamber  70 . The path along which the atomized lubricant oil Y in the second oil chamber  71  moves is bent by the second stopper  68  located in the second oil chamber  71 . Further, the path along which the atomized lubricant oil Y in the third oil chamber  73  moves is bent by the third stopper  72  located in the third oil chamber  73 . The first, second, and third stoppers  67 ,  68 ,  72  each corresponding to one of the oil chambers  70 ,  71 ,  73  prevents the atomized lubricant oil Y from easily flowing toward the fifth pump chamber  43 . 
     (1-2) The gear accommodating chamber  331  is communicated with the first oil chamber  70  with a first oil entering passage including the through hole  691  and the space g 1  between the end surface  672  of the first stopper  67  and the end surface  701  of the first oil chamber  70 . The first stopper  67  is arranged to narrow the space g 1 , which serves as the outlet of the first oil entering passage. 
     The gear accommodating chamber  331  is communicated with the second oil chamber  71  with a second oil entering passage including the first oil chamber  70  and the space g 2  between the first end surface  682  of the second stopper  68  and the end surface  711  of the second oil chamber  71 . The second stopper  68  is arranged to narrow the space g 2 , which serves as the outlet of the second oil entering passage. 
     The gear accommodating chamber  331  is communicated with the third oil chamber  73  with an third oil entering passage including the second oil chamber  71  and the space g 3  between the first end surface  601  of the third stopper  72  and the first end surface  731  of the third oil chamber  73 . The third stopper  72  is arranged to narrow the space g 3 , which serves as the outlet of the third oil entering passage. 
     The outlet of the first oil entering passage (space g 1 ), the outlet of the second oil entering passage (space g 2 ), and the outlet of the third oil entering passage (space g 3 ) are narrowed to effectively prevent the atomized lubricant oil Y in the gear accommodating chamber  331  from entering the corresponding oil chamber  70 ,  71 ,  73 . 
     (1-3) The lubricant oil Y on the surfaces  701 ,  702 ,  711 ,  712 ,  731 ,  732 ,  733  of the first, second, and third oil chambers  70 ,  71 ,  73  falls toward the lowest area of the third oil chambers  73  by the self weight. The lowest area of the third oil chamber  73  is an area at which the oil Y on the surfaces  701 ,  702 ,  711 ,  712 ,  731 ,  732 ,  733  is collected. Therefore, the oil Y on the surfaces  701 ,  702 ,  711 ,  712 ,  731 ,  732 ,  733  is readily sent to the gear accommodating chamber  331  via the drainage channel  74  connected to the lowest area of the third oil chamber  73 . 
     (1-4) The first oil chamber  70  and the second oil chamber  71  are defined by the front end portion  69  of the bearing holder  45 , which supports the radial bearing  37 . This structure easily forms highly sealed oil chambers  70 ,  71 . 
     (1-5) The diameters of the end surfaces  492 ,  502  of the shaft seals  49 ,  50  fitted about the first and second rotary shafts  19 ,  20  are greater than the diameters of the circumferential surfaces  192 ,  202  of the rotary shafts  19 ,  20 . Therefore, the diameter of each of the first and second labyrinth seals  57 ,  58  located between the end surface  492 ,  502  of each shaft seal  49 ,  50  and the bottom surface  472 ,  482  of the corresponding recess  472 ,  482  is greater than the diameter of the labyrinth seal (not shown) located between the circumferential surface  192 ,  202  of each rotary shaft  19 ,  20  and the through hole  141 ,  142 . As the diameter of each labyrinth seal  57 ,  58  is increased, the volume of each labyrinth chamber  551 ,  552 ,  561 ,  562  for preventing pressure fluctuations from spreading is increased. This structure improves the sealing performance of each labyrinth seal  57 ,  58 . That is, the space between the end surface  492 ,  502  of each shaft seal  49 ,  50  and the bottom surface  472 ,  482  of the associated recess  47 ,  48  is suitable for accommodating the labyrinth seal  57 ,  58  for improving the sealing performance by increasing the volume of each labyrinth chamber  551 ,  552 ,  561 ,  562 . 
     (1-6) As the space between each recess  47 ,  48  and the corresponding shaft seal  49 ,  50  is decreased, it is harder for the oil Y to enter the space. The bottom surface  472 ,  482  of each recess  47 ,  48 , which has the circumferential surface  471 ,  481 , and the end surface  492 ,  502  of the corresponding shaft seal  49 ,  50  are easily formed to be close to each other. Therefore, the space between the end of each annular projection  53 ,  54  and the bottom of the corresponding annular groove  55 ,  56  and the space between the bottom surface  472 ,  482  of each recess  47 ,  48  and the end surface  492 ,  502  of the corresponding shaft seal  49 ,  50  can be easily decreased. As the spaces are decreased, the sealing performance of the labyrinth seals  57 ,  58  is improved. That is, the bottom surface  472 ,  482  of each recess  47 ,  48  is suitable for accommodating the labyrinth seals  57 ,  58 . 
     (1-7) The labyrinth seals  57 ,  58  sufficiently blocks flow of gas. When the Roots pump  11  is started, the pressures in the five pump chambers  39 - 43  are higher than the atmospheric pressure. However, each labyrinth seal  57 ,  58  prevents gas from leaking from the fifth pump chamber  43  to the gear accommodating chamber  331  along the surface of the associated shaft seal  49 ,  50 . That is, the labyrinth seals  57 ,  58  stop both oil leak and gas leak and are optimal non-contact type seals. 
     (1-8) Although the sealing performance of a non-contact type seal does not deteriorate over time unlike a contact type seal such as a lip seal, the sealing performance of a non-contact type seal is inferior to the sealing performance of a contact type seal. However, in the above described embodiment, the first, second and third stoppers  67 ,  68 ,  72  compensate for the sealing performance. Each circumferential surface  671 ,  681 ,  721  corresponds to the projecting portion of the associated stopper  67 ,  68 ,  72  and is defined in the corresponding oil chamber  70 ,  71 ,  73 . The circumferential surfaces  671 ,  681 ,  721  further compensate for the sealing performance. 
     (1-9) As the first rotary shaft  19  rotates, the oil Y in the first helical groove  61  is guided from the side corresponding to the fifth pump chamber  43  to the side corresponding to the gear accommodating chamber  331 . As the second rotary shaft  20  rotates, the oil Y in the second helical groove  62  is guided from the side corresponding to the fifth pump chamber  43  to the side corresponding to the gear accommodating chamber  331 . That is, the shaft seals  49 ,  50 , which have the first and second helical grooves  61 ,  62  functioning as pumping means, positively prevent leakage of the oil Y. 
     (1-10) The outer circumferential surfaces  491 ,  501 , on which the helical grooves  61 ,  62  are formed, coincide with the outer surface of the large diameter portions  60 ,  80  of the first and second shafts  49 ,  50 . At these parts, the velocity is maximum when the shaft seals  49 ,  50  rotate. Gas located between the outer circumferential surface  491 ,  501  of each shaft seal  49 ,  50  and the circumferential surface  471 ,  481  of the corresponding recess  47 ,  48  is effectively urged from the side corresponding to the fifth pump chamber  43  to the side corresponding to the gear accommodating chamber  331  through the first and second helical grooves  61 ,  62 , which are moving at a high speed. The lubricant oil Y located between the outer circumferential surface  491 ,  501  of each shaft seal  49 ,  50  and the circumferential surface  471 ,  481  of the corresponding recess  47 ,  48  flows with gas that is effectively urged from the side corresponding to the fifth pump chamber  43  to the side corresponding to the gear accommodating chamber  331 . The helical grooves  61 ,  62  formed in the outer circumferential surface  491 ,  501  of the shaft seals  49 ,  50  effectively prevent the oil Y from leaking into the fifth pump chamber  43  from the recesses  47 ,  48  via the spaces between the outer circumferential surfaces  491 ,  501  and the circumferential surfaces  471 ,  481 . 
     (1-11) Part of the lubricant oil Y guided from the side corresponding to the fifth pump chamber  43  toward the side corresponding to the gear accommodating chamber  331  with the helical grooves  61 ,  62  reaches the second end surface  722  of the third stopper  72 . The lubricant oil Y on the second end surface  722  is thrown to the third end surface  733  of the third oil chamber  73  by the centrifugal force generated by the rotation of the third stopper  72 . The thrown lubricant oil Y then reaches the third end surface  733 . That is, the third stopper  72  returns the lubricant oil Y, which is guided from the side corresponding to the fifth pump chamber  43  to the side corresponding to the gear accommodating chamber  331  by the helical grooves  61 ,  62 , to the gear accommodating chamber  331  via the third oil chamber  73 . 
     (1-12) A small space is created between the circumferential surface  192  of the first rotary shaft  19  and the through hole  141 . Also, a small space is created between each rotor  27 ,  32  and the wall forming surface  143  of the rear housing member  14 . Therefore, the labyrinth seal  57  is exposed to the pressure in the fifth pump chamber  43  introduced through the narrow spaces. Likewise, a small space is created between the circumferential surface  202  of the second rotary shaft  20  and the through hole  142 . Therefore, the second labyrinth seal  58  is exposed to the pressure in the fifth pump chamber  43  through the space. If there are no channels  63 ,  64 , the labyrinth seals  57 ,  58  are equally exposed to the pressure in the suction pressure zone  431  and to the pressure in the maximum pressure zone  432 . 
     The first and second discharge pressure introducing channels  63 ,  64  readily expose the labyrinth seals  57 ,  58  to the pressure in the maximum pressure zone  432 . That is, the labyrinth seals  57 ,  58  are influenced more by the pressure in the maximum pressure zone  432  via the introducing channels  63 ,  64  than by the pressure in the suction pressure zone  431 . Thus, compared to a case where no discharge pressure introducing channels  63 ,  64  are formed, the labyrinth seals  57 ,  58  of the illustrated embodiment receive higher pressure. As a result, compared to a case where no discharge pressure introducing channels  63 ,  64  are formed, the difference between the pressure acting on the front surface of the labyrinth seals  57 ,  58  and the pressure acting on the rear surface of the labyrinth seals  57 ,  58  is significantly small. In other words, the discharge pressure introducing channels  63 ,  64  significantly improves the oil leakage preventing performance of the labyrinth seals  57 ,  58 . 
     (1-13) Since the Roots pump  11  is a dry type, no lubricant oil Y is used in the five pump chambers  39 ,  40 ,  41 ,  42 ,  43 . Therefore, the present invention is suitable for the Roots pump  11 . 
     A second embodiment according to the present invention will now be described with reference to FIG.  9 . Mainly, the differences from the embodiment of FIGS. 1 to  8  will be discussed below. Since the sealing of the first and second rotary shafts  19 ,  20  have the same structure, only the sealing of the first rotary shaft  19  will be described. 
     As shown in FIG. 9, the leak prevention ring  75  is fitted about the small diameter portion  59  of the first shaft seal  49 . The circumferential surface  751  of the leak prevention ring  75  is defined at the portion projecting into the third oil chamber  73 . 
     A third embodiment according to the present invention is shown in FIG.  10 . Since the sealing of the first and second rotary shafts  19 ,  20  have the same structure, only the sealing of the first rotary shaft  19  will be described. The first shaft seal  49 A is integrally formed with the end surface of the first rotary shaft  19  and the fifth rotor  27 . The first shaft seal  49 A is fitted to a recess  76 , which is formed on the end surface of the rear housing  14  facing the rotor housing  12 . The labyrinth seal  77  is provided between the end surface of the first shaft seal  49 A and the bottom surface  761  of the recess  76 . 
     The leak prevention ring  78  is fitted about the first rotary shaft  19 . The annular oil chamber  79  is defined between the bottom surface  472  of the first recess  47  and the front end portion  69  of the bearing holder  45 . 
     The illustrated embodiments may be modified as follows. 
     (1) In the embodiment shown in FIGS. 1 to  8 , each shaft seal  49 ,  50  may be integrally formed with the corresponding leak prevention ring  66 . 
     (2) The present invention may be applied to other types of vacuum pumps than Roots types. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.