Patent Publication Number: US-6659747-B2

Title: Shaft seal structure of vacuum pumps

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to shaft seal structures of vacuum pumps that draw gas by operating a gas conveying body in a pump chamber through rotation of a rotary shaft. 
     Japanese Laid-open Patent Publication Nos. 60-145475, 3-89080, 6-101674 describe a vacuum pump that includes a plurality of rotors. Each rotor functions as a gas conveying body. Two rotors rotate as engaged with each other, thus conveying gas through a pump chamber. More specifically, one rotor is connected to a first rotary shaft and the other is connected to a second rotary shaft. A motor drives the first rotary shaft. A gear mechanism transmits the rotation of the first rotary shaft to the second rotary shaft. 
     The gear mechanism is located in an oil chamber that retains lubricant oil. The pump of Japanese Laid-open Patent Publication No. 60-145475 uses a labyrinth seal that seals the space between the oil chamber and the pump chamber to prevent the lubricant oil from leaking from the oil chamber to the pump chamber. More specifically, a partition separates the oil chamber from the pump chamber and has a through hole through which a rotary shaft extends. The labyrinth seal is fitted between the wall of the through hole and the corresponding portion of the rotary shaft. The pump of Japanese Laid-open Patent Publication No. 3-89080 includes a bearing chamber for accommodating a bearing that supports a rotary shaft. An intermediate chamber is formed between the bearing chamber and the pump chamber. A partition separates the bearing chamber from the intermediate chamber and has a through hole through which a rotary shaft extends. A labyrinth seal is fitted between the wall of the through hole and the rotary shaft. The pump of Japanese Laid-open Patent Publication No. 6-101674 includes a lip seal and a labyrinth seal. The seals are fitted between the wall of a through hole of a partition that separates the oil chamber from the pump chamber and a rotary shaft that extends through the through hole. 
     If the labyrinth seal includes a plurality of annular grooves, seal performance is maintained over time. Further, if the volume of each annular groove is relatively large, the seal performance of the labyrinth seal is improved. However, in the aforementioned vacuum pumps, it is difficult to increase the volume of each annular groove due to limited space. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to improve seal performance of a labyrinth seal that prevents oil from leaking to 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 present invention provides a vacuum pump that draws gas by operating a gas conveying body in a pump chamber through rotation of a rotary shaft. The vacuum pump includes an oil housing member, which forms 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. An annular shaft seal is located around the projecting section to rotate integrally with the rotary shaft. The shaft seal has a first seal forming surface that extends in a radial direction of the shaft seal. A second seal forming surface is formed on the oil housing member. The second seal forming surface opposes the first seal forming surface and is substantially parallel with the first seal forming surface. A labyrinth seal is located between the first and second seal forming surfaces. 
     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 objectives 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 showing a multiple-stage Roots pump of a first embodiment according to the present invention; 
     FIG.  1 ( b ) is an enlarged cross-sectional view showing a seal structure around a first rotary shaft of the pump of FIG.  1 ( a ); 
     FIG.  1 ( c ) is an enlarged cross-sectional view showing a seal structure around a second rotary shaft of the pump of FIG.  1 ( a ); 
     FIG.  2 ( a ) is a cross-sectional view taken along line  2   a — 2   a  of FIG.  1 ( a ); 
     FIG.  2 ( b ) is a cross-sectional view taken along line  2   b — 2   b  of FIG.  1 ( a ); 
     FIG.  3 ( a ) is a cross-sectional view taken along line  3   a — 3   a  of FIG.  1 ( a ); 
     FIG.  3 ( b ) is a cross-sectional view taken along line  3   b — 3   b  of FIG.  1 ( a ); 
     FIG.  4 ( a ) is a cross-sectional view taken along line  4   a — 4   a  of FIG.  3 ( b ); 
     FIG.  4 ( b ) is an enlarged cross-sectional view showing a major portion of FIG.  4 ( a ); 
     FIG.  4 ( c ) is a further enlarged cross-sectional view showing a portion of the seal structure of FIG.  4 ( b ); 
     FIG.  5 ( a ) is a cross-sectional view taken along line  5   a — 5   a  of FIG.  3 ( b ); 
     FIG.  5 ( b ) is an enlarged cross-sectional view showing a major portion of FIG.  5 ( a ); 
     FIG.  5 ( c ) is a further enlarged cross-sectional view showing a portion of the seal structure of FIG.  5 ( b ); 
     FIG. 6 is a perspective view showing a first annular shaft seal; 
     FIG. 7 is a perspective view showing a second annular shaft seal; 
     FIG. 8 is a cross-sectional view showing a major portion of a seal structure of a second embodiment according to the present invention; 
     FIG. 9 is a cross-sectional view showing a major portion of a seal structure of a third embodiment according to the present invention; 
     FIG. 10 is a cross-sectional view showing a major portion of a seal structure of a fourth embodiment according to the present invention; 
     FIG. 11 is a cross-sectional view showing a major portion of a seal structure of a fifth embodiment according to the present invention; 
     FIG. 12 is a cross-sectional view showing a major portion of a seal structure of a sixth embodiment according to the present invention; 
     FIG. 13 is a cross-sectional view showing a major portion of a seal structure of a seventh embodiment according to the present invention; and 
     FIG. 14 is a cross-sectional view showing a major portion of a seal structure of an eighth embodiment according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of a multiple-stage Roots pump  11  according to the present invention will now be described with reference to FIGS.  1 ( a ) to  7 . 
     As shown in FIG.  1 ( a ), the pump  11 , or a vacuum pump, includes a rotor housing member  12  and a front housing member  13 . The housing members  12 ,  13  are joined together. A lid  36  closes the front side of the front housing member  13 . A rear housing member  14  is connected to the rear side of the rotor housing member  12 . The rotor housing member  12  includes a cylinder block  15  and a plurality of (in this embodiment, four) chamber forming walls  16 . As shown in FIG.  2 ( b ), the cylinder block  15  includes a pair of block sections  17 ,  18 , and each chamber forming wall  16  includes a pair of wall sections  161 ,  162 . The chamber forming walls  16  are identical to one another. 
     As shown in FIG.  1 ( a ), a first pump chamber  39  is formed between the front housing member  13  and the leftmost chamber forming wall  16 , as viewed in the drawing. Second, third, and fourth pump chambers  40 ,  41 ,  42  are respectively formed between two adjacent chamber forming walls  16  in this order, as viewed from the left to the right in the drawing. A fifth pump chamber  43  is formed between the rear housing member  14  and the rightmost chamber forming wall  16 . 
     A first rotary shaft  19  is rotationally supported by the front housing member  13  and the rear housing member  14  through a pair of radial bearings  21 ,  37 . A second rotary shaft  20  is rotationally supported by the front housing member  13  and the rear housing member  14  through a pair of radial bearings  22 ,  38 . The first and second rotary shafts  19 ,  20  are parallel with each other and extend through the chamber forming walls  16 . The radial bearings  37 ,  38  are supported respectively by a pair of bearing holders  45 ,  46  that are installed in the rear housing member  14 . The bearing holders  45 ,  46  are fitted respectively in a pair of recesses  47 ,  48  that are formed in the rear side of the rear housing member  14 . 
     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 directions of 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  as engaged with each other. The second rotors  24 ,  29  are accommodated in the second pump chamber  40  as engaged with each other. The third rotors  25 ,  30  are accommodated in the third pump chamber  41  as engaged with each other. The fourth rotors  26 ,  31  are accommodated in the fourth pump chamber  42  as engaged with each other. The fifth rotors  27 ,  32  are accommodated in the fifth pump chamber  43  as engaged with each other. The first to fifth pump chambers  3943  are non-lubricated. Thus, the rotors  23 - 32  are maintained in a non-contact state with any of the cylinder block  15 , the chamber forming walls  16 , the front housing member  13 , and the rear housing member  14 . Further, the engaged rotors do not slide against each other. 
     As shown in FIG.  2 ( a ), the first rotors  23 ,  28  form 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 . The second to fourth rotors  24 - 26 ,  29 - 31  form similar suction zones and pressure zones in the associated pump chambers  40 - 42 . As shown in FIG.  3 ( a ), the fifth rotors  27 ,  32  form 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 with the rear housing member  14 . A pair of through holes  141 ,  142  are formed in the rear housing member  14 . The rotary shafts  19 ,  20  extend respectively through the through holes  141 ,  142  and the associated recesses  47 ,  48 . The rotary shafts  19 ,  20  thus project into the gear housing member  33  to form projecting portions  193 ,  203 , respectively. A pair of gears  34 ,  35  are secured respectively to the projecting portions  193 ,  203  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  4 ( b ), 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  is 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 rear housing member  14  functions as a partition that separates the fifth pump chamber  43  from the oil zone. The gears  34 ,  35  rotate to agitate the lubricant oil Y in the gear accommodating chamber  331 . The lubricant oil Y thus lubricates the radial bearings  37 ,  38 . A gap  371 ,  381  of each radial bearing  37 ,  38  allows the lubricant oil Y to enter a portion of the associated recess  47 ,  48  that is located inward from the gap  371 ,  381 . The recesses  47 ,  48  are thus connected to the gear accommodating chamber  331  through the gaps  371 ,  381  and form part of the oil zone. 
     As shown in FIG.  2 ( b ), a passage  163  is formed in the interior of each chamber forming wall  16 . Each chamber forming wall  16  has an inlet  164  and an outlet  165  that are connected to the passage  163 . The adjacent pump chambers  39 - 43  are connected to each other by the passage  163  of the associated chamber forming wall  16 . 
     As shown in FIG.  2 ( a ), an inlet  181  extends through the block section  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  extends through the block section  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  sends the gas to the pressure zone  392 . The gas is compressed in the pressure zone  392  and enters the passage  163  of the adjacent chamber forming wall  16  from the inlet  164 . The gas thus reaches the suction zone of the second pump chamber  40  from the outlet  165  of the passage  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, as repeating the above-described procedure. 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  sends 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 maximum pressure acts in the pressure zone  432  of the fifth pump chamber  43  such that 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 around the first and second rotary shafts  19 ,  20 , respectively. The shaft seals  49 ,  50  are located in the associated recesses  47 ,  48  and rotate integrally with the associated rotary shafts  19 ,  20 . A seal ring  51  is located between the inner circumferential side of the shaft seal  49  and a circumferential side  192  of the first rotary shaft  19 . In the same manner, a seal ring  52  is located between the inner circumferential side of the shaft seal  50  and a circumferential side  202  of the second rotary shaft  20 . Each seal ring  51 ,  52  prevents the lubricant oil Y from leaking from the associated recess  47 ,  48  to the fifth pump chamber  43  along the circumferential side  192 ,  202  of the associated rotary shaft  19 ,  20 . 
     As shown in FIGS.  4 ( b ),  4 ( c ),  5 ( b ), and  5 ( c ), there is a gap between an outer circumferential side  491 ,  501  of a portion with a maximum diameter of each shaft seal  49 ,  50  and the circumferential wall  471 ,  481  of the associated recess  47 ,  48 . Likewise, there is a gap between a front side  492 ,  502  of each shaft seal  49 ,  50  and a bottom  472 ,  482  of the associated recess  47 ,  48 . 
     A plurality of annular projections  53  coaxially project from the bottom  472  of the recess  47 . In the same manner, a plurality of annular projections  54  coaxially project from the bottom  482  of the recess  48 . Further, a plurality of annular grooves  55  are coaxially formed in the front side  492  of the shaft seal  49  that opposes the bottom  472  of the recess  47 . In the same manner, a plurality of annular grooves  56  are coaxially formed in the front side  502  of the shaft seal  50  that opposes the bottom  482  of the 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 front sides  492 ,  502  and the bottoms  472 ,  482  each form a plane perpendicular to the axis  191 ,  201  of the associated rotary shaft  19 ,  20 . In other words, the front sides  492 ,  502  and the bottoms  472 ,  482  are seal forming surfaces that extend in a radial direction of the associated shaft seals  49 ,  50 . 
     As shown in FIG.  4 ( c ), a resin layer  59  is securely applied on the front side  492  of the first shaft seal  49 . As shown in FIG.  5 ( c ), a resin layer  60  is securely applied on the front side  502  of the second shaft seal  50 . A gap g 1  between the resin layer  59  and the bottom  472  is smaller than a gap G 1  between the distal end of each projection  53  and the bottom of the associated groove  55 . A gap g 2  between the resin layer  60  and the bottom  482  is smaller than a gap G 2  between the distal end of each projection  54  and the bottom of the associated groove  56 . Each gap G 1 , G 2  is substantially equal to the gap between the outer circumferential side  491 ,  502  of the associated shaft seal  49 ,  50  and the circumferential wall  471 ,  481  of the recesses  47 ,  48 . The gap g 1  is a minimum gap between the first shaft seal  49  and the rear housing member  14 . The gap g 2  is a minimum gap between the second shaft seal  50  and the rear housing member  14 . In the present invention, the term “minimum gap” refers to a gap with a dimension that improves sealing of the labyrinth chambers. 
     As shown in FIGS.  1 ( b ),  4 ( b ), and  6 , a first helical groove  61  is formed in the outer circumferential side  491  of the first shaft seal  49 . As shown in FIGS.  1 ( c ),  5 ( b ), and  7 , a second helical groove  62  is formed in the outer circumferential side  501  of the second shaft seal  50 . The first helical groove  61  forms a path from a side corresponding to the gear accommodating chamber  331  toward the fifth pump chamber  43  as viewed in the rotational direction R 1  of the first rotary shaft  19 . The second helical groove  62  forms a path from a side corresponding to the gear accommodating chamber  331  toward the fifth pump chamber  43  as viewed in the rotational direction R 2  of the second rotary shaft  20 . In this manner, each helical groove  61 ,  62  brings out a pumping effect that conveys 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 a pumping means that urges the lubricant oil Y between the outer circumferential side  491 ,  501  of the associated shaft seal  49 ,  50  and the circumferential wall  471 ,  481  of the recess  47 ,  48  to move from a side corresponding to the fifth pump chamber  43  toward the oil zone. 
     As shown in FIG.  3 ( b ), first and second discharge pressure introducing lines  63 ,  64  are formed in a chamber forming wall surface  143  of the rear housing member  14  that forms the final-stage fifth pump chamber  43 . As shown in FIG.  4 ( a ), the first discharge pressure introducing line  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 line  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 line  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 ), an annular cooling chamber  65  is formed in the rear housing member  14  to surround the shaft seals  49 ,  50 . Coolant water circulates in the cooling chamber  65  to cool the lubricant oil Y in the recesses  47 ,  48 . 
     The first embodiment has the following effects. 
     The front side  492 ,  502  of each shaft seal  49 ,  50 , which is fitted around the associated rotary shaft  19 ,  20 , has a diameter larger than that of the circumferential side  192 ,  202  of the rotary shaft  19 ,  20 . In this embodiment, each labyrinth seal  57 ,  58  is located between the front side  492 ,  502  of the associated shaft seal  49 ,  50  and the bottom  472 ,  482  of the recess  47 ,  48 . Thus, as compared to the case in which a labyrinth seal is located between the circumferential side  192 ,  202  of each rotary shaft  19 ,  20  and the rear housing member  14 , the diameter of each labyrinth seal  57 ,  58  is relatively large. The larger the diameter of each labyrinth seal  57 ,  58  is, the greater the volume of each labyrinth chamber  551 ,  552 ,  561 ,  562  is. This improves the seal performance of the labyrinth seals  57 ,  58 . Thus, arrangement of each labyrinth seal  57 ,  58  of this embodiment is preferable in increasing the volume of each labyrinth chamber  551 ,  552 ,  561 ,  562  for improving the seal performance of the labyrinth seals  57 ,  58 . 
     The smaller the gap between the wall of each recess  47 ,  48  and the associated shaft seal  49 ,  50  is, the less likely it is for the lubricant oil Y to enter this gap. In this embodiment, the bottom  472 ,  482  of each recess  47 ,  48  and the front side  492 ,  502  of the associated shaft seal  49 ,  50  can be located close to each other in a uniform manner at the substantially entire area. This makes it easy to minimize the minimum gaps g 1 , g 2 . The smaller each minimum gap g 1 , g 2  is, the greater the seal performance of the associated labyrinth seal  57 ,  58  is. Accordingly, the location of each labyrinth seal  57 ,  58  of this embodiment is preferable. 
     When the Roots pump  11  is completely assembled, the resin layer  59 ,  60  of each shaft seal  49 ,  50  is in contact with the bottom  472 ,  482  of the associated recess  47 ,  48 . The recesses  47 ,  48  are located in the rear housing member  14  that is formed of metal. When the Roots pump  11  operates, the resin layers  59 ,  60  simply slide along the bottoms  472 ,  482  of the associated recesses  47 ,  48  without affecting rotation of each rotary shaft  19 ,  20 . 
     More specifically, when manufacturing the Roots pump  11 , the total (F 1 +d 1 ) of the depth F 1  of each annular groove  55  (see FIG.  4 ( c )) and the thickness d 1  of the resin layer  59  (see FIG.  4 ( c )) is selected to be slightly larger than the projecting amount H 1  of each annular projection  53  (see FIG.  4 ( c )). The first rotary shaft  19  and the first shaft seal  49  are then assembled together such that the resin layer  59  contacts the bottom  472  of the recess  47 . In this state, the first rotary shaft  19  is allowed to rotate smoothly. Likewise, the total (F 2 +d 2 ) of the depth F 2  of each annular groove  56  (see FIG.  5 ( c )) and the thickness d 2  of the resin layer  60  (see FIG.  5 ( c )) is selected to be slightly larger than the projecting amount H 2  of each annular projection  54  (see FIG.  5 ( c )). The second rotary shaft  20  and the second shaft seal  50  are then assembled together such that the resin layer  60  contacts the bottom  482  of the recess  48 . In this state, the second rotary shaft  20  is allowed to rotate smoothly. 
     Accordingly, each resin layer  59 ,  60  minimizes the minimum gap g 1 , g 2  between the shaft seal  49 ,  50  and the rear housing member  14 . If sealing of each labyrinth chamber  551 ,  552 ,  561 ,  562  is improved, the seal performance of each labyrinth seal  57 ,  58  is also improved. The improved sealing of the labyrinth chambers  551 ,  552 ,  562 ,  562  can be achieved by reducing the volume of each minimum gap g 1 , g 2 . That is, each resin layer  59 ,  60  of this embodiment improves the seal performance of the labyrinth seals  57 ,  58 . 
     As described, each resin layer  59 ,  50  contacts the bottom  472 ,  482  of the associated recess  47 ,  48  without hampering the rotation of each rotary shaft  19 ,  20 . Thus, locating each resin layer  59 ,  60  at the front side  492 ,  502  of the associated shaft seal  49 ,  50  is preferable in minimizing the minimum gaps g 1 , g 2 . 
     The labyrinth seals  57 ,  58  also stop gas leak. More specifically, when the Roots pump  11  operates, the pressure in each pump chamber  39 - 43  exceeds 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. 
     During the rotation of the first rotary shaft  19 , the first helical groove  61  of the first shaft seal  49  forms a path along the circumferential wall  471  of the recess  47 . This sends the lubricant oil Y corresponding to the path of the first helical groove  61  from a side corresponding to the fifth pump chamber  43  toward the gear accommodating chamber  331 . In the same manner, the second helical groove  62  of the second shaft seal  50  forms a path along the circumferential wall  481  of the recess  48  during the rotation of the second rotary shaft  20 . The lubricant oil Y corresponding to the path of the second helical groove  62  thus flows from a side corresponding to the fifth pump chamber  43  toward the gear accommodating chamber  331 . Accordingly, the shaft seals  49 ,  50  with the helical grooves  61 ,  62 , each of which functions as the pumping means, have an improved seal performance against the lubricant oil Y. 
     Each helical groove  61 ,  62  is located along the outer circumferential side  491 ,  501  of the associated shaft seal  49 ,  50 , or the outer circumferential side of the portion with the maximum diameter of the shaft seal  49 ,  50 . The circumferential speed thus becomes maximum at the portion at which each helical groove  61 ,  62  is located. Accordingly, each helical groove  61 ,  62  rotates at a relatively high speed. This efficiently urges the gas between the outer circumferential side  491 ,  501  of each shaft seal  49 ,  50  and the circumferential wall  471 ,  481  of the associated recess  47 ,  48  to move from a side corresponding to the fifth pump chamber  43  toward the gear accommodating chamber  331 . The lubricant oil Y between the outer circumferential side of  491 ,  501  of each shaft seal  49 ,  50  and the circumferential wall  471 ,  481  of the associated recess  47 ,  48  follows the movement of the gas, thus efficiently moving from a side corresponding to the fifth pump chamber  43  toward the gear accommodating chamber  331 . The location of each helical groove  61 ,  62  of this embodiment is thus preferable in preventing oil from leaking from the recesses  47 ,  48  to the fifth pump chamber  43 . 
     If the number of the rotation cycles of each helical groove  61 ,  62  increases, the seal performance of each shaft seal  49 ,  50  improves. Since it is relatively easy to increase the number of the rotation cycles of the each helical groove  61 ,  62 , the helical grooves  61 ,  62  are preferable pumping means. 
     Each rotary shaft  19 ,  20  includes a plurality of rotors that are formed integrally with the rotary shaft  19 ,  20 . Thus, if each shaft seal  49 ,  50  is formed integrally with the associated rotary shaft  19 ,  20 , the maximum diameter of the shaft seal  49 ,  50  must be selected with reference to the diameter of each through hole  141 ,  142  of the rear housing member  14 . However, in this embodiment, each shaft seal  49 ,  50  is formed separately from the associated rotary shaft  19 ,  20 . It is thus possible to shape and size the shaft seals  49 ,  50  to advantageously improve the pumping effect of the pumping means. 
     The circumferential side  192  of the first rotary shaft  19  forms a slight gap with respect to the wall of the through hole  141 . Also, each fifth rotor  27 ,  32  forms a slight gap with respect to the chamber forming wall surface  143  of the rear housing member  14 . These gaps introduce the pressure in the final-stage, fifth pump chamber  43  to the first labyrinth seal  57 . Further, the circumferential side  202  of the second rotary shaft  20  forms a slight gap with respect to the wall of the through hole  142 . The pressure in the fifth pump chamber  43  is thus introduced to the second labyrinth seal  58 . 
     Without the discharge pressure introducing lines  63 ,  64 , the labyrinth seals  57 ,  58  are equally affected by the pressure in the suction zone  431  and the pressure in the pressure zone  432  of the fifth pump chamber  43 . More specifically, if the pressure in the suction zone  431  is P 1  and the pressure in the maximum pressure zone  432  is P 2  (P 2 &gt;P 1 ), each labyrinth seal  57 ,  58  receives about half the total of the pressures P 1 , P 2  ((P 2 +P 1 )/2) from the fifth pump chamber  43 . 
     The pressure in each recess  47 ,  48 , which is connected to the gear accommodating chamber  331 , corresponds to the atmospheric pressure (approximately 1000 Torr) that remains non-affected by operation of each rotor  23 - 32 . The pumping effect of the helical grooves  61 ,  62  reduces the pressure in the space between each shaft seal  49 ,  50  and the wall of the associated recess  47 ,  48  to a level P 3  lower than the atmospheric pressure at the portion between each helical groove  61 ,  62  and the associated labyrinth seal  57 ,  58 . Accordingly, if the pump  11  does not have the discharge pressure introducing lines  63 ,  64 , the pressure difference between the radial inner end and the radial outer end of each labyrinth seal  57 ,  58  becomes approximately P 3 −(P 2 +P 1 )/2. 
     Each discharge pressure introducing line  63 ,  64  of this embodiment improves the effect of introducing the pressure in the maximum pressure zone  432  to the associated labyrinth seals  57 ,  58 . That is, the effect of introducing the pressure in the maximum pressure zone  432  to the labyrinth seals  57 ,  58  through the discharge pressure introducing lines  63 ,  64  dominates the effect of introducing the pressure in the suction zone  431  to the labyrinth seals  57 ,  58 . Thus, the pressure received by each labyrinth seal  57 ,  58  becomes much larger than the aforementioned value (P 2 +P 1 )/2. Accordingly, the pressure difference between the radial inner end and the radial outer end of each labyrinth seal  57 ,  58  becomes much smaller than the value P 3 −(P 2 +P 1 )/2. As a result, the oil leak preventing effect of each labyrinth seal  57 ,  58  is improved. 
     The effect of introducing the pressure in the maximum pressure zone  432  to each labyrinth seal  57 ,  58  depends on the communication area of each discharge pressure introducing line  63 ,  64 . Since the discharge pressure introducing line  63 ,  64  with a desired communication area is easy to accomplish, the discharge pressure introducing lines  63 ,  64  optimally introduce the pressure in the maximum pressure zone  432  to the labyrinth seals  57 ,  58 . 
     The discharge pressure introducing lines  63 ,  64  are located in the chamber forming wall surface  143  that forms the fifth pump chamber  43 . Each through hole  141 ,  142 , through which the associated rotary shaft  19 ,  20  extends, is formed in the chamber forming wall surface  143 . The maximum pressure zone  432  of the fifth pump chamber  43  faces the chamber forming wall surface  143 . Accordingly, each discharge pressure introducing line  63 ,  64  is readily formed in the chamber forming wall surface  143  such that the line  63 ,  64  is connected to the maximum pressure zone  432  and the associated through hole  141 ,  142 . 
     If the Roots pump  11  is a dry type, the lubricant oil Y does not circulate in any pump chamber  39 - 43 . It is preferred that the present invention be applied to this type of pump. 
     The present invention may be modified, as shown in second to eight embodiments of FIGS. 8 to  14 . Although only the labyrinth seal for the first rotary shaft  19  is illustrated in FIGS. 8 to  13 , an identical labyrinth seal is provided for the second rotary shaft  20  of these embodiments. 
     In the second embodiment, as shown in FIG. 8, a plurality of annular projections  66  that project from the front side  492  of the shaft seal  49  oppose the annular projections  53 , which project from the bottom  472  of the recess  47 . A resin layer  67  is formed at the distal end of each projection  66 . The annular projections  66 ,  53  form a labyrinth seal. 
     As shown in FIG. 9, the third embodiment does not include the annular projections  53  that otherwise project from the bottom  472  of the recess  47 , unlike the first embodiment. Instead, the annular grooves  55  formed in the shaft seal  49  form a labyrinth seal. 
     As shown in FIG. 10, the fourth embodiment does not include the annular grooves  55  that are otherwise formed in the shaft seal  49 , unlike the first embodiment. Instead, the annular projections  53  projecting from the bottom  472  of the recess  47  form a labyrinth seal. A resin layer  68  is formed at the distal end of each projection  53 . 
     As shown in FIG. 11, the fifth embodiment does not include the annular projections  53  that otherwise project from the bottom  472  of the recess  47 , unlike the first embodiment. Instead, the annular grooves  55  of the shaft seal  49  form a labyrinth seal. A resin layer  69  is formed on the bottom  472  of the recess  47 . 
     As shown in FIG. 12, the sixth embodiment does not include the annular grooves  55  that are otherwise formed in the shaft seal  49 , unlike the first embodiment. Instead, the annular projections  53  projecting from the bottom  472  of the recess  47  form a labyrinth seal. A resin layer  70  is formed at the front side  492  of the shaft seal  49 . 
     In the seventh embodiment, as shown in FIG. 13, a shaft seal  49 A is formed integrally with the rotary shaft  19  and is connected to the fifth rotor  27 . The shaft seal  49 A is accommodated in a recess  71  formed in the side of the rear housing member  14  that opposes the rotor housing member  12 . A labyrinth seal  72  is located between the rear side of the shaft seal  49 A and a bottom  711  of the recess  71 . 
     As shown in FIG. 14, the eighth embodiment includes a pair of shaft seals  49 B,  50 B. A pair of rubber sliding rings  73 ,  74  are respectively fitted around the shaft seals  49 B,  50 B. A plurality of leak preventing projections  731  are formed around the sliding ring  73 , and a plurality of leak preventing projections  741  are formed around the sliding ring  74 . When the first rotary shaft  19  rotates, the leak preventing projections  731  slide along the circumferential wall  471  of the recess  47  in a contact manner. Likewise, when the second rotary shaft  20  rotates, the leak preventing projections  741  slide along the circumferential wall  481  of the recess  48  in a contact manner. Each leak preventing projection  731 ,  741  does not cover the entire circumference around the axis of the associated shaft seal  49 B,  50 B, or the axis  191 ,  201  of the associated rotary shaft  19 ,  20 , and is formed diagonally with respect to the axis  191 ,  201 . Each leak preventing projection  731 ,  741  forms a path from a side corresponding to the gear accommodating chamber  331  toward the fifth pump chamber  43 , as viewed in the rotational direction R 1 , R 2  of the associated rotary shaft  19 ,  20 . 
     When the first rotary shaft  19  rotates, the leak preventing projections  731  urge the lubricant oil Y between the circumferential wall  471  of the recess  47  and the outer circumferential side of the first shaft seal  49 B to move from a side corresponding to the fifth pump chamber  43  toward the gear accommodating chamber  331 . In the same manner, when the second rotary shaft  20  rotates, the leak preventing projections  741  urge the lubricant oil Y between the circumferential wall  481  of the recess  48  and the outer circumferential side of the second shaft seal  50 B to move from a side corresponding to the fifth pump chamber  43  toward the gear accommodating chamber  331 . 
     If a single leak preventing projection is formed around the entire circumference around the axis  191 ,  201  of each rotary shaft  19 ,  20 , the axial dimension of each sliding ring  73 ,  74  needs to be enlarged. In this case, the resistance to the sliding of each sliding ring  73 ,  74  becomes relatively large, which is not preferable. In contrast, the leak preventing projections  731 ,  741  of the eighth embodiment do not require the enlargement of the axial dimensions of the sliding rings  73 ,  74 . 
     The present invention may be modified as follows. 
     The bottom of each recess  47 ,  48  and the front side of the associated shaft seal  49 ,  50  may be tapered such that a labyrinth seal is located between the opposed tapered surfaces. 
     In the first embodiment, a resin layer may be applied at the distal end of each projection  53 ,  54 . 
     A resin plate may be located between the bottom  472 ,  482  of each recess  47 ,  48  and the front side  492 ,  502  of the associated shaft seal  49 ,  50 , thus forming a resin layer. 
     The present invention may be applied to other types of vacuum pumps than Roots types. 
     The present example 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.