Patent Publication Number: US-6663367-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, 2-157490, 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. 2-157490 employs a lip seal that seals the space between an oil chamber and a pump chamber. 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. 
     However, it is difficult to reliably stop an oil leak only with a lip seal or a labyrinth seal. For example, in the pump of Japanese Laid-open Publication No. 6-101674, which uses the lip seal and the labyrinth seal, if the life of the lip seal comes to an end, the oil leak must be stopped only by the labyrinth seal. The stopping of the oil leak thus becomes less reliable. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to improve an effect of a vacuum pump of preventing oil from leaking to a pump chamber. 
     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 opposes the oil housing member. A second seal forming surface is formed on the oil housing member. The second seal forming surface opposes the first seal forming surface. A pumping means is formed at the first seal forming surface. The pumping means urges oil between the first and second seal forming surfaces to move from a side corresponding to the pump chamber toward the oil zone when the rotary shaft rotates. 
     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.  2 ( c ) is a cross-sectional view taken along line  2   c — 2   c  of FIG.  1 ( a ); 
     FIG. 3 is an enlarged cross-sectional view showing a main portion of the Roots pump of FIG.  1 ( a ); 
     FIG.  4 ( a ) is an enlarged plan view showing a main portion of a seal structure fitted around a first rotary shaft; 
     FIG.  4 ( b ) is an enlarged plan view showing a main portion of a seal structure fitted around a second rotary shaft; 
     FIG. 5 is an enlarged cross-sectional view showing a main portion of a seal structure of a second embodiment according to the present invention; 
     FIG. 6 is an enlarged cross-sectional view showing a main portion of a seal structure of a third embodiment according to the present invention; 
     FIG. 7 is an enlarged cross-sectional view showing a main portion of a seal structure of a fourth embodiment according to the present invention; 
     FIG. 8 is an enlarged cross-sectional view showing a main portion of a seal structure of a fifth embodiment according to the present invention; 
     FIG.  9 ( a ) is a cross-sectional view showing a sixth embodiment of the present invention and corresponding to FIG.  2 ( c ); 
     FIG.  9 ( b ) is a cross-sectional view showing the Roots pump of the sixth embodiment, as taken along the boundary between a cylinder block and a rear housing member; 
     FIG.  10 ( a ) is a cross-sectional view taken along line  10   a — 10   a  of FIG.  9 ( b ); and 
     FIG.  10 ( b ) is a cross-sectional view taken along line  10   b — 10   b  of FIG.  9 ( b ). 
    
    
     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 to  4 ( b ). 
     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. Each pump chamber  39 - 43  is divided by the associated rotors  23 - 32  into a suction zone and a pressure zone. The pressure in the pressure zone is higher than the pressure in the suction zone. 
     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  (see FIG.  3 ). 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  2 ( c ). 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  2 ( c ). 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. 
     A gear accommodating chamber  331  is formed in the gear housing member  33  and retains lubricant oil (not shown) 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 in the gear accommodating chamber  331 . The lubricant oil thus lubricates the radial bearings  37 ,  38 . A gap  371 ,  381  of each radial bearing  37 ,  38  allows the lubricant oil 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 of the first pump chamber  39 . As shown in FIG.  2 ( c ), an outlet  171  extends through the block section  17  of the cylinder block  15  and is connected to the pressure zone of the fifth pump chamber  43 . When gas enters the first pump chamber  39  from the inlet  181 , rotation of the first rotors  23 ,  28  sends the gas to 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. After the gas reaches the fifth pump chamber  43 , the gas is 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. 
     As shown in FIGS.  1 ( a ) and  3 , 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 . 
     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 . 
     As shown in FIGS.  3  and  4 ( a ), a first helical groove  55  is formed in the outer circumferential side  491  of the first shaft seal  49 . As shown in FIGS.  3  and  4 ( b ), a second helical groove  56  is formed in the outer circumferential side  501  of the second shaft seal  50 . The first helical groove  55  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  56  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  55 ,  56  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  55 ,  56  forms a pumping means that urges the lubricant oil 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. The outer circumferential side  491 ,  501  of each shaft seal  49 ,  50  and the circumferential wall  471 ,  481  of the associated recess  47 ,  48  form opposed seal forming surfaces. 
     As shown in FIGS. 3,  4 ( a ), and  4 ( b ), a labyrinth seal  53  is formed between the wall of the through hole  141  of the rear housing member  14  and the circumferential side  192  of the first rotary shaft  19 . Further, a labyrinth seal  54  is formed between the wall of the through hole  142  of the rear housing member  14  and the circumferential side  202  of the second rotary shaft  20 . A plurality of annular grooves  531 ,  541  are formed respectively around the circumferential sides  192 ,  202  of the rotary shafts  19 ,  20 . Each labyrinth seal  53 ,  54  is formed by the associated annular grooves  531 ,  541 . The annular grooves  531 ,  541  are aligned along the axis of the associated rotary shaft  19 ,  20 . 
     The first embodiment has the following effects. 
     Each seal ring  51 ,  52 , which is located between the shaft seal  49 ,  50  and the associated rotary shaft  19 ,  20 , prevents lubricant oil from leaking from the associated recess  47 ,  48  to the fifth pump chamber  43  along the circumferential side  192 ,  202  of the rotary shaft  19 ,  20 . Further, during the rotation of the first rotary shaft  19 , the first helical groove  55  of the first shaft seal  49  forms a path along the circumferential wall  471  of the recess  47 . This sends the lubricant oil corresponding to the path of the first helical groove  55  from a side corresponding to the fifth pump chamber  43  toward the gear accommodating chamber  331 . In the same manner, the second helical groove  56  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 corresponding to the path of the second helical groove  56  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  55 ,  56 , each of which functions as the pumping means, have an improved seal performance against the lubricant oil. 
     Each helical groove  55 ,  56  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  55 ,  56  is located. Accordingly, each helical groove  55 ,  56  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 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  55 ,  56  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  55 ,  56  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  55 ,  56 , the helical grooves  55 ,  56  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. 
     If lubricant oil leaks from the space 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 the through hole  141 ,  142 , each labyrinth seal  53 ,  54  prevents the lubricant oil from entering the fifth pump chamber  43 . 
     The labyrinth seals  53 ,  54  also function as gas seals. More specifically, the pressure in each pump chamber  39 - 43  becomes higher than the atmospheric pressure immediately after the Roots pump  11  is started. In this state, the labyrinth seals  53 ,  54  prevent gas from leaking from the fifth pump chamber  43  to the gear accommodating chamber  331  along the circumferential sides of the rotary shafts  19 ,  20 . The labyrinth seals  53 ,  54  thus function as oil seals and gas seals and are optimal non-contact type seal means. 
     If the Roots pump  11  is a dry type, the lubricant oil does not circulate in any pump chamber  39 - 43 . It is preferred that the present invention be applied to this type of pump. 
     Next, a second embodiment of the present invention will be described with reference to FIG.  5 . The description focuses on the difference between the first embodiment, which is illustrated in FIGS. 1 to  4 ( b ), and the second embodiment. 
     In the second embodiment, a pair of rubber lip seals  57 ,  58  replace the labyrinth seals  53 ,  54  of FIG.  3 . The lip seals  57 ,  58  are fitted respectively in the through holes  141 ,  142 . Each lip seal  57 ,  58  contacts and slide along the circumferential side  192 ,  202  of the associated rotary shaft  19 ,  20 . If lubricant oil leaks from the space 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 the through hole  141 ,  142 , each lip seal  57 ,  58  prevents the lubricant oil from entering the fifth pump chamber  43 . 
     A third embodiment of the present invention will be described with reference to FIG.  6 . The description focuses on the difference between the first embodiment, which is illustrated in FIGS. 1 to  4 ( b ), and the third embodiment. 
     In the third embodiment, a portion of a recess  47 A forms a tapered surface  471 A and a portion of a recess  48 A forms a tapered surface  481 A. Further, the outer circumferential sides of a pair of shaft seals  49 A,  50 A form tapered surfaces  491 A,  501 A, respectively. A pair of helical grooves  55 A,  56 A are formed respectively in the tapered surfaces  491 A,  501 A. The diameter of each tapered surface  491 A,  501 A, or each helical groove  55 A,  56 A, becomes gradually larger, as viewed from the fifth pump chamber  43  toward the gear accommodating camber  331 . Thus, when the helical grooves  55 A,  56 A rotate, centrifugal force acts advantageously to urge lubricant oil to move from a side corresponding to the fifth pump chamber  43  toward the gear accommodating chamber  331 . 
     Next, a fourth embodiment of the present invention will be described with reference to FIG.  7 . The description focuses on the difference between the first embodiment, which is illustrated in FIGS. 1 to  4 ( b ), and the fourth embodiment. 
     This embodiment includes a pair of shaft seals  49 B,  50 B. A pair of rubber sliding rings  59 ,  60  are respectively fitted around the shaft seals  49 B,  50 B. A plurality of leak preventing projections  591  are formed around the sliding ring  59 , and a plurality of leak preventing projections  601  are formed around the sliding ring  60 . When the first rotary shaft  19  rotates, the leak preventing projections  591  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  601  slide along the circumferential wall  481  of the recess  48  in a contact manner. Each leak preventing projection  591 ,  601  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  591 ,  601  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 Rl, R 2  of the associated rotary shaft  19 ,  20 . 
     When the first rotary shaft  19  rotates, the leak preventing projections  591  urge the lubricant oil 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  601  urge the lubricant oil 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  59 ,  60  needs to be enlarged. In this case, the resistance to the sliding of each sliding ring  59 ,  60  becomes relatively large, which is not preferable. In contrast, the leak preventing projections  591 ,  601  of the fourth embodiment do not require the enlargement of the axial dimensions of the sliding rings  59 ,  60 . 
     A fifth embodiment of the present invention will hereafter be described with reference to FIG.  8 . The description focuses on the difference between the first embodiment, which is illustrated in FIGS. 1 to  4 ( b ), and the fifth embodiment. 
     A shaft seal  49 C is formed integrally with the first rotary shaft  19  and is connected to the fifth rotor  27 . In the same manner, a shaft seal  50 C is formed integrally with the second rotary shaft  20  and is connected to the fifth rotor  32 . A pair of recesses  61 ,  62  are formed in a wall of the rear housing member  14  that opposes the rotor housing member  12 . The shaft seals  49 C,  50 C are fitted respectively in the recesses  61 ,  62 . A labyrinth seal  53  is formed between the outer circumferential side of the shaft seal  49 C and a circumferential wall  611  of the recess  61 . A labyrinth seal  54  is formed between the outer circumferential side of the shaft seal  50 C and a circumferential wall  621  of the recess  62 . A first helical groove  63  is formed in a side of the shaft seal  49 C that opposes a bottom  612  of the recess  61 , and a second helical groove  64  is formed in a side of the shaft seal  50 C that opposes a bottom  622  of the recess  62 . 
     Each helical groove  63 ,  64  defines a path toward the axis of the associated shaft seal  49 C,  50 C, as viewed in the rotational direction R 1 , R 2  of the associated rotary shaft  19 ,  20 . Thus, when the rotary shafts  19 ,  20  rotate, the helical grooves  63 ,  64  bring out a pumping effect, or send fluid from a side corresponding to the fifth pump chamber  43  toward the gear accommodating chamber  331 . 
     A sixth embodiment of the present invention will hereafter be described with reference to FIGS.  9 ( a ) to  10 ( b ). The description focuses on the difference between the first embodiment, which is illustrated in FIGS. 1 to  4 ( b ), and the sixth embodiment. 
     As shown in FIG.  9 ( a ), after having been sent from the fourth pump chamber  42  to the suction zone  431  of the fifth pump chamber  43 , refrigerant gas reaches the pressure zone  432  and is discharged to the exterior from the outlet  171  through rotation of the fifth rotors  27 ,  32 . 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 FIGS.  9 ( a ) to  10 ( b ), first and second discharge pressure introducing lines  65 ,  66  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 FIGS.  9 ( b ) and  10 ( a ), the first discharge pressure introducing line  65  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  65  is connected also to the through hole  141  through which the first rotary shaft  19  extends. As shown in FIGS.  9 ( b ) and  10 ( b ), the second discharge pressure introducing line  66  is connected to the maximum pressure zone  432  and the through hole  142  through which the second rotary shaft  20  extends. 
     The sixth embodiment has the following effects. 
     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 helical groove  55 . 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 helical groove  56 . 
     Without the discharge pressure introducing lines  65 ,  66 , the helical grooves  55 ,  56  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 helical groove  55 ,  56  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 . 
     Each discharge pressure introducing line  65 ,  66  of this embodiment improves the effect of introducing the pressure in the maximum pressure zone  432  to the associated helical grooves  55 ,  56 . That is, the effect of introducing the pressure in the maximum pressure zone  432  to the helical grooves  55 ,  56  through the discharge pressure introducing lines  65 ,  66  dominates the effect of introducing the pressure in the suction zone  431  to the helical grooves  55 ,  56 . Thus, the pressure received by each helical groove  55 ,  56  becomes much larger than the aforementioned value (P 2 +P 1 )/ 2 . Accordingly, the pressure difference between an end closest to the fifth pump chamber  43  and an end closest to the gear accommodating chamber  331  of each helical groove  55 ,  56  becomes much smaller than the value [1000−(P 2 +P 1 )/ 2 ]Torr. As a result, the oil leak preventing effect of each helical groove  55 ,  56  is improved. 
     The effect of introducing the pressure in the maximum pressure zone  432  to each helical groove  55 ,  56  depends on the communication area of each discharge pressure introducing line  65 ,  66 . Since the discharge pressure introducing line  65 ,  66  with a desired communication area is easy to accomplish, the discharge pressure introducing lines  65 ,  66  optimally introduce the pressure in the maximum pressure zone  432  to the helical grooves  55 ,  56 . 
     The discharge pressure introducing lines  65 ,  66  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  65 ,  66  is connected to the maximum pressure zone  432  and the associated through hole  141 ,  142 . 
     The present invention may be modified as follows. 
     In the fourth embodiment of FIG. 7, the shaft seals  49 B,  50 B may be formed of rubber. Further, a leak preventing projection may be formed integrally with each seal  49 B,  50 B at the circumferential side of the shaft seal  49 B,  50 B. 
     In the fifth embodiment of FIG. 8, each labyrinth seal  53 ,  54  may be replaced by a helical groove formed in the circumferential side of the associated shaft seal  49 C,  50 C. 
     A helical groove may be formed in a side of the rear housing member  14  that opposes the rotor housing member  12 . 
     The present invention may be applied to other types of vacuum pumps than the Roots type. 
     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.