Oil leak prevention structure of vacuum pump

Abstract

A 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. The oil housing member defines an oil zone adjacent to the pump chamber. The rotary shaft has a projecting portion that projects from the pump chamber into the oil zone through the oil housing member. Stoppers are located on the rotary shaft to integrally rotate with the rotary shaft and prevent oil from entering the pump chamber. The stoppers are located along the axial direction of the rotary shaft.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an oil leak prevention structure of vacuum pumps that draw gas by operating a gas conveying body in a pump chamber through rotation of a rotary shaft.

In a typical vacuum pump, lubricant oil is used for lubricating moving parts. Japanese Laid-Open Patent Publications No. 63-129829 and No. 3-11193 disclose vacuum pumps having structures for preventing oil from entering zones where presence of lubricant oil is undesirable.

In the vacuum pump disclosed in Publication No. 63-129829, a plate for preventing oil from entering a generator chamber is attached to a rotary shaft. Specifically, when moving along the surface of the rotary shaft toward the generator chamber, oil reaches the plate. The centrifugal force generated by rotation 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 a drain passage connected to the lower portion.

The vacuum pump disclosed in 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), which rotates integrally with the rotary shaft, is a mechanism that prevents oil from entering undesirable zones. When centrifugal force generated by rotation of a plate (slinger) is used for preventing oil from entering a certain zone, the effectiveness is influenced by the shapes of the plate (slinger) and 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. The oil housing member defines an oil zone adjacent to the pump chamber. The rotary shaft has a projecting portion that projects from the pump chamber into the oil zone through the oil housing member. Stoppers are located on the rotary shaft to integrally rotate with the rotary shaft and prevent oil from entering the pump chamber. The stoppers are located along the axial direction of the rotary shaft.

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 partial cross-sectional view of the pump shown in FIG. 4 ( a ); FIG. 4 ( c ) is an enlarged partial cross-sectional view of the pump shown in FIG. 4 ( b );

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 partial cross-sectional view of the pump shown in FIG. 5 ( a ); FIG. 5 ( c ) is an enlarged partial cross-sectional view of the pump shown in FIG. 5 ( b );

FIG. 6 a is an enlarged cross-sectional view of the pump shown in FIG. 1 ( a ); FIG. 6 ( b ) is an enlarged partial cross-sectional view of the pump shown in FIG. 6 ( a );

FIG. 7 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. 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;

FIG. 10 is an enlarged cross-sectional view illustrating a third embodiment of the present invention; and

FIG. 11 is an enlarged cross-sectional view illustrating a fourth embodiment of the present invention;

FIG. 12 is an enlarged cross-sectional view illustrating a fifth embodiment of the present invention; and

FIG. 13 is an enlarged cross-sectional view illustrating a sixth 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, a 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 to each other. The rotary shafts 19 , 20 extend through the chamber defining walls 16 . The radial bearings 37 are supported by bearing holders 45 . Two bearing receptacles 47 , 48 are formed in end 144 of the rear housing member 14 . The bearings holders 45 are fitted in the bearing receptacles 47 , 48 , 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. 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. The third rotors 23 , 28 are accommodated in the third pump chamber 39 and are engaged with each other. The fourth rotors 24 , 29 are accommodated in the fourth 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 pressurization zone 392 in the first pump chamber 39 . The pressure in the pressurization 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 391 and pressurization zones 392 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 pressurization zone 432 , which are similar to the suction zone 391 and the pressurization 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 bearing receptacles 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 rotates the first rotary shaft 19 in the direction indicated by arrow RI 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 cause the rotary shafts 19 , 20 to rotate integrally.

As shown in FIGS. 4 ( a ) and 5 ( a ), a gear accommodating chamber 331 is defined in the gear housing member 33 . The gear accommodating chamber 331 retains lubricant oil Y for lubricating the gears 34 , 35 . The gears 34 , 35 form a gear mechanism, which is accommodated in the gear accommodating chamber 331 . The gear accommodating chamber 331 and the bearing receptacles 47 , 48 form a sealed oil zone. The gear housing member 33 and the rear housing member 14 form an oil housing, or an oil zone adjacent to the fifth pump chamber 43 . 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 .

As shown in FIG. 2 ( b ), a passage 163 is formed in the interior of each chamber defining wall 16 . Each chamber defining wall 16 has an inlet 164 and an outlet 165 that are connected to the passage 163 . Each adjacent pair of the pump chambers 39 - 43 are connected to each other by the passage 163 of the associated chamber defining 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 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 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 pressurization zone 392 . In the pressurization zone 392 , the gas is compressed and its pressure is higher than in the suction zone 391 . Thereafter, the gas is sent to the suction zone of the second pump chamber 40 through the inlet 164 , the passage 163 , and the outlet 165 in the corresponding wall defining wall 16 . 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 pressurization 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 pressurization zones of the first to fifth pump chambers 39 - 43 , the pressure in the pressurization zone 432 of the fifth pump chamber 43 is the highest, and the pressurization zone 432 functions as a maximum pressurization zone. The outlet 171 is connected to the maximum pressurization zone 432 defined by the fifth rotors 27 , 32 in the fifth pump chamber 43 .

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. The shaft seals 49 , 50 are located in the first and second bearing receptacles 47 , 48 , respectively. A seal ring 51 is located between the inner circumferential surface of the first shaft seal 49 and the circumferential surface 192 of the first rotary shaft 19 . Likewise, a seal ring 52 is located between the inner circumferential surface of the second shaft seal 50 and the circumferential surface 202 of the second rotary shaft 20 . Each seal ring 51 , 52 prevents lubricant oil Y from leaking from the associated receptacle 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 ( b ), space exists between the outer circumferential surface 491 of the large diameter portion 60 of the first shaft seal 49 and the circumferential wall 471 of the first receptacle 47 . Also, as shown in FIG. 5 ( b ), space exists between the outer circumferential surface 501 of the large diameter portion 80 of the second shaft seal 50 and the circumferential wall 481 of the second receptacle 48 . Also, space exists between the front surface 492 of the first shaft seal 49 and the bottom 472 of the first receptacle 47 , and space exists between the front surface 502 of the second shaft seal 50 and the bottom 482 of the second receptacle 48 . The shaft seals 49 , 50 rotate integrally with the rotary shafts 19 , 20 , respectively.

Annular projections 53 coaxially project from the bottom 472 of the first receptacle 47 . In the same manner, annular projections 54 coaxially project from the bottom 482 of the second receptacle 48 . Annular grooves 55 are coaxially formed in the front surface 492 of the first shaft seal 49 , which faces the bottom 472 of the first receptacle 47 . In the same manner, annular grooves 56 are coaxially formed in the front surface 502 of the second shaft seal 50 , which faces the bottom 482 of the second receptacle 48 . Each annular projection 53 , 54 projects in the associated groove 55 , 56 . 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 . The front surfaces 492 , 502 of the shaft seals 49 , 50 function as sealing surface of the shaft seals 49 , 50 . The bottoms 472 , 482 of the bearing receptacles 47 , 48 function as sealing surface of the rear housing member 14 . In this embodiment, the front surface 492 and the bottom 472 are formed along a plane perpendicular to the axis 191 of the first rotary shaft 19 . Likewise, the front 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 front surface 492 and the bottom 472 are seal forming surfaces that extend in a radial direction of the first shaft seal 49 . Likewise, the front surface 502 and the bottom 482 are seal forming surfaces that extend in a radial direction of the second shaft seal 50 .

As shown in FIGS. 4 ( b ) and 7 , a second 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 60 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 exerts a pumping effect and 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 pumping means that urges the lubricant oil between the outer circumferential surface 491 , 501 of the associated shaft seal 49 , 50 and the circumferential wall 471 , 481 of the associated receptacle 47 , 48 to move from a side corresponding to the fifth pump chamber 43 toward the oil zone. The circumferential walls 471 , 481 of the bearing receptacles 47 , 48 function as sealing surfaces. The outer circumferential surfaces 491 , 501 face the sealing surfaces.

As shown in FIG. 3 ( b ), first and second discharge pressure introducing channels 63 , 64 are formed in a chamber defining wall 143 of the rear housing member 14 . The chamber defining wall 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 pressurization zone 432 , the volume of which is varied by rotation of the fifth rotors 27 , 32 . The first discharge pressure introducing channel 63 is also connected to the through hole 141 . As shown in FIG. 5 ( a ), the second discharge pressure introducing channel 64 is connected to the maximum pressurization zone 432 and the through hole 142 .

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 circulates in the loop chamber 65 . Coolant in the loop chamber 65 cools the lubricant oil Y in the bearing receptacles 47 , 48 . This prevents the lubricant oil Y from evaporating.

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. A front end portion of the bearing holder 45 has an annular projection 69 projecting inward and 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 .

The first oil stopper 67 has a tapered circumferential surface 671 . The distance between the tapered circumferential surface 671 and the axis 191 of the first rotary shaft 19 increases from the side corresponding to the fifth pump chamber 43 toward the side corresponding to the gear accommodating chamber 331 .

A circumferential surface 671 of the first stopper 67 is located in the first oil chamber 70 , and a circumferential surface 681 of the second stopper 68 is located in the second oil chamber 71 . The circumferential surface 671 faces a circumferential wall surface 702 , which defines the first oil chamber 70 . The circumferential surface 681 of the second stopper 68 faces a circumferential wall surface 712 , which defines the second oil chamber 71 .

The rear surface 672 of the first stopper 67 faces a wall surface 701 , which defines the first oil chamber 70 . The rear surface 682 , which is located at the right side as viewed in FIG. 6 , of the second stopper 68 faces a end surface 711 , which defines the second oil chamber 71 . The front surface 683 of the second stopper 68 faces and is widely separated from the rear surface 601 of the large diameter portion 60 of the first shaft seal 49 .

The rear surface 682 is perpendicular to the axis 191 of the rotary shaft 19 and blocks flow of oil. The tapered circumferential surface 671 is located adjacent to the rear surface 682 at the side closer to the gear accommodating chamber 331 . The tapered circumferential surface 671 starts from the proximal end 684 of the rear surface 682 . The surface of an imaginary cone that includes the tapered circumferential surface 671 intersects the end surface 701 of the first oil chamber 70 .

The third stopper 72 is integrally formed with the large diameter portion 60 of the first shaft seal 49 . A third annular oil chamber 73 is defined in the first receptacle 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 wall surface 733 defining the third oil chamber 73 . The rear surface 601 of the third stopper 72 faces and is located in the vicinity of an end surface 731 defining the third oil chamber 73 . The front surface 722 of the third stopper 72 faces and is located in the vicinity of a wall 732 defining the third oil chamber 73 .

The radiuses of the stoppers 67 , 68 , 72 decrease from the side corresponding to the fifth pump chamber 43 toward the gear accommodating chamber 331 . Likewise, the radiuses of the oil chambers 70 , 71 , 73 decrease from the side corresponding to the fifth pump chamber 43 toward the gear accommodating chamber 331 . The second stopper 68 is located adjacent to the first stopper 67 and is closer to the fifth pump chamber 43 than the first stopper 67 is. The radially central portion of the rear surface 682 of the second stopper 68 is exposed to the first oil chamber 70 , which corresponds to the first stopper 67 . The third stopper 72 is located adjacent to the second stopper 68 and is closer to the fifth pump chamber 43 than the second stopper 68 is. The radially central portion of the rear surface 601 of the third stopper 72 is exposed to the second oil chamber 71 , which corresponds to the first stopper 67 . That is, the rear surface 682 of the second stopper 68 is part of the walls defining the first oil chamber 70 . The rear surface 601 of the third stopper 72 is part of the walls defining the second oil chamber 71 .

A drainage channel 74 is defined in the lowest portion of the first receptacle 47 and the end 144 of the rear housing 14 to return the lubricant oil Y to the gear accommodation chamber 331 . The drainage channel 74 has an axial portion 741 , which is formed in the lowest part of the receptacle 47 , and a radial portion 742 , which is formed in the end 144 . 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 .

An annular leak prevention ring 66 is fitted about the small diameter portion 59 of the second shaft seal 50 to block flow of oil. A third stopper 72 is formed on the large diameter portion 80 of the second shaft seal 50 . The first and second oil chambers 70 , 71 are defined in the bearing holder 45 , and the third oil chamber 73 is defined in the second receptacle 48 . A drainage channel 74 is formed in the lowest part of the receptacle 48 . Part of the third oil chamber 73 corresponding to the second shaft seal 50 is connected to the gear accommodating chamber 331 by the drainage channel 74 corresponding to the second shaft seal 50 .

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 , lubricant oil Y enters a through hole 691 formed in the projection 69 of each bearing holder 45 through space 371 , 382 in each radial bearing 37 . Then, the lubricant oil Y moves toward the corresponding first oil chamber 70 via a space between the circumference of the small diameter portion 59 of the shaft seal 49 , 50 and the circumference of the through hole 691 , and a space g 1 between the rear 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 lubricant oil Y that reaches the rear surface 672 of the first stopper 67 is thrown to the circumferential wall 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 lubricant oil Y thrown to the circumferential wall surface 702 or the end surface 701 remains on the wall 702 or the surface 701 . The remaining oil Y falls along the walls 701 , 702 by the self weight and reaches the lowest part of the first oil chamber 70 . After reaching the lowest part of the first oil chamber 70 , the lubricant oil Y moves to the lowest part of the second oil chamber 71 .

After entering the first oil chamber 70 , the lubricant oil Y moves toward the second oil chamber 71 through a space g 2 between the rear surface 682 of the second stopper 68 and the end surface 711 of the second oil chamber 71 . At this time, the lubricant oil Y on the rear surface 682 is thrown to the circumferential wall 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 lubricant oil Y thrown to the circumferential wall surface 712 or the end surface 711 remains on the circumferential wall surface 712 or the surface 711 . The remaining oil Y falls along the surfaces 712 , 711 by the self weight and reaches the lowest part of the second oil chamber 71 .

After reaching the lowest part of the second oil chamber 71 , the lubricant oil Y moves to the lowest part of the third oil chamber 73 .

After entering the second oil chamber 71 , the lubricant oil Y moves toward the third oil chamber 73 through a space g 3 between the rear surface 601 of the third stopper 72 and the end surface 731 of the third chamber 73 . At this time, the lubricant oil Y on the rear surface 601 is thrown to the circumferential wall surface 733 or the 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 lubricant oil Y thrown to the circumferential wall surface 733 or the end surface 731 remains on the wall 733 or the surface 731 . The remaining oil Y falls along the wall 733 and the surface 731 by the self weight and reaches the lowest part of the third oil chamber 73 .

After being thrown from the rear surface 672 of the first stopper 67 to part of the circumferential wall surface 702 or the end surface 701 that is above the rotary shafts 19 , 20 , part of the oil may drop on the tapered circumferential surface 671 . Also, after being thrown from the rear surface 682 to the circumferential wall surface 712 or the end surface 711 , part of the oil Y drops on the tapered circumferential surface 671 . After dropping on the tapered circumferential surface 671 , the oil Y is thrown toward the circumferential wall surface 702 by the centrifugal force generated by rotation of the leak prevention ring 66 or moves from the side corresponding to the rear surface 682 toward the end surface 701 along the surface 671 . When moving on the tapered circumferential surface 671 toward the end surface 701 , the oil Y is thrown to the end surface 701 or moves to the rear surface 672 of the first stepper 672 . Therefore, after reaching the tapered circumferential surface 671 , the oil Y moves to the lowest part of the second oil chamber 71 .

After reaching the lowest part of the third oil chamber 73 , the lubricant oil Y is returned to the gear accommodating chamber 331 by the corresponding drainage channel 74 .

The first 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 . To prevent the atomized lubricant oil Y from entering the fifth pump chamber 43 , the lubricant oil Y is preferably liquefied on a stationary wall. Also, the lubricant oil Y on the rotary shafts 19 , 20 or on the members integrally rotating with the rotary shaft 19 , 20 is preferably moved to the stationary wall.

The stoppers 67 , 68 , 72 effectively moves the lubricant oil Y to the walls defining the oil chambers 70 , 71 , 73 . As the number of the stoppers is increased, the area for receiving oil in the stoppers is increased. As the area for receiving oil is increased, the amount of oil that is thrown by the centrifugal force generated by rotation of the stoppers is increased. That is, the stoppers 67 , 68 , 72 , which are arranged on each rotary shaft 19 , 20 , effectively blocks flow of oil.

(1-2) The oil Y on the stoppers 67 , 68 , 72 is thrown into the oil chambers 70 , 71 , 73 surrounding the stoppers 67 , 68 , 72 . The oil Y thrown into the oil chambers 70 , 71 , 73 reaches the walls defining the oil chambers 70 , 71 , 73 . Ultimately, the oil Y on the walls defining the oil chambers 70 , 71 , 73 reaches the drainage channel 74 . Since the stoppers 67 , 68 , 72 are surrounded by the oil chambers 70 , 71 , 73 , respectively, the oil Y thrown by the stoppers 67 , 68 , 72 is easily guided to the gear accommodating chamber 331 .

(1-3) The atomized lubricant oil Y moves through the oil chambers from the side corresponding to the gear accommodating chamber 331 to the fifth pump chamber 43 . The enclosing property of each oil chamber 70 , 71 , 73 is important for preventing the movement of the atomized oil Y.

The first stopper 67 is located closer to the gear accommodating chamber 331 than the second stopper 68 is. The rear surface 682 of the second stopper 68 functions to define the first oil chamber 70 , which corresponds to the first stopper 67 . Likewise, the second stopper 68 is located closer to the gear accommodating chamber 331 than the third stopper 72 is. The rear surface 601 of the third stopper 72 functions to define the second oil chamber 71 , which corresponds to the second stopper 68 . This structure is relatively simple for retaining independence of the oil chamber 70 , 71 , 73 from one another and for improving the enclosing property of each oil chamber 70 , 71 , 73 .

(1-4) The first and second oil chambers 70 , 71 are formed about the projections 69 of the bearing holders 45 , respectively. Since the oil chambers 70 , 71 are formed in the bearing holders 45 supporting the radial bearings 37 , the sealing property of the oil chambers 70 , 71 are improved.

(1-5) 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 first stopper 67 has the tapered circumferential surface 671 located in the first oil chamber 70 . 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 second stopper 68 has the circumferential surface 681 located in the second oil chamber 71 . 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 .

The third stopper 72 has the circumferential surface 721 located in the third oil chamber 73 . 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 . Since the tapered circumferential surfaces 671 , 681 , 721 of the stoppers 67 , 68 , 72 are located in the oil chambers 70 , 71 , 73 , respectively, the atomized oil Y in the oil chambers 70 , 71 , 73 scarcely reaches the fifth pump chamber 43 .

(1-6) The path from the through hole 691 of each bearing holder 45 to the space g 1 between the rear surface 672 of the first stopper 67 and the end surface 701 functions as an oil passage from the side corresponding to the gear accommodating chamber 331 to the first oil chamber 70 . The first stopper 67 narrows the space g 1 , which is at the end of the oil passage.

The path from the first oil chamber 70 to the space g 2 between the rear surface 682 of the second stopper 68 and the end surface 711 functions as an oil passage from the side corresponding to the gear accommodating chamber 331 to the second oil chamber 71 via the first oil chamber 70 . The second stopper 68 narrows the space g 2 , which is at the end of the oil passage.

The path from the second oil chamber 71 to the space g 3 between the front surface 722 of the third stopper 72 and the end surface 731 functions as an oil passage from the side corresponding to the gear accommodating chamber 331 to the third oil chamber 73 via the first oil chamber 70 and the second oil chamber 71 . The third stopper 72 narrows the space g 3 , which is at the end of the oil passage.

The end portions of the oil passage (the spaces g 1 , g 2 , g 3 ) are narrowed. This structure is advantages in preventing atomized lubricant oil Y from entering each the oil chambers 70 , 71 , 73 from the side corresponding to the gear accommodating chamber 331 .

(1-7) The 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 . Oil on the rear surface 682 is thrown in the radial direction by the centrifugal force generated by rotation of the oil leak prevention ring 66 . Lubricant Y is thrown from the rear surface 682 to the tapered circumferential surface 671 . At least part of this oil is moved from the small diameter side to the large diameter side of the tapered circumferential surface 671 by the centrifugal force generated by rotation of the oil leak prevention ring 66 . That is, the oil Y moves away the fifth pump chamber 43 . This is advantageous in preventing oil from entering the fifth pump chamber 43 . That is, since the tapered circumferential surface 671 is adjacent to the rear surface 682 , the oil pump Y is prevented from moving toward the fifth pump chamber 43 .

(1-8) The smallest diameter portion of the tapered circumferential surface 671 is directly connected to the proximal end 684 of the rear surface 682 of the second oil stopper 68 . If a circumferential surface that is parallel to the axis of the rotary shaft 19 , 20 is connected to the proximal end 684 of the rear surface 682 , part of the oil Y thrown from the rear surface 682 reaches the circumferential surface. The oil on the circumferential surface may return to the rear surface 682 of the second stopper 68 . This is disadvantages in preventing oil from entering the fifth pump chamber 43 . However, in the first embodiment, the tapered circumferential surface 671 is directly connected to the rear surface 682 of the second stopper 68 . This structure prevent lubricant oil Y thrown from the rear surface 682 from returning to the rear surface 682 .

(1-9) Above the axes 191 , 201 of the rotary shafts 19 , 20 , lubricant oil Y flows downward along the front surfaces 492 , 502 of the shaft seals 49 , 50 from the circumferential surface 491 of the shaft seal 49 , 50 to the fifth pump chamber 43 . Below the axes 191 , 201 of the rotary shafts 19 , 20 , lubricant oil Y flows upward along the front surfaces 492 , 502 of the shaft seals 49 , 50 from the circumferential surface 491 of the shaft seal 49 , 50 to the fifth pump chamber 43 . Therefore, the lubricant oil Y is more likely to enter the fifth chamber 43 along the shaft seals 49 , 50 above the axes 191 , 201 .

At least part of the lubricant oil Y thrown to the circumferential wall surfaces 702 , 712 remains on the circumferential wall surfaces 702 , 712 . Above the rotary shafts 19 , 20 , the circumferential wall surfaces 702 , 712 are tapered downward from the side corresponding to the fifth pump chambers 43 toward the side corresponding to the gear accommodating chamber 331 . That is, the lubricant oil Y on the part of the circumferential wall surfaces 702 , 712 above the rotary shafts 19 , 20 flows downward in relation with the rotary shafts 19 , 20 while flowing away from the fifth pump chamber 43 . Since the circumferential wall surfaces 702 , 712 permit the lubricant oil Y to flow downward in relation to the rotary shafts 19 , 20 and away from the fifth pump chambers 43 , the lubricant oil Y is effectively prevented from entering the fifth pump chambers 43 .

(1-10) The lubricant oil Y on part of the circumferential wall surfaces 702 , 712 above the rotary shafts 19 , 20 flows downward along the walls 701 , 711 , which are perpendicular to the axes 191 , 201 of the rotary shafts 19 , 20 . Thereafter, the lubricant oil Y smoothly flows downward along the walls 701 , 711 to the portion below the rotary shafts 19 , 20 . The walls 701 , 711 , which are connected to and perpendicular to the circumferential wall surfaces 702 , 712 , permits the lubricant oil Y on the area above the rotary shafts 19 , 20 to smoothly flow downward to the area below the rotary shafts 19 , 20 .

(1-11) In the Roots pump 11 having the laterally arranged rotary shafts 19 , 20 , the lubricant oil Y on the walls of the oil chambers 70 , 71 , 73 falls to the third oil chamber 73 by the self weight. In other words, the lubricant oil Y on the walls of the oil chambers 70 , 71 , 73 is collected to the lowest part of the third oil chamber 73 along the walls. Therefore, the oil on the walls of the oil chambers 70 , 71 , 73 reliably flows to the gear accommodating chamber 331 via the drainage channel 74 connected to the lowest part of the third oil chamber 73 .

(1-12) The diameters of the shaft seals 49 , 50 fitted about the rotary shafts 19 , 20 are larger than the diameter of the circumferential surface of the rotary shafts 19 , 20 . Therefore, the diameters of the labyrinth seals 57 , 58 between the front surfaces 492 , 502 of the shaft seals 49 , 50 and the bottom 472 , 482 of the bearing receptacles 47 , 48 are larger than the diameters of the labyrinth seals located between the circumferential surface 192 , 202 of the rotary shafts 19 , 20 and the rear housing member 14 . As the diameters of the labyrinth seals 57 , 58 increase, the volumes of the labyrinth chambers 551 , 552 , 561 , 562 for preventing pressure fluctuation are increased, which improves the sealing performance of the labyrinth seals 57 , 58 . That is, the spaces between the front surface 492 , 502 of each shaft seals 49 , 50 and the bottom 472 , 482 of the corresponding bearing receptacle 47 , 48 is suitable for retaining the labyrinth seal 57 , 58 in terms of increasing the volumes of the labyrinth chambers 551 , 552 , 561 , 562 to improve the sealing property.

(1-13) As the space between each bearing receptacle 47 , 48 and the corresponding shaft seal 49 , 50 is decreased, it is harder for the lubricant oil Y to enter the space between the bearing receptacle 47 , 48 and the shaft seal 49 , 50 . The bottom surface 472 , 482 of each receptacle 47 , 48 , which has the circumferential wall 471 , 481 , and the front 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 receptacle 47 , 48 and the front 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 receptacle 47 , 48 is suitable for accommodating the labyrinth seal 57 , 58 .

(1-14) 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.

(1-15) 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. The stoppers 67 , 68 , 72 compensate for the sealing performance. Each circumferential surface 671 , 681 , 721 is located in the oil chambers 70 , 72 , 73 , respectively. This structure further compensates for the sealing performance.

(1-16) The tapered circumferential surface 671 is adjacent to the rear surface 682 of the second stopper 68 further compensates the sealing performance.

(1-17) As the first rotary shaft 19 rotates, the lubricant 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 . The lubricant oil Y in the helical groove 61 is moved from the side corresponding to the fifth chamber 43 to the gear accommodating chamber 331 . As the second rotary shaft 20 rotates, the lubricant 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 . The lubricant oil Y in the helical groove 62 is moved from the side corresponding to the fifth chamber 43 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 lubricant oil Y.

(1-18) 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 of the first and second shaft seals 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 wall 471 , 481 of the corresponding receptacle 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 wall 471 , 481 of the corresponding receptacle 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 each shaft seal 49 , 50 effectively prevent the lubricant oil Y from leaking into the fifth pump chamber 43 from the corresponding bearing receptacle 47 , 48 via the spaces between the outer circumferential surface 491 , 501 and the circumferential wall 471 , 481 .

(1-19) The lubricant oil Y is moved from the side corresponding to the pump chamber 43 to the gear accommodating chamber 331 by the helical grooves 61 , 62 . Part of this oil reaches the front surface 722 of third stopper 72 . At this time, the lubricant oil Y on the front surface 722 is thrown to the circumferential wall surface 733 of the third oil chamber 73 by the centrifugal force generated by rotation of the third stopper 72 . The oil Y thrown toward the circumferential wall surface 733 reaches the circumferential wall surface 733 . That is, the lubricant Y is moved from the side corresponding to the fifth pump chamber 43 by each helical groove 61 , 62 to the side corresponding to the gear accommodating chamber 331 . The third stopper 72 then guides the lubricant oil Y to the gear accommodating chamber 331 via the third oil chamber 73 .

(1-20) 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 chamber defining wall 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 zone 431 and to the pressure in the maximum pressurization zone 432 .

The first and second discharge pressure introducing channels 63 , 64 expose the labyrinth seals 57 , 58 to the pressure in the maximum pressurization zone 432 . That is, the labyrinth seals 57 , 58 are influenced more by the pressure in the maximum pressurization zone 432 via the introducing channels 63 , 64 than by the pressure in the suction zone 431 . Thus, compared to a case where no discharge pressure introducing channels 63 , 64 are formed, the labyrinth seals 57 , 58 of the first 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 pressures acting on the front surface and the rear surface of the labyrinth seals 57 , 58 is significantly small. In other words, the discharge pressure introducing channels 63 , 64 significantly improve the oil leakage preventing performance of the labyrinth seals 57 , 58 .

(1-21) 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 .

The present invention may be embodied in other forms. For example, the present invention may be embodied as second to sixth embodiments, which are illustrated in FIGS. 9 to 13 , respectively. In the second to fourth embodiments, like or the same reference numerals are given to those components that are like or the same as the corresponding components of the first embodiment. Since the first and second rotary shafts 19 , 20 have the same structure, only the first rotary shaft 19 will be described in the second to sixth embodiments.

In the second embodiment shown in FIG. 9 , a recess 493 is formed in the large diameter portion 60 of the shaft seal 49 . The circumferential surface 494 of the recess 493 is tapered such that the recess 493 widens from the side corresponding to the fifth pump chamber 43 to the gear accommodating chamber 331 . The drainage channel 74 is inclined downward toward the gear accommodating chamber 331 .

The lubricant oil Y on the circumferential surface 494 is moved toward the gear accommodating chamber 331 by the centrifugal force generated by rotation of the shaft seal 49 . Thereafter, the lubricant oil Y reaches the end surface 731 . Then, the oil Y is thrown to the circumferential wall surface 733 of the third oil chamber 73 . The recess 493 reduces the weight of the shaft seal 49 . The recess 493 also increases the amount of oil received by the shaft seal 49 before the third oil chamber 73 .

In the third embodiment shown in FIG. 10 , a pair of stopper rings 75 , 76 are fitted about the small diameter portion 59 of the shaft seal 49 . Separation rings 77 , 78 are fitted in the receptacle 47 . The stopper rings 75 , 76 define three oil chambers 79 , 80 , 81 in the space between the projection 69 of the bearing holder 45 and the bottom 472 of the receptacle 47 .

In the fourth embodiment shown in FIG. 11 , stoppers 82 , 83 , 72 are integrally formed with the shaft seal 49 .

In the fifth embodiment shown in FIG. 12 , stoppers 84 , 85 , 72 are integrally formed with the shaft seal 49 . The radial dimensions of the stoppers 84 , 85 , 72 increase in this order. The stoppers 84 , 85 , 72 are surrounded by oil chambers 86 , 87 , 88 , respectively. The radiuses of the oil chambers 86 , 87 , 88 increase in this order. Circumferential walls 861 , 871 , 881 of the oil chambers 86 , 87 , 88 are not tapered. The fifth embodiment has the same advantages as the advantages (1-1) to (1-5), (1-8) to (1-14), and (1-15) to (1-20).

In the sixth embodiment shown in FIG. 13 , a shaft seal 49 A is integrally formed with the end surfaces of the rotary shaft 19 and the rotor 27 . The shaft seal 49 A is located in a receptacle 89 formed in the front wall of the rear housing member 14 , which faces the rotor housing member 12 . A labyrinth seal 90 is located between the rear surface of the first shaft seal 49 A and the bottom 891 of the receptacle 89 .

An oil leak prevention rings 91 , 92 are fitted about the rotary shaft 19 . An annular oil chamber 93 is defined between the bottom 472 of the receptacle 47 and the projection 69 of the bearing holder 45 .

It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms.

(1) Four or more stoppers may be arranged along the axis of each rotary shaft.

(2) In the first embodiment, each shaft seal 49 , 50 may be integrally formed with the corresponding leak prevention ring 66 .

(3) In the third embodiment, each shaft seal ring 77 , 78 may be integrally formed.

(4) 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.

Claims

1. 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 comprising:an oil housing member, wherein the oil housing member defines an oil zone adjacent to the pump chamber, and the rotary shaft has a projecting portion that projects from pump chamber into the oil zone through the oil housing member; a plurality of stoppers, which are located on the rotary shaft to integrally rotate with the rotary shaft and prevent oil from entering the pump chamber, wherein the stoppers are located along the axial direction of the rotary shaft, wherein each of the stoppers has a circumferential surface; and a plurality of annular oil chambers, each of which surrounds one of the circumferential surfaces, wherein the stoppers are arranged in decreasing order of diameter from the side closer to the pump chamber toward the oil zone, wherein the oil chambers are arranged in decreasing order of diameter from the side closer to the pump chamber to the oil zone, wherein one of an adjacent pair of the stoppers is a first stopper, which is closer to the oil zone, and the other stopper of the pair is a second stopper, which is closer to the pump chamber, wherein the second stopper has an end surface that is perpendicular to an axis of the rotary shaft and faces toward the oil zone, and wherein the end surface of the second stopper is part of the walls defining the oil chamber in which the first stopper is located.

2. The pump according to claim 1, wherein each stopper has an end surface that is perpendicular to the axis of the rotary shaft, wherein a tapered circumferential surface is located about the rotary shaft, wherein the tapered circumferential surface is adjacent to at least one of the end surfaces of the stoppers and is closer to the oil zone than the adjacent end surface, and wherein the diameter of the tapered circumferential surface gradually increases from the side closer to the pump chamber toward the oil zone.

3. The pump according to claim 1, wherein the oil zone accommodates a bearing, which rotatably supports the rotary shaft.

4. The pump according to claim 1, further comprising:an annular shaft seal, which is located about the projecting portion to rotate integrally with the rotary shaft, wherein the shaft seal is located closer to the pump chamber than the stoppers are and has a first seal forming surface that extends in a radial direction of the shaft seal; a second seal forming surface formed on the oil housing member, wherein the second seal forming surface faces the first seal forming surface and is substantially parallel with the first seal forming surface; and a non-contact type seal located between the first and second seal forming surfaces.

5. The pump according to claim 1, further comprising:a seal surface located on the oil housing; and an annular shaft seal, which is located about the projecting portion to rotate integrally with the rotary shaft, wherein the shaft seal is located closer to the pump chamber than the stoppers are, wherein the shaft seal includes pumping means located on a surface of the shaft seal that faces the seal surface, wherein the pumping means guides oil between a surface of the shaft seal and the seal surface from the side closer to the pump chamber toward the side closer to the oil zone.

6. The pump according to claim 1, further comprising a drainage channel, which connects the oil chambers to the oil zone to conduct oil to the oil zone.

7. The pump according to claim 6, wherein the drainage channel is connected to the lowest parts of the oil chambers.

8. The pump according to claim 7, wherein the drainage channel is substantially horizontal or is inclined downward toward the oil zone.

9. The pump according to claim 8 further comprising a plurality of circumferential wall surfaces, the center of curvature of each coinciding with that of the rotary shaft, wherein each circumferential wall surface surrounds at least a part of one of the circumferential surfaces of the stoppers that is above the rotary shaft, and wherein at least one of the circumferential wall surfaces is inclined such that the distance between the wall and the rotary shaft decreases toward the oil zone.

10. The pump according to claim 1, wherein a peripheral portion of each stopper protrudes into the corresponding oil chamber.

11. The pump according to claim 10, wherein the oil chambers form a bent path extending from the side closer to the pump chamber to the side closer to the oil zone.

12. The pump according to claim 10, wherein the bent path has a radially extending oil passage, wherein the oil passage connects each adjacent pair of the oil chambers, and wherein the oil passage is narrower than the oil chamber in the axial direction of the rotary shaft.

13. The vacuum pump according to claim 1, wherein the rotary shaft is one of a plurality of parallel rotary shafts, wherein the rotary shafts are connected to one another by a gear mechanism such that the rotary shafts rotate synchronously, and wherein the gear mechanism is located in the oil zone.

14. The vacuum pump according to claim 13, wherein a plurality of rotors are located about each rotary shaft such that each rotor functions as the gas conveying body, and wherein the rotors of one rotary shaft are engaged with the rotors of another rotary shaft.

15. 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 comprising:an oil housing member, wherein the oil housing member defines an oil zone adjacent to the pump chamber, and the rotary shaft has a projecting portion that projects from the pump chamber into the oil zone through the oil housing member; a plurality of annular stoppers, which are located on the rotary shaft to integrally rotate with the rotary shaft and prevent oil from entering the pump chamber, wherein each stopper has a circumferential surface, which has a greater diameter than that of the rotary shaft, and wherein the stoppers are arranged along the axis of the rotary shaft in decreasing order of diameter from the side closer to the pump chamber toward the oil zone, wherein each of the stoppers has a circumferential surface; and a plurality of annular oil chambers, each of which surrounds one of the circumferential surfaces, wherein the stoppers are arranged in decreasing order of diameter from the side closer to the pump chamber toward the oil zone, wherein the oil chambers are arranged in decreasing order of diameter from the side closer to the pump chamber to the oil zone, wherein one of an adjacent pair of the stoppers is a first stopper, which is closer to the oil zone, and the other stopper of the pair is a second stopper, which is closer to the pump chamber, wherein the second stopper has an end surface that is perpendicular to an axis of the rotary shaft and faces toward the oil zone, and wherein the end surface of the second stopper is part of the walls defining the oil chamber in which the first stopper is located.