US3525578A

US3525578A - Vacuum pump

Info

Publication number: US3525578A
Application number: US779779A
Authority: US
Inventors: Joseph A Le Blanc Jr
Original assignee: Precision Scientific Co
Current assignee: Precision Scientific Co
Priority date: 1968-11-29
Filing date: 1968-11-29
Publication date: 1970-08-25
Anticipated expiration: 1987-08-25
Also published as: JPS5026769B1; DE1948804A1; GB1277470A; FR2024945A1

Description

Aug. 25, 1'9'70 J. A. LE BLANC, JR 3,

' VACUUM PUMP Filed Nov. 29, 1968 5 Sheets-Sheet l IIVl EIVTOR. JOSEPH A. Le BLA/VQJR.

l {WW 1970 J. A. LE BLANC, JR 3,525,578

VACUUM PUMP Filed Nov. 29, 1968 5 Sheets-Sheet 2 4 7 E I JOSEPH A INVENTOR.

YHHQ/ J. A. LE BLANC, JR

Aug. 25, I970 VACUUM PUMP 5 Sheets-Sheet 5 Filed Nov. 29, 1968 /A/VENTOR. AQLeBLA/VC, JR

JOSEPH Aug. 25, 1970 J, LE BL c, JR 3,525,578

VACUUM PUMP Filed Nov. 29, 1968 5 SheetS-ShGet 4 '//A 1 I I42 INVEN'TOR. JOSEPH 4L6 BLA/VG, JR

/7 6 /72 By W 2 )1 Aug. 25, 1970 J. A. LE BLANC, JR 2 VACUUM PUMP Filed Nov. 29, 1968 .5 Sheets-s hfiet 5 l/VVE/VTOR JOSEPH A. Le ELAN/6J5 United States Patent Oflice 3,525,578 Patented Aug. 25, 1970 3,525,578 VACUUM PUMP Joseph A. Le Blane, Jr., Chicago, Ill., assignor to Precision Scientific Co., Chicago, 111., a corporation of Illinois Filed Nov. 29, 1968, Ser. No. 779,779 Int. Cl. F04c 23/00; F04b 41/06; F04d 25/16 US. Cl. 418-13 20 Claims ABSTRACT OF THE DISCLOSURE An improved multistage rotary mechanical vacuum pump of the oil sealed, internal vane type includes a single rotor passing through each stage of the pump provided with respective sets of vanes for each stage of the pump. The single rotor has an axial opening extending therethrough for providing stage-to-stage gas conductance.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to mechanical vacuum pumps and more particularly to an improved multistage, rotary vacuum pump of the oil sealed, internal vane type.

Description of the prior art Multistage vacuum pumps of the oil-sealed, rotary internal vane (i.e., Gaede) type have long been known. However, such prior art multistage high vacuum pumps have typically embodied separate rotors for each pump stage, and they have been relatively bulky and cumbersome. Because of the arrangement whereby stage-to-stage gas conductance was effected by a passageway external of the pump chambers, such structures were not always satisfactory.

Accordingly, a prime object of the present invention is to provide a multistage oil-sealed, rotary internal vane vacuum pump utilizing a single hollow rotor to effect stage-to-stage gas conductance.

Further, there has for some time been a need for a reduced size, miniature high vacuum pump. Through the use of the novel structural arrangement employed in the present invention, this need has now been met, and, accordingly, a related object of the present invention is to provide a miniature high vacuum pump of reduced size, yet capable of producing a high vacuum.

In addition to the principal object of the present invention, other objects include the provision of a miniature high vacuum pump of the character described in which improved sealing means are employed for minimizing the leakage about the shaft as it enters the exhaust stage of the pump.

Another object is to provide an arrangement in which the various constituents of the pump may be quickly and easily bolted together during assembly yet which permits the easy removal and replacement of the vanes and rotor without complete disassembly of the pump.

SUMMARY OF THE INVENTION The foregoing and other objects, advantages, and fea tures of the present invention are achieved with a mechanical vacuum pump of the rotary, oil sealed, internal vane type comprising a pair of stators arranged side-byside with a center plate interposed between the two stators and a pair of end plates disposed at opposite ends of the two stators, thereby defining a pair of pumping chambers. The stators and center plate each have bores therein, and a single hollow rotor is rotatably journaled in the center plate and projects into the bore of each stator, with each projecting end of the rotor being adapted to slidably carry at least one vane. The rotor is open at both ends so as to provide an axial passageway through the stators and center plate in order to provide for stage-to-stage gas conductance. Intake means conduct gas into the first pumping chamber, and first gas transfer means conduct gas from the first pump chamber into the axial passageway of the rotor. Second gas transfer means conduct gas from the axial passageway of the rotor to the second pump chamber, and outlet means conduct gas from the second pumping chamber to the exterior of the pump. The rotor is preferably end-mounted on a motor drive shaft journaled for rotation in the end plate adjacent the other pump chamber.

DESCRIPTION OF THE DRAWINGS The foregoing and other objects, advantages, and features of the present invention will hereinafter appear, and, for purposes of illustration, but not of limitation, exemplary embodiments of the present invention are shown in the accompanying drawings, in which:

FIG. 1 is a top plan view, partially in section, of a mechanical vacuum pump produced in accordance with the present invention;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a fragmentary right side elevational view thereof, partially in section;

FIG. 4 is a sectional view taken substantially along line 4-4 in FIG. 3 showing the main casting of the P p;

FIG. 5 is a sectional view taken substantially along line 5-5 in FIG. 3 showing the exhaust stage end plate;

FIG. 6 is a sectional view taken substantially along line 66 in FIG. 3 showing the exhaust stage stator;

FIG. 7 is a sectional view taken substantially along line 77 in FIG. 3 and illustrates the center plate;

FIG. 8 is a sectional view taken substantially along line 8-8 in FIG. 3 showing the intake stage stator;

FIG. 9 is a sectional view taken substantially along line 99 in FIG. 3 illustrating the intake stage end plate;

FIG. 10 is a front elevational view of the rotor;

FIG. 11 is a sectional view taken along line 11-11 in FIG. 10;

FIG. 12 is a sectional view taken along line 1212 in FIG. 10;

FIG. 13 is an elevational view of the shaft;

FIG. 14 is a perspective view of one of the pump vanes; and

FIG. 15 is an exploded perspective view of the various elements of the pump.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With particular reference to the drawings, a mechanical vacuum pump 20 is shown, with pump 20 being mounted on a suitable base 22 (see FIGS. 2 and 3). Pump 20 comprises a main casting 24 which is secured to base 22 by a mounting screw 26 (see FIG. 3.) An outer housing 28 is fixed to main casting 24, and housing 28 defines a hollow space 30 in which the operating elements of the pump 20 are disposed. A supply of suitable sealing oil, the level of which is denoted by the reference character L in FIG. 3, is maintained in space 30 so as to cover and seal the pump elements. A transparent bubble 32 of plexiglass or the like may be provided at an appropriate point in housing element 28 so that the oil level within space 30 may be visually checked.

Main casting 24 is provided with a transverse central opening 34 and further contains a cavity 36. Cavity 36 communicates with the oil bath in space 30 by means of a groove 38 (see FIGS. 3 and 4) which is provided in the face of main casting 24. As will hereinafter be described in greater detail, cavity 36 and groove 38 are provided so that the oil seal may be provided about the point at which the shaft passes into the interior of the pump stages.

As best shown in FIGS. 1, 3, and 4, main casting 24 is provided with four vertical openings in the top thereof. The first opening is an intake port 40 which communicates with an intake cavity 42 provided in casting 24. A transverse intake passageway 44 (see FIG. 4) is pr vided in casting 24 and communicates with cavity 42.

The second vertical opening in casting 24 is an opening 46 (see FIG. 3) into which a plug 48 is fitted. Opening 46 provides a convenient route for the addition of sealing oil to the interior of the pump.

The third vertical opening provided in the casting 24 is an opening 50 into which is threaded a gas ballast valve 51 for a purpose that will hereinafter appear. Opening 50 communicates with a passageway 52 extending downwardly toward the interior of the pump, with the bottom portion of passageway 52 communicating with a transverse port 54 (see FIG. 4) for a purpose that will herein after appear.

The fourth vertical opening in casting 24 is an exhaust port 56 which communicates with a cavity 58 in casting 24 which in turn communicates with the space 30.

Suitable couplings such as a pair of

couplings

60, 62 may be threaded into inlet port 40 and outlet port 56, and

fittings

60, 62 serve as the attachment points for the pump in the system in which it is to be operated.

An electric motor 64 is mounted on main casting 24, and a motor shaft 66 (see especially FIGS. 3 and 13) is driven by the motor 64. Shaft 66 is journaled for rotation in a bushing 68 which is force-fitted in opening 34 in casting 24, and shaft 66 projects through cavity 36 into space 30 and is operatively linked to the pump stages in a manner hereinafter to be described.

The stages of the pump 20 are bolted to main casting 24, as best shown in FIGS. 1, 3, and 15. The pump stages are formed by an exhaust stage end plate 70, an exhaust stage stator 72, a center plate 74, an intake stage stator 76, and an intake stage end plate 78. Each of these elements is respectively shown in greater detail in FIGS. -9.

As best shown in FIG. 5, exhaust stage end plate 70 comprises a generally circular disc having a flattened bottom portion 80. A number of ports and cavities are provided in plate 70 for a purpose that will hereinafter appear. A centrally disposed opening 82 is provided, and the forward end of shaft 66 is designed to pass through opening 82. A pair of first mounting bores 84 are provided, as are a second set of mounting bores 86 for purposes that will hereinafter appear. Another pair of mounting openings 88 are provided in end plate 70 (which as will hereinafter appear, is somewhat oversized relative to the other elements of the pump) and openings 88 are countersunk in order to provide seats for a pair of bolts 90 (see FIG. 1) which are employed to bolt end plate 70 to main casting 24 (see especially FIG. 1). A pair of vertical bores 92 are provided in the upper portion of end plate 70 for a purpose that will hereinafter appear.

A transverse intake opening 94 is provided in end plate 70, and, when end plate 70 is fixed in position adjacent main casting 24, opening 94 is aligned with passageway 44 in casting 24 which in turn is in communication with the intake cavity 42 in main casting 24. I

In the face of end plate 70 opposite main casting 24 are provided a pair of cavities, namely, a generally U-shaped exhaust stage inlet cavity 96 and a generally L-shaped exhaust stage outlet cavity 98. As will be explained in greater detail hereinafter, cavity 96 provides for the transfer of gas from the hollow interior of the rotor of the pump to the pump chamber of the exhaust stage, and cavity 98 provides for the conductance of gas from the pump chamber of the exhaust stage to the outlet of pump 20. A transverse opening 99 (See FIG. 5) is provided in plate 70 so as to provide fluid communication between the cavity 98 and gas ballast port 54.

Exhaust stage stator 72 is generally disc shaped and has a center opening which serves to define an exhaust stage pumping chamber 100. Stator 72 is provided with a first set of openings 102 corresponding with the first pair of openings 84 provided in end plate 70, and stator 72 is likewise provided with a second set of mounting openings 104 which correspond to openings 86 provided in end plate 70. Stator 72 further has a transverse opening 106 adapted to provide gas communication with port 94 in end plate 70 as a part of the pump intake passageway, and likewise has a transverse opening 108 which communicates with the L-shaped cavity 98 in end plate 70 and thus provides a portion of the outlet passageway for the pump 20.

As best shown in FIGS. 3, 7, and 15, center plate 74 comprises a generally disc-shaped member having a fiattened surface along the top portion thereof. Center plate 74 has an opening 122 in the center thereof. An inlet opening 124 is provided transversely therein, and opening 124 communicates with opening 106 in exhaust stator 72. A transverse exhaust passage 126 is provided in center plate 74 part way therethrough, and a vertical exhaust port 128 in the flattened portion 120 communicates with passageway 126. A first pair of mounting holes 130 are provided in center plate 74 as are a second set of mounting openings 132.

Openings

130, 132 are adapted for alignment, respectively, with

openings

102, 104 in exhaust stator 72. In addition, each of the openings 13!], 132 is provided with a

radial bore

131, 133 for a purpose that will hereinafter appear.

Intake stage stator 76 (see especially FIGS. 1, 8, and 15) is substantially identical to exhaust stage stator 72. It comprises a generally disc-shaped member having a center opening provided therein which defines an intake stage pumping chamber 140. A first set of mounting openings 142, which are countersunk, are provided in stator 76, and openings 142 are adapted for alignment with openings 130 in center plate 74. Similarly, a second set of mounting openings 144 are provided and these openings are adapted for alignment with openings 132 in center plate 74. An inlet opening 146 is provided transversely through intake stator 76 and opening 146 is adapted to provide gas communication with the intake opening 124 provided in center plate 74.

Intake stage end plate 78 comprises a disc-shaped member having a series of mounting openings 150 provided therein which openings are adapted for alignment with openings 144 in stator 76. A pair of

cavities

152, 154 are provided in the inner face of end plate 78. Cavity 152 serves as an intake stage inlet cavity and thus provides communication between the pumping chamber 140 and the intake opening 146 in the intake stator. Cavity 154 provides for gas conductance between pump chamber 140 and the hollow center of the stage. End plate 78 does not have a center opening through which the shaft passes or is journaled for rotation.

A single hollow rotor (best shown in FIGS. 10- 12) is provided in the large center openings provided in intake stator 76, center plate 74, and exhaust stator 72. The diameter of rotor 160 is substantially similar to the diameter of opening 122 in center plate 74 so that rotor 160 may sealingly rotate therein. As best shown in FIGS. 11 and 12, an annular groove 162 is provided in rotor 160 for a purpose hereinafter described. An opening is provided axially in the center of rotor 160 in order to provide a rotor hollow 164, with the intake stage portion 166 being of slightly reduced diameter. The ends 170, 172 of the rotor hollow 164 are flared outwardly so as to facilitate the gas communication between the rotor hollow and the

transfer cavities

96 and 154 in the

end plates

70, 78.

A transverse bore 174 is provided through the center of rotor 160 and provides fluid communication between rotor hollow 164 and annular groove 162 for a purpose that will hereinafter appear. A pair of radial grooves 176 are provided in the exhaust stage side of rotor 160 in order to receive a locking pin which may pass through the shaft 66 in order to lock the rotor onto the shaft. Respective pairs of vane slots 180 in the exhaust side and 182 in the intake side are provided in the rotor 160, and a vane 184 slides back and forth in its vane slot as rotor 160 is rotated, with the vane being urged centrifugally outwardly so that the outer end thereof, which is rounded (see FIG. 14) maintains constant contact with the stator (see, e.g., FIG. 8).

As best shown in FIGS. 3 and 13, shaft 66 has an enlarged diameter portion having a flattened surface 190 adapted to be rotatably connected to motor 64. Shaft 66 has a reduced diameter section 192 adapted to rotatably pass through bushing 68 in main casting 24 (see FIG. 3). Shaft 66 also has an elongated reduced diameter section 194 provided with an annular groove 196, and a transverse bore 198 is provided adjacent the small diameter end of reduced diameter portion 194. Shaft section 194 passes through opening 82 in exhaust stage end plate 70. A locking pin 200 (see FIG. 3) is force-fitted in opening 198, and the ends of pin 200 are received in locking slots 176 and rotor 160 so as to provide for a positive drive of rotor 160 by shaft 66. Shaft section 194 passes only through the exhaust stage end plate 70 and extends into but not through the exhaust stage and neither through nor into the intake stage. Intake end plate 78 has no openings, ports, or other communication with the interior of the pump, thereby avoiding leakage of nondegassed oil into the intake stage.

As previously mentioned, an oil bath is maintained in cavity 36 in main casting 24 in order to provide an oil seal about the point at which the shaft 66 passes through end plate 70. In order to minimize the passage of oil from cavity 36 along the shaft reduced diameter portion 194 and into the rotor hollow 164, as best shown in FIG. 3, a rotating graphite ring sealing arrangement is provided about shaft 66. A member 202 is provided in a recess in end plate 70, and an O-ring seal 204 is provided in a groove in member 202. A graphite ring 206 is provided about shaft portion 194, and a cap 208, which encloses ring 206 in part, is urged against ring 206, which in turn is urged against the surface of bearing 202, by a spring 210. A rubber sleeve 211 serves to prevent leakage of oil along the shaft, and sleeve 211, spring 210, cap 208, graphite ring 206, and shaft 66 all rotate as a unit with respect to member 202. A similar rotating graphite ring sealing arrangement 212 having identical elements is provided in order to seal the oil bath in cavity 36 from the shaft 66 as it passes through opening 34 in casting 24. By use of the described double graphite ring oil seals, the possibility of oil leaking from cavity 36 along the shaft and through the end plate 70 and into the interior of the pumping stages is absolutely minimized.

The unique design of the present invention obviates the necessity for end caps covering the points at which the shaft passes through the end plates of the pump stages, such as were required by pumps of the prior art. Instead, the shaft of the present invention does not even pass through the intake end plate, and because of the shaft seal employed, and end cap is not required at the exhaust stage end plate in order to isolate the point of shaft passage through the end plate from the surrounding oil path.

As is well known to those skilled in the art, it is desired for a controlled, minor amount of oil to be regularly introduced into the interior of a rotary high vacuum pump of this character in order to provide a wet film of oil on the surfaces in the interior of the pump in order to seal the various moving parts. In the pump of the present invention, this controlled oiling is accomplished by means of a leaky valve plate 220 (see FIG. 1) which is clamped over exhaust port 12-8 by means of a suitable bolt 222. Valve plate 220 has an elongated slit 224 provided therein which passes over port 128, and this slit permits oil to leak into port 128 in a controlled fashion. A valve of the type described is set forth and described in greater detail in applicants US. Pat. No. 3,326,456.

The various operative elements of the pump may be assembled in the following manner. With the motor 64 and main casting 24 properly positioned on base 22, with the shaft 66 properly positioned, and with the sealing arrangements positioned on the shaft, the exhaust end plate 70 is bolted to main casting 24 via openings 58. The exhaust stage stator 72, center plate 74, and intake stage stator 76 are all assembled in aligned position, and a pair r of bolts 230 are passed through the respective aligned pairs of

openings

142, 130, and 102 and similar aligned openings 84 in end plate 70 and in main casting 42 thereby permitting the elements to be fixed in proper aligned position. The rotor 160 is then positioned on shaft 66 so that the pin 200 engages the locking grooves 176. As best shown in FIGS. 6 and 8, the openings in

stators

72, 7 6 are larger than the opening in the center plate 74 and than the diameter of rotor 160. The rotor is thus eccentrically mounted with reference to the openings in the

stators

72, 76. Thus, the rotor and the pumping chambers 100, function to provide crescent-shaped pumping chambers which are swept by the vanes 184 carried by the rotor as it rotates.

Fine adjustments in the alignment of the various elements are achieved and the bolts 230 are then tightened. The intake stage end plate 78 is then placed in position and the elements are all tightly bolted into position, with a series of bolts 250 passing through the aligned sets of

openings

150, 144, 132, 104, and 86 and into a similar plurality of aligned threaded openings 260 provided in casting 24. A splash guard 270 may be bolted in position to the end plate in order to cover the exhaust valve end point.

With the pump properly assembled in the foregoing manner, and with oil introduced through opening 46 to the level L so that the entire pump is immersed and space 30 and cavity 36 filled with oil, the operation of the device may be described as follows. With special reference to FIG. 15, an exploded perspective view of the present invention in which a series of arrows have been shown in order to trace the gas at flow through the pump, gas enters the pump from the area to be evacuated, through fitting 62, intake port 40 through the intake cavity 42 and intake passageway 44 and thence through the series of aligned

inlet openings

94, 106, 124, and 146 in the exhaust end plate, exhaust stator, center plate, and intake stator, thence through inlet cavity 152 in end plate 78 wherein the gas is passed into the pump chamber 140 of the intake stage.

The gas is swept around by a vane 184 carried by rotor 160 and compressed and forced out through the intake stage outlet cavity 154 provide in end plate 78 wherein the gas passes into the rotor hollow 164. The gas is transferred from the intake stage through the exhaust stage to the exhaust stage intake cavity 96 in end plate 70, which serves to transfer gas from the rotor hollow 164 to the pump chamber 100 of the exhaust stage wherein it is once again swept around by vanes 184 and forced outwardly through the exhaust stage exhaust outlet cavity 98. The aligned opening 108 in the exhaust stator 72 and opening 126 provided in the center plate provide the exhaust passageway and, as previously described. an exhaust valve plate 220 attached to the top of center plate 74 is disposed over exhaust port 128 provided in center plate 74. Gas thus is expelled out through valve plate 220 wherein it bubbles to the surface of the oil level L and thence outwardly through opening 56 and outlet fitting 60.

As previously noted, lubrication of the pump is accomplished through the controlled admittance of minor amounts of oil into the exhaust port 128, with the oil passing backwardly to the interior of the pump whereby the surfaces of the pump are Wetted within a thin film of oil so as to seal them. As previously noted, a transverse bore 174 is provided in rotor 160 so as to provide a communication between groove 162 and rotor hollow 164. This groove serves to permit oil in groove 162 migrating along the outer surface of rotor 164 from the exhaust stage pump chamber 100 towards the intake stage pumping chamber 140 to be degassed.

As previously noted, by merely removing the bolts 250, the intake stage end plate may be removed without the other fixed elements being removed so that the rotor may simply be slid out and vanes replaced.

As further noted, in accordance with the present invention, a single rotor is employed for both stages of the twostage pump of the present invention. Because of the unique cantilever mounting of the rotor at the very end of the shaft, and the relatively loose interlock between the shaft and the rotor, it is unnecessary to precisely align the motor shaft in the operative pump elements. Instead, the rotor itself is precisely aligned in the center plate, with independently slidable vanes serving to accomplish complete alignment in the stators of the pump. Moreover, since the shaft does not pass through intake stage and plate 78, the possibility of oil from space 30 containing occluded gas leaking into the intake stage, with the consequent effect on pump efficiency, is minimized. Furthermore, oil from the surrounding bath is precluded from entering the pump along shaft reduced diameter portion 194 by the double graphite ring sealing arrangements. As noted, oil enters the pump only through the check valve 220 wherein it is outgassed, and oil migrating between the rotor 164 and the center plate opening 122 is likewise outgassed by

grooves

162, 174 so the oil reaching the intake stage of the pump is completely outgassed.

By suitable adjustment of gas ballast valve 51, atmospheric air in small amounts may be bled into the pump exhaust passageway via opening 50, passageway 52, port 54, opening 99, and cavity 98.

Accordingly, the present invention involves a uniquely designed highly efficient rotary, oil-sealed, two-stage mechanical vacuum pump which, because of the unique relationship of its elements can provide in miniaturized fashion a very efficient high vacuum pump.

I claim:

1. A multistage, mechanical vacuum pump comprising:

a pair of stators arranged side-by-side With a center plate interposed between the two stators and a pair of end plates disposed at opposite ends of the two stators whereby to define a first pumping chamber and a second pumping chamber,

a single unitary hollow rotor journaled for rotation in the center plate and projecting into each stator, each projecting end of the rotor being adapted to slidably carry at least one vane,

the rotor being open at both ends so as to provide an axial passageway through the stators and center plate in order to provide for stage-to-stage gas conductance;

intake means for conducting gas into the first pumping chamber;

first gas transfer means for conducting gas from the first pumping chamber into the axial passageway of the rotor;

second gas transfer means for conducting gas from the axial passageway of the rotor into the second pumping chamber; and

outlet means for conducting gas from the second pumping chamber to the exterior of the pump.

2. A vacuum pump, as claimed in claim 1, and further comprising an annular degassing groove in the outer surface of the rotor, with at least one radial passageway being 8 formed in the rotor between the degassing groove and the hollow interior of the rotor.

3. A vacuum pump, as claimed in claim 1, wherein the rotor is end-mounted on a shaft journaled for rotation in the end plate disposed adjacent the second pumping chamber.

4. A vacuum pump, as claimed in claim 1, and further comprising enclosure means enclosing the end plates, stators, and center plates and providing a cavity adapted to contain an oil bath in which the end plates, stators, and center plate are immersed.

5. A vacuum pump, as claimed in claim 4, wherein the rotor is mounted on a shaft journaled for rotation in the end plate disposed adjacent the second pumping chamber and wherein spring-mounted, graphite shaft seal means is provided at the point the shaft passes through the end plate.

6. A vacuum pump, as claimed in claim 4, wherein the enclosure means comprises a main casting and a housing member, with the housing member being mounted on the main casting so as to surround the end plates, stators, and center plate.

7. A vacuum pump, as claimed in claim 6, wherein the end plate adjacent the second pumping chamber, the stators, and the center plate are mounted as a unit on the main casting, with the end plate adjacent the first pumping chamber being independently and removably secured in position so as to provide access to the interior of the pumping chambers.

8. A vacuum pump, as claimed in claim 6, wherein the shaft is journaled for rotation in the main casting, with a spring mounted, graphite shaft seal means being provided at the point the shaft passes through the main casting.

9. A vacuum pump, as claimed in claim 1, wherein:

the intake means comprises a first passageway in the end plate adjacent the first pumping chamber and a transverse passageway provided in the stators, center plate, and end plate adjacent the second pumping chamber;

the first gas transfer means comprises a second passage way in the end plate adjacent the first pumping chamber;

the second gas transfer means comprises a first passageway in the end plate adjacent the second pumping chamber; and

the outlet means comprises a second passageway in the end plate adjacent the second pumping chamber, a transverse passageway in the center plate and the stator forming the second pumping chamber, and an exhaust port in the center plate.

10. A vacuum pump, as claimed in claim 9, and further comprising exhaust valve means associated with the exhaust port in the center plate, the said valve means being adapted to permit gas to be expelled from the exhaust port and to normally permit the passage of a controlled amount of lubricating oil from the oil bath into the exhaust port.

11. A vacuum pump, as claimed in claim 1, and further comprising gas ballast passageway means in communication with the outlet means; and

adjustable gas ballast valve operatively associated with gas ballast passageway means for permitting the contolled admittance of atmospheric air into the pump.

12. In a multistage mechanical vacuum pump of the rotary, oil-sealed, internal vane type including an intake stator and an exhaust stator assembled in abutting relationship on opposite sides of a center plate, with each stator having an end plate for closing the stator at an end thereof opposite the center plate and with a housing assembly enclosing the end plates, stators, and center plate and providing a volume of space for retaining an oil bath, the improvement comprising:

a single unitary hollow rotor rotatably journaled in the center plate and extending into each stator, each projecting end of the rotor being adapted to slidably carry at least one vane, the said hollow rotor being open at both ends so as to provide an axial opening for stage-to-stage gas conductance.

13. The improvement, as claimed in claim 12, wherein the rotor is end-mounted on a shaft journaled for rotation in one of the end plates, with spring-mounted graphite shaft seal means being provided at the point the shaft passes through the end plate.

14. The improvement, as claimed in claim 12, wherein the rotor comprises an annular degassing groove in its outer surface with at least one radial interior passageway formed in the rotor between the degassing groove and the hollow interior of the rotor.

15. A vacuum pump comprising:

a center plate;

a first stator disposed on one side of the center plate;

a second stator disposed on the other side of the center plate;

a first end plate adjacent the side of the first stator opposite the center plate;

a second end plate adjacent the side of the second stator opposite the center plate;

a single unitary rotor journaled for rotation in the center plate and extending into each stator, the said rotor being open at both ends and having a hollow interior;

the rotor, first end plate, first stator, and the center plate forming a first pumping chamber and the rotor,

second end plate, second stator, and center plate forming a second pumping chamber, communication between said chambers being provided by the hollow interior of said rotor;

slidable vane means carried by the rotor in each of the pumping chambers, with the vane means being adapted to sweep the pumping chambers as the rotor is rotated;

shaft means extending through the second end plate, the said rotor being end-mounted on the shaft for rotation therewith; and

drive means for causing the shaft to rotate.

16. A pump, as claimed in claim 15, and further comprising a housing and a main member, with the end plates, center plate, and stators being mounted on the main sup port member and with the housing being secured to the main member so as to enclose the end plates, stators, and center plate and provide a cavity adapted to contain an oil bath.

17. A vacuum pump, as claimed in claim 16, wherein the end plate adjacent the second pumping chamber, the stators, and the center plate are mounted as a unit on the main support member, with the end plate adjacent the first pumping chamber being independently and removably secured in position so as to provide access to the interior of the pumping chambers.

18. A vacuum pump, as claimed in claim 15, and further comprising:

intake means forming a gas transfer path between the first pumping chamber and the space to be evacuated;

exhaust means forming a gas transfer path between the second pumping chamber and the pump surroundings;

first means forming a gas transfer path between the first pumping chamber and the hollow interior of the rotor; and

second means forming a gas transfer path between the hollow interior of the rotor and second pumping chamber. 19. A vacuum pump, as claimed in claim 18, wherein: the intake means comprises a first passageway in the first end plate and a transverse passageway provided in the stators, center plate, and the second end plate;

the first gas transfer means comprises a second passageway in the second end plate; and

the second gas transfer means comprises a first passageway in the second end plate; and

the outlet means comprises a second passageway in the second end plate, a transverse passageway in the center plate and second stator, and an exhaust port in the center plate.

20. A vacuum pump, as claimed in claim 19, and further comprising exhaust valve means associated with the exhaust port in the center plate, the said valve means being adapted to permit gas to be expelled from the exhaust port and to normally permit the passage of a controlled amount of lubricating oil from the oil bath into the exhaust port.

References Cited UNITED STATES PATENTS 2,902,210 9/1959 Power 230-45 X 3,226,014 12/ 1965 Ianenkov 230-158 3,344,745 10/1967 Scognamillo 230-158 X 3,371,857 3/1968 Le Blanc 230-158 X 3,399,826 9/1968 Andriulis 230158 X 3,438,570 4/1969 Bode et a1. 230-45 X 3,455,245 7/1969 Reichling 230-158 X DONLEY J. STOCKING, Primary Examiner W. J. KRAUSS, Assistant Examiner US. Cl. X.R.