CA1196615A - Gerotor vacuum pump - Google Patents

Abstract

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GEROTOR VACUUM PUMP
David E. Hanson ABSTRACT OF THE DISCLOSURE

A unique gerotor-type pump is disclosed for pumping gases. The pump has an inner gear-type rotor mounted within and rotating off-center with respect to an outer gear-type rotor ring, about a parallel axis. Surfaces of the teeth of the inner rotor are in continuous contact with the outer rotor to define at least one pumping chamber which alternately expands and contracts as the inner and outer rotors turn. An elongated inlet port is positioned to communicate with the pumping chamber during a substantial portion of the cycle when the pumping chamber is expanding, (the intake cycle) and an outlet port is positioned to communicate with the pumping chamber only just prior to and at the end of the contraction or exhaust cycle. Oil for sealing and lubricating between moving parts of the pump is provided by employing differential pressure created by the pump's own opera-tion to draw oil through passageways which communicate between an oil reservoir and the pumping chamber.

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Description

( SPECIFICATION

The present inveniion relates in genexal to pumps for pumping gases and more specifically to vacuum pumps and the like~
Pump5 for compressiing or transferring gases, such as vacuum pumps, are used in a wide variety of industrial and labora-tory applications~ Depending on the particular application, typical features desired in a vacuum p~mp include a long opera-tin~ life or durability, high pump capacity to transfer relatively large quantities of gas in a short time and the capability to pump down to pressure levels of less than or equal to about 10 3 ~0 Torr. In industrial applications~ it is especially desirable thal the pump be resis~ant to excessive wear and blockage due to contaminants such as dirtr water or water vapor in the gas being pumped. For example, vacuum pumps are often used to evacuate ~efri~eration systems before freon or other coolant is added. In ~his r.ype of application, the gas being pumped may carry water droplets, water vapor or dust, as well as other contaminants which may impair the effectiveness of lubricaring oil in the pump and result in increased wear and potential leakage, espe-cially at the pressure levels set forth above.
One type of vacuum pump which has been used in such industrial applications is a rotary vane pump r such as the one illustrated in U S. Patent No. 3,782,868, granted January 1, 1974.
Typically, rotary vane pumps employ an off-center rotor wilhin a cylindrical chamber. The rotor usually has a pair of radially ~h~

slidable vanes which are in continuous contact with -the surface of the chamber, to define a pumping chamber between the rotor and the cylindrical chamber wall, that alternately expands and contracts as the rotor ~urns. Although such pumps generally h~ve worked satisfactorily, the continuous high-speed wear between the rotary vanes and chamber wall require continuous and generous lubrication, and may be subject to wear from contaminants entering the oil and reducing its lubrication efficiency.
Another type of pump which has been used for pumping at pressures levels described here is commonly referred to as a rotary pision pumpO That pump employs an eccen~rically mounted element which turns with a base and carries an oscillating vane.
Because o~ the eccen~ric moun~ing, vibration levels may be sufficient to ha~e detrimental effects in the drive components and thus reduce the useful life o~ the pump, as well as being noisy and difficuli to attach to a rigid sysiem~
Another iype of pump heretofore known, but noi for P~Pin~ gases, is referred to as a gerotor pump~ This type oE
pu~p employs an inner geAr-type rotor which rotates within an outer gear~type ring. The teeth of the inner rotor are in continuous contact with the surface of the outer rotor to deEine a pumping chamber between each pair of teeth, which chamber aljternately expands and coniracts as the rotors turn. Gerotor pumpsj as such, are well known and have been specifically used for pumping oils, hydraulic fluids and other liquidsO ~s compared io oiher pumps, eOg., the rotary vane pu~p, it has relatively few $

moving parts, it is easy ~o fabricate and assemble, and has low differential rotational speed as between the rotors,which xeduces wear. However, the gerotor pumps currently available have a variety of shortcomings when used for pumping gases. For in-stance, gero~or pumps r.ypically depend on the oil or fluid being pumped or lubrication and don~t have separate lubrication capability as is required when gas is being pumped. This, com-hined with the usual inlet and outlet port design for gerotor pumps, permirs gas to bypass between moving parts of the pump and p:revents the pump from being used to pump gases at low pressures.
Accordinglyg it is a general object of the present nvention to provide a geroror-type gas pump which does not suffer from the deficiencies descri~ed above.
It is another object of the present-invention to pro- -yide a gerotor type vacuum pump which has a port design and suffic.ien~ lubrication t.o reduce wear and provide sealing between moving surfaces so as co permit the pumping of gas a~ very low pressure.
2Ci These and orher objects of the present invention are set for~h in the following detailed description of the preferred embodiment of the present invention as shown in the attached.
j~rawinss, of which:
~ Figure l is a perspective view of a gerotor pump lemploying rhe present invention. I
Figul^e 2 is an exploded perspective view o the gerocor !
pump of Figure 1.
Figure 2a is a ront elevational. view of the cenier piece of the rhree-piece pump block shown in Figure 2.

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¢ --j Figure 2b is a horizontal sectional view taken along 'line 2b-2b of Figure 2a.
Figure 3 is an elevational view of the assembled gerotor rotor elements, taken along line 3-3 of Figure 2~
Figure 4 is an elevational view of the assembled gerotor rotor elements, taken along line 4-4 of Figure 2.
Figure 5 is an elevational view of the insude surface of the pump end plate, taken along line 5-5 of Figure 2.
Figure 6 is a vertical sec~ional view taken along line 6-G of Figure 5O
Figures 7-10 are sequen-tial vertical plan views o~
assembled gerotor rolor elements illustraiing, in pari, ihe operation of the gerotor pump embodying the present invention.
~ igure 11 is a schematic of speed control and protective cirCuii employed in the present invention.
~ ,eferring to the arawings for the purpose of illustra-tion only, the present invention is embodied in a two-stage vacuu~ pu~p 20 employing a gerotor rotor assembly 22 for pumping gaseous malerials and the like. For each pumping stage,' the rotor assembly 27 émploys an interior gear-type rotor 24 and an outer gear-type ro~or ring 26 mounted within one of two ~xially parallel but off-set rotor chambers 28 in ea~h end of p~mp block 30. Hereinafter the numeral designations pertaining to ~he first and second pumping stages shiall be respectively ~ollowed by the numerals 1 or 2, e . g O, rotor chamber 28~

i The gerotor rotor assembly 22 and pump block 30 are mounted on one side oE a mounting plate 34 and within a cover 36. The cover 36 contains an oil bath in which the rotor assembly and block are submerged during operation. The rotor assembly 22 f is driven by an electric motor 38 which is attached to the other side of the mounting plale 34, and drives the assembly chrough a sealed cen~er shaft opening 40.
Re~erring briefly to Figures 7-10, which illustrate che gerotor elements in differen-c rotational positions, it may be seen thac the inner rocor 24-1 is mounted on a drive shafc 42 which is off-center within the outer rotor 26-lo As the innex rotor ~4--1 is turned by the electric motor 33, via shaft 42, intermeshing o~ the inner and outer rotor teeth, 44 and 46 re-i spectively, causes ihe outer rotor also to rotate within the rotor chamber 28-1. The inner rocor 24-1 has one less tooth than .
che outer rotor 26-1 r SO that the teeth of the inner rotor are in 1, continuous contact with the surface of the oucer rotor and define a pumping chamber 48-1 between each pair of rotor teeth, as shown by the shaded or cross hatched area. As the inner rotor 24-1

2.0 jrotates, the pumping chamber 48-1 alternately expands and con-t:racts during each revolution o~ the inner and outer rotors, as shown in sequence in Figures 7-10.
1 In accordance with the present invencion, the advantages o~ the gerotor principle in general may ~e used for pumping gases 2~5 and the like, by employing an elongated gas inlet port 50-1 which i 6~
spa~s a relatively large angle so as ~o communicate with each pumping chamber 48-1 during mosi of the rotational cycle when the chamher is expanding, and a discharge port 52-1, angularly llspaced from the inlet port 50-1 spanning a substantially smaller angle 13 and posi-~ioned so as to communicate with the pumping I chamber 48-1 only ~ust prior to and/or at the end of the compres-sion (contraction) cycle. It should be noied that ~he inlet and outlet ports 50-1 and 52-1 are preferably at opposite ends of the rotor set, and ihe inlet port shown in dashed lines in Figures 7-10 is actually in the pump mounting plate 34 ~see Figure 2) and is therefore actually above the surface of the paper. Although not normally part of a plan view, the inlet port is shown in Figures 7~10 for the purpose of e~plana~ion and to better illus-trate the relative angular spacing between the inlet and outlet ports. This may be more clearly understood by referring briefly ~o Figure 2, which shows the actual apparatus and inlet and outlet ports in a perspective rather than a plan view.
For lubricating as well as sealing between relative moving parts of the rotor assemhly 22, oil is introduced into the ~0 pumpin~ chamber 48-1 by using differential pressures created hy the rotation of ihe pump iiselfO The oil sealing cooperates with the relative shape of and spacing be~ween the inlet~and outlet E~FtS to permit the pump to be used for pumping gases at very low pressures. In this aspect of the present invention~ oil is ~ 6 . drawn into the pumping chamber from the surrounding oil bath through a channel 54 in the mounting plate, or a like channel 54 in ~he wear pla~e 56, which channel communicates between the oil bath at one end and the sealed shafi opening 40 at the other.
Suction created during the expansion of the pumping chamber 48-1, draws oil into the shaft area, from the shaft area through a minute space be~ween the rotor element 24-1 and the wear plate 56 to the inlet port 50-1, and into the chamber 48-1. The lubri-cating oil coats the surfaces of the moving parts and provides a seal between them, permitting ihe pumping of relatively low p~essure levels. In otner words, the.oil is drawn through the inlet port 50-1 at one end of the pumping chamber ~8-1, and gradually moves along the length o~ the rotor gear 24-1, as ihe rotor gear is also turning, and exits ~hrough the outlet port 52-1 at the other end, thereby following a generally spiral path ihrough the rotor assembly as it lubrica~es and seals. The flow p~th through the second ~umping stage is similar.
Turning now to a more detailed description of the attached drawings, which show the present invention in its preferred embodimeni for the purpose of illustration only, the pump 20 is compact and relatively lightweight, making it especially portable, and ideal for servicing equipmënt in the f.ileld, for example, refrigeration systems and the like~ As shown in Figure 1, the pump 20 has a handle 58 which may be i attached, as an example to the moun~ing plate 34, which permits the pump to be carried about~
The pump is driven by direct connection between the rotor assembly drive shaft 42 and the electric motor 38, through the mounting plate 34. Although different types oE motors may be used, a brush type motor, as opposed to a conventional induc- ' ~ion ~otor, is preferred because it permits the use of multiple drlve speeds for the pump.
The various elemenls of a gerotor pump assembly embody-ing the present invention are best shown in Figure 2, which de-picts a dual or two-stage pump, with two sets, 24-1, 26-1 and 24-2, 26-2, of gerotor pumping elements connected in series to achieve higher pumping efficiency and lower pressure levels. Each set of rotor elements rotate within one of a pair of cylindrical ro,tor chambers 28-1 and 28-2 provided in the pump block 30. The pump block may be of one piece construction, but a stacking or build-up arrangement of three separate pieces, as shown in Fig. 2, iS preferred because it reduces fabrication and machining cost.
In this arrangement, a center piece (shown in Figures 2a and 2b) is mounted between-two end pieces with bores to form the rotor ,cham~exs 28-1 and 28-2. Both of the inner rotors 24-1 and 24-2 lare turned by ihe common drive shaft 42,,which exte~ds through s~aft oepning 60 in the center piece o~ the pump block between the rotor chambers. The other end of the shaf~ 42 extends through a bearing (not shown) in shaft opening 40 in the mounting ~ _ plate 34 to the motor 38.
The rotor elemenis 24-1, 26-1 and 24-2, 26-2 are mounted wi~hin rotor chambers 28-1 and 28-2, respectively~ sub-, stantially flush with the end surfaces of ~he pumping block 30, but with sufficient clearance for rotation and oil sealing.
Because the mounting plate 34 is preferably made of aluminum, the pump block 30 is preferably spaced from the mounting p]ate by the steel wear plate, although other wear surface or coatings ' may be used. The other end of the pump block is closed by a steel~
end plate 62. The entire assembly of the end plate, pump block, rotor elements and wear plate are secured to the mounting plate 34 by bolts not shown. As noted earlier, this entire assembly is submerged in an oil bath contained within the pump housing 36.
When Ihe pump is operating, gas is drawn from the yQlume to be evacuated through a conduit or hose attached to an air inlet opening 61 in the mounting plate 34; which com~
municates with the crescent-shaped elongated curved inlet port 50-1. The wear plate 56 has a matching crescent-shaped opening, to permit the air or other gas that is being pumped to enter the pumping chamber 48-1 defined between the first stage rotor elements 24-1 and 26-1.
Turning briefly to Figures 7-10, which-depict the ~umping sequence for the first stage pumping chamber 48 1. As `~he inner rotor 24-1 turns in the illustra~ed embodiment in a I clockwise direction~ it drives the outer rotor 26-1, which has .

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one more tooth than the inner rotor, in a clockwise direc~ion at a sligh~ly slower rotational speed than the inner rotor. This Il is one advantage of a gerotor pump -- slow differential rota-j, tional speed between the inner rotor 24-1 and the outer rotor 26-1. In Figure 7j the shaded area, which represents the pumping chamber 48-1, is beginning to expand and draw in gas from the inlet port 50-1. It should be noted that ~he pumping chamber 48-1 defined between the rotor elements is closed at one end by the wear plate 56 and at the other end by ihe inside surEace of the rotor chamber. The inlei port is shown in dashed lines for purposes of explanation, but as noied earlier, is actually part of the spacer plate and mountin~ plate, and in actuality is above the level of the paper in the Figure 7 plan view.
As the pump continues to rotate in a clockwise direc-~5 tion, the pump chamber 48-1, which is beginning to expand when it is first in communication with the leading edge of the inlet port (Figure 7), has substantially completed ~he expansion cycle when it passes our. of communication with the end edge of the inlet pox~ 5Figure 8)o To accommodate this communication during most of the expansion cycle, the elongated inlet port 50-1 ls posi-tioned so that ils leading edge (in the direction of rotor rotation) is spaced as close as 3 from the position-or point a~
whtch the contract:ion ox compression cycle is complete,and spans an angle A (Figs. 7, 10~ which is greater than the angle C between adjacenl ieeih of the inner rotor 24~1. Preferably) the angle A does not exceed the quantity (180 - ~)~

After passing.o.u~ of cornmunication with the inlet port ! 50-1, continued rolation of the inner rolor 24-1, causes the pumping chamber 48-1 ~o contrac~ (Fi.gure g) compressing the g~s ~ithin ihe chamber. Ii is only when the chamber is nearly completely contracted, and the volume of gas is almost compressed to its minimum, that the chamber moves in to communicati.on with the outlet port 52-1 (Figure 10). Tlle oullet port is at the opposite end of the rotor chamber 28-1 from the inlet port 50-1, and is preferably spaced (anyle ~) belween 5 and 38 from ihe inlet port -- ~he smalleJ~the rotor diameter, the larger the anyular spacing required. The outlet port 52-1 is automatically smaller than the inlet port 50-1, in that it spans an angle B
Iwhich is lsss than the angle C between adjacent inner rotor teeth,' and is preferably less than or equal -~o one-h~lf the angle C, i.e., C C/2. The outlet port may be of any desired cross-sectional shape or geome~ry within the ran~e set forth above, but the illus-trated embodiment employs a circular outlet port 52-1. When the pumping chamber conles into communica~ion with ihe outlet port the compressed gas is forced rapidly into the port which, re~erring back IO Figure 2, communicates directly wi~h ~he inleL. pc~rt .~0-2 of the second pumping stage.
By comparing the relative sizes of thè rotor elements . ~etween stages 1 and 2, it is apparent thai stage 1 has a much larger pumping capacity than the second stage of the pump. I~llen lar~er volumes of gas are being pumped by ~he first stage than can be handled by the second stage, such as durihg ini~ial evac-uation of a volume of gas, the excess gas is permitted -to escape through a bypass port 63 (Figs. 2a and 2b) tha-t communicates with the crescen~.-s~aped inlet port 50~2 of the second stage. The b~pass port 63, which is drille~ or otherwise forme~ ~ c~
piece of ihe pump :`

block, is normally closed by a relief valve, for eY~ample, a poppet valve of the type shown in Figure 5, which is set ~o open under the pressure caused by the pumping of large quantities of gas.
After the gas exits from the pump block, it is allowed io escape into the ambient atmosphere through a standard vent 65 in the housing 36~
As shown in Figure 2, the ouile~ port 52-1 of the firs~ stage communicates directly wi~h the elongated, crescent-l shaped inle~ pori 50-2 for the second pumping stage. The second staye rotor chamber 28~2 is narrower than the first siage, and rotatably receives the second slage outex rotor 26-2 and inner rotor 24-2, which is driven by ~he common drive shaft 42 extend-ing through p~mp bloc~ 30. I'he end of the pump block is covered by the end plate 62 which; as best seen in Figures 5 and 6, provides ihe outlei port 52-2 for ihe second pumping stage.
This outlet port, as shown in Figure 6, is normally closed by a spring loaded poppet valve 64 mounted on the exterior of the end plate. This poppet valve, which may be of a variety of shapes, is held against the port 52-2 in ihe normally closed position by a coil spring 69 and overlying leaf or spring retainer 71, and i serves to prevent gas and lubricating oil from leakiny in-to the pumping chambers. Other types of one-way valves, for example, apper or reed valves~ may also be used without departing from ilthe present invention. Although ~he outlet port 52-2 in the end 2S plate is circular, it includes a small recessed area 66, on the . ~
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insi.de surface of the pla~e, which extends from ~he outlet port at an angle to communicate with the pumping chamber 4~2 in the second s~age sligh~ly ~arlier in ~he compressio~ cycle than the outlet por~ in the firs~ stage, but still substan~ially when the compression or con~rac~ion cycle is complete. This is understood to permit better exhaust from the second stage when higher vacuum levels or lower gas pressures are being pumped.
Accordingly, after gas enters the second pumping stage inlet 50-2, the operation is substan~ially the same as the first sta~e, and the port geometry and loca~ion similar~ The pumping chamber 48-2 which is defined between ~he inner rotor 24-2 and outer rotor 2~-2, expands substantially completely as it moves pas~ the inlet port 50-2 and then contracts so that it communicates with the outlet port 52-2 in the end pla~e 62 just prior to and/o.r.
the end o~ the compression cycle. After ~he initial evacuation of the large quantities of gas, and when there is not sufficient gas remaining at the source to require operation of the intermediate pressure relief valve, all the gas being pumped passes through the second stage~ and exits through the spring loaded poppet valve 64 mounted in the end plate 62.
An important aspect of the present invention, which enhances its use as a pump for gaseous materials.an~ for pumping gTses at relatively low pressures resides in a novel oil lubrica-tion and sealing system embodied in the present inven~ion. As descrlbed bri.e~ly earlier~ the entire pumping block 30 and ~ 13 -rotor assembly 22 are submerged in an oil bath contained within the pump housing ~6. Referring back to Figure 2, oil passage is provided along the llnear groove or channel 54 in the mounting plate 34, or alternati~ely in the wear plate 56, which extends ~angentially from the drive shaft opening ~0~ The end of the channel 54 communicates wi~h the oil bath through a small opening 68 in ihe wear plate. That is, thP opening 68 is beyond the edge of t~e pump block 30 and airectly accessible to the lubricant surroun~ing it. The pressure differential created by the expand-ing pump chamber 48-1 draws oil through the small opening 68 and alony the tangential channel 54 to Ihe shaft opening 40, ~d from there, through the minute clearance between the rotor elements 2~-1, 26-1 and the surface of the wear plate 56, in~o the inlet ! port 50-1 and from there into the pumping chamber. This small !~ quantity of oil coats the contacting surfaces of the inner rotor and outer rotox and seals the minuie clearances between them to reduce leakage of gas therebetween and permit more efficient and lower pressure levels io be achievedO Moving in the same general direction as gas flow, the oil moves from inlet 50 1~ along the inner rotor 24-1 to ihe outlet 52-1 and into the second stage for lubricating and sealing there also. As the rotor ~urns, this oil traces a generally spiral path through each pumpiny stage.
In accordance with a further aspect of this oil sealing arrallgemenl, the diamerer of the drive shaft 42 is preferably - 14 ~

substantially smaller ~han the minor root diameler of the inner rotorS 24-1 and 24-2. This provides a relati~ely wide uninter-rupted area which, when sealed by an oil film, helps prevent the I bypass of gas between the end suxface of the inner rotor and the i facing surface o~ the end plate or rotor chamber. Substantially shor~er or narrower surfaces would not provide a sufficiently wide oil film and would permit gas to bypass (sometimes referred to as "blowby" or "leakage") between the moving parts and thus impair the ability of the pump to obtain low pressure levels.
Preferably the ratio of inner rotor minor root diameter to drive shaft dia~eter which is believed io provide the bes~ sealing arrangement is between and includes 2/1 and 4/1.
To further enhance the seal and sealing area between adjacent pump parts in accordance with the present invention, the inlet and outlet ports are preferably located substantially between and have a wid-th preferably less ihan the difference between the minor root ra~ius of the inner rotor and the major rQot radius of the outer rotor (Ro - Ri~ (Fig. 93 so as to maximize the sealing area between the drive sha~t 42 and the ~0 inside periphexal edges of the ports. Further, it should be nQted that in ihe preferred embodiment of the present inventîon~
the lnlet and oullet ports are at opposlte ends o~ t~e pumping c~amber which serVes to increase the distance between them for im~roved sealing and to reduce "blowby'l between the ports.
~5 An auxiliary oil port 53 (Fig. 4) is provided in the second stage o~ the pumping block to improve lubrication ~d , and sealing in relatively high pressure conditions, when much of the oil from the first stage is being exhausted through the intermediate by-pass valve. The oil is drawn through port 53 into the second slage by viscous drag and pressure differential created by the rotation of the outer rotor.
~n alternative technique for introducing lubricating and sealing oil into the pumping chamber, is to provide a series of small depressions in the end surfaces of the inner and/or outer rotors which would communicate during rotation, with oil channeling grooves in the wear pla~e 56, which grooves would extend beyond the edge of the pump block to communicate with the oil bath in which the pump is immersed. Thus, as the rotors rotate, ihey will pick up a selected or pre-measurecl supply of oil as they move past the oil supply channels in the wear plate. The pockets would then discharge the oil into the pumping char~er by way of -the suction created at the inlet port 50~1. When the ;pump is stopped, this arrangement would prevent the vacuum in the sysiem from drawing or sucking oil from the pump back into that system ox source.
The gerotor pump 20 of the present invention is prefexably controlled by the multi-speed electric circuit shown in Figure 11. A multi-speed pump switch 70 has ~ig~ and low spl~ed positions 72 and 74 for varying the p~np speed. For example, high speed may be used during initial evacuation or pump down. The switch 70 varies the pump speed by connecting either ,, i ~ ~ f one or both of resistors 76 and 78 in series with capacitor ~0.
The resistor-capaci~or combination is in parallel with triac 82 and ~he diac 84, and the different charging rates of the capacitor at the switch positions 72 and 74 provide different switch-on 1 intervals for the triac, which energizes the motor 38. As a ' unique protection againsl overhealing, e.g., due to excessive high speed operating time, thermal s~i~ch 68 is connected in parallel with switch 70 and upon overheating operates to connect resistor77 into the circuit to change the charging time constant of the circuit to shift the pump into a low speed mode for cooling.
In summary, with the features described above, a gerotor type pump, which is normally used only for pumping liquids such as hydraulic fluids, and the advan~ages attendant with such I a pump, i.e., Ihe low relative moving speeds between parts, I durability and reliability, may be used for pumping gases at very low pressures, even at the molecular level~
Although the present invention has been described in terms of the preferred embodiment, the scope of the present inventlon, as set forth in the attached claims, is intended to 2n include those equivalent structures, some of which may be im~ediately apparent upon reading chis description and others of which may become apparent only after some study7--I

Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. In a gerotor vacuum pump adapted for evacuating gases from a container or the like at relatively low pressure levels, wherein the gases are drawn from the container into said pump through a vacuum connection and into a pumping chamber, said pump including first and second walls defining rotor chamber therebetween, an outer rotor having a plurality of teeth disposed on the inner axial surface thereof, an inner rotor mounted on a shaft for rotation within said outer rotor and having one less tooth on its outer axial surface than said outer rotor, the inner axial surface of said outer rotor and the outer axial surface of said inner rotor defining at least one pumping chamber which expands and contracts as said inner rotor rotates relative to said outer rotor, the improvement comprising: a gas inlet port in said first wall disposed between said vacuum connection and said pumping chamber, said gas inlet port communicating with said pumping chamber during a substantial portion of the time when said pumping chamber is expanding, a gas outlet port in said second wall axially disposed and angularly spaced from said inlet port, said gas inlet port communicating with said pumping chamber only just prior to complete contraction of the pumping chamber, and oil inlet means communicating an oil source with said gas inlet port, said oil communicating with said gas inlet port and following a generally spiral path through said pump, said oil source having a pressure greater than the pressure within said gas inlet port and said pumping chamber during operation of said pump, whereby the expansion of said pumping chamber during operation of said pump creates a pressure differential between said oil source and said gas inlet port which draws oil from said oil inlet means through said gas inlet port with said gas being evacuated from said container into said pumping chamber and coating the contacting surfaces of said inner rotor and outer rotor upon communication of said gas inlet port with said pumping chamber to provide an oil coating between said contacting surfaces so as to seal the gas in said pumping chamber during the expansion and contraction of said pumping chamber and permit the pumping of relatively low pressure levels, the inner rotor minor root diameter being sufficiently larger than the diameter of said shaft to provide a relatively wide uninterrupted area which receives an oil film during operation of the pump which thereby effectively minimizes bypass of gas between an endface of said inner rotor and a surface of the first wall adjacent thereto,

2. A pump in accordance with claim 2, wherein the ratio of root diameter of said inner rotor to the shaft diameter is at least 2:1.

3. A pump in accordance with claim 2, wherein the ratio of root diameter of said inner rotor to the shaft diameter is from approximately 2:1 to 4:1.

4. A pump in accordance with claim 2, wherein the leading edge of said gas inlet port is angularly spaced from 5° to 38° from the nearest edge of said gas outlet port.

5. A pump in accordance with claim 2, wherein the gas inlet port spans an angle which is greater than the angle between adjacent teeth of said inner rotor.

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6. A pump in accordance with claim 1, wherein said gas outlet port spans an angle which is less than the angle between adjacent teeth of said inner rotor.

7, A pump in accordance with claim 1, comprising a pair of rotor chambers, said outlet port of one of said chambers communicating with the inlet port of the other of said chambers, and the outlet of said other of said chambers communicates with the ambient atmosphere.

8. A pump in accordance with claim 1, further comprising a spring biased valve normally closing said outlet port.

9. A pump in accordance with claim 1, further comprising an electric motor for turning said rotor gear, and control means for deenergizing said motor when a selected amount of gas is pumped.

10. A pump in accordance with claim 9, wherein said control means includes a temperature sensing element disposed to control operation of said motor.

11. A gerotor vacuum pump adapted for evacuating gases from a container or the like at relatively low pressure levels comprising:
first and second walls defining a cylindrical rotor chamber therebetween;
an outer rotor having a plurality of teeth disposed on the inner axial surface thereof;

an inner rotor mounted on a shaft for rotation within said outer rotor, and having one less tooth on its outer axial surface than said outer rotor, the inner axial surface of said outer rotor and the outer surface of said inner rotor defining at least one pumping chamber which expands and contracts as said inner rotor rotates relative to said outer rotor;
a vacuum connection providing a passageway for gas flow from said container to said pump;
a gas inlet port in said first wall disposed between said vacuum connection and said pumping chamber, said gas inlet port communicating with said pumping chamber during a substantial portion of the time when the pumping chamber is expanding, said gas inlet port further spanning an angle which is greater than the angle between adjacent teeth of said inner rotor;
a gas outlet port in said second wall axially disposed and angularly spaced from said inlet port, said gas inlet port communicating with said pumping chamber only just prior to complete contraction of the chamber, said gas outlet port further spanning an angle which is less than the angle between adjacent tooth divisions of said inner rotor, whereby gas flowing through said pump follows a spiral path between said inlet and said outlet as said rotor turns; and oil inlet means communicating an oil source with said gas inlet port, said oil source having a pressure greater than the pressure within said gas inlet port and said pumping chamber during operation of said pump, whereby the expansion of said pumping chamber during operation of said pump creates a pressure differential between said oil source and said gas inlet port which draws oil from said oil inlet means through said gas inlet port with said gas being evacuated from said container into said pumping chamber and coating the contacting surfaces of said inner rotor and outer rotor upon communica-tion of said gas inlet port with said pumping chamber to pro-vide an oil coating between said contacting surfaces so as to seal the gas in said pumping chamber during the expansion and contraction of said pumping chamber and permit the pumping of relatively low pressure levels, the inner rotor minor root diameter being sufficiently larger than the diameter of said shaft to provide a relatively wide uninterrupted area which receives an oil film during operation of the pump which there-by effectively minimizes bypass of said gas between an endface of said inner rotor and a surface of the first wall adjacent thereto.

12. A pump in accordance with claim 11, wherein the ratio of root diameter of said inner rotor to the axial shaft diameter is at least 2:1.

13. A pump in accordance with claim 11, wherein the rotor of root diameter of said inner rotor to the shaft diameter is from approximately 2:1 to 4:1.

14. A pump in accordance with claim 11, wherein the leading edge of said gas inlet port is angularly spaced from 5° to 38° from the nearest edge of said gas outlet port.

15. A pump in accordance with claim 11, comprising a pair of rotor chambers, said outlet port of one of said chambers communicating with the inlet port of the other of said chambers, and the outlet of said other of said chambers communicates with the ambient atmosphere.

16. A pump in accordance with claim 11, further comprising a spring biased valve normally closing said outlet port.

17. A pump in accordance with claim 11, further comprising an electric motor for turning said rotor gear, and control means for deenergizing said motor when a selected amount of gas is pumped.

18. A pump in accordance with claim 17, wherein said control means includes a temperature sensing element disposed to control operation of said motor.