US2628014A - Multiple stage nonslip vacuum pump - Google Patents

Description

Feb. l0, 1953 M. PEMMERI. 2,628,014

MULTIPLE STAGE NoNsLIP VACUUM VPUMP Filed Aug. 17, 1950 3 Sheets-Sheet 1 Feb. l0, 1953 M PEMMERL 2,628,014

MULTIPLE STAGE NONSLIP VACUUM PUMP Filed Aug. 17, 1950 5 Sheets-Sheet 2 Feb. 10, 1953 M. PEMMERL MULTIPLE STAGE MoNsLIP VACUUM PUMP 3 Sheets-Sheet 5 Filed Aug. 17 1950 Patented Feb. 10, 1953 MULTIPLE STAGE NONSLIP VACUUM PUMP Michael Pemmerl, Chatham, N.,'J., assignor of one-half to Max Raslfin, Chatham, N. J.

Application August 1111950, serial No. 179,981

9 Claims.

The invention relates to rotary pumps of the type wherein two segmental pistons rotate in the same direction about a common axis in a cylindrical working chamber having intake and outlet ports and are driven alternately at high and low angular speeds in such relation to the chamber ports that, during each cycle of operation, fluid is sucked through the intake port into the space between two separating working faces of the pistons at the same time that nuid previously entrained between the other working faces of the pistons is expelled through the outlet ports as these latter faces close upon each other.

Prior to the present invention, rotary pumps of this type have involved a single stage of pumpagain will produceI a state of imbalance.

ing action, during which retrograde slip of Huid Consequently, it is another object to include in past the frictional faces of the pistons and chamthe driving mechanism for the pistons special ber walls occurs and is especially aggravated means for restoring the balance thereof to the when the iiuid being pumped is air or other gases. maximum degree practicable.

Consequently, when the single stage principle has A still further object of the invention is to probeen adopted for vacuum pumps, it has not been vide a specific. construction of multiple stage possible to produce a vacuum of sulciently high pump which is characterized by extreme simdegree for certain industrial purposes, such as in plicity, compactness, economy of materials, and the evacuation of radio and television tubes. smooth operational eiciency.

It, therefore, is the principal object of the' 25 Other objects and advantages will appear as present invention to provide an improved vacuum the following specic description is read in conpump wherein slip is neutralized by adding at nection with the accompanying drawings, in least one additional stage of pumping action folwhich: lowing the heretofore practiced single, or basic, Fig. 1 is a side elevation of the complete pump, stage. To be more explicit, the improvement with the concealed piston assembly stopped in consists in subjecting the uid which is being mid-cycle condition; Fig. 2 is an end elevation of expelled from the working chamber of the basic the same, partly in section; Fig. 3 is a top plan pumping stage to suction created in the working view partly in section; ,and Fig. 4 is a large scale chamber provided for the additional pumping longitudinal vertical section on line li-Jl of Fig. 2. stage before there has been any opportunity for Fig. 4A is a large scale fragmentary longitudislip to occur. Successful achievement of the slipnal vertical section of the casing unit and piston neutralization action is evidenced by the inf unit alone taken on line 4-4 of Fig. 2, showing creased rate and smoothness of pull on the uid the piston unit stopped at the end of an operabeing evacuated. tional cycle.

In a single stage rotary pump, the two pistons 40 Fig. 5 is a transverse vertical section on line come together at one side of the rotational axis ei-5 of Fig. 4A; and Fig. 5A is a similar view on in dead-center condition once in each operational line 5A5A of Fig. 4. cycle and thus create a state of imbalance which, Fig. 6 is 'a'transverse vertical section on line though only momentary, is quite objectionable. 5--5 of Fig. 4A', showing the mid-cycle piston Consequently, in applying an additional pumping positions in broken lines. stage for slip-neutralization, the pistons in the 7 is a, Similar View on une 1 1 of Fig ,M additional WOlkng Chamber have been S0 CS- Fig. 8 is a transverse vertical section on line posed in coordination with the pistons ci the basic s g of Fig, 4; and Fig. 8A is a similar view Dumping Stage that the pistons 0f both Chambers showing the linkage arrangement corresponding will assume their dead-center conditions when to the dead center piston Condition represented located at diametrically opposite sides of the in Figs. 4A, 5, 6 and 7, rotational axis in balancing relation 'to each pig 9 i5 a 'diagrammatic View embracing the other. The achievement of this state of balance, three pumping stages to demonstrate the sliphowever. has the disadvantage Of Causing' ill'- neutralization effect of the primary and secondchamber slip between certain areas of the fricary Stages of neutralization.

tional surfaces and thus impairs the eiiiciency of both stages.

Due' to this situation, it is a. further object of 'i the invention to restore the full efficiency of the basic and primary slip-neutralization stages by adding a secondary slip-neutralization stage embodied in a third working chamber and set of l pistons therefor.

position in axial alignment with the pistons of the basic stage, which means that these two sets of pistons will be diametrically opposite to the pistons of the primary slip-neutralization and Referring nowl in detail to the drawings, wherein like reference characters 'designate corresponding parts in the several views, the improved pump comprises three major assembly units, viz.: casing unit A, piston unit B and differential drive unit C.

Casing unit A provides three distinct working g ble, so a one-piece cylindrical body I3, which may be supported in a horizontal position 'on base 13", has been adopted to constitute the principal element of said casing unit. For reasons which will become apparent as the description progresses, body I3 has been bored longitudinally to provide working chambers whichare stepped 11p-in diameter from one end .of .said body to the other but are of substantially the-same length. Working chamber IQ, which is Yat the intake end of the pump, as will be specified in greater detail later, has the smallest cubical capacity of the .series of chambers and working chamber I2, which is at the discharge end, ,has the .greatest capacity. The annular shoulder I4 formed at the juncture of working chamber .III with working chamber I I and the similar shoulder I5 at the juncture of working chamber II with working `chamber I2 serve a special purpose lwhich will be .described later in .connection with .specific description of piston unit B.

The open ends .of the casing bore are adapted to be closed by end plates .IS and 4I1.respectively, with plate I5 forming .the outer end wall of working chamber IB and plate I1 -forming the outer end wall of working chamber I2. .End plates I6 and AI1 preferably are detachably secured to body I3 by screws I6' and I1. However, it is a useful feature of Vthe invention that end .plate I1 shall normally form a part of piston .unit B so that said plateis always in .correct position to be applied to casing body I3 wheneversaid piston unitv is insertedinto the bore thereof. Division walls between working Achambers are required, but itis a feature of the present invention that, instead .of being provided in vcasing unit A, the necessary vwalls for .this purpose are incorporated in piston unit B to attain the simplicity of construction which yhas been advanced as one of the objects of the invention, .'Iherefore, detailed description ofthe division or-partitioning walls will be postponed until the Vsubject of piston unit B has been reached herein.

Body I3 is provided with intake ports I8, I3 and I8" and with outletports I-9, IS 'and IB, respectively, for working chambers ID, I.I .and I2. Intake port I- and-outlet'port I9 'are adapted to be connected to the 'suction 'and discharge pipes of the pumping or air evacuating system for which the Ypump is designed. Referring particularly to Figs-and 7, it -willbe observ-ed that intake port I8 and outlet port I9 for working chamber I2 and intake port I8" and outlet port I9 for working chamber I2 vare all disposed in communication with the upper areas of said working chambera'whereas Fig. '6 discloses the fact that intake port I8 and outlet port IQ' vfor working chamber II are in communication with the lower area of the latter. The reasonfor this staggered arrangement will appear as the description progresses.

In accordance with the concept of slip-neutralization advanced herein, outlet port I3 for Working chamber Iii is connected by a communicating duct 20 with intake port I3' for working chamber II. Similarly, outlet port I9' for working chamber II is connected by communicating duct 2| with intake port I8" for working chamber I2. It is within the scope of the invention to employ any suitable means for providing the two communicating ducts 2i) and 2| but it is preferred to use externally located tubing 22 and 22' for this purpose.

Fndplates i6 and I1 of casing unit A, when in appliedpositions are located suitably to sup- ,port journaling means for both ends of piston 'unit 'B,'which will -now be described.

Referring particularly to Fig. 4, it will be observed that piston unit B includes a central driven shaft 23, which has a driven sleeve 24 rotatably enclosing .the left end portion thereof. The right end of driven shaft 23 is adapted to be journaled in a bearing boss 25 provided centrally on end plate AIE of casing vunit A, whereas driven sleeve 24 is adapted to be journaled in a. bearing boss 26 provided centrally on end plate I1, Driven sleeve 24 thus is rotatable in endplate I'I and in turn .serves to journal the left end of driven shaft 23. Driven sleeve 2li is integral with the hub of a preferably hollow segmental piston 21 which is disposed in working chamber I2 in cooperation with a structurally mated piston 28, which also is preferably hollow. The hub of piston 21 is rotatable with driven sleeve 24 on driven shaft 23 and extends throughout nearly half the length of working chamber I2, Whereas the head of the piston extendsfrom the outer endof said chamber .nearly to the other end. The inwardly projecting half of piston .2.1 overlies and bears against the hub of piston28, whose projecting half Vsimilarly overlies andbearsagainst the hub of piston 21, all in the manner well known in the art. Piston28 is amxed by suitable removable means, such as cross-pin 29, to driven shaft 23 for rotational movement therewith. The external dimensions of pistons 21 and 2B should be such that they frictionally bear against the end and peripheral surfaces of working chamber i2 in as nearly duid-tight a manner as may be eiected between any moving parts. The inner end wall of working chamber I2 .is formed by a centrally perforated partitioning disc 3G which separates the said workingchamber from adjoining Working chamber I I andis rotatably mounted on driven shaft 23 in abutting relation to the flush inner (right) end faces of pistons 21 and 28. The thickness of partitioning disc 3G should -be substantially equal to the distance remaining between the inner end faces of pistons 21 and 23 and shoulder I5 cf the outer chamber wall. The external diameter of partitioning disc 3i) should be such that it will frictionally bear against the peripheral surface of workingchamber I2.

Working chamber 1li is provided simiiarly to working chamber i2 with a pair of coasting pistons 2i" and of proportionately smaller size than pistons '2? and 'and which preferably are solid in construction. Piston 2S is aiiixed to driven shaft 23 for rotation therewith, by cross-pin 29', but it is intended that piston 2? shall rotate with piston?? of working chamber i2, so partitioning disc 3:3 is afiined to said pistons 21 and 2 by removable means such as stud bolts 3 I. it will be observed that piston is set diametrically opposite to piston 2 with respect to the axis of driven shaft 23. The'reason for this arrangement will appear when the operation of the pump is described later herein.

Working chamber It is provided with a pair of hollow coacting pistons 21 and 28 of proportionately smaller size than pistons 27 and 23 of working chamber II. In like manner, piston 28 is anixed to driven shaft 23 by removable means, such as cross-pin 29", and piston 2'1" is joined to piston 2 for movement therewith through the medium of a second partitioning disc 32 and by the use of attaching means such as stud bolts 33. This second partitioning disc 32 is proportioned to rotate in working chamber II in substantially air-tight abutting relation to the peripheral surface thereof and to shoulder I4. In this instance, piston 2l" is set diametrically opposite to piston 2l and in axial alignment with piston 21.

Referring now to Figs. 5, 6 and 7, it will be observed that each piston subtends an arc of substantially 90 degrees between its leading and trailing axial working faces 34 and 35, respectively; also that the associated intake and outlet ports for each working chamber are disposed in substantially horizontal alignment either above or below the rotational axis of the piston unit and that they open into the working chamber through arcuate grooves 33. Each groove 35 extends through an are of substantially 90 degrees and both grooves of the pair associated with any one working chamber are separated at their ends most remote from the horizontal plane in which the rotational axis lies by a narrow land 37 which is bisected b-y the vertical plane through said rotational axis. In the case of working chambers IQ and I2, the intake and outlet ports are in communication with the upper areas thereof, so land 3l is located at the uppermost point. On the contrary, the intake and outlet ports for working chamber I I communicate with the lower area thereof, so land 3l is at the lowest point. En either case, the land 3l occupies a dead-center position, or starting and stopping point, for each cycle of pumping operation within the working chamber. Due to the specic piston and port relation just described, when both pistons in any working chamber are in the dead-center condition represented in Figs. 5, 6 and 'I in solid lines, adjacent working faces of the pistons will be slightly separated and the space therebetween will be masked by land 3l; also the pistons will substantially span the arcuate grooved portions of the respective ports.

By comparing the structural proportions disclosed in Figs. 5, 6 and '7, it will be apparent that the piston displacement is graduated through working chambers Iii, Ii a-nd I2. The difference in displacement between working chamber I and working chamber I l is substantially equal to the interior fluid volume of communicating duct 23, so that each operational cycle of the pistons in working chamber II will cause the iiuid in said duct to be sucked into working chamber II rst and then the fluid displaced by the pistons of working chamber Ill will be sucked in. In this way, all of the fluid evacuated by the basic pumping stage in working chamber IU will be accommodated in working chamber I I without the usual back pressure and slip. In like manner, the displacement difference between working chambers II and i2 is substantially equal to the interior fluid volume of communicating duct 2i which leads from the outlet port of the former chamber to the intake port of the latter.

Various types of differential driving mechanisms generally suitable for the operation of' forms part of the present invention, the linkage type of driving mechanism has been adopted.

Referring now to Figs. 1, 2, 8 and 8A in particular, it will be observed that driving unit C includes a cylindrical drum 38 which has central apertures 39 and 43 in its inner and cuter end walls 4I and 42, respectively, in concentric relation to the drum axis. Inner aperture 39 has an exteriorly projecting bearing boss 43, which is adapted to rotatably engage an external peripheral :bearing surface 44 provided on bearing boss 24 of end plate II in eccentric relation to the axis of driven shaft 23 of piston unit B. The axis of bearing surface 44 is disposed directly beneath the axis of driven shaft 23. When drum 38 of differential driving unit C' is thus mounted for rotation on an axis eccentric to the axis of rotation of piston unit B, bearing boss 43 of the former will frictionally abut the adjacent vertical face of end plate Il.

Driving linkage 45 interconnects drum 38 with driven shaft 23 and driven sleeve 24 of piston unit B, and includes a crank 46 whose hub 4l is removably axed, as by cross-pin 48, to the outer (left) end of driven shaft 23. A cooperative crank 49 has its hub 53 removably affixed, as by key 5I, to driven sleeve 24. Hub 5t of crank 49 is confined between hub 4i of crank 4S and bearing boss 26 of end plate Il and thus is in substantial axial alignment with said hub 6l. Roughly, crank 48 may be described as lying in the plane of its hub 55, whereas crank 46 is axiallyoffset inwardly to cause it to overlie hub 55 of crank 45 in the plane of the latter. Each of said cranks 46 and 49 is bifurcated radially outward from its hub. The bifurcated porti-on of crank 46 stra-oldies the inner end of a preferably bowed link 52 and is pivotally connected to the latter by a removable axially disposed screw pin 53, which is adapted to be operated in application or detachment by insertion of anysuitable turning tool (not shown) through aperture 46. The outer end of link 52 is pivotally connected to drum 33 by means of a removable screw pin 54, which is terminally engaged with both end walls 4I and 42 in axial spanning relation thereto. In like manner, a similar link 55 has its inner end pivctally connected to crank 49 by screw pin 56 and its outer end connected to the end walls of drum 38 by screw pin 57. The specific construction of cranks 45 and 49 and the arrangement of said cranks and links 52 and 55 in relation to the dead-center positions of the pistons of piston unit B is such that each leading piston of the pair in each working chamber will be driven through the complete 360 degrees of one operational cycle while the trailing piston is driven 90 degrees, or one-fourth as fast in angular speed. Upon completion of each cycle, the order of fast and slow movement of the pistons of each pair will be reversed, this alternating differential driving motion being in conformity with the well known mode of operation of single stage rotary pumps.

It has been stated previously herein that pistons 27-28 and 27"-28 are hollow in structure whereas pistons 2?'-28 are solid. This difference serves to reduce the degree of imbalance of the diametrically opposed sets of pistons to a minimum. Attention is now invited to the fact i that cranks 4E and 49 of differential driving unit C are enlarged in size and mass beyond what aces-,m4

would seem to be necessary. The reasonfor this enlargement is to make the 'cranks So heavy that, when they are disposed below the rotational'axis of piston unit B in the positions shown in Fig. 8A and all of the pistons thereof are in deadcenter condition, the weight of said cranks will counter-balance the excess weight of hollow pistons 21-28 and ZTL-28 over 'solid pistons 2'l28. The weight may be increased for a given size of crank by making it of metal that is heavier than that of which the pistons are made. This balancing feature is very important and makes the pump vibrationless and quiet in opera-- tion.

The pump is adapted to be driven by any suitable prime mover, such as an individual electric motor (not shown). To this end, a cupped face plate 58 is provided for fixed -connection to shaft 59 of the motor and detachable connection to outer end wall @l2 of drum 3S by such means as screws Eil. it will be noted that drum 38 is well adapted to serve as a pulley for engagement by the drive belt (not shown) of some other type of power system.

Due to the unit assembly feature, assembling the three principal units and dismounting the same is greatly simplied. For instance, assuming that the pump is completely assembled as illustrated in Figs. l to 4, inclusive, it may be separated into its component units in the following manner. First, face plate 58 is detached from its connection with drum 23S of differential drive unit C and said face plate and the motor are moved out of the way. Then, differential driving unit C is disconnected from piston unit B by simply knocking out cross-pin 48 and then pulling drum 38 axially outward until it is clear of the end of driven shaft23. After differential driving unit C has been moved out of the wat', screws il may be unscrewed to free end plate i'I from body i3 of casing unit A, whereupon the entire piston unit B, together with the integral partitioning means for working chambers is, il and I2 constituted by partitioning discs 39 and 32, may be extracted bodily from the left end of the casing bore. The advantages of this unit assembly feature should be self-evident,

The operation ofthe pump is likewise extremely simple though productive of Aresults heretofore unknown. Referring now to diagrammatic Fig. 9, it will be seen that, as the pistons in working chamber Iii Vof the basic pumping stageadvance through each cycle of operation, the air'or other fluid is sucked through intake port I3 from the suction pipe of the system into said chamber (see arrows) as the space between the trailing working face 35 of fast-moving pistonl" separates from the leading working face Sli of 'slowmoving piston 2S". at the same time, fluid previously entrained in this manner between leading working face 3:3 of piston 2l" and trailing working face 35 of pist-on 28 is being expelled through outlet port le and communicating duct 20 into working chamber Il as these last-mexitioned piston faces close on each other. It is during this cycle of pumping operation in working chamber le that compression of thefluid being evacuated therefrom into working chamber i I ordinarily would cause retrograde leakage between the frictional faces of the pistons and casing walls back into the suction pipe. Due, however, to the addition of working chamber li and its pistons 21' and 28', the tendency toward slip will be neutralized to a great degree, because the fluid displaced from working chamber il] plus the Afluid forced ahead of the vsame into communicating ductv2li will be sucked into the larger interior .of working chamber il, as indicated by lthe arrows. This is the primary stage of slip neutralization to which previous reference has been made. It also has been explained that neutralization of slip by this stage is not quite perfect, because there is an area between the frictional faces where rearward suction is opposed to forward pull of the pistons in working chamber il to such a degree that slight slip will occur. It is for this reason that the secondary slip-neutralization stage has been applied by the pumping action in working chamber l2. It will be seen that fluid evacuated by the pistons in working chamber il through outlet port le' and communicating duct 2l into working chamber l2 will be accommodated, to include the contents of said duct, by the still further increased piston displacement in said chamber vl 2. lIn fact, such perfect neutralization of slip has been achieved as a result of the combined primary and secondary stages thereof that a Vacuum pump constructed in accordance with 'the invention consistently produces a vacuum of 2 microns of mercury when operating at a shaft velocity of 800 R. P. M.

Having thus described the invention, I claim:

l. A multiple stage non-slip vacuum pump comprising: casing means providing a plurality of cylindrical working chambers which include a basic pumping chamber and a slip-neutralizing chamber of greater cubic capacity than said basic pumping chamber, each of said working chambers being closed by end walls and having intake and outlet ports disposed substantially degrecs apart, means providing a communicating duct leading from the outlet port of the basic pumping chamber to the intake port of the slipneutralizing chamber, a pair of relatively movable segmental pistons rotatable in each working chamber on a concentric axis and being of length and diameter corresponding to those dimensions of the respective working chambers in which they are located, the angle between the leading and trailing faces of each piston being such that greater piston displacement will occur in the slipneutralizing chamber than in the basic pumping chamber, and driving means to cause the pistons in each working chamber to move with variable velocities and to approach and recede from each other alternately in such manner that when the space between approaching faces is in communication with one port of the corresponding working chamber the space between receding faces is in communication with the other port, said driving means being constructed and arranged to cause the space between receding faces of the pistons in the slip-neutralizing chamber to be in communication with the intake port of said chamber while the approaching faces of the pistons in the basic pumping chamber are in communication with the outlet port of the latter chamber, whereby fluid will be sucked into the slip neutralization chamber simultaneously with expulsion of fluid from the basic pumping chamber.

2. A multiple stage non-slip vacuum pump as defined in claim 1, wherein the piston displacement in the slip-neutralizing chamber exceeds the piston displacement in the basic pumping chamber by an amount substantially equal to the interior fluid volume of the communicating duct.

3. In a multiple stage non-slip vacuum pump, the combination of a, casing unit and a piston 9 unit removably mounted in said casing unit, said lcasing unit including a cylindrical body having a longitudinal bore of circular cross-section providing at least two aligned concentric working charnbers of successively stepped up diameters having individual intake and outlet ports and a communicating duct leading from the outlet port of one chamber to the intake port of the other, and a closure plate for one end of the casing bore having shaft bearing means ycentrally disposed therein, said piston unit comprising a closure plate` for the opposite end of the casing bore, means for removably attaching the last-mentioned closure plate to the casing body, said last mentioned-closure plate having a central sleeve bearing therein, a driven sleeve journaled in the said sleeve bearing, a driven shaft having one end portion relatively permanently journaled in said driven sleeve and its opposite end removably journaled in the bearing means of the first-mentioned closure plate of the casing unit, a pair of coacting rotary pistons of appropriate size rotatable in each working chamber, one piston of each pair being affixed to the driven shaft for rotation therewith, a partitioning disc rotatably mounted on said driven shaft in interposed relation to the respective pairs of pistons and being adapted to rotatably bear against the peripheral wall of the casing bore in uid sealing relation thereto," one of the pistons not affixed to the driven shaft being connected to the driven sleeve for rotation therewith, and means connecting both of the pistons not affixed to the driven shaft xedly to the partioning disc for rotation therewith.

4. In a multiple stage non-slip vacuum pump as defined in claim 3, the assembly-facilitation feature wherein the working chamber further from the casing end closed by the rst-mentioned closure plate is the larger of the two and a shoulder is formed at the juncture of both working chambers, the partitioning disc being of a size and form to flt the larger diameter working chamber in abutting relation to said shoulder when the units are assembled.

5. A multiple stage non-slip vacuum pump comprising, in combination, casing means providing three concentric aligned working chambers of circular cross-section which include a basic pumping chamber and primary and secondary slipneutralizing chambers, each of said working chambers having intake and outlet ports, means providing separate communicating ducts leading from the outlet port of the basic pumping chamber to the intake port of the primary slip-neutralizing chamber and from the outlet port of the last-mentioned chamber to the intake port of the secondary slip-neutralizing chamber, a pair of cooperative rotary pistons operatively mounted within each of said working chambers and adapted to suck fluid into the chamber through the intake port thereof and to discharge previously entrained fluid simultaneously through its outlet port, a driven sleeve journaled in said casing means in exteriorly projecting relation thereto and being xedly connected to one piston in each working chamber, a driven shaft journaled in said driven sleeve in eXteriorly projecting relation thereto and being flxedly connected to the pistons that are not iixedly connected to the sleeve, the respective pistons being arranged angularly on the driven sleeve and shaft in such manner that the pistons in two of the working chambers will be diametrically opposed to the pistons in the third working chamber when the pistons of each pair 4are in relatively closed deadcenter condition, external cylindrical bearing means provided on the casing means adjacent t-o the projecting portions of said driven sleeve and shaft in eccentric relation to the rotational axis thereof, and differential driving means for the pistons comprising a drum adapted to be connected operatively with a source of power for rotation in one direction and having apertured end walls, said drum having a concentric bearing in one of said end Walls adapted to slidably and rotatably engage the eccentric bearing of the casing means, a crank aiiixcd to the driven sleevey a second crank aixed to the driven shaft, separate links pivotally connected at opposite ends to the-respective cranks and to the said drum walls in such relation to the pistons when in deadcenter condition that the cranks will be disposed in closely abutting relation diametrically opposite to the two pairs of pistons that are on the same side of the rotational axis with respect to each other, said cranks being weighted in relation to the pistons so as to counter-balance the same and being located on the same side of the driven shaft axis as the drum axis, the link connected to the crank which is leading the other in relation to the direction of drum rotation being connected to the drum at a point substantially diametrically opposite to the driven shaft axis and the link for the other crank being connected to the drum at a point close to the said driven shaft axis and substantially diametrically opposite to the drum connection of the first-mentioned crank, whereby the pistons in each working chamber are driven alternately at high and low speed.

-6. A multiple stage non-slip vacuum pump as dened in claim 5, wherein the differential driving means constitutes a removable unit wherein the means for affixing one crank to the driven sleeve is a longitudinally slidable key and the means for afxing the other crank to the driven shaft is a cross-pin engageable through the aperture in the outer end wall of the drum for detachment, whereby the complete unit may be withdrawn axially from the casing means.

'7. A multiple stage non-slip vacuum pump comprising: casing means providing a plurality of cylindrical working cham-bers lincluding a basic pumping chamber and a slip-neutralizing chamber of greater cubic capacity than said basic pumping chamber, each of said chambers having intake and outlet ports; means providing a communicating duct leading from the outlet port of the basic pumping chamber to the intake port of the slip-neutralizing chamber; piston means rotatable in each of said working chambers and comprising a pair of independently movable segmental pistons mounted for coacting rotation on an axis concentric to the working chamber in which located, each piston of each pair having leading and trailing faces lying substantially in planes intersecting on the rotational axis and which are separated by an included angle of less than degrees whereby compression and expansion spaces are provided between adjacent leading and trailing faces of both pistons; operating means adapted to drive both pistons of the pair in each working chamber alternately at high and low speed whereby as the leading face of a piston operating at high speed closes on the trailing face of the coacting piston compression of gas in the working chamber space between said faces will occur and conversely expansion -will occur in the space between the trailing face of the high speed piston and the leading face of the low speed piston; and means adapted to coordiu 11 Y nate the motion of the pairs of pistons in the respective basic pumping chamber and slip-neutralizing chamber in such a manner that in the basic pumping chamber the space under expansion between receding piston faces will register with the intake port thereof simultaneously with registration of the compression space between closing piston faces with the outlet port, and in the slip-neutralizing chamber the space between piston faces under expansion will register with the intake port of said chamber simultaneously with registration of the space under compression with the outlet port.

8. A multiple stage non-slip vacuum pump as dened in claim "I, wherein the slip-neutralizing chamber and the face areas of the pistons therein are dimensioned to aord higher piston displacement than in the basic pumping chamber.

9. A multiple stage non-slip vacuum pump as defined in claim '7, wherein the slip neutralizing chamber and the face areas of the pistons therein 112 are dimensioned to aiord higher piston displacement than in the basic pumping chamber by a. Volume substantially equal to that of tne said communicating duct which extends between chambers.

MICHAEL PEMMERL.

REFERENCES CITED The following references are of record in the ie of this patenti'l UNITED STATES PATENTS Number Name Date 1,443,764 Smith Jan. 30, 1923 1,607,383 Aurand Nov. 16, 1926 2,189,674 Parker Feb. 6, 1940 FOREIGN PATENTS Number Country Date 2,083 Great Britain 1908 713,099 France 1931

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