US3227093A

US3227093A - Piston pump having rolling diaphragm

Info

Publication number: US3227093A
Application number: US341889A
Authority: US
Inventors: John F Taplin
Original assignee: Individual
Current assignee: Bellofram Corp
Priority date: 1964-02-03
Filing date: 1964-02-03
Publication date: 1966-01-04
Anticipated expiration: 1983-01-04

Description

Jan. 4, 1966 J. F. TAPLIN 3,227,093

PISTON PUMP HAVING ROLLING DIAPHRAGM Filed Feb. 3, 1964 2 Sheets-Sheet 1 PRESSURE lawma- J 622 17. Tupiz'za,

by MW kiibvney Jan. 4, 1966 J. F. TAPLIN PISTON PUMP HAVING ROLLING DIAPHRAGM 2 Sheets-Sheet 2 Filed Feb. 5, 1964 1820633303223 Jami Tap .9 Wm W United States Patent 3,227,093 PISTON PUMP HAVKNG RGLLING DIAPHRAGM .l'ohn F. Taplin, l Sewall St., West Newton, Mass. Filed Feb. 3, 1964, Ser. No. 341,889 (llaims. -(Cl. ltlS-ISO) This invention is concerned with piston pumps, and more particularly with piston pumps having rolling diaphragrns.

The rolling diaphragm of piston pumps of the aforementioned description has a radially outer fixed portion clamped to the cylinder of the pump and a radially inner portion which is secured to the top of the piston of the pump. The rolling diaphragm further includes an intermediate portion known as the rollingwall thereof. During the suction stroke of the piston pump the rolling wall of the diaphragm rolls off the piston side wall onto the cylinder side wall. Thereafter during the compression stroke of the piston pump the rolling wall performs a reverse movement, i.e. it rolls oil" the side wall of the cylinder onto the side wall of the piston. The rolling diaphragm of a single action piston pump subdivides the cylinder into two chambers. One of these two chambers is the pump chamber into which a fluid or liquidis sucked during the suction stroke of the piston pump, and out of which the fluid or liquid is pressed during the following compression stroke of the piston of the pump. The other of two chambers into which thecylinder is subdivided by the rolling diaphragnrmay be referred to as the inactive chamber since the fluid or liquid to be moved by the pump never enters into that chamber.

in order for a piston pump having a rolling diaphragm to properly perform the pressure in the pump chamber must always exceed the pressure in the inactive chamber. The term pressure reversal refers to a condition wherein the pressure in the inactive chamber exceeds the pressure in the pump chamber. Pressure reversal may result in a collapse of the rolling diaphragm which, in turn, renders the pump inoperative. 7

It is, therefore, a principal object of this invention to provide reciprocating piston pumps having rolling diaphragms which pumps are not subject to pressure reversal and to collapse of their diaphragm.

Double acting reciprocating piston pumps having a pair of rolling diaphragms rather than but one single rolling diaphragm are not subject to pressure reversal and collapse of their diaphragms. Such pumps are, however, bulky and relatively expensive since they require a duplication of many parts thereof and in particular a duplication of their rolling diaphragms and of their inlet check valves and of their outlet check valves.

It is, therefore, another object of this invention to pro vide relatively compact and inexpensive. single action rolling diaphragm type piston pumps which pumps do not require any duplication of their parts, and which pumps are not subject to pressure reversal and diaphragm collapse. I

.In prior art reciprocating single action piston pumps the inactive chamber of the cylinder is generally freely vented to atmosphere in order to prevent the build-up of pressure therein, and thus to avoid pressure reversal and diaphragm collapse.

I have found that free venting of the inactive chamber to atmosphere is notor in many instances may not be a sufficiently effective means for avoiding pressure reversal and collapse of the rolling diaphragm.

It is, therefore, another object of this invention to proother limit position;

3,227,093 Patented Jan. 4, 1956 vide single action piston pumps having a rolling diaphragm which are not subject even under the most adverse conditions to pressure reversal and collapse of their rolling diaphragm.

In a single acting piston pump having a rolling diaphragm wherein the inactive chamber is freely vented the dgree of pressure build-up in the inactive chamber depends upon the size of the latter, the velocity of the reciprocating movement of the piston, the area of the venting or air dumping orifice, etc. One or more of these parameters may unavoidably or necessarily be such as to establish a tendency of pressure reversal and collapse. of the rolling diaphragm.

It is, therefore, another object of the invention to provide single acting piston pumps having rolling diaphragms which pumps depart from the heretofore adhered-to principle of freely venting the inactive chamber to atmosphere and wherein another more effective principle is substituted. therefor, namely that of constituting the reciprocating piston of the pump a means for establishing a vacuum in the inactive chamber, i.e. of reducing the pressure in the inactive chamber to and below atmospheris pressure. i

The foregoing and other generaland specialobjects of the invention will appear from the ensuing description of the invention, as illustrated in the accompanying drawings, wherein r FIG. 1 is a vertical section through a piston pump embodying this invention showing the reciprocating piston thereof during a suction stroke midway between the two limit positions thereof;

FIG. 2 is a vertical section through another piston pump embodying this invention showing the reciprocating piston thereof at the end of its suction stroke;

FIG. 3 is a vertical section through the structure of FIG. 3 showing the reciprocating piston thereof in its FIG. 4a is a vertical section through a portion of a prior art piston. pump and illustrates the condition known as collapse of the rolling diaphragm; and

FIG. 4b is a section similar to that illustrated in FIG. 4a showing another form of a collapse of the rolling diaphragm of a piston pump.

Referring now to the drawings, and more particularly to FIG. 1 thereof, reference numeral 1 has been applied to generally indicate a cylinder body comprising an upper cylinder body portion 1a and a lower cylinder body portion lib held together by screws 10. The aforementioned cylinder body portions 1a, 1b may be formed, for instance, of aluminum castings. The upper portion 1a of the cylinder body 1 defines a suction inlet 3 including a spring biased. inlet check valve 4 and a pressure outlet 5 including a spring biased outlet check valve 6. Reciprocating piston '7 is arranged inside of cylinder body 1 and has a top surface hand a bottom surface 7b. The lower portion 11) of cylinder body 1 defines an internal bottom surface 8 which includes an upstanding collar 9 arranged in coaxial relation to, and inside of, cylinder body ll. Collar 9 accommodates a slide hearing it) guiding a piston rod 11 secured to piston 7. The bottom surface 7b of piston 7 has a cylindrical recess adapted to re ceive collar 9 when piston 7 is in its lowest position, i.e. is completing the suction stroke thereof. Speaking more generally, the bottom surface 7b of piston 7 is shaped to conform substantially with the internal bottom surface 8 of cylinder body 1 in order to minimize the volume of the gap formed between surfaces 71) and 8 when piston 7 is at the end of the suction stroke thereof, i.e. in its lowest position. Reference numeral 12 has been applied to generally indicate a rolling diaphragm. Rolling diaphragm 12 includes a radially outer portion 12a clamped between portions 1a, 1b of cylinder body 1, an intermediate rolling wall 12b and a radially inner portion 120 secured to the top surface 70 of piston 7. To this end the radially inner portion 120 of rolling diaphragm 12 is covered by a clamping plate 13 screwed against the top surface 7a of piston 7 by means of nut 14 riding on the screw-threaded top end 11a of piston rod 11. Screw-nut 15 is arranged inside of the center recess 7c of piston 7 and clamps the latter against the radially inner portion 2c of rolling diaphragm 12 and against clamping plate 13. Rolling diaphragm 12 subdivides the interior of cylinder body 1 into the upper pump chamber 16 and a lower chamber 17 referred to as the inactive chamber. The lower portion 1b of cylinder body 1 defines a duct 18 connecting inactive chamber 17 with the outer atmospher'e. Duct 18 is under the control of a spring-biased check valve 19 venting inactive chamber 17 to the outer atmosphere when the pressure inside chamber 17 exceeds atmospheric pressure. On the other hand, check valve 19 precludes at any time air from the outside of cylinder body 1 to enter inactive chamber 17. The lower portion 1b of cylinder body 1 is provided with a circular groove 20 receiving a so-called O-ring 21, or an equivalent annular elastic seal closely surrounding piston rod 11. Seal 21 in conjunction with diaphragm 12 limits the inflow of air into, and the outflow of air from, inactive chamber 17 to duct 18, and the aforementioned check valve 19 precludes air from the outside of cylinder body 1 from entering into chamber 17.

When piston 7 is moved downward by motor means (not shown in FIG. 1) operating piston rod 11, i.e. during the suction stroke of piston 7, liquid enters through suction inlet 3 into pump chamber 16 as indicated by arrow R until the latter chamber is filled with liquid. Upon reversal of the motion of piston 7, i.e. during the compression stroke thereof, liquid contained in pump chamber 16 is expelled out of it through pressure outlet 5 and check valve 6. Check valve 19 limits the build-up of pressure inside of chamber 17 during the first suction stroke of piston 7 and precludes outside air from entering into chamber 17 during the subsequent compression stroke of piston 7. Thus the air in inactive chamber 17 is rarified and the pressure of air in chamber 17 is maintained at any point of time below the pressure prevailing in pump chamber 16. In other words, so-called pressure reversal condition is effectively precluded, thus preventing collapse of the rolling diaphragm 12. The lower portion of the pump structure performs as a vacuum pump, evacuating inactive chamber 17. Therefore, as long as the pump of FIG. 1 operates sucking liquid through inlet 3 into chamber 16 and expelling or discharging liquid from chamber 16 through outlet 5, a vacuum is established and maintained in chamber 17 which precludes the collapse of rolling diaphragm 12.

Because of the geometry or configuration of the surface 761 of piston 7 and that of the juxtaposed surface 8 of cylinder body 1 the volume of chamber 17 at the end of the downward stroke of piston 7 is minimized. Consequently the amount of residual air remaining in chamber 17 at the end of each downward stroke of piston 7 is likewise minimized. This, in turn, results in establishing a relatively high vacuum in chamber 17 during the ensuing upward stroke or compression stroke of piston 7.

The entire volume of piston 7 consists of a homogeneous mass, e.g. aluminum. In other words, piston 7 is not hollow, and this precludes air contained in chamber 17 from entering into the piston 7, being temporarily trapped therein and later, when discharged from piston 7, to degrade the vacuum prevailing in chamber 17.

The structure shown in FIGS. 2 and 3 is very similar to that shown in FIG. 1, except for the fact that in the former venting of the inactive chamber is not effected by means responsive to the amount of pressure above atmospheric pressure prevailing in the inactive chamber but by venting means which are positively controlled by the piston of the pump, and more particularly by the position of the piston of the pump relative to the cylinder body thereof.

In FIGS. 2 and 3 like parts as in FIG. 1 have been designated by the same reference characters with a prime added. Thus reference character 1' has been applied to indicate a cylinder body comprising an upper portion 1a and a lower portion 1b. The upper portion 1a defines suction inlet 3' and pressure outlet 5' controlled by check valves 4' and 6', respectively. Piston '7 has an upper surface 7a and a lower surface 7b, the latter having the same shape or configuration as the internal bottom surface 8' of cylinder body 1. Collar 9 of cylinder body 1' receives a slide bearing 10 of bronze arranged above circular groove 20 for sealing ring 21' closely surrounding piston rod 11. The geometry of recess 70 in piston 7 corresponds to that of collar 9' and, therefore, the latter fits tightly into recess 70. Rolling diaphragm 12' includes the radially outer portion 12a secured to cylinder body 1', and intermediate rolling Wall 1212 arranged in the gap established by virtue of the clearance prevailing between the bore of the cylinder body 1' and the outer diameter of piston 7, and rolling diaphragm 12' further comprises the radially inner portion 12c. The latter is provided with an annular bead 22 engaging a circular groove 23 in the side wall of piston 7' immediately adjacent the top surface 7a thereof. Bead 22' and groove 23 are means for securing rolling diaphragm 12 to piston 7. Rolling diaphragm 12 subdivides the interior of cylinder body 1' into pump chamber 16' and inactive chamber 17'. Piston rod 11 defines a duct 24' having an intake opening 25 always situated inside chamber 17 and having an exhaust opening 26' which is blocked in certain positions of piston 7' and unobstructed in other positions of piston 7. The exhaust opening or orifice 26' is obstructed during the preponderant portion of the compression stroke of piston 7, i.e. the movement of piston 7 from its lowest position to its highest position. Thus no external air is admitted into chamber 17' during that time of increasing volume of chamber 17. Exhaust opening or orifice 26' is freed only when piston 7 reaches the full suction stroke position thereof and when piston 7 reverses the direction of its movement. At this time chamber 17 communicates with the outer atmosphere and, therefore, the pressure inside of chamber 17 tends to be equal to atmospheric pressure. During the ensuing compression stroke of piston 7 the volume of chamber 17' decreases progressively, and since chamber 17 is tightly sealed from the outer atmosphere, the pressure in chamber 17' decreases progressively during the compression stroke of piston 7'. During the following suction stroke of piston 7 the volume of chamber 17' decreases progressively and reaches a minimum at the time exhaust opening or orifice 26' is uncovered. At that time the pressure in the small gap formed between surfaces 7!) and 8' may be equal to atmospheric pressure and this is the maximum pressure which ever can occur inside of chamber 17' during any full operating cycle of the piston pump.

In FIGS. 4a and 4b reference character 1" has been applied to indicate a portion of the cylinder body of a prior art piston pump having a piston 7" and a rolling diaphragm 12". FIGS. 4a and 4b indicate two diaphragm configurations resulting from pressure reversal and known as collapse of the rolling diaphragm. It will be apparent from the foregoing that pressure reversal and collapse of the rolling diaphragm are effectively avoided in the structures shown in FIGS. l-3 and described in connection therewith.

While this invention is not limited to any particular kind of rolling diaphragm, best results will be achieved by using a rolling diaphragm made in accordance with the teachings set forth in United States Patent 2,849,026 to John F. Taplin, issued on Aug. 26, 1958 for Flexible Fluid Sealing Diaphragm.

While, in accordance with the patent statutes, I have disclosed the specific details of two embodiments of my invention, it is to be understood that these details are merely illustrative and that many variations thereof may be made without departing from the spirit and scope of the invention. It is, therefore, my desire that the language of the accompanying claims shall be accorded the broadest reasonable construction, and shall be limited only by what is expressly stated therein, and by the prior art.

I claim as my invention:

1. A reciprocating single action piston pump comprising in combination:

(a) a cylinder body defining a suction inlet and a pressure outlet and having a bottom surface;

(b) a reciprocating piston arranged inside said cylinder body and having a bottom surface juxtaposed to said bottom surface of said cylinder body;

(c) a rolling diaphragm having a radially outer portion secured to said cylinder body, an intermediate rolling wall and a radially inner portion secured to said piston and subdividing the interior of said cylinder body into a pump chamber and an inactive chamber;

(d) means for exhausting to the outer atmosphere air contained in said inactive chamber during the suction stroke of said piston without an auxiliary vacuum pump, said exhausting means including means for minimizing the clearance between said bottom surface of said cylinder body and said bottom surface of said piston; and

(e) means effective during a preponderant portion of the compression stroke of said piston for precluding the backflow of air from the outer atmosphere into said inactive chamber.

2. A reciprocating single action piston pump comprising in combination:

(a) a cylinder body defining a suction inlet and a pressure outlet and having a bottom surface having a predetermined geometry;

(b) a reciprocating piston arranged inside said cylinder body and having a bottom surface juxtaposed to said bottom surface of said cylinder body and matching the geometry thereof to minimize the volume of the space between said bottom surface of said cylinder body and said bottom surface of said piston at the end of the suction stroke of said piston;

(c) a rolling diaphragm having a radially outer portion secured to said cylinder body, an intermediate rolling wall and a radially inner portion secured to the top surface of said piston and subdividing the interior of said cylinder body into a pump chamber and an inactive chamber;

((1) duct means connecting said inactive chamber with the outer atmosphere;

(e) a piston rod secured to said piston and project ing transversely through said bottom surface of said cylinder body;

(f) sealing means surrounding said piston rod for sealing said inactive chamber to limit the inflow of air into and the outflow of air from said inactive chamher to said duct means; and

(g) automatic means for controlling the flow of air through said duct means allowing the outflow of air from said inactive chamber during the suction stroke of said piston and substantially precluding the inflow of air into said inactive chamber during the compression stroke of said piston.

3. A reciprocating single action piston pump comprising in combination:

(a) a cylinder body defining a suction inlet and a pressure outlet and having a bottom surface including a flat radially outer substantially annular portion and an inwardly projecting radially inner portion forming a bearing coaxial to said cylinder body;

(b) a reciprocating piston arranged inside said cylinder body and having a bottom surface juxtaposed to said bottom surface of said cylinder body and including a flat radially outer substantially annular portion substantially matching with the shape of said radially outer portion of bottom. surface of said cylinder body, said bottom surface of said piston further including a radially inner recess substantially matching with the shape of said inwardly projecting portion of said bottom surface of said cylinder body;

(d) duct means defined by said cylinder body connecting said inactive chamber with the outer atmosphere;

(e) a piston rod secured to said piston and sliding in said bearing formed by said bottom surface of said cylinder body;

(g) a check valve controlling the flow of air through said duct means to cause the outflow of air above atmospheric pressure from said inactive chamber and to preclude the inflow of air under atmospheric pressure into said inactive chamber.

t. A reciprocating single action piston pump comprising in combination:

(a) a cylinder body defining a suction inlet and a pressure outlet and having a bottom surface including a fiat radially outer substantially annular portion and an inwardly projecting portion forming a bearing coaxial to said cylinder body;

(b) a reciprocating piston arranged inside said cylinder body and having a bottom surface juxtaposed to said bottom surface of said cylinder body and including a flat radially outer substantially annular portion substantially matching with the shape of said radially outer portion of said bottom surface of said cylinder body, said bottom surface of said piston further including a radially inner recess substantially matching with the shape of said inwardly projecting portion of said bottom surface of said cylinder body;

(d) a piston rod secured to said piston and sliding in said bearing formed by said bottom surface of said cylinder body;

(e) duct means connecting said inactive chamber with the outer atmosphere, said duct means including a passageway defined by said piston rod; and

(f) means integral with said cylinder body for opening said duct means at least during a portion of the suction stroke of said piston and for closing said duct means during the preponderant portion of the con pression stroke of said piston.

5. A reciprocating single action piston pump comprising in combination:

(a) a cylinder body defining a suction inlet and a pressure outlet and further defining an internal bottom surface;

(b) a reciprocating piston arranged in said cylinder body, said piston having a top surface and a bottom surface, said bottom surface of said piston being shaped to conform substantially with said internal bottom surface of said cylinder body to minimize the volume of the gap formed between said bottom surface of said piston and said internal bottom surface of said cylinder body when said piston is at the end of the suction stroke thereof;

(c) a rolling diaphragm having a radially outer portion secured to said cylinder body, an intermediate rolling wall and a radially inner portion secured to said top surface of said piston and subdividing the interior of said cylinder body into a pump chamber and an inactive chamber;

(d) duct means for connecting said inactive chamber with the outer atmosphere; and

(e) automatic means for controlling the flow of air through said duct means allowing the outflow of air from said inactive chamber during the suction stroke of said piston and substantially precluding the inflow of air into said inactive chamber during a preponderant portion of the compression stroke of said piston.

6. A reciprocating single action piston pump as defined in claim wherein substantially the entire volume of said piston consists of a homogeneous mass, whereby trapping in said piston of air contained in said inactive chamber and subsequent discharge of air trapped inside said piston into said chamber is positively precluded.

7. A reciprocating piston pump comprising in combination:

(a) a cylinder body defining a suction inlet and a pressure outlet;

(b) a reciprocating piston arranged inside said cylinder body;

(0) a rolling diaphragm having a radially outer portion secured to said cylinder body, an intermediate rolling wall and a radially inner portion secured to the top surface of said piston and subdividing the interior of said cylinder body into a pump chamber and an inactive chamber;

((1) a piston rod secured to said piston;

(e) duct means extending through said piston rod and having a venting orifice for discharging air from said inactive chamber to the outer atmosphere;

(f) means for sealing said inactive chamber to limit the inflow of air into and the outflow of air from said inactive chamber to said duct means, said sealing means including an annular seal surrounding said piston rod; and

(g) means integral with said cylinder body for obstructing said orifice during the preponderant por tion of the compression stroke of said piston and for freeing said orifice at substantially the full-suctionstroke-position of said piston.

8. A reciprocating single action piston pump comprising in combination:

(a) a cylinder body defining a suction inlet and a pressure outlet and further defining an internal bottom surface including an inwardly projecting coaxial collar;

(b) a reciprocating piston arranged in said cylinder body, said piston having a top surface and a bottom surface and said bottom surface of said piston including a coaxial recess conforming with said collar of said internal bottom surface of said cylinder body and said bottom surface of said piston being adapted to closely hug at the end of the suction stroke of said piston said internal bottom surface of said cylinder body;

(d) a slide bearing arranged in coaxial relation inside said collar of said bottom surface of said cylinder body;

(e) a piston rod attached to said piston, guided in said slide bearing and defining a duct including a venting opening for connecting said inactive chamber with the outer atmosphere;

(f) an O-ring seal for said piston rod arranged inside said collar adjacent the axially outer end thereof; and

(g) means integral with said cylinder body for 0hstructing said venting opening except when said piston is adjacent the full suction stroke position thereof, and said venting opening obstructing means inciuding said slide bearing.

9. A reciprocating single action piston pump comprising in combination:

(b) a reciprocating piston arranged inside said cylinder body and having a bottom surface juxtaposed to said bottom surface of said cylinder body and including a fiat radially outer substantially annular portion substantially matching with the shape of said radially outer portion of said bottom surface of said cylinder body, said bottom surface of said piston further including a radially inner recess substantially matching with the shape of said inwardly projecting portion of said bottom surface of said cylinder body;

(d) a piston rod connected to said piston and projecting out of said cylinder body;

(e) means for exhausting air contained in said inactive chamber from said cylinder body including a duct defined by said cylinder body connecting said pump chamber with the outer atmosphere; and

(f) means effective during a preponderant portion of the compression stroke of said piston substantially precluding the backfio-w of air from the outside of said cylinder body into said inactive chamber, said backfiow-precluding-means including a check valve arranged in said duct, and said back-iiow-precludingmeans further including an annular seal interposed between said cylinder body and said piston rod.

10. A reciprocating single action piston pump comprising in combination:

(a) a cylinder body defining a suction inlet and a pressure outlet;

(b) a reciprocating piston arranged inside said cylinder body;

(c) a rolling diaphragm having a radially outer portion secured to said cylinder body, an intermediate rolling wall and a radially inner portion secured to said piston and subdividing the interior of said cylinder body into a pump chamber and an inactive chamher;

((1) a piston rod connected to said piston and projecting out of said cylinder body;

(e) means for exhausting air contained in said inactive chamber from said cylinder body including a duct defined by said piston rod, said duct having an inlet at a point relatively close to the top surface of said piston and an outlet relatively remote from the top surface of said piston; and

(f) means effective during a preponderant portion of the compression stroke of said piston substantially precluding the back-flow of air from the outside of 9 10 said cylinder body into said inactive chamber, said 2,945,451 7/1960 Griswold 103-174 back-flow-prccluding-means including means for ob- 3,027,848 4/1962 Merkle 103-150 structing said outlet of said duct and said back-flow- FOREIGN PATENTS precluding-means further includlng an annular seal interposed between said cylinder body and said piston 5 11303475 3/1962 Fffincerod, 1,308,008 9/1962 France.

8,851 1/1897 Great Britain. References Cited by the Examiner UNITED STATES PATENTS 2,919,650 1/1960 Wiggermann 103450 10 DGNLEY J'STOCKINGEMMM 2,928,351 3/1960 Klinger 103--150 W. L. FREEH,AssislantExaminer.

SAMUEL LEVINE, Primary Examiner.

Claims

1. A RECIPROCATING SINGLE ACTION PISTON PUMP COMPRISING IN COMBINATION: (A) A CYLINDER BODY DEFINING A SUCTION INLET AND A PRESSURE OUTLET AND HAVING A BOTTOM SURFACE; (B) A RECIPROCATING PISTON ARRANGED INSIDE SAID CYLINDER BODY AND HAVING A BOTTOM SURFACE JUXTAPOSED TO SAID BOTTOM SURFACE OF SAID CYLINDER BODY; (C) A ROLLING DIAPHRAGM HAVING A RADIALLY OUTER PORTION SECURED TO SAID CYLINDER BODY, AN INTERMEDIATE ROLLING WALL AND A RADIALLY INNER PORTION SECURED TO SAID PISTON AND SUBDIVIDING THE INTERIOR OF SAID CYLINDER BODY INTO A PUMP CHAMBER AND AN INACTIVE CHAMBER; (D) MEANS FOR EXHAUSTING TO THE OUTER ATMOSPHERE AIR CONTAINED IN SAID INACTIVE CHAMBER DURING THE SUCTION STROKE OF SAID PISTON WITHOUT AN AUXILIARY VACUUM PUMP, SAID EXHAUSTING MEANS INCLUDING MEANS FOR MINIMIZING THE CLEARANCE BETWEEN SAID BOTTOM SURFACE OF SAID CYLINDER BODY AND SAID BOTTOM SURFACE OF SAID PISTON; AND (E) MEANS EFFECTIVE DURING A PREPONDERANT PORTION OF THE COMPRESSION STROKE OF SAID PISTON FOR PRECLUDING THE BACKFLOW OF AIR FROM THE OUTER ATMOSPHERE INTO SAID INACTIVE CHAMBER.