US3749527A

US3749527A - Positive displacement pump for dispensing a metered quantity of liquid

Info

Publication number: US3749527A
Application number: US00235153A
Authority: US
Inventors: W Slagle
Original assignee: LINCOLN HALL RESEARCH CO
Current assignee: GAMBLE NATIONAL Inc; LINCOLN HALL RESEARCH CO; LINCOLN HALL RESEARCH CO US
Priority date: 1972-03-16
Filing date: 1972-03-16
Publication date: 1973-07-31
Anticipated expiration: 1990-07-31

Abstract

A liquid dispensing pump is disclosed which is driven by compressed gas and which withdraws liquid from a source of supply under partial vacuum. The pump includes means for positively purging the driving gas at the end of the dispensing stroke, so that the pump will quickly be ready for another dispensing action if desired. The pump includes a provision for volume adjustment, and the volume adjustment mechanism includes a special anti-rebound feature.

Description

United States Patent 1 i1 1 Slagle [451 July 31,1973

[ POSITIVE DISPLACEMENT PUMP FOR DISPENSING A METERED QUANTITY OF LIQUID Inventor:

William E. Slagle, Barrington, Ill.

Lincoln-Hall Research Company, Lincolnwood, Ill.

Mar. 16, 1972 Assignee:

Filed:

Appl. No.:

U.S. Cl 417/395, 91/234, 91/402 Int. Cl. F04b 43/10, F04b 45/06 Field of Search 417/402, 392, 401,

Primary Examiner-William L. Freeh Assistant Examiner-Richard Sher Attorney-Vernon D. Beehler. Gene W. Arant et a1.

[57] ABSTRACT A liquid dispensing pump is disclosed which is driven by compressed gas and which withdraws liquid from a source of supply under partial vacuum.

The pump includes means for positively purging the driving gas at the end of the dispensing stroke, so that the pump will quickly be ready for another dispensing action if desired.

The pump includes a provision for volume adjustment, and the volume adjustment mechanism includes a special anti-rebound feature.

12 Claims, 7 Drawing Figures 50 fi /60 40 5 55 62 /09 6 I //7 A25 #6 f in e7: 205 /2/ 224 3 5 a //5 /2 i //Z I 5 e 1 .5 20, I: /9/ [i //9 E63 r- T 7 T: %5 l L. T

2 n 1, /70 u ii /92 l /4, 495 /4 f /44 /45 r /4/ /4d 59 A48 6 l" 1 6 M3 M2 Patented July 31, 1973 3,749,527

4 Sheets-Sheet 1 70 CASH 1956/572 Patented July 31, 1973 3,749,527

4 Shouts-Sheet i5 Patented July 31, 1973 3,749,527

4 Sheets-Sheet 4 POSITIVE DISPLACEMENT PUMP FOR DISPENSING A METERED QUANTITY OF LIQUID BACKGROUND OF THE INVENTION The present invention is an improvement on Beverage Dispensing Apparatus which is disclosed in the co-pending application of Wilbert .l. Jaeger, Ser. No. 38,800, filed May 19, 1970, which is assigned to the same assignee as the present application.

One aspect of the present invention is the provision of novel means for purging the driving gas from the gas expansion chamber as a direct result of the forward movement of the piston.

Another feature of the present invention is the provision of two separate and independent means for purging the driving gas, which operate in parallel and hence supplement each other.

A third aspect of the invention is the incorporation of an anti-rebound feature into the volume adjustment means.

It is therefore the object and purpose of the present invention to provide a new and improved and more reliable positive displacement pump for dispensing a precisely metered quantity of a liquid.

DRAWING SUMMARY FIG. 1 is a perspective view of a liquid dispensing unit in accordance with the invention;

FIG. 2 is a cross-sectional view of the pump and its associated control housing, shown in one position of the working parts;

FIG. 3 is a view like FIG. 2 but showing a different position of the working parts;

FIG. 4 is an enlarged fragmentary cross-sectional view ofa two-part valve mechanism utilized in the control housing;

FIG. 5 is a transverse cross-sectional view taken on the line 5-5 of FIG. 4;

FIG. 6 is a transverse cross-sectional view taken in the line 6-6 of FIG. 2; and

FIG. 7 is a cross-sectional view of the pump showing an alternate position of the volume adjustment mechamsm.

PREFERRED EMBODIMENT Reference is now made to FIGS. 1 to 7, inclusive which illustrates a complete liquor dispensing unit in accordance with the present invention. FIG. 1 shows a dispensing nozzle assembly N, a liquor bottle 20, a liquor dispensing line 30 which is coupled to the nozzle assembly, a pressure relief line 40 coupled to the nozzle assembly for controlling the dispensing of liquor, and a pump P for the liquor.

The pump P shown generally in FIG. 1 (its internal construction being shown in detail in FIGS. 2, 3 and 4) includes a main housing 100 within which the dispensing operation is performed, and an auxiliary or control housing. The control housing includes an upper block 150, a middle block 160, and a lower block 170. A tube section 31 extending from liquor bottle is coupled to the upper end of dispensing housing 100 through a valve unit 102. A tube section 32 supplies liquor at atmospheric pressure to the interior of bottle '20. Dispensing tube leading to the nozzle N is also coupled to the upper end of dispensing housing 100, through a valve unit 104. Adjusting handle 145, which is used for adjusting the precise amount of liquor to be dispensed in each shot, protrudes from the bottom of housing 100.

The pressure relief line 40 from the nozzle assembly N is coupled into the center section 160 of the control housing. Gas pressure line 205 receives pressurized gas from a tank C for energizing the dispensing operations, and is coupled into the lower section 170 of the control housing.

FIG. 1 also illustrates an indicator system which may be used as optional equipment. A pulse transducer 195 is coupled to the lower section 170 of the control housing. The pulse transducer is energized from an electrical cable 196 and supplies counting pulses via electrical cable 198 to a cash register. The operation of lower section 170 of the control housing (FIGS. 2, 3 and 4) is such that each time a shot of liquor is dispensed a pulse of gas pressure is applied to the pulse transducer 195.

The operation of the displacement pump P is such as to draw a vacuum on the line 31. The manner in which liquor is drawn by vacuum from bottle 20 is described in detail in the copending application of William E. Slagle, Ser. No. l93,l37, filed Oct. 27, 1971 and assigned to the same assignee as the present application.

The dispensing housing contains a chamber having forward end portion and rearward end portion 112, and a piston 130 which reciprocates between the end portions of the chamber. In the retracted position of piston 130 as shown in FIG. 2 the chamber portion 110 holds a precisely measured quantity of liquor. As the piston 130 travels. forward the liquor is dispensed through valve fitting 104 into output line 30. FIG. 3 shows the piston as it is moving forward and prior to striking the end wall 113 of the chamber.

More specifically, the dispensing housing 100 on its upper end has a passageway 101 which opens through the end wall 113 of the

chamber

110, 112. A standard check valve unit 102 is screwed into the outer end of passageway 101, and receives the end of tube 31. Valve fitting 102 contains a check valve which permits liquor to flow from the tube 31 into the chamber 1 10, but will not permit flow in the opposite direction.

In similar fashion a passageway 103 communicates from the upper end of housing 100 through the wall 1 13 of the chamber. A valve fitting 104 is screwed into the outer end of passageway 103, and receives the output tubing line 30. Valve fitting 104 contains a check valve which permits liquor to flow from the chamber out into 30, but not in the opposite direction.

A separate piston housing provides the chamber portion 112 within which piston may be partially retracted. Piston housing 1 15 is inserted into the otherwise open lower end of dispensing housing 100 to confine the piston 130 in the chamber thereby provided. The piston 130 is of generally cylindrical configuration and is covered by a flexible bellofram 132 which covers the lower end of the piston as well as its cylindrical sidewall and extends peripherally outwardly from the piston. The peripheral extremity of bellofram 132 is retained between annular shoulder 109 of housing 100 and the upper circumferential end face of the piston housing 1 15.

A metal end cap 131 fits over the lower end of piston 130 to retain bellofram 132 in place. A piston stem or rod 137 has its upper end threaded at 135 and screwed into a threaded opening in the piston 130. A nut 136 carried on the threaded portion of stem 137 is tightened against the metal cap 131 for retaining the piston and pistion rod in assembled relationship, and at the same time firmly pressing the end cap 131 against the bellofram. When the piston 130 traveis forwardly the bellofram 132 is progressively peeled back from the circumference of the piston, and hence serves to maintain a tight pressure seal between the chamber portion 110 and the chamber portion 112 (see FIGS. 3 and 4).

Pressurized gas flows into compartment 112 through a horizontal passageway 106, annular space 111 surrounding the piston housing, and a passageway 116. The pressurized gas is supplied to passageway 106 through a communicating passageway 193 of the lower control housing block 170. Whenever piston 130 is to be driven in an upward stroke pressurized gas from the line 205 is permitted to pass through the control mechanism into passageway 193 and hence into compartment 112.

The dispensing housing 100 also has a sensing passageway 105 formed therein, which communicates from the upper end of chamber portion 1 to the control housing 150. The purpose of passageway 105 is to sense the liquid pressure in the chamber portion 110, the response to this liquid pressure being utilized in the control system in a manner which will subsequently be described. The net flow of liquor back and forth in the sensing passageway 105 is of miniscule proportions, however, and it therefore does not affect the precision metering action achieved by the chamber 110.

The operation of pump P in dispensing a precisely measured amount of liquor has been described in the copending application of Wilbert J. Jaeger, Ser. No. 38,800, filed May 19, l970,and which is assigned to the same assignee as the present application. According to the present invention, however, a somewhat different control action is utilized. Furthermore, according to the present invention a novel method is provided for purging the pressurized gas when the drive stroke is completed.

CONTROL OF PUMP P The method of controlling the operation of dispensing pump P is, in general, as follows:

The supply of compressed gas from the supply line 205 to the chamber 112 of the pump is controlled by a pilot-operated valve. This valve including

movable members

174 and 180 is shown in its closed position in FIG. 2, while in FIG. 3 it is shown in its open position. The pilot-operated valve is in turn controlled by a pressure-sensitive diaphragm 153. FIG. 2 shows the position of diaphragm 153 when the system has been pressurized and is ready to operate, while FIG. 3 shows the alternate position which the diaphragm assumes after pressure has been relieved through pressure relief line 40 by actuation of a button on nonle N.

The

control housing

150, 160, 170 and mechanisms contained therein will now be described in detail.

In the upper control housing section 150 a sensing passage 151 communicates between sensing passage 105 of housing 100, and-a chamber 152 which opens onto the upper side of a diaphragm 153.

Control housing section 160 has a raised surface portion 162 which is spaced only a short distance away from the under surface of diaphragm 153, when diaphragm 153 is in its raised position as shown in FIG. 2. Both the pressure line 205 and pressure reiief line 40 communicate with the under side of diaphragm 153.

Pressure line 205 communicates through an opening formed in the raised surface portion 162. When the button is pushed and the pressure in line 40 is relieved. there is a drop in pressure on the under side of diaphragm 153, permitting that diaphragm to close on the raised portion 162.

The

separate blocks

150 and 160 are conveniently arranged so that diaphragm 153 is supported between the under side of biock 150 and the upper side of block 160.

A second diaphragm 167 is supported between the under side of block 160 and the upper side of block 170. Pressure from line 205 is applied to the under side of diaphragm 167, but is bled off when line 40 is open.

The under side of diaphragm 167 is coupled to the upper member 174 of a

twopart valve mechanism

174, 180. When line 40 is opened (by pushing a liquor dispensing button on the nozzle assembly N), after the movement of diaphragm 153 takes place as previously described the pressure on the under side of diaphragm 167 rises and overcomes the counterbaiancing effect of associated spring mechanisms. Diaphragm 167 and member 174 then move upward, admitting gas from line 205 into drive passageway 193. Piston 130 is then driven upward in a dispensing stroke.

Upon completion of the dispensing stroke of piston 130 there is a pressure decay in liquor chamber 110, and a consequent pressure drop in sensing passageway which is communicated to diaphragm 153. The resulting movement of diaphragm 153 causes diaphragm 167 to move, driving valve member 174 downward and shutting off the supply of pressure gas to compartment 112.

The downward movement of valve member 174 supplies a pressure pulse through passageway 192 to transducer device 195, which then registers the dispensing of one shot of iiquor.

The downward movement of valve member 174, returning it to its position shown in FIG. 2, also provides a pathway for purging gas from compartment 1 12. The gas flows through

passageways

116, 111, 106, 193, around the valve member 180, and through passageway 191 to atmosphere. The decay of pressure in compartment 112 permits the piston to be returned to its starting position by action of its return spring.

Moreover, according to the present invention a positive mechanical purging action is provided, which acts in parallel and in addition to the purging action just described. The mechanical purging action will be described in conjunction with a more detailed description of the piston mechanism.

PISTON HOUSING AND MECHANICAL PURGE Having now established the background of the invention, in respect to which its operation is essentiaily identical to what is described in the previously referenced Jaeger application, the novel features of the invention will now be described in detail.

The piston housing is generally cylinderical in configuration, but somewhat above its center it has a transverse horizontal wall 121 formed therein. A forward or upper recess 117 is thereby formed, which is of cylindrical configuration and is adapted to partially receive the piston when the same is withdrawn downwardly from the liquor chamber. In one side wall of the forward recess 117 a passageway 116 is formed for admitting compressed gas for driving the piston forseal 123 as the piston 130 is driven forward and retracted. A central hub 125 is formed on the under side of transverse wall 121 and depends downwardly therefrom. The vertical height of the central hub 125 is about two-thirds the vertical height of the piston housing 115, and its lower end is coterminous with the lower end of the piston housing. Central hub 125 has a cylindrical center hole 126 formed therein, which extends downwardly from the central recess 122, and the piston rod 137 (although other portions of its length are designated differently) slides within the full length of the hole 126. Near the upper end of hub 125, and only a short distance below the transverse wall 121 and O-ring seal 123, a horizontal passageway 127 is formed which communicates between the external surface of the hub 125 and its center hole 126.

Beneath the transverse wall 121 the piston housing 115 has a rearward or lower recess 120 formed therein, which is also generally cylindrical and which has a threaded wall 119. The threaded wall 119 receives the end cap or adjustment handle 145, for purpose of volume adjustment.

The piston rod 137 has one portion of its length, approximately intermediate its two ends, which has a reduced diameter and is designated by numeral 138. The distance of reduced diameter portion 138 of the piston rod from the piston 130 is such that, in the extreme forward position of the piston as shown in FIG. 3, the reduced diameter portion of the piston rod extends from a point just below the passageway 127 in hub 125 to a point just above the O-ring seal 123. In this position of the piston and piston rod, as would be evident from FIG. 3, compressed gas contained within expansion chamber 112 is permitted to freely pass through the central recess 122 and through the O-ring seal 123 and a short section of the hub center hole 126 and thence horizontally through the passageway 127 into the lower recess 120 of the piston housing. Thus, a positive mechanical purge of the gas expansion compartment 112 is provided.

The purged gas passes downward from chamber 120 into the interior of adjusting handle or cap 145, where it escapes into atmosphere through a slot or opening 148 which is provided specifically for that purpose. The opening 148 is located near the lower end of the cap 145 so that it operates for its intended purpose even when the pump is adjusted for dispensing smaller quantitles of liquor, as shown in drawing FIG. 7.

A lock nut 146 is utilized to fasten the cap 145 in a selected position of adjustment relative to piston housing 115. The return spring for the piston is designated 141, and has its upper end in engagement with the lower surface of the transverse wall 121 and immediately surrounding the central hub 125. Spring 141 extends below the hub 125 and its lower end is held by a spring retainer 142 which is in turn attached to the lower extremity 140 of the piston rod by means of a lock washer 143.

Below the reduced diameter portion 138 the piston rod is designated 139, and the portion 139 has the same size or outer diameter as the forward or upper portion 137. A positive support for the piston and its piston rod is provided by the central hub 125, since the length of reduced diameter portion 138 of the piston rod is quite small compared to the length of the hub.

Although it would not be necessary, the lower extremity of the piston rod is also formed with a reduced diameter, being designated as 140. Spring retainer 142, although held onto the piston rod by lock washer 143, may slide a short distance lengthwise on the end portion 140 of the piston rod. The adjusting handle or end cap has a central recess 147 formed in its lower end, immediately beneath the piston rod. When the piston is withdrawn from the liquor chamber it is done so by the action of spring 141 which forces the spring retainer 142 downward. Spring retainer 142 bottoms on the end wall of the end cap 145. Piston 130 and the piston rod may over travel a small distance, however, in which case the lower extremity 140 of the piston rod dips momentarily into recess 147 of the end cap. This action avoids a hard rebound of the piston rod and also avoids an undesired secondary dispensing action. This feature of the pump is particularly advantageous where, as shown in FIG. 7, for example, the adjustment cap has been set inward to provide for dispensing of a relatively small quantity of liquid. Where the chamber is set for its maximum volume, as shown in FIG. 2, the rearward travel of the piston is stopped by the transverse wall 121 and there is no rebound problem of observable magnitude.

Another feature of the apparatus is shown in FIG. 6 where it will be seen that a number of longitudinally extending ribs 144 are provided within the end cap 145. Ribs 144 fit relatively closely around the spring 141 and prevent it from getting out of alignment. At the same time the flow of purged gas toward the exit port 148 is not impeded.

A significant feature of the present invention is that driving gas is purged from the gas expansion chamber 112 by two entirely separate and independent means. One means is dependent upon the piston 130 striking the forward wall 113 of the liquor chamber 110, with resulting pressure change which is sensed in the passageway 105. Movement of the diaphragms 153 and 167 then results, permitting gas to flow through a relatively long passageway including the passageway 191 to atmosphere. The other and separate purging action, however, is initiated even before the pistion strikes the end wall 113. As soon as the reduced diameter portion 138 of the piston rod couples passageway 127 to the gas expansion chamber 112, the purging action commences. The forward velocity of the piston and its associated piston rod is sufficient to carry it through the samll remaining distance that it needs to travel.

Although the volume of the metering chamber is adjustable, it will be noted that such adjustments do not affect the mechanical purging through the passageway 127. The reason is that this purging action occurs as the piston nears the forward extreme of its travel, while volume adjustment is accomplished by changing the rearward limit of piston movement.

It will also be noted that the considerable length of pistion rod rearward section 139 is sufficient to provide a rigid support for the piston 130 even at the time when no support is being achieved at the O-ring seal 123. it will further be noted that the relatively short pathway through the O-ring seal and passageway 127 assures a positive purging action for the compartment 112.

As will be understood by those skilled in the art, what has been described are preferred embodiments in which modifications and changes may be made without departing from the spirit and scope of the accompanying claims.

What is claimed in support of Letters Patent is:

1. A positive displacement pump for selectively dispensing a liquid, comprising:

a cylinder having a reciprocating piston, a piston rod extending rearwardly from said piston, and sliding bearing means supporting said piston rod;

a concentric cylinder resilient seal coupled between said piston and the cylinder wall so as to divide said cylinder into a forward chamber for containing a liquid to be dispensed, and a rearward chamber;

means for allowing the liquid to exit from said forward chamber through an outlet port during the forward travel of said piston, and to enter said forward chamber through an inlet port during the return travel of said piston;

inlet vent means for selectively admitting pressurized gas into said rearward chamber for driving said piston forward;

control means operable when said piston reaches the forward limit of its travel for closing said inlet vent means; 7

said sliding bearing means normally providing a seal with said piston rod, and said bearing means and piston rod cooperatively providing an outlet vent for exhausting driving gas from said rearward chamber when said piston closely approaches the forward limit of its travel;

and drive means operable for returning said piston to its rearward position when the driving gas has been exhausted.

2. A pump as claimed in claim 1 wherein said control means is operable in response to a drop in liquid pressure at the forward end of said forward chamber which results from exhausting said driving gas from said rearward chamber through said outlet vent and releasing the liquid from said forward chamber through said outlet part.

3. A pump as in claim 1 wherein said sliding bearing means consists of an elongated cylindrical member extending rearward from the back side of said rear chamber and having a concentric center bore matching, in close fitting relationship, the piston rod outer diameter;

said elongated member having an exhaust port extending radially from said center bore through said elongated member wall;

said seal consisting of an O-ring seal acting between said elongated cylindrical member and said piston rod, and in sliding contact with said piston rod;

said piston rod having a section of reduced diameter;

and

said outlet vent being provided through the cooperative action of said piston rod, bearing means, and seal, said rod reduced section simultaneously engaging said seal and said exhaust port as the piston closely approaches the forward limit of its travel.

4. A pump as in claim 3 wherein:

said O-ring seal and said exhaust port are located at the piston rod entrance to the rear chamber thereby providing an exhaust path of minimum restriction.

S. A positive displacement pump for selectively dispensing a liquid, comprising:

a concentric cylinder resilient seal coupled between said piston and the cylinder wall which separates said cylinder into a forward chamber for containing a liguid to be dispensed, and a rear chamber;

means for allowing said liquid to exit from said forward chamber through an outlet part during the forward travel of said piston and enter said forward chamber through an inlet part during the return travel of said piston;

a source of pressurized gas for driving said piston forward;

normally closed inlet vent means for admitting the pressurized gas from said source into said rear chamber; I

nonnally open outlet vent means for exhausting the pressurized gas from said rear chamber;

manually actuable control means for selectively opening said inlet vent means and simultaneously closing said outlet vent means to drive the piston forward;

separate control means operable when said piston reaches the forward limit of its travel for simultaneously closing said inlet vent means and opening said outlet vent means;

said sliding bearing means normally providing a seal with said piston rod, and said bearing means and piston rod cooperatively providing an outlet vent for exhausting driving gas from said rear chamber only when said piston closely approaches the forward limit of its travel;

6. A pump as in claim 5 wherein said separate control means is operable in response to a drop in liquid pressure at the forward end of said forward chamber, said drop in liquid pressure resulting from the combined effects of the exhausting of said driving gas from said rear chamber through said outlet vent and the release of the liquid from said forward chamber through said outlet part. 1

7. A pump as in claim 5 with a highly restricted bleeder hole consisting of a small diameter hole passing through the wall of said rear chamber, for slowly venting the driving gas from said rear chamber and thereby allowing said piston to return to its rearward limit.

8. A pump as in claim I with an adjustable sleeve for limiting the piston return thereby providing an adjust able control of the piston displacement without affecting the cooperative action of said piston rod, bearing means, and seal.

9. A pump as in claim 5 with an adjustable sleeve for limiting the piston return thereby providing an adjustable control of the piston displacement without affecting the cooperative action of said piston rod, bearing means, and seal.

10. A pump as in claim wherein the said drive said piston rod slidingly passes; means operable for returning said piston consists of a and a retainer member secured to the end of said pisspring driven piston return. ton rod for preventing said piston rod from being 11. A pump as in claim 5 wherein said drive means withdrawn f Said spring retainer; Operable for 'f' l Said Piston isaspring driven P 5 the action being such that when the piston is rereturn :ompnsmgZ turned, the piston'rod is free to overtravel beyond an adjustable sleeve for limiting the return travel of said piston and thereby providing displacement control, composed of a cylindrical housing having a counter-bored interior end wall; a cylindrical spring retainer disposed within the end of said housing and adapted to engage the raised the normal fully compressed position of the spring, temporarily extending into the recessed central portion of said housing end wall, thereby avoiding an undesired rebound of the piston rod from the housing end wall.

circumferential portion of said housing end wall; A P as i claim wherein f j a Spring disposed i hi i housing and having one able sleeve has radial flbS pro ecting from its inside surend received b said s ri t i 5 face thereby providing guidance and stability for the a piston rod concentric with said spring, said spring said spring and piston rod.

retainer having a central opening through which

Claims

1. A positive displacement pump for selectively dispensing a liquid, comprising: a cylinder having a reciprocating piston, a piston rod extending rearwardly from said piston, and sliding bearing means supporting said piston rod; a concentric cylinder resilient seal coupled between said piston and the cylinder wall so as to divide said cylinder into a forward chamber for containing a liquid to be dispensed, and a rearward chamber; means for allowing the liquid to exit from said forward chamber through an outlet port during the forward travel of said piston, and to enter said forward chamber through an inlet port during the return travel of said piston; inlet vent means for selectively admitting pressurized gas into said rearward chamber for driving said piston forward; control means operable when said piston reaches the forward limit of its travel for closing said inlet vent means; said sliding bearing means normally providing a seal with said piston rod, and said bearing means and piston rod cooperatively providing an outlet vent for exhausting driving gas from said rearward chamber when said piston closely approaches the forward limit of its travel; and drive means operable for returning said piston to its rearward position when the driving gas has been exhausted.

3. A pump as in claim 1 wherein said sliding bearing means consists of an elongated cylindrical member extending rearward from the back side of said rear chamber and having a concentric center bore matching, in close fitting relationship, the piston rod outer diameter; said elongated member having an exhaust port extending radially from said center bore through said elongated member wall; said seal consisting of an O-ring seal acting between said elongated cylindrical member and said piston rod, and in sliding contact with said piston rod; said piston rod having a section of reduced diameter; and said outlet vent being provided through the cooperative action of said piston rod, bearing means, and seal, said rod reduced secTion simultaneously engaging said seal and said exhaust port as the piston closely approaches the forward limit of its travel.

4. A pump as in claim 3 wherein: said O-ring seal and said exhaust port are located at the piston rod entrance to the rear chamber thereby providing an exhaust path of minimum restriction.

5. A positive displacement pump for selectively dispensing a liquid, comprising: a cylinder having a reciprocating piston, a piston rod extending rearwardly from said piston, and sliding bearing means supporting said piston rod; a concentric cylinder resilient seal coupled between said piston and the cylinder wall which separates said cylinder into a forward chamber for containing a liguid to be dispensed, and a rear chamber; means for allowing said liquid to exit from said forward chamber through an outlet part during the forward travel of said piston and enter said forward chamber through an inlet part during the return travel of said piston; a source of pressurized gas for driving said piston forward; normally closed inlet vent means for admitting the pressurized gas from said source into said rear chamber; normally open outlet vent means for exhausting the pressurized gas from said rear chamber; manually actuable control means for selectively opening said inlet vent means and simultaneously closing said outlet vent means to drive the piston forward; separate control means operable when said piston reaches the forward limit of its travel for simultaneously closing said inlet vent means and opening said outlet vent means; said sliding bearing means normally providing a seal with said piston rod, and said bearing means and piston rod cooperatively providing an outlet vent for exhausting driving gas from said rear chamber only when said piston closely approaches the forward limit of its travel; and drive means operable for returning said piston to its rearward position when the driving gas has been exhausted.

6. A pump as in claim 5 wherein said separate control means is operable in response to a drop in liquid pressure at the forward end of said forward chamber, said drop in liquid pressure resulting from the combined effects of the exhausting of said driving gas from said rear chamber through said outlet vent and the release of the liquid from said forward chamber through said outlet part.

8. A pump as in claim 1 with an adjustable sleeve for limiting the piston return thereby providing an adjustable control of the piston displacement without affecting the cooperative action of said piston rod, bearing means, and seal.

10. A pump as in claim 5 wherein the said drive means operable for returning said piston consists of a spring driven piston return.

11. A pump as in claim 5 wherein said drive means operable for returning said piston is a spring driven piston return comprising: an adjustable sleeve for limiting the return travel of said piston and thereby providing displacement control, composed of a cylindrical housing having a counter-bored interior end wall; a cylindrical spring retainer disposed within the end of said housing and adapted to engage the raised circumferential portion of said housing end wall; a spring disposed within said housing and having one end received by said spring retainer; a piston rod concentric with said spring, said spring retainer having a central opening through which said piston rod slidingly passes; and a retainer member secured to tHe end of said piston rod for preventing said piston rod from being withdrawn from said spring retainer; the action being such that when the piston is returned, the piston rod is free to overtravel beyond the normal fully compressed position of the spring, temporarily extending into the recessed central portion of said housing end wall, thereby avoiding an undesired rebound of the piston rod from the housing end wall.

12. A pump as in claim 11 wherein the said adjustable sleeve has radial ribs projecting from its inside surface thereby providing guidance and stability for the said spring and piston rod.