US3301197A

US3301197A - Pump

Info

Publication number: US3301197A
Application number: US377008A
Authority: US
Inventors: Osborne C Dodson; Loughry Horton Hampton
Original assignee: LOUGHBORNE Inc
Current assignee: LOUGHBORNE Inc
Priority date: 1964-06-22
Filing date: 1964-06-22
Publication date: 1967-01-31
Anticipated expiration: 1984-01-31

Description

31, 1967 o. c. DODSON ETAL 3,3

PUMP

3 Sheets-Shed 1 Filed June 22, 1964 INVENTORS. OSBORNE C. DODSON H HAMPTON LOUG'HRY ATTORNEY o. c. obnsom ETAL 3,301,197

Jan. 31, 1967 PUMP 3 Sheets-Sheet 2 Filed June 22, 1964 46 45 HIIH NH" W INVENTORS. OSBORNE G. DODSON H. HAMPTON LOUG'HRY ATTORNEY o. c. DODSON ETAL 3,301,197

Jan. 31, 1967 PUMP 5 Sheets-Sheet 5 Filed June 22, 1964 5AT CONSTANT RADIUS INVENTORS. OSBORNE 6'. DODSON H HAMPTON LOUG'HRY ATTORNEY United States Fatent Cfifice 3,3fil,l97 Patented Jan. 31, 1967 3,391,197 PUMP Osborne C. Dodson, Chagrin Falls, and Horton Hampton Longhry, Chesterland, Ohio, assignors to Longhborne, Inc., Chesterland, Ohio, a corporation of Ohio Filed June 22, 1964, Ser. No. 377,008 13 Claims. (Cl. 103169) The present invention relates to a pump and, more particularly, to an improved pump adapted to provide substantially constant flow at a relatively low volume.

A pump of the type indicated has many industrial and laboratory applications where minute quantities flowing at a relatively steady pressure are necessary for carrying out an experiment or the like. To illustrate, in certain experimental research, a radioactive solution is pumped into the blood stream of an animal to observe by X-ray treatment the effect of the solution on certain organs such as the kidneys. Unless the fiow of the solution is fairly steady and at a desired volume, variables are introduced which render the results unreliable. Although for a given operation, the volume and pressure should be fairly constant, these variables should be adjustable through a substantial range to meet diverse conditions. For example, in a pump of the type herein contemplated, the volume of flow may range from 0.01 cubic centimeter per minute to 40 cubic centimeters per minute at a pressure within the range of about 15 to about 20 inches of mercury.

A reciprocating pump has not previously been adapted to these rather stringent steady-state operating conditions with possible variable ranges. With a reversal pump it is extremely difiicult to obtain reversal of motion without some dwell. constant delivery desired in the present pump. As indicated, even a slight deviation can be significant for certain applications.

The present pump, even though of the reciprocating order, has been adapted to provide substantially constant fiow at a relatively low volume by employing a plurality of relatively small cylinder and piston structures and regulating the out-of-phase operation of the structures by cam-actuated means, so as virtually to eliminate pulsating flow and bridge the reversal of strokes by the pistons with a minimum effect on the steady state of flow.

It is, therefore, a principal object of the invention to provide an improved pump structure.

Another object is to provide a reciprocating pump which discharges at low volume and a substantially constant flow.

A further object is to provide a reciprocating pump whose delivery is free from characteristics inherent in crank or eccentric drives, that is, a pulsing flow and/or appreciable dwell at the time of stroke reversal.

A still further object is to provide a pump of the type described having a flow-through piston to minimize entrapment of gas bubbles and the like.

Additional objects will be apparent from the following drawings and description wherein:

FIGURE 1 is a semi-diagrammatic, side elevational view of one form of the present pump;

FIGURES 2 and 3 are front and side elevational views, respectively, partially in section, of a preferred form of the present pump;

FIGURE 4 is an offset section of FIGURE 2 taken on the line 44;

FIGURE 5 is a view taken on the plane of the line 55 of FIGURE 4;

FIGURE 6 is a section of FIGURE 2 taken on the line 66;

FIGURES 7 and 8 are front and side elevational views, respectively, of an adjustable cam that may be used in the preferred form of the present pump;

This in turn affects the relatively FIGURE 9 is a fragmentary view of an end of the plunger or piston, illustrating a modified structure; and

FIGURE 10 is a profile development of a cam which may be used in the embodiment of FIGURES 2 and 3.

Referring initially to the embodiment of FIGURE 1, this form of the invention includes a pair of cylinders 10 and 11 mounted in opposed relation. As is the case with the preferred embodiment hereinafter described, it has been found that syringes, such as the ordinary hypodermic syringe, are uniquely suited to the purposes of the present pump. Such syringes are normally of small volumetric displacement and formed from glass or plastic. In the form of FIGURE 1, each of the cylinders or syringes has a relatively small piston 12 (as to length) and a relatively long connecting rod 13 terminating in a flange 14. The opposed syringes have a common entry and discharge, and for this purpose each syringe has its nozzle connected to a separate conduit 15 and 15a, the two conduits being a branched section, arranged in parallel, of a larger main conduit 16. Each of the conduits 15 and 15a has conventional check valves 17, so that the flow is always in the direction of the arrows 13. The pistons 12 are positioned in an out-of-phase relation so that one discharges its cylinder while the other piston fills its associated cylinder and vice versa, thereby to achieve such substantially constant flow.

The cylinders or syringes 1t) and 11 are suitably mounted in stationary chucks 19 having axially extending threaded studs 29. Elastomeric cylinders 21 disposed between the chucks 19 and cylinders 10 or 11 cushion the latter against jarring, slight misalignment, and the like.

Bearings 22 havin axially extending wear sleeves 23 support a drive bar 24 for reciprocation in the direction of its length. This direction generally parallels the opposed alignment of the syringes 10 and 11. Two yokes 25 fixed to the bar 24 have openings to receive the studs 29 and are fixed to the latter by knurled nuts 26. A cam mounted for rotation about an axis 28

contacts cam followers

29 and 30 of which at least one may be adjusted axially of the drive bar 24 as illustrated. The cam 27 has a specially designed periphery to minimize the time required for reversal of the reciprocation of the drive bar 24. More particularly, the profile of cam 27 is modified so as to produce a slight rise in delivery over the last two or three'degrees of the delivery stroke of the bar 24 in one direction in order to compensate for a minute dwell at the reversal of the stroke of the bar 24. Additionally, there is a compensating undercut on the diametrical opposite side of the cam 27 which produces the same desirable result when the bar 24 is at the other end of its linear stroke and affecting once more a reversal in direction.

In operation, power means (not shown) such as a conventional synchronous electric motor uniformly rotates cam 27 about axis 28.

Cam followers

29 and 30 alternatively take the brunt of the driving force so as to reciprocate the drive bar 24 in a linear direction. As illustrated, one of the cam followers may, if desired, be spring-loaded for short travel. Because of the design of cam 27, there is a minimum nonsignificant dwell of the bar 24 as it reverses its linear direction. Meanwhile, there is a continuous intake and discharge as indicated by arrows 18 through the main conduit 16 due to the out-of-phase, cooperating action of the pistons 12 wherein one cylinder or syringe is discharging while the other is filling.

The embodiment of FIGURES 2 through 10 differs from the embodiment just described in several respects, most notably in the preferred use of a flow-through piston, dual cams, and certain structural features designed to facilitate readying the piston or plunger for its return stroke, as hereinafter more fully described, and to accom- -rnodate automatically slight variances in alignment or positioning of the various parts.

As illustrated especially by FIGURES 2 and 3, cylinders or

syringes

31 and 32, here mounted in side-by-side relation, are shorter in length than the corresponding syringes of FIGURE 1 but have much longer pistons in the form of

plungers

33 and 34. Conveniently, the lower end of each syringe (as viewed in FIGURES 2 and 3) is both sealed and supported by a clamp. The structure for each side of the dual cylinder and piston construction is the same, and therefore only one side will be described in detail. An elastomeric bushing 35 and washer 36 butt against a flange 37 on the syringe or cylinder and are held in place by a collet chuck 38 and knurled pressure nut 39 which threadably engage one another. The sides of the collet chuck are slotted (FIGURE 6) to receive the forked arms 40 of a clamp 41. Set screws 4?; fix the arms to the collet chuck 38, and the clamp itself is fixed to a support post 43 by a set screw 44. An additional set screw 45 threadably engages a suitable opening crossing a slotted end of the clamp, further to tighten the clamp about the post 43. A cross piece 45 joins the two posts 43 and may, if desired, have a still further mounting post 46 for attaching still other apparatus.

The two

cylinders

31 and 32 preferably have a common entry and discharge port or conduit as in the embodiment of FIGURE 1. Also, these ports or conduits may be on the same side of the reciprocating pistons or plungers as in FIGURE 1. Preferably, however, to reduce or eliminate formation of entrapped air and/ or gas bubbles and, further, to facilitate purging of the cylinders, a flowthrough plunger is employed. Such a plunger has a communicating passage extending from the upper face of the plunger longitudinally thereof to a lateral outlet as shown at 33a in FIGURE 2. In the form illustrated, each plunger has a tubular elastomeric tip 48 at the face of the plunger to minimize leakage therearound. An inlet checkvalve 49 of conventional design permits fluid only to enter the plunger 33 from a conduit or line 50. This check-valve may be located at the lateral outlet as illustrated in FIGURE 2; or a like checkvalve 49a may be located at the face of the plunger itself as illustrated in FIGURE 9. In either case, the valve is of standard construction and is not therefore described in detail, except briefly to note that a spring loaded float 51 (FIGURE 9') may be used. The spring normally urges the float against its seat, but the float rises to permit flow through an apertured plug 52 under the urging of pressurized fluid. FIG- URE 9 illustrates a still further modification that may be employed in the present embodiment in that the face of a piston 3311 may be cupped or concaved as at 53. This construction permits a sweeping action of the flow-through plunger as a further guard against entrapment of air bubbles or the like.

A similar check-valve 54 is located at the discharge end of each cylinder to permit outward flow only through a line 55. As in the embodiment of FIGURE 1, the plungers of the cooperating

cylinders

31 and 32 are in an outof-phase relation with respect to each other, so that one discharges its cylinder while the other is filling its associated cylinder. To this end, the lines 50 preferably have a common entry port or supply line, and the lines 55 are similarly related with respect to a discharge port or exit line to realize the substantially constant volume flow.

Suitable drive means is provided both uniformly to perate each plunger and to maintain the desired out-ofphase relation between the plungers during their reciprocation. Such means may take the form of a housing generally indicated at 56 which anchors the posts 43 and includes a motor 57. Preferably, a synchronous motor is used as an aid to uniform operation. The motor is conventionally wired and has a switch 58, fuse 59, pilot light 60, and a capacitor 61. The output shaft 62 of the motor carries a gear 63 meshing with a gear 64. A shaft 65 supports gear 64 and is journaled for rotation in bearings 66 which are carried in wall sections 67 of the housing. A pinion 68 on shaft 65 drives a gear 69 fixed to a crossshaft 76), also journaled in bearings 71 carried by wall sections of the housing 56. A cardioid cam 72 is secured to each end of the cross-shaft '70. These cams are offset with respect to each other circumferentially of the shaft 78 so as to provide mutually the desired out-of-phase operation of the

plungers

33 and 34 and thereby achieve the substantially constant volume flow.

Push

rods

73 and 74 each ride a follower 75 journaled for rotation within a slotted end of each rod against the periphery of a cam disposed directly beneath the push rod. The double-follower construction of FIGURE 1 can be used, if desired, for this embodiment as well. The structure for each push rod and attendant parts is the same. A flexible wire 76 joins the upper end of each push rod to a plunger 33 of 34. An enlarged end of the wire 76 is held against a push rodby a threaded cap 77, and the other end of the wire is pinned within a suitable axial opening in the lower end of a plunger. The wire 76 may be made of a heat-treated, spirally wound steel. It is sufliciently stiff to transmit the working stroke of the push rod with no bending. An arm 78 fixed by a set screw to a push rod slides along a post 43 to guide the rod during its movement and prevent it from twisting. A sleeve '79 mounted on the housing 56 is concentric with a push rod and contains a bearing 80 for the vertical reciprocal movement of the rod. A ring 81 fixed to the push rod may be adjustably positioned thereon to limit the descent of the rod by striking the upper end of a sleeve 79. Within each sleeve, the rod has a flange 82 pinned thereto and a coil spring 83 extending from the flange to the upper end of the sleeve 79.

Reverting to the cams 72, in the case of the double cam design, the cam profile is modified to provide for a slight and preferably adjustable overlap of the strokes of the dual plungers in addition to the described out-ofphase relation. This additionally allows the cam profiles to be so formed that the followers for the upcoming delivery stroke meet a rising curve the moment of the cessation of the previous stroke (or momentarily in advance thereof) instead of being at a point of zero rise at that particular moment. In general, each cam has the same peripheral construction. One cam may have a rise or pumping action for 181 of revolution. At the 180 mark, the other cam begins its similar rise or pumping action for 181. The one degree differential or overlap compensates for slack and otherwise insures a more uniform operation of the pump. Following the indicated rise of 181, the fall of the cam should be fairly constant for the balance of 360 of revolution. FIGURE 10 illustrates one cam profile that may be used. Reading in counterclockwise direction, there is a rise for 181 followed by a constant radius for three degrees. After this dwell, there is a constant fall for about 161 and a con stant radius dwell for about 15. Preferably, a small hollow indicated at 84 is provided to insure meeting tolerances and obtaining a proper pick-up point.

To provide a fine regulation on the operation of the cams, they may be adjustably mounted as shown in FIG- URES 7 and 8. The shaft 70 has a back-plate 8S behind a cam 72a provided with a vertical slot 86, while the cam 72a has a slanting slot 87 partially overlying the slot 86. The plate 85 is preferably fixed with respect to the shaft '70. A nut 88 and bolt 89 combination and a cap screw normally hold the cam, plate, and shaft tightly together. However, upon loosening the nut 88 and screw 90, the cam 72a can be slightly rotated about the shaft 70 and relatively to the plate 85, in either direction, to make desired adjustments. During this adjustment, the bolt 89 slides along both slots 86 and 87, after which the nut and

bolt assembly

88 and 89 and screw 90 may again be tightened.

In operation, the motor 57 drives the shaft 70 through the indicated gearing to rotate the cams 72, circumferentially offset with respect to each other, and thereby impart out-of-phase, alternate up and down strokes of the

plungers

33 and 34. The plungers are always moving in opposite directions and are never stationary except for a very slight momentary dwell at reversal of direction due to mechanical clearances or material elasticity which can be compensated for by the adjustable features described or modification of the cam profile. As previously indicated, one plunger begins its upward travel just slightly before, for example, one degree of cam movement, the other plunger reaches its maximum height of travel as viewed in the figures.

On the upstroke of each plunger, a check-valve 49 closes and check-valve 54 opens. The fluid above the plunger or piston is thus forced out of the cylinder and into one of the lines 55 which, as previously indicated, may be connected in parallel. On the downstroke of each plunger, check-valve 54 closes and check-valve 4-9 opens. Additional fluid, due to hydrostatic head or otherwise, is thus free to enter a cylinder through one of the lines 50. Also, during the downstroke, the entering fluid passes through the passage 47 of a plunger, flushing all before it so that the fluid flow is always uni-directional through the syringe or cylinder. This has the advantage of sweeping out any air bubbles or the like trapped within the cylinder. The rubber tip 48 is especially useful in assisting this function.

It is important to obtain a straight-line motion while accommodating for slight deviation or misalignment of the various parts without breaking the air seal or, indeed, breaking of the glass cylinder itself. The stiff wire 76 materially contributes to achieving these ends, since it can adjust itself to some lateral displacement and is still sufficiently stiff to transmit upward movement of the

push rods

73 and 74.

In order further to smooth out reversals of linear motion of the plungers, aids are included to bridge the turna-round for each stroke. Ring 81 limits descent of a pushrod and thereby its overlying plunger and prevents a follower 75, for example, from seating within the hollow 84 of a cam periphery. Also, by taking the brunt of the downward movement on the sleeve 79, there is a minimum of this force felt on the smooth operation of the cams 72. Conversely, coil spring 83 opposes the upstroke of each push rod, so that as soon as the high point of a cam is reached, there is no delay but an immediate retreat or descent of the plunger under the urging of the compressed spring 83.

Although the foregoing describes and illustrates several embodiments of the present invention, it is understood that the invention may be practiced in still other forms within the scope of the following claims.

What is claimed is:

1. A syringe pump adapted to deliver a substantially constant flow at relatively low volume comprising substantially opposed, piston-operated cylinders of the sy ringe type, suction and discharge conduits connected to said cylinders and having common entry and discharge ports, the piston of each cylinder having a connecting piston rod and being positioned within its cylinder and with respect to the other piston within the other cylinder to provide inter se opposite driving and suction strokes and thereby realize said substantially constant flow, and single means both to operate said pistons in unison and to lock the respective positions of the pistons inter se within their associated cylinders, said single means including a drive bar mounted for reciprocable movement in the di rection of its length, yoke means connecting the drive bar to each of the piston rods, and cam-actuated means to reciprocate the drive bar, the profile of the came being constructed to minimize pulsating flow and dwell at periods of reversal of said reciprocable drive bar.

2. A syringe pump adapted to deliver a substantially constant flow at relatively low volume comprising substantially opposed, piston-operated cylinders of the hypodermic syringe type, suction and discharge conduits connected to said cylinders and having common entry and discharge ports, the piston of each cylinder having a connecting rod and being positioned within its cylinder and with respect to the piston within the other cylinder to provide inter se opposite driving and suction strokes and thereby realize said substantially constant flow, and single means both to operate said pistons in unison and to lock the respective positions of the pistons inter se within their associated cylinder, said single means including a drive bar mounted for linear reciprocation in the direction of its length, yoke means connecting the drive bar to each of the piston rods, a cam follower on the drive bar, a cardioid cam to engage the follower, and power means to rotate the came and thereby reciprocate the drive bar, the profile of the cam having a slight rise as the drive bar nears one end of its linear stroke to compensate for the reversal in direction of the bar, and a compensating undercut on the diametrical opposite side of the cam to provide a commensurate result when the drive bar nears the other end of its stroke.

3. A pump adapted to deliver substantially constant flow comprising cylinders mounted in substantially sideby-side relation, a reciprocable piston for each cylinder having a communicating passage extending from the face of the piston longitudinally thereof to an outlet, an inlet check-valve for said communicating passage, an outlet check-valve for each cylinder adapted to be connected to a common discharge, the pistons of each cylinder being positioned in an out-of-phase relation so that one discharges its cylinder while the other piston fills its cylinder and vice versa, thereby to deliver said substantially constant flow, a push rod mounted for reciprocable movement in the direction of its length engaging each piston, a cam contacting each push rod, and power means adapted to rotate the cams and reciprocate the push rods and pistons engaged thereby.

4. The pump of claim 3 wherein said inlet check-valve is located on the face of said piston.

5. The pump of claim 3 wherein said communicating passage terminates laterally of at least one of the pistons, and the inlet check-valve for that piston is located at said lateral termination.

6. A syringe pump adapted 'to deliver a substantially constant flow at relatively low volume including a pair of cylinders of the syringe type, suction and discharge conduits connected to said cylinders and having common entry and discharge ports, a plunger partially telescoping each cylinder and having a communicating passage extending from the telescoping face of the plunger longitudinally thereof to a lateral outlet on the plunger removed from the cylinder, an inlet check-valve for each cylinder in the suction conduit, an outlet check-valve for each cylinder in the discharge conduit, the plunger of each cylinder being positioned in its cylinder and with respect to the other plunger in the companion cylinder to provide inter se opposite driving and suction strokes and thereby realize said substantially constant flow, a push rod mounted for reciprocable movement in the direction of its length engaging each plunger, flexible wire means connecting each push rod to its plunger, a cam contacting each push rod, and power means adapted to rotate the cams simultaneously and reciprocate the push rods and pistons engaged thereby, the profile of the cams being constructed to minimize pulsating flow and dwell at periods of reversal of said push rods.

7. A syringe pump adapted to deliver a substantially constant flow at relatively low volume including a pair of piston-operated cylinders of the syringe type mounted substantially vertically in side-by-side relation, suction and discharge conduits connected to said cylinders and having common entry and discharge ports, the piston of each cylinder having a communicating passage extending from the face of the piston longitudinally thereof to a lateral outlet, an inlet check-valve for each cylinder in the suction conduit, an outlet check-valve for each cylinder in the discharge conduit, the piston of each cylinder being positioned in its cylinder and with respect to the other piston in the companion cylinder to provide out-of-phase opposite driving and suction strokes and thereby realize said substantially constant flow, and drive means both to operate said piston and maintain said out-of-phase relation, said drive means including a push rod vertically mounted for reciprocation in the direction of its length engageable With each piston, a cam follower at the free end of each push rod, a substantially horizontally disposed crossshaft extending beneath each push rod, a cardioid cam carried by the cross-shaft and engaging each cam follower, said cams being circumferentially displaced about said cross-shaft to maintain the defined out-of-phase relations of the pistons, and power means to rotate said cross-shaft, the profile of each of said cams having a slight rise as its associated push rod nears one end of its vertical stroke to compensate for the reversal in direction of said stroke, and a compensating undercut on the diametrical opposite side of the cam to provide a commensurate result when the push rod nears the reverse end of such stroke.

8. The pump of claim 7 wherein at least one of said piston faces has an elastomeric section contacting the sides of its cylinder to minimize leakage therearound.

9. The pump of claim 7 further including stop means to limit the movement of at least one of the push rods toward its cam to facilitate readying the piston for its return stroke.

10. The pump of claim 7 further including spring means resiliently opposing the movement of at least one of the push rods away from its cam to facilitate readying the piston for its return stroke.

11. A pump adapted to deliver a substantially constant flow at relatively low volume comprising substantially opposed cylinders each having a reciprocating piston therein,

suction and discharge conduits connected to said cylinders and having common entry and discharge ports, the piston of each cylinder being so positioned Within its cylinder and with respect to another piston within a companion cylinder as to provide inter se a desired driving and suction relationship by the strokes of said pistons, and single means both to operate said pistons simultaneously and to lock the respective positions of the pistons inter se Within their associated cylinders, said single means including a drive bar mounted for reciprocal movement in the direction of its length, yoke means connecting the drive bar to each of the pistons, and cam-actuated means to reciprocate the drive bar, the profile of the cam being constructed to minimize pulsating flow and dwell for periods of reversal of said reciprocal drive bar.

12. A pump adapted to deliver substantially constant flow comprising cooperating cylinders, a reciprocal piston for each cylinder having a communicating passage extending longitudinally through said piston, an inletcheckvalve for each of said communicating passages, an outlet check-valve for each cylinder adapted to be connected to a discharge, the pistons of each cylinder being positioned in a desired inter se phase relationship so as to discharge and fill said cylinders substantially at different times, a push rod mounted for reciprocal movement in the direction of its length engaging each piston, a cam contacting each push rod, and power means adapted to rotate the cams and reciprocate the push rods and pistons engaged thereby.

13. A pump adapted to deliver substantially constant flow comprising cooperating cylinders, a reciprocal piston for each cylinder having a communicating passage extending longitudinally through said piston, an inlet check-valve for each of said communicating passages, an outlet checkvalve for each cylinder adapted to be connected to a discharge, the pistons of each cylinder being positioned in a desired inter se phase relationship so as to discharge and fill said cylinders substantially at difierent times, and means adapted to reciprocate the pistons in the cylinders while preserving said desired phase relationship.

References Cited by the Examiner UNITED STATES PATENTS 195,375 9/1877 Judkins 103-183 661,259 11/1900 Elam 103-17l 2,654,326 10/1953 Sheen et a1 103-170 DONLEY J. STOCKING, Primary Examiner.

WARREN E. COLEMAN, MARTIN P, SCHWADRON,

Examiners.

H. F. RADUAZO, Assistant Examiner.

Claims

1. A SYRINGE PUMP ADAPTED TO DELIVER A SUBSTANTIALLY CONSTANT FLOW AT RELATIVELY LOW VOLUME COMPRISING SUBSTANTIALLY OPPOSED, PISTON-OPERATED CYLINDERS OF THE SYRINGE TYPE, SUCTION AND DISCHARGE CONDUITS CONNECTED TO SAID CYLINDERS AND HAVING COMMON ENTRY AND DISCHARGE PORTS, THE PISTON OF EACH CYLINDER HAVING A CONNECTING PISTON ROD AND BEING POSITIONED WITHIN ITS CYLINDER AND WITH RESPECT TO THE OTHER PISTON WITHIN THE OTHER CYLINDER TO PROVIDE INTER SE OPPOSITE DRIVING AND SUCTION STROKES AND THEREBY REALIZE SAID SUBSTANTIALLY CONSTANT FLOW, AND SINGLE MEANS BOTH TO OPERATE SAID PISTONS IN UNISON AND TO LOCK THE RESPECTIVE POSITIONS OF THE PISTONS INTER SE WITHIN THEIR ASSOCIATED CYLINDERS, SAID SINGLE MEANS INCLUDING A DRIVE BAR MOUNTED FOR RECIPROCABLE MOVEMENT IN THE DIRECTION OF ITS LENGTH, YOKE MEANS CONNECTING THE DRIVE BAR TO EACH OF THE PISTON RODS, AND CAM-ACTUATED MEANS TO RECIPROCATE THE DRIVE BAR, THE PROFILE OF THE CAME BEING CONSTRUCTED TO MINIMIZE PULSATING FLOW AND DWELL AT PERIODS OF REVERSAL OF SAID RECIPROCABLE DRIVE BAR.