CA1155711A

CA1155711A - Reciprocating pump and reversing mechanism therefor

Info

Publication number: CA1155711A
Application number: CA000359599A
Authority: CA
Inventors: William S. Credle, Jr.
Original assignee: Coca Cola Co
Current assignee: Coca Cola Co
Priority date: 1979-09-21
Filing date: 1980-09-05
Publication date: 1983-10-25
Also published as: ZA805694B; NZ194940A; KR850000419B1; IT1193974B; MX151144A; PH18178A; JPS595785U; GR70209B; IT8024802A0; BR8006025A; AR224781A1; GB2060086A; ES8105833A1; FR2465905A1; GB2060086B; AU6250080A; NL8005213A; FR2465905B1; NL182742B; BE885317A

Abstract

ABSTRACT OF THE DISCLOSURE
A reciprocating, pneumatically operated pump apparatus is described for use in a post-mix syrup dispensing operation which comprises a fail-safe pump reversing system which includes a valve actuating member for reversing the valve mechanism which controls the reciprocating pump action. An overcenter, snap-acting mechanism is provided to preclude the valve actuating member from sticking in a central position. The snap-acting mechanism is configured to substantially preclude bearing load in a direction transversely of the direction of movement of the valve actuating member. The pump housing is molded plastic with all fluid passages and pump connections formed therein, to minimize the number of external connections. The output valves of the pump are positioned at the highest point in the discharge sections of the piston chamber to minimize the accumulation of air bubbles.

Description

r j 1155711 This invention relates to a pneumatically operated diaphragm pun P
utilized in a Post-mix beverage syrup dispensing system and more specifically to a reciprocating pump including a spring actuated reversing means for reversing the direction of a reci-procating pump at the end of its respective strokes.
Diaphragm pumps are widely used particularly for pumping liquid solutions and highly viscous materials and are frequently 10 used under conditions such that the viscosity of the fluid being pumped, the head of the suction side of the pump and the back pressure on the pump discharge may all vary as conditions under ¦which the pump is operating vary. The speed of such pumps has ¦generally been controlled by inserting an adjustable valve in the 15 ¦air line leading to the pump. However, this approach requires ¦that the operation of the pump be kept under continuous observa-¦tion and the valve adjusted to suit varying conditions, otherwise ¦the speed of the pump will vary substantially depending upon the ¦conditions of operation. For example, if the back pressure on 20 ¦the pump should increase or decrease for any particular reason,or ¦if the viscosity of the liquid being pumped should vary, then the ¦speed of operation and the quantity of liquid being pumped per ¦ unit of time will accordingly be affected. Therefore, it is ¦highly desirable that the pump be controlled such that it operates 251 at a substantially constant speed under varying conditions.
Furthermore, it is essential that the entire pumpin~ cycle be ¦completed so as to ensure continuous delivery of the medium being pumped at a constant consistency or concentration. In order to l ensure the latter, means have been suggested such as disclosed in 301 U.S. Patent 4,008,9&4 wherein opposed coil springs are provided l'l 1155711 1 ~for assisting the respective valve member in the completion of its pumping cycle. The coil compression springs of identical force under the pressurized gas system assist in completion of the pumping cycle first in one direction, and then by asserting a positive reversing effect when either of the springs becomes fully compressed. Although providing a reversing mechanism for the double acting pump disclosed there are inherent disadvantages with such a system. For example, if for some reason the pressur-ized system is effected in such a way that a back pressure is 10 created or established so as to inhibit or reverse the pumping cycle before it is completed, there is no means for overcoming the undesirable effect, and the fully compressed state of the spring is not reached. Thus, it is possible that the pumping cycle could be reversed regardless of the presence of the com-15 pression springs, before the cycle is completed, thus effectingthe efficiency if not the complete purpose of the reciprocating pump.
It is, therefore, an object of the present invention to pro-vide a; reciprocating diaphragm pump for delivering, under constant 20 pressure, syrup to a Post-mix beverage dispensing system which will overcome the above noted disadvantages.
It is a further object of the present invention to provide a double acting reciprocating pump for syrup in a Post-mix beverage dispensing system wherein a reversing means is provided 25 for reversing the direction of the pump at the end of each re-spective strokes.
Yet, still a further object of the present invention is to provide a gas operated diaphragm pump including a specialized valve, actuated bv a spring-loaded member attache~ to a common 30 shaft, which alternates the supply of pressurized gas to the q ~--1 respective diaphragms.
It is still a further object of the present invention to provide a double acting reciprocating pneumatic pump for dis-pensing syrup to a dispensing outlet wherein the pump cycle re-versing system includes a snap-acting reversing means which en-sures the completion of the pumping cycle.
Yet~ still another object of the present invention is to provide a pneumatic double acting reciprocating pump having a reversing system which includes a valve, a valve actu-10 acting member, and a snap-acting spring member which reliably directs the supply of pressurized gas to the surface of either one of the two diaphrag~s in a cyclic manner.
A further object of the present invention is to provide a reciprocating pneumatic diaphragm pump including a reversing 15 means which allows for the dispersing of fluid from either one of two diaphragm chambers at the respective ends of the pump in a systematic, controlled manner.
Other objects and further scope of applicability of the present invention will become apparent from the detailed descrip-20 tion given hereinafter. It should be understood, however, thatthe detailed description and accompanying drawings, while indicat-ing preferred embodiments of the present invention, are given by way of illustration only since various changes and modifications within the spirit and scope of the invention will become apparent 25 to those skilled in the art. Any such changes and modifications should be considered to be within the scope of this invention.

GENERAL DESCRIPTION OF THE INVENTION

The foregoing objects and others are accomplished in 1 11557~1 accordance with the present invention generally speaking by pro-viding a pumping device comprising a pair of flexible diaphragms mounted on the respective ends of a common shaft. The outer sur-face of the diaphragms are in contac~ with the liquid to be dis-pensed by the system, more particularly syrup for a Post-mix beverage dispensing system. The chamber within the pump housing contains an inner wall in which passages are provided for direct-ing compressed air, introduced into the reciprocating pump, to the surfaces of the diaphragms. The flow of air is controlled by a reversing valve adapted so as to redirect the flow of compressed air to the respective diaphragm at the com-pletion of each stroke of the pump in a cyclic manner. A valve actuating member or yoke is provided which engages the shaft within the inner chamber of the pump housing and travels with the pumping action of the shaft. The yoke is designed so as to engage the reversing valve during the terminal phase of the pump-ing stroke thus activating the valve and reversing the piston action of the pump. To complete the pump reversing system, a snap-acting spring actuating means interconnected with the yoke of the shaft, is centered within the inner chamber of the housing of the pump, pivotably mounted beneath the shaft connecting the diaphragms. The valve is provided with ~-rings posi-tioned within the valve body with respect to the air passages of the valve such that during the first half of the reciprocating cycle pressurized gas is introduced through the respective passage ways and directed to the air chamber of one of the diaphragms.
At the same time a passage is provided for exhaust gases to be released from the air chamber of the remaining diaphragm. Upon interaction with the shaft yoke and the spring mounted actuating means the relationship of the valve openings to the pressurized Il 1155711 1 gas acting on the surface of the respective diaphragm is changed at the completion of the pumping stroke so as to reverse the action of the pump~ The snap-action mechanism provided precludes the sticking of the pneumatic reversing system in an intermediate position.
In operation, pressurized gas is introduced through a passage way into a valve member and is directed via a passageway within the inner wall of the pump housing to the air chamber of one of the diaphragms within the pump. As the piston action of the dia-10 phragm forces syrup from the diaphragm chamber out the appropriatepassage to the dispensing outlet, movement of the shaft also moves the remaining diaphragm in a non-pressurizing direction. This same shaft movement also engages the shaft yoke. As the shaft yoke moves it initiates the pivotal action of a pair of 15 snap-acting compression springs which, prior to rotating off center, are pushing against each other. As the springs rotate off-center they uncoil and push the shaft and yoke along in the direction of the established movement. ~he action of the spring mechanism ensures that the movement of the diaphragm, initiated 20 by the air pressure, is taken to completion by the snap action of the compression springs while at the same time reversing the flow of press~rized air within the valve member. This procedure is then repeated as long as the dispensing outlet is open and the syrup is being dispensed as a pressurized stream. When the dis-25 pensing outlet is closed sufficient back pressure is exerted onthe diaphragms to prevent shaft movement.
It has been determined in the course of the present invention that a reciprocating diaphragm pump for syrup in a Post-mix beverage dispensing system can be provided such that the liquid 30 can be delivered under controlled pressure conditions in a I
1 ,~reliable manner. A reversing valve is provided which includes j~a pair of compression springs bearing one on the other so as not to apply pressure of the bearing surfaces on the pump shaft.
~,As the pistons of the diaphragm pump are driven by the pressurized !Igas there is always the possibility that a back pressure or some lother malfunction might occur which could result in a premature ¦reversing of the pump cycle before it is completed thus interrupt-¦
~ing delivery of the necessary syrup at the dispensing outlet.
IUpon the introduction of the sprin~ actuated means of the instant iinvention, this premature cycle reversal is eliminated by the snap ¦action resulting from the specific positioning and interrelation-¦¦ship of the compression springs within the valve reversing system.¦
Thus the present invention provides a self-contained reciprocating pump and reversing mechanism therefor comprising in combination a housing having laterally spaced chambers with diaphragm members therein dividing each of said chambers into a driving section and a discharge section, said diaphragm members being inter-connected by a common shaft, a pin extending through said common shaft at approximately the mid-point of the longitudinal dimen-sion thereof, manifold means in said housing for interconnecting said driving sections and discharge sections of said chambers with fluid inlet and outlet ports, respectively, inlet and outlet valves in said ports for controlling the flow of liquid to be pumped to and from said discharge sections with respect to said manifold means, said discharge sections having end walls parallel to major surfaces of said diaphragm members and top walls perpendicular thereto, said inlet and outlet ports being disposed in said end walls at the highest possi~le positions therein adjacent said top walls, and control valve means for directing drivin~ fluid alternately to a selected one of said driving sections defined by said diaphragms in said lateral cham~ers, a valve actuating member mounted on said shaft for sliding movement thereon between first and second positions partially in response to engagement by said pin, said valve actuating member constraining said control valve means to alternately direct said driving fluid to the respective driving sections in said first and second positions of said actuating member, and snap-acting means for accelerating and precluding the stopping of said valve actuating member between said first and second positions, said Snap-acting means including a pair of opposed compression springs attached to the bottom of said actuator and disposed on opposite sides of an axis which is parallel to a longitudinal axis of said shaft, said compression springs exerting equal and opposite forces on said valve actuating member in directions transverse to said axis throughout the positions of movement on said shaft.
BRIEF DESCRIPTION OF THE DRAWINGS
_ ____ __ _ __ _ _ _ __ ___ __ __, The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only and thus are not limitative of the present invention.
Figure 1 is a cross-sectional view of the pump of the present invention representing the initial position of a pressure stro~e in the direction indicated;
Figure lA is a top view of the pump of Figure 1, illustrating the details of the fluid input and output manifolds and the inlet and outlet valves of the pump of the present invention;

Figures 2A and 2B are partial side and bottom views respective ly, of the pump of Figure 1 illustrating the spring reversing system of the present invention at the snap-over center toward the right;
j Figures 3A an~ 3B are partial side and bottom views respcctive-ly, of the pump of Figure 1, illustrating the spring reversing mechanism of the present invention immediately after the snap-over a ., !

1 I,position of Figures 2A, 2B, which causes the pump shaft to reverse !
¦Idirections and move to the left;
j, Figure 4 is a cross-sectional view of the reversing valve of .
Ilthe present invention in the position that it occupies when the 5 ¦~pump shaft of Fig~re 1 is driven to the right;
! Figure 5 is a cross-sectional view of the reversing Yalve of ¦~lthe present invention in the position that it occupies when the ¦Ipump shaft of Figure 1 is driven to the left;
lll Fi~ure 6 is an exploded view illustrating the details of how 10 ¦Ithe yoke of the present invention is mounted on the pump shaft;
iand Figure 7 is a partial view illustrating another embodiment of the pump diaphragm of the present invention, on page with ¦~ Fi~ure 3B.
DETAILED DESCRIPTION OF THE DRAWINGS
i' l ,l Referring now to Figures 1 and lA, there is seen a cross-15 !i sectional side and top view, respectively, of the reciprocating I pump of the present invention generally designated 10, comprising ¦ a housing 11 having an input manifold 12A and an output manifold 12B in its top wall for carrying the syrup to be pumped from the I inlet SI through the respective chambers discussed below tc the I pump outlet SO. Within an inner chamber 13 of the pump is pos-¦ itioned a shaft 14 interconnecting diaphra~ms 16A and 16B. An I actuating member or yo~e 17 with protrusions or arms 17A is ¦¦ slidably supported on the shaft 14 by the longitudinal bore 17B, ¦~ Figure 6, passing therethrough. A reversing valve 40 is attached 2S ll to the inner wall 21 of housing 11 within the inner chamber 13 !1 of the pump. The shaft 14 is press-fit with a pin 25, which Il l!

-. .

1 l~upon operation of the pump, travels with the movement of the shaft ! predetermined ~istance before engaging an end of slot 26provided in the yoke 17. Shaft 14 is mounted for sli~ing move-Iment in 0-ring seals OR at its respective ends. Pivotally mountedl Ibeneath the yoke and interconnected therewith is a spring actuating Imember 27, (Figures 2A, 2B, 3A, 3B) within the housing chamber ¦13. The reversing effect of the valve 40 is facilitated as a i~esult of the interrelationship between the actuating yoke member ¦
117 and the sprlng actuating means 27 and alternately directs pres-Isurized gas introduced through passageway 22 to the respective air chambers 15~ and 15B, through passageways 23 and 24, to al~ply pressure to the respective dia~hragms 16A and 16B. The reversing ¦valve 40 comprises a valve bod~- 41 and s~ool element 42 with I!O-rings 43. A more complete discussion of the o~eration of the l5 ¦ reversing valve can be found below with respect to Figurcs ¦! 2A, 2B, 3A, 3B, 4, and 5. Each diaphragm of the pump is constructfd ¦¦ of a flexible material, such as rubber, secured to the inner ¦ walls of the pump housing at positions 20.
l In a preferred embodiment of thc present invention, the 20 I diaphragms further include a metal piston on the outer face of the respective diaphragm and a metal retaining ca~ on the inner ¦ surface of the respective diaphragm, as illu~trated in Figure 7 to be ~iscussed hereinafter.
The pumping cycle of the pump of the present invention an~ j the flow of fluid therethrough can ~e best illustrated by referenc~
to Figure lA. Fluid to be pumped is introduced through an inlet SI to input manifol.l 12A which extends across thc to~ of the pum and communicates with fluid chambers 28 ~nd 29 via normally c~ose~l l 9 " 1155711 1 ¦I,check valves 31L, 31R. When the fluid pressure in input manifold ¦
12A exceeds the pressure in either chamber 28 or 29, check valves 31L, 31R open. Since the pump of the present invention is a reciprocating pump, the fluid pressures in chambers 28, 29 are always in the opposite state. That is, if the pump shaft in Figure lA is moving to the right chamber 28 has a higher fluid pressure than manifold 12A, and chamber 29 has a lower fluid pres-~sure than manifold 12A. Under these conditions check valve 31L
¦opens introducing fluid into chamber 29 and check valve 31R is ¦closed. Thus, as the pump cycles, check valves 31L, 31R alternatel jopen and close.
j Outlet check valves 32L, 32R, disposed in an output manifold ! 12B, function in substantially the same manner. That is, when ¦¦the pressure output manifold 12B is less than the pressure in one 15 Ilof the respective chambers 28, 29 the check valve in that chamber ¦I opens, discharging fluid therefrom to pump outlet SO. In the above example, with the pump shaft 14 moving to the right, the ¦pressure in chamber 28 is high, thus opening valve 32R and per-II miting the fluid therein to discharge via manifold 12B and pump 20 1¦ outlet SO.
I' The check valves 31L, 31R, 321" 32R are substantially iden-II tical except for the respective orientations thereof. Each is l formed from rubber and includes a central stem fixedly mounted Il in the pump wall, and a disc-shaped seat B, which normally seats 25 ~I on fluid ports C. When biased by fluid pressure to open, li disc-shaped seat B flexes away from ports C, permiting fluid to ¦¦ pass therethrough.
The above-descrihed check valves are disposed at the highest ! - 1 0 -1~557~
'!
1.

1 I positions of chambers 28, 29 to preclude the formation of air pockets which could be sucked out through pump outlet SO, result-ing in an uneven flow of fluid.
ll Figure 6 illustrates the details of actuating member or yoke ¦¦17, which is mounted for movement on shaft 14. Yoke 17 includes lla pair of upstanding arms 17A described hereinbefore for engaging ilthe valve 40 and switching the same from one state to another.
I,A longitudinal bore 17B is provided in yoke 17 for receiving llpump shaft 14. ~fter pump shaft 14 is inserted in bore 17B, 10 ¦,pin 25, described hereinbefore is press-fit into aperature 14A
I in shaft 14. A bottom plate 17C is suitably attached to the ¦Ibottom of yoke 17, thus supporting a pair of pins 39 therein.
! As will be discussed hereinafter pins 39 support one pair of ends j !lof spring members of the snap acting mechanism illustrated in 15 , Figures 2B and 3B.
Referring now to Figure 2A, 2B, there is seen in cross-section Il the pump mechanism set forth in Figure 1 rcprcsenting a pressure !I stroke of the pump in the direction indicatcd at the point of I engagement of the pin 25 of shaft 14 with an end of slot 26 in the 20 I shaft yoke 17. ~t this instant the yoke is picked up by pin 25 ~¦ and begins to move with the shaft and the s~ring actuating memher 1 27, connected to the yoke, begins to pass over centcr. lhe di-¦ aphragm 16 applies pressure to the liquid present in the chamber ¦ 28, whicll is released ~ia check valve 32R into passageway 12 and 25 li directed out through the puml- outlet SO to the respective ¦I dischar~e stations. Figures 2B, 3B represent the postion of thc ¦¦ diaphragm, shaft and yoke at the completion of the strokc. ~s ¦I the reversing mechanism, ~encrally indicated 27, movcs ovcr ccnter Il I
ll l ' 11557~
j!

! I
1 llthere is produced a snap action effect which thrusts one arm 17A
¦~of the yoke against the protruding end of the spool 42 thus ¦¦changing immediately the positions of the O-rings of the valve so ¦as to suddenly reverse the flow of pressurized air through the ¦valve 40 at the completion of the stroke,and reverse the piston ¦action of the pump.
Figures 2B and 3B illustrate the details of the spring rever-jsing mechanism 27. The sprin~ reversing mechanism in one embodi-jment comprises a coil spring 36 wrapped about a pin 37 and pivot-10 l¦ally attached by way of pin 38 to the housing and pin 39 to the!I yoke 17. Upon engagement by the pump shaft, the yoke 17 will ~move in the direction of the stroke of the pump which in turn rotates pins 37 over center about pins 38'such that the springs 1¦36 take over and push the yoke in the direction of the established lS llmovement at a speed faster than the shaft movement, until the yoke !Ihits against the spool 42 of the valve mechanism so as to reverse ¦I the direction of the flow of pressurized air within the system ¦ and establish the piston action of the pump in the opposite direc-¦ tion. The position of the compression springs and yoke at the ¦ ends of the stroke are represented in Figure 3B. The presence of the pins 37 within the coil spring 36 prevents the s~ring mem-ber from buckling during the movement of the piston during the operation of the pump. Alternately, torsion springs 36T in I phantom may be substituted for the coil springs herein illustrated 25 ¦ to privide the snap-acting actuating means of the prescnt inven-tion. lhe yoke 17 slides or is pushed along by the shaft and !I spring mechanism 27 of the pump first in one direction then in a reverse direction according to the reversi~g ~ction of the valve ~1 1155711 1 40.
In Figures 4 and 5 there is illustrated a simplified enlarged cross-sectional view of the reversing valve 40 of the present in-vention which is represented herein as a spool valve comprising S a valve body 41, the spool 42 having three O-rings 43 intermit-tently positioned thereon within the valve cavity 44.
Within the upper area of the valve body are îocated air passages 45 coupled to passage 22 of Figure 1, for introducing the pressur-ized gas into the valve cavity 44, and 46 and 47 coupled to passages 23,24 of Figure 1, for directing air through the valve to the surface of the respective diaphragms of the pump. The valv 40 herein represented shows air under pressure being introduced to the valve cavity 44 through passageway 45 such that during the first half of the reciprocating cycle the air is directed to the respective air chamber 15B, through passageway 46 and passageway 24 (See Figure 1), while at the same time remaining passageway 47 provides for exhaust gases to be released as illustrated from the air chamber of the remaining or opposite diaphragm air chamber 15A. Upon contact by the left protru~ing end of the spool 42 with the yoke 17 as discussed above, the spool 42 is thrust to the right such that at the end of the pumping action the O-rings 43 shift their position as illustrated in ~igure 4, and the pressur-ized gas is now directed in the opposite direction so as to be introduced into the air chamber 15A of the diaphragm 16A, thus driving the pump in the opposite direction. In this position, the left end of the spool now projects fron~ the valve cavity 44 and awaits to be engaged by an arm 17A of the shaft yoke in the reverse action of the piston.

'.
'l 1 il In operation the valve 40 alternates the air flow through ,the respective passa~es 23, 24 to the air chambers lSA, 15B of ¦the diaphragms 16A, 16B. The compression springs 36 or 36T inteT-¦connected to the yoke continuously urge the shaft of the diaphragm ¦pump first in one direction then the other, responsive to the lo-cation of the yoke 17 along the shaft. The pressurized air is introduced into the air chambers 15A, 15B behind the respective ! diaphragms 16A, 16B and drives the diaphragms so as to discharge l¦the liquid from the diaphragm chambers. As stated above~ the 10 Ilyoke 17 on the shaft 14 initially moves in conjunction with the ¦Imovement of the shaft upon engagement of an end of slot 2~ with llthe pin 25 in shaft 14. The compression springs 36 or 36T, which ¦¦at the time of engagement aré pushing against each other, with ¦¦substantially no net force in a direction transverse to the pump 15 !' shaft, pivot over center and apply a further driving force to the ¦yoke which is then caused to move quickly b~ the snaP-action of the springs 36 to seat the projecting portions or arms 17A
¦ of the yoke 17 against the protruding spool 42 of the valve 41.
¦ This changes the positions of the 0-rings within the valve body ¦ and reverses the flow of pressurized air therein thus completing the first half of the cycle of the diaphragm pump. The continuous introdu~tion of pressurized air into the valve 40 initiates the ¦I pumping action of the shaft mounted piston in the opposite direc-i! tion, first compressing the springs 36 or 36T and then repeating 25 ¦! the action described above in the opposite direction, the compres-sed springs now pushing in the opposite direction. The spring reversing mechanism ensures that the movement of either of thc A

Ij C~
I

1 ~diaphragms initiated by the air pressure, is completed thus pre-¦¦venting prematuire reversal of the pumpin~ stroke or sticking of ¦ithe valve 40 in a central position.
l Re~erring now to Figure 7, there is seen in cross-section a pump construction similar to that discussed above with respect to Fi~ures 1 and lA, except with respect to the structure of diaphra~ms 16A, 16B. The diaphragms 16A and 16B further include cup-shaped metal plates 52 on the outer face of the respective diaphragm surface and cup-shaped retaining cap 54 on the inner surface of the respective diaphragms. This configuration elimin-ates the formation of crevices in the flexible diaphragm.
Preferably, the pump housing is constructed of a molded plasti~ :, as herein represented in Figure 1, such that the valves are mounted through the pump and all the li-nes or passageways run inside the plastic housing. This construction eliminates unneces-sary joints and external lines which contributes to a more reliabl system. As is seen in Figure 1, the inner wall of the housing comprises one continuous member which surrounds the pump reversing system components. l'he outer walls of ~he housing 11 are also fabricated of molded plastic ~hich provides for an overall more desirable construction of the diaphragm pump of the present invention.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the pre-sent invention and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of th following claims

Claims

Claim:

1. A self-contained reciprocating pump and reversing mechanism therefor comprising in combination a housing having laterally spaced chambers with diaphragm members therein dividing each of said chambers into a driving section and a discharge section, said diaphragm members being interconnected by a common shaft, a pin extending through said common shaft at approximately the mid-point of the longitudinal dimension thereof, manifold means in said housing for interconnecting said driving sections and discharge sections of said chambers with fluid inlet and outlet ports, respectively, inlet and outlet valves in selected ones of said ports for controlling the flow of fluid to be pumped to and from said discharge sections with respect to said manifold means, said discharge sections having end walls parallel to major surfaces of said diaphragm members and top walls perpendicular thereto, said inlet and outlet ports being disposed in said end walls at the highest possible positions therein adjacent said top walls, and control valve means having first and second positions for directing driving fluid alternately to a selected one of said driving sections defined by said diaphragms in said lateral chambers, a valve actuating member mounted on said shaft for sliding movement thereon between first and second positions partially in response to engagement by said pin, said valve actuating member constraining said control valve means to alternately direct said driving fluid to the respective driving sections in said first and second positions of said actuating member, and snap-acting means for accelera-ting and precluding the stopping of said control valve means between said first and second positions, said snap-acting means including a pair of opposed compression springs attached to the bottom of said actuator and disposed in the housing, on opposite sides of an axis which is parallel to a longitudinal axis of said shaft, said compression springs exerting forces on said valve actuating member which have an equal and opposite component in directions transverse to said axis and components combining to exert longitudinal forces in respective longitudinal directions beyond a central position.