US3645648A

US3645648A - Hydraulic pulse generator

Info

Publication number: US3645648A
Application number: US809685A
Authority: US
Inventors: Sture Anders Backman; Knut Ludvig Winquist
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-08-23
Filing date: 1969-03-24
Publication date: 1972-02-29
Anticipated expiration: 1989-02-28
Also published as: ES370791A1; BE737239A; GB1277775A; JPS4844689B1; NL6912902A; DE1941882A1; FR2016261A1

Abstract

A device for creating a flow of liquid and for imparting a pulsating pressure condition thereto, such as, for example, a device for pumping liquid fuel to the nozzle of an injection-type carburetor.

Description

United States Patem Backman et al.

HYDRAULIC PULSE GENERATOR .Sture Anders Backman; Knut Ludvig Winquist, both of Orebro, Sweden Johan H. Graffman, Benicasim, Castellon, Spain Mar. 24, 1969 Inventors:

Assignee:

Filed:

Appl. No.:

Foreign Application Priority Data Aug. 23, 1968 Sweden l 1,405/68 Dec. 6, 1968 Sweden ...l6,743/68 U.S.C1 .4l7/2l4,4l7/22l,417/471,

' 417/552 1111. C1 ..F04b 19/00, F04b 21/04, F041) 49/00 Field otSearch ..l03/38, 178, 225, 203;

[56] References Cited UNITED STATES PATENTS 408,461 8/1889 Capitaine 103/38 1,218,903 3/1917 Scovel .230/190 1,586,307 5/1926 Hildebrand 103/38 2,007,197 7/1935 Hcdblom "103/153 3,118,383 1/1964 Woodward ..l03/178 3,307.49l 3/1967 Wolff 1 03/203 3.314.365 4/1967 Ritchie 103/38 Primary Examiner-Carlton R. Croyle Assistant Examiner-John J. Vrablik Attorney-Stevens, Davis, Miller & Moshcr [5 7] ABSTRACT A device for creating a flow of liquid and for imparting a pulsating pressure condition thereto, such as, for example, a device for pumping liquid fuel to the nozzle of an injectiontype carburetor.

6 Claims, 2 Drawing Figures PAIENTEUFEB29 I972 SHEET 1 [1F 2 W M M mm .w v .u M? m M Nun ATTORNCY-S HYDRAULIC PULSE GENERATOR The present invention relates to a hydraulic pulse generator, i.e., a device which from an undefined flow of liquid creates a flow of pulsating liquid. One requirement placed upon such pulsating liquid flows is that the flow comprises distinct and well defined pulses of the desired magnitude.

Pulse generators known heretofore have not fulfilled this requirement to the extent desired, especially when concerned with volatile liquids. When volatile liquids flow through valves, etc., small bubbles of gas appear in the liquid rendering the liquid highly compressible and to varying degrees. This variable compressibility makes it impossible to obtain well defined pulses of the desired magnitude. Another disadvantage associated with previously known pulse generators is their inability to adapt themselves to varying flow requirements. Known pulse generators usually function at a constant full flow output with a nonconsumed portion of the flow recirculating via a bypass line provided with an overflow valve, a method which is both uneconomical and which affects the distinctiveness of the pulses deleteriously.

One field of use in which the aforementioned problems are relevant concerns a certain type of injection carburetor for internal combustion engines, where engine fuel is supplied to an injection nozzle at a pulsating pressure, i.e., in the form of pulses, to initiate and maintain vibratory movements of a valve body. A more detailed description of such a carburetor is given in U.S. Pat. application, Ser. No. 536,550, Swedish Pat. specification No. 305,980 and Pat. application No. 9179/68, these carburetors being of the type to which the present invention is applicable. It is important in these carburetors that the fuel be pumped to the injection nozzle in well defined and distinct pulses. The problem, however, is that petrol, for instance, is a very volatile liquid and contains certain quantities of gaseous hydrocarbon in solution. This means that the risk of gas bubbles appearing in the petrol is high, especially given the pressure drops to which the liquid is subjected while passing through the narrow passages existing in known pressure and suction valves, particularly when such valves are spring loaded. These gas bubblesrender the liquid compressible to varying degrees, thereby deleteriously affecting the functioning of the pulse generator and thereby the functioning of the carburetor.

A further disadvantage in known pulse generators resides in their being incapable to immediately respond to varying flow requirements, such as, varying fuel requirements of an internal combustion engine. An automobile engine, for instance, operates at maximum capacity (corresponding to maximum fuel flow requirements) only during a small proportion of its total operating span. Consequently, it is particularly uneconomical to employ a pulse generator which functions at a constant maximum capacity in conjunction with a carburetor. In this regard, the power required to drive the pulse generator itself constitutes a significant power loss.

It is, therefore, an object of this invention to provide a novel pulse generator for liquids which overcomes the disadvantages of known pulse generators, with particular regard to the aforementioned disadvantages which, however, are not limitative of the scope of this invention.

It is a specific object to provide an improved pulse generator for pumping liquid fuels to a carburetor of the fuel injection type.

It is a further specific object to provide a pulse generator which is especially adapted to pump a liquid in accordance with distinct, well defined pulses of a particular magnitude which itself may be varied to suit different requirements.

It is a further specific object to provide a pulse generator which itself consumes a minimum of power.

It is a further specific object to provide a pulse generator for the carburetor of an internal combustion engine and which generator may be driven by an electric motor drawing power from the electrical system associated with said engine.

Other objects are those which are inherent in the herein presented disclosure of two preferred embodiments of realization of the invention, a detailed description of which follows with reference to the accompanying drawing wherein:

FIG. 1 is a sectional view of a pulse generator according to a first embodiment of this invention; and,

FIG. 2 is a sectional view of a pulse generator according to a second embodiment of this invention.

The objects of this invention are realized generally through an arrangement comprising a driven reciprocable piston means which is constructed to function as a nonreturn valve for liquid being pumped thereby, said piston means being interconnected with a drive means therefor through an axially resilient coupling.

A first embodiment of the hydraulic pulse generator is shown in FIG. I of the drawing and includes a housing 1 providing a cylindrical space 2 in which a pulse piston 3 is reciprocably displaceably arranged. The piston 3 includes longitudinal or axially extending through passages 4 and an annular recess 5 defining an inlet port 6. Leading to the inlet port 6 is a feed port 7. Mounted in a central bottom opening 8 in the piston is a valve body 9. The valve body, which comprises a valve disk portion 10 and a stem portion 11, constitutes together with a valve seat 12 on the piston a-nonretum valve and is, for this purpose, movable relative to the piston along its direction of axial movement. The axial displaceability of the valve body results from the fact that it is secured to the piston 3 by means of a pin 14 which is fixedly secured to the piston and which extends through transverse opening 13 in the stem portion 11 of said valve body, there being a certain degree of clearance between the outer periphery of the pin 14 and the wall defining opening 13, this clearance being exaggerated in the drawing for purposes of illustration only. Valve body 9, therefore, is capable of being displaced axially away from piston 3 to an extent equal to said clearance, such displacement resulting in an annular passage being formed between the outer periphery of valve body 9 and seat 12.

The axially endmost radial surface of valve body 9 constitutes the working end of the piston 3 and serves to delimit the pulse or compression chamber 15 from the remainder of cylindrical space 2. An outlet passage 16 leads from pulse chamber 15 to externally of housing 1.

Piston 3 comprises a piston rod means generally denoted 17 which in turn is made up of two axially

successive rod portions

18 and 19 which are axially coupled together by means of a resilient coupling 20.

Coupling 20 comprises a sleeve 21 in whose bore rod portion 19 is guidingly slidably arranged, sleeve 21 in turn being guidingly slidably arranged within a bore 30 of housing 1. Rod portion 19 comprises a pair of longitudinally spaced apart

abutment members

23 and 24, the first of which is abuttable against one end of the bore in sleeve 21 and the second of which, 24, serves to confine one end of a compression-type spring such as cup spring means 22 whose other end abuts against a shoulder 25 of sleeve 21.

The other rod portion 18 is integral at one end thereof with piston 3 and includes an abutment flange 36 at its other end which confines one end of compression spring 35 whose other end abuts against shoulder 37 of the housing 1. The extreme outer end of rod portion 18 abuts against a flat solid disk 27 which rests against shoulder 28 of sleeve 21. Rod portion 18 is guidingly slidably fitted within bore 29 in housing 1.

Abutment member 24 is axially adjustable along the length of rod portion 19 by means of threaded nut means 26 in order to vary the compressive force in spring means 22.

The external or free end of rod portion 19 is provided with a follower shoe 31 which in turn cooperates with a drive means to impart a periodic axial thrust to the rod portion 19. Such a drive means comprises a motor 34 on whose shaft (which runs perpendicular to the reciprocation axis of piston 3) is mounted an eccentric or cam 33 which bears against the surface of shoe 31. The axial position of rod portion 19 relative to shoe 31 can be varied through threaded means 32 in order to vary the length of stroke of the piston 3. The speed of motor 34 determines the frequency with which rod piston 19 is driven by follower 33 and, consequently, the frequency of the liquid pulses discharged from chamber 15 into passage 16.

In summary, therefore, it is seenthat piston 3 is normally urged, with reference to the drawing figure, in a downward direction by compression spring 35, and in turn said piston acting through rod portion 18 urges sleeve 21 downwardly which, acting through spring 22, urges rod portion 19 downwardly. The two

rod portions

18 and 19 are, therefore, resiliently interconnected in that they may be displaced in either axial direction relative to each other pursuant to a corresponding flexing of the spring means 35 or 22.

The outlet or discharge passage 16 can lead to an injector nozzle 39 of a fuel injection-type carburetor such as disclosed in the aforementioned patent applications. For the purposes of this application, it is sufficient to note that the orifice portion of nozzle 39 which feeds into the carburetor is closed by an oscillating valve 40 which is normally urged to a closed position by a spring means 41 whose tension can be varied in accordance with varying requirements of fuel flow. In order to achieve proper fuel atomization, it is desired to have valve 40 oscillate between open and closed positions relative to nozzle 39, such oscillations being obtainable if a pulsating fuel flow is supplied to nozzle 39. In order for the valve 40 to be able to oscillate smoothly and in order for the flow through nozzle 39 to be controllable solely by spring 41, it is necessary that the pulses be distinct and be of a constant magnitude.

In operation, therefore, liquid fuel is pumped from its storage tank and into passage 7 which leads into cylindrical space 2 in correspondence to the annular recess 5. This fuel then passes from recess axially through passages'4 to the inlet side of nonretum valve body 9.

While piston 3 is being moved along its compression stroke (upwards in the figure) by means 33, 34, etc., valve body 9 is forced to closed position against seat 12 by the liquid pressure in chamber 15. However, when piston 3 is accelerated in a downward direction (its suction stroke) from its top dead center position under the action of spring 35, the valve body 9 lags behind said piston to an extent equal to the clearance between hole 13 and pin 14, this clearance being in the order of only a few tenths of a millimeter (e.g., 0.2 mm. for a stroke of 0.8 mm.) but being sufficient for fuel to pass along seat 12 from passages 4 and into compression chamber 15. When piston 3 reaches its bottom dead center position, valve body 9 still has a downwardly directed momentum and thereby closes against seat 12 whereupon the piston 3 commences its upward compression stroke and valve body 9 then is maintained tightly against seat 12 by the increasing pressure in chamber 15. The pressure in chamber 15, passage 16, and nozzle 39 then builds up to a point at which valve 40 then opens.

In order to obtain sufficiently distinct pulses and small pressure losses, it is necessary that the inoperative or dead space in the pulse chamber 15 be reduced as much as possible, and that the discharge line to nozzle 39 be as short as possible. The first condition is fulfilled by having the radial width of the pulse chamber 15 equal to the width of the cylinder and its axial length only slightly larger than the axial length of the piston stroke, whereby the piston comes very close to the upper end of space 2 when it is at its top dead center position. In fact, according to an actual embodiment, it is possible to reduce this axial height of chamber 15 to almost zero by adjusting shoe 31 so that the piston is in contact with the upper end of space 2 when said piston is at said top dead center position.

The length of the piston stroke is varied in dependence on the desired rate of fuel flow and, during operation, is automatically adjusted in accordance with the back pressure which develops in nozzle 39, which back pressure depends upon the tension in spring 41.

The automatic adjustment of the length of stroke in correspondence to the back pressure in the pulse chamber is effected through the resilient coupling 20. When the back pressure increases the cup spring 22 is compressed to a corresponding degree, depending upon its tension and characteristics. Its tension can, for this purpose, be set by means of the nut 26. The length of the piston rod 1.7, and thus the compression clearance between the piston and the pulse chamber wall, can be adjusted by means of the nut 32.

The major portion of the liquid flowing into port 6 continues through annular passage 5 and out passage 38 and recirculates to the storage tank where entrained gas bubbles may be liberated therefrom. This continuous recirculation of liquid prevents any gas bubbles from being entrained within the liquid which flows through passages 4 into pulse chamber 15, since, despite all. precautions, some gas bubbles will unavoidably develop within port 6 as the result of the rapid reciprocation of the piston, leakage, etc. It is to be noted, in this regard, that the recirculation of liquid according to the invention occurs on the suction side of the piston and not on the pressure side, that is: from passage 16, whereby the recirculation according to the invention does not entail any power loss insofar as the pulse generator itself is concerned.

The embodiment of FIG. 2 is analogous to that of FIG. 1 in many respects and, therefore, various heretofore presented explanations relative to the first embodiment will not necessarily be repeated relative to FIG. 2 in instances in which the same basic principles of operation obviously apply. It will be noted that many elements in FIG. 2 which are analogous to elements in FIG. 1 have been identified by the same numbers as in FIG. 1 but in the hundred series.

In FIG. 2, housing 101 defines a piston space 102 in which a piston means including pulse piston 103 and valve body 109 is reciprocably displaceably arranged. A flexible diaphragm 104 is clamped in a sealing manner respectively to the piston 103 and to the housing 101 so as to define a continuous radial seal therebetween to thereby sealingly separate the compression chamber 115 from the remainder of space 102, said diaphragm in the meantime permitting axial displacement of the piston relative to the housing. Housing 101 includes a feed port 107 leading into an inlet chamber 106 which in turn connects with axially extending passageway means along valve body 109.

Valve body 109 includes a perforated plate 111 at one end thereof and a valve disk portion 1 10 at the other end thereof. Said valve body 109 is axially reciprocable relative to the housing 101 between respective opposite axial positions thereof defined respectively by abutment of plate 111 with housing shoulder 113 and seating of valve disk portion 110 against'housing valve seat 112. The extent of such reciprocation, that is the length of stroke, is determined by the extent to which the distance between plate 111 and disk 110 exceeds the distance between shoulder 113 and seat 112.

Piston 103 includes an axially extending pin member 108 received within a bore 114 in valve body 109, a friction ring 121 mounted on member 108 serving to frictionally connect pin 108 and body 109 together.

It is seen, therefore, that valve body 109 comprises a nonretum valve acting between compression chamber 115 and passageways 105, permitting flow of liquid fuel from chamber 106 and along passageways 105 and into chamber 115 when disk portion 110 is spaced from seat 112 but preventing backflow from chamber 1 15 into passageways 105 when said disk portion 1 10 is seated against said seat.

An outlet passageway 116 leads from pulse chamber 115 to externally of housing 101 to

elements

139, 140, and 141 which are the same as 39, 40, and 41 in FIG. 1.

Piston 103 is provided with a piston rod means 117 which in turn is made up of two axially

successive rod portions

118 and 119 which are axially coupled together by means of a resilient coupling means 120.

Coupling means 120 serves to resiliently interconnect the two

separate members

118 and 119 which constitute a piston rod means for piston 103.'ln this regard, sleeve 118 is integral with the piston 103 and rod piston 119 is slidingly fitted within the bore of sleeve member 118. Rod portion 119 comprises a pair of axially spaced apart abutment members 123 an 124, the first of which is abutable against one end of said bore and the second of which, 124, serves to confine one end of a compression spring, such as cup spring means 122, whose other end abuts against a shoulder 125 of sleeve 118. Compression spring 135 is mounted externally of housing 101 but is analogous to spring 35 in the first embodiment since it also is confined between housing shoulder 137 and shoulder 136 in the piston rod member 118. This member 118 is axially slidably fitted within bore 130 in housing 101.

In this second embodiment, the abutments shoulder 124 and the follower shoe 131 constitute one integral member threadedly adjustably fitted onto one end of rod portion 119.

The motor 134 and cam 133 are analogous to the

corresponding elements

34 and 33 in FIG. 1 and function in the same manner relative to shoe 131 and the other elements of the pulse generator.

In the FIG. 2 embodiment, the tension on spring 122 and the axial position of rod portion 119 relative to shoe 131 are both variable integrally rather than separately as is the case in the FIG. 1 embodiment. Further, the FIG. 2 embodiment includes an equalizing means comprising a diaphragm 126 extending across one end of chamber 106 opposite to valve body 109, said diaphragm being biased towards said valve member by a coil spring 128 compressed between the housing 101 and a thrust plate 127 which bears against one side of the diaphragm.

The pulse generator functions in the following manner. Fuel, e.g., petrol, is pumped from a fuel tank, in the direction of the arrow in FIG. 2, through the inlet 107 to the inlet chamber 106. The fuel then passes through perforations in the plate 111 and along the passages 105 to the nonreturn valve means 109, 112. The piston means 103 is driven alternately along a pumping and suction stroke by means 133. During the suction stroke the piston carries along with it the valve body 109 by virtue of the friction provided therebetween by the friction ring 121, until the plate 111 comes into contact with abutment 113. The length of stroke of the valve body 109 has been exaggerated in the drawing, and in actual fact only amounts to a fraction of the length of stroke of the piston 103, e.g., about 0.2 mm. in a stroke length of about 0.8 mm. As the valve disk 110 leaves its seat 112, fuel passes into the pulse chamber 115; and when the piston has reached the end of its suction stroke and returns along its pumping stroke, the valve 110 closes rapidly by virtue of the pressure build up in chamber 115. During its pumping or compression stroke, the piston 103 forces out a jet of liquid fuel from the pulse chamber 115 through the outlet 116, as shown by the dotted arrow, to a means requiring same, in this instance the nozzle 139.

In order to obtain sufficiently distinct pulses and to keep pressure losses at a minimum, it is necessary that the inoperative or dead space in the pulse chamber 115 be as small as possible, as already explained with reference to FIG. 1, and that the line 139 to the consumer means be as short as possible. The first condition is fulfilled by making the width of the pulse chamber 115 substantially equal to the width of the cylinder and its axial length only slightly greater than the length of the piston plus the stroke of the piston 103, whereby at the end of the pumping stroke the clearance between the end surface of the piston 103 and the valve disk portion 110 is so small that the piston almost comes into contact with the disk. The length of stroke of the piston varies in dependence on the desired fuel flow, and is adjusted automatically accord-' ing to the back pressure in the injector nozzle 139, which depends upon the tension of spring 141, as has already been explained with reference to FIG. 1.

The clearance between the piston 103 and the disk 110 can be adjusted by changing the axial disposition of shoe 131 along rod member 119 analogously to the arrangement of FIG. 1.

In the FIG. 1 embodiment, the valve 9 is arranged in the pulse piston 3 and the drop in pressure occurring when the fuel is conveyed from the inlet chamber 6 to the pulse chamber is very slight. This results because the energy required to move the valve body 9 is transferred to said body from the piston partly through the pin 14 which connects them and partly through the valve seat 12 arranged in the piston. In this way the valve is positively controlled by the piston, as opposed to conventional nonreturn valves controlled by the pressure conditions in the liquid. In the FIG. 2 embodiment, the same effect is obtained by transferring energy through the friction ring 121. Another important feature of the FIG. 2 embodiment is that the described construction of the valve results in a relatively large valve opening occurring at 112 thus resolving the problems mentioned in the presented herein introduction, among them the drop in pressure experienced in known devices.

Recirculating line 138 functions in the same manner set forth with regard to passageway 38 in F IG. 1.

The herein given details of a preferred embodiment are given by way of illustration only and are not intended to be limitative of the scope of the invention; it being understood that all modifications and substitutions of either an obvious nature or well within the purview of one skilled in the art are intended to be covered by the scope of this invention. For instance, the pulse piston in FIG. 2, instead of being connected with the housing by a diaphragm 104, may sealingly abut the walls of the cylinder chamber, as in the embodiment of FIG. 1. Furthermore, instead of comprising a motor driven eccentric the drive means may be an electromagnet whose armature is connected with the piston rod and which is supplied with pulsating electric current, whose frequency thus determines the pulse frequency of the generator.

What is claimed is:

1. A hydraulic pulse generator for imparting distinct pressure pulses to a liquid fuel being pumped to an internal combustion engine, comprising: a housing having a piston space therein, a piston reciprocally mounted in said space, a compression chamber being defined between a sidewall and a rigid end wall of said housing and an opposed rigid radial face of said piston, a first liquid passageway extending axially along said piston and opening into said compression chamber, a nonreturn valve member freely mounted on said piston cooperating with a valve seat in said passageway whereby said valve member may freely reciprocate to a limited extent relative to said piston and said seat along the same axial direction as that along which said piston reciprocates, cooperating rigid abutment means on said piston and said valve member and a clearance between the respective abutment means which defines the extent of axial travel of said valve member relative to said piston, a second liquid passageway in said housing leading from a liquid fuel supply to said first passageway, a third liquid passageway in said housing leading from said compression chamber to externally of said housing, a drive means for reciprocating said piston along respective suction and pumping strokes, said valve member, because of inertia lagging behind said piston during said suction stroke whereby said member detaches from said seat to the extent of said clearance and permits liquid fuel to How from said first passageway into said compression chamber, and said valve member snapping shut against said seat at the beginning of said pumping stroke because of reverse inertia and remaining so shut during said pumping stroke as a result of the pressure in said compression chamber acting against said valve member whereby a sharply distinct pressure pulse is imparted to the fuel in said compression chamber during said pumping stroke, said drive means comprising an elongate piston rod means for reciprocably driving said piston, said rod means comprising first and second axially successive elongate portions separate from each other and interconnected by an axially resiliently deformable coupling means arranged to transmit axial force from one to the other of said portions, whereby said portions are axially displaceable relative to each other pursuant to deformation of said coupling means, said first elongate portion being rigid with said piston and said second portion being adapted to be driven axially.

2. The pulse generator of claim 1, said valve member having an opening extending transversely to the reciprocation axis of said piston, a fixed rigid element on said piston extending into said opening with a clearance therebetween which constitutes the first mentioned clearance.

3. The pulse generator of claim 1, said valve member and said valve seat being concentrically arranged relative to the reciprocatory axis of said piston, said valve seat being defined by substantially the outermost radial periphery of said piston, said valve member having an end face which constitutes substantially all of said piston rigid radial face but spaced from the chamber sidewall to allow liquid flow therepast.

4. The pulse generator of claim 1, said valve member having a frustoconical cross section along an extent thereof which cooperates with said seat and an elongate parallel-sided portion extending from said extent and being guidingly fitted within a bored portion of said piston member.

5. The pulse generator of claim 1, the width of said compression chamber being equal to the width of said piston space, the axial length of said compression chamber being in the order of 0.01 mm. when said piston is at the extreme end of its pumping stroke.

6. The pulse generator of claim 5, said clearance being in the order of a few tenths of a millimeter.

Claims

1. A hydraulic pulse generator for imparting distinct pressure pulses to a liquid fuel being pumped to an internal combustion engine, comprising: a housing having a piston space therein, a piston reciprocally mounted in said space, a compression chamber being defined between a sidewall and a rigid end wall of said housing and an opposed rigid radial face of said piston, a first liquid passageway extending axially along said piston and opening into said compression chamber, a nonreturn valve member freely mounted on said piston cooperating with a valve seat in said passageway whereby said valve member may freely reciprocate to a limited extent relative to said piston and said seat along the same axial direction as that along which said piston reciprocates, cooperating rigid abutment means on said piston and said valve member and a clearance between the respective abutment means which defines the extent of axial travel of said valve member relative to said piston, a second liquid passageway in said housing leading from a liquid fuel supply to said first passageway, a third liquid passageway in said housing leading from said compression chamber to externally of said housing, a drive means for reciprocating said piston along respective suction and pumping strokes, said valve member, because of inertia lagging behind said piston during said suction stroke whereby said member detaches from said seat to the extent of said clearance and permits liquid fuel to flow from said first passageway into said compression chamber, and said valve member snapping shut against said seat at the beginning of said pumping stroke because of reverse inertia and remaining so shut during said pumping stroke as a result of the pressure in said compression chamber acting against said valve member whereby a sharply distinct pressure pulse is imparted to the fuel in said compression chamber during said pumping stroke, said drive means comprising an elongate piston rod means for reciprocably driving said piston, said rod means comprising first and second axially successive elongate portions separate from each other and interconnected by an axially resiliently deformable coupling means arranged to transmit axial force from one to the other of said portions, whereby said portions are axially displaceable relative to each other pursuant to deformation of said coupling means, said first elongate portion being rigid with said piston and said second portion being adapted to be driven axially.