GB2369411A

GB2369411A - Double-acting pump with pressure equalisation between chambers

Info

Publication number: GB2369411A
Application number: GB0028585A
Authority: GB
Inventors: Carmeli Adahan
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-11-23
Filing date: 2000-11-23
Publication date: 2002-05-29
Anticipated expiration: 2020-11-23
Also published as: GB2369411B; GB0028585D0

Abstract

A double-acting gas vacuum pump 10 or compressor comprises a reciprocating piston 20 separating two working chambers 70,72 in a cylinder 26 which has a pressure equalising notch 66 provided in its interior surface 68 to enable pressure in both chambers 70,72 to equalise when the piston 20 moves over the notch 66 in the vicinity of an extremity of its travel within the cylinder 26. This provides a pump output characteristic of the two chambers 70,72 changing between operating in parallel and operating in series, ie between a single stage pump operation and a two stage operation. Also disclosed is a sealing arrangement 54 between working chamber 72 and a piston rod 30, using a rigid seal portion 56 held within a flexible sleeve portion 58 to seal around the piston rod 30 to enable angular and reciprocating travel of the rod 30.

Description

FIELD OF THE INVENTION The present invention relates to pumps which are utilized for the compression or evacuation of gas.

BACKGROUND OF THE INVENTION As known, pumps may be used as compressors, or as vacuum pumps. A pump which is used as a compressor, generally draws a working gas, such as air, through an inlet, at atmosphere pressure, and compresses it to a higher pressure prior to outflow of the gas through an outlet. A vacuum pump, however, generally draws a working gas through an inlet, and discharges it to the atmosphere through an outlet, creating a lower pressure, or vacuum at the inlet. The same pump can function both as a vacuum pump or a compressor, depending on the manner in which it is applied. While the description below relates specifically, where applicable, to vacuum pumps, it also intended to apply equally to compressors.

In operation, a vacuum pump displaces gas, until it reaches a predetermined vacuum levels beyond which it no longer pumps. When higher vacuum levels are needed, another pump may be connected to the vacuum pump outlet. This is called a series connection. While in the first pump, the original vacuum differential between its inlet and outlet is maintained, the additional pump, enables a higher vacuum level to be obtained at the first pump's inlet.

When the two pumps are driven by the same motor, and connected in series, a two-stage pump is obtained. One version of a two-stage pump is a dual pump, in which a piston reciprocates within a cylinder while pumping on both sides of the piston. When one of the pump's inlets is connected to the other pumps outlet, the dual pump becomes a two-stage pump.

When higher vacuum levels or pressures are not required, the two stages of the

pump are ineffective, as they perform a function, which a single stage can perform alone. Accordingly, it is often desirable to connect the two pumps or stages in parallel and thus double the flow capacity in comparison to series connected pumps or stages.

This is accomplished by a valving system, which change the connections of the ports of the two pumps, from series to parallel, as required. The valving system constitutes an undesired addition of hardware, which also requires its own control system.

For the dual pump to function with a single cylinder and a piston pumping on both of its sides, it is necessary to seal the piston rod. This task is fairly conventional, when the piston rod translates along the cylinder axis. In the case of a pump employing a wobble piston, however, this is somewhat problematic, due to the angular translation of the piston rod, during operation.

SUMMARY OF THE JNVBNTION The present invention seeks to provide a dual pump, having a single cylinder. and functioning, at alltimes, in parallel as well as in series, without the provision of valves or other external controls, and without requiring additional hardware.

The present invention seeks to provide, yet further, a dual pump, having a single cylinder and a wobble piston, which pumps on both of its sides There is thus provided, in accordance with a preferred embodiment of the

invention, a double action pump which includes a cylinder having a first, rear end and a second, forward end ; and a housing for the cylinder which includes a gas intake manifold associated with the first and second ends of the cylinder ; and a gas outlet manifold associated with the first and second ends of the cylinder.

The gas intake manifold includes a gas inlet port for intake of a working gas into the gas intake manifold, and first and second valved cylinder inlet ports arranged in communicative association with the first and second ends of the cylinder, respectively, for permitting an inflow of the working gas into the cylinder, from the gas intake manifold ; similarly, the gas outlet manifold includes first and second valved cylinder outlet ports arranged in communicative association with the first and second ends of the cylinder, respectively, for permitting an outflow of the working gas from the cylinder into the gas outlet manifold of the housing, and a gas outlet port for permitting outtlow ofworking gas from the gas outlet manifold of the housing.

The pump further includes a piston arranged for reciprocating motion within

the cylinder ; a piston rod attached to the piston and extending externally of the cylinder and of the housing ; a drive assembly for imparting a reciprocating motion to the piston, so as to cause alternate forward and rearward working strokes of the piston between the first and second ends of the cylinder ; and a sealing arrangement provided between the piston rod and the cylinder housing, thereby to effectively prevent passage of the working gas between the piston rod and the cylinder housing, such that

J each forward stroke and each rearward stroke of the piston causes a pressure drop on an upstream side thereof and a pressure increase on a downstream side thereof thereby also to cause intake of the working gas into the cylinder via a predetermined single one of the valved inlet ports, and outflow of the working gas from the cylinder via. a predetermined single one of the valved outlet ports, thereby to provide a double action ; and wherein the piston is operative to divide the cylinder into mutually sealed, upstream and downstream working chambers, the respective magnitudes of which vary in relation to the stroke position of the piston, so as to impart to the pump a double action, parallel characteristic.

In accordance with a preferred embodiment of the invention, the improvement is characterized by provision of pressure equalization apparatus operative, when the piston is located in the vicinity of the extremity of its travel within the cylinder, to permit gas communication between the upstream and downstream volumes, thereby to cause a reduction in a pressure gradient therebetween ; such that, when the pressure differential across the piston is at least a predetermined magnitude, a double action, series characteristic is imparted to the pump, so as to boost the performance thereof Additionally in accordance with a preferred embodiment of the invention. the piston is a wobble piston, such as to undergo both axial and angular translation, and the sealing arrangement includes a floating seal having formed therein an opening through which the piston rod extends, the floating seal including a first portion operative to seal against the piston rod, thereby to substantially prevent escape of

working gas therepast, and further having a second, flexible portion operative to undergo angular translation together with the piston rod, and so as to support the first portion In sealing relation with the piston rod.

Preferably, the sealing arrangement has a generally elongate, nipple configuration, extending along a portion of the piston rod.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more fully understood and appreciated from the following detailed description, taken in conjunction with the drawings, in which: Fig. lA is a partially cut-away plan view of a single cylinder dual pump

having a wobble piston, constructed and operative in accordance with a preferred embodiment of the present invention, and showing the piston in a rearward extreme position ; Fig. IB is a view similar to that of Fig. lA, but showing the piston in a forward extreme position ; Fig. IC is a cross-sectional, partial view of the forward end of the apparatus as seen in Fig. IB ;

Fig. ID is a cross-sectional view of the cylinder and piston seen in Fig. 1C, taken along line 1D-1D therein, particularly showing the lateral extent of the pressure equalizing notch formed therein; Fig. 2 is a partially cut-away plan view of a single cylinder two-stage pump having a wobble piston, constructed and operative in accordance with an alternative embodiment of the present invention ; and Fig. 3 is a partially cut-away plan view of a single cylinder two-stage pump having a linear, reciprocating piston, constructed and operative in accordance with yet a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring initially to Figs. 1A - 3, while the present invention relates to pumps, in general, the description refers principally to a vacuum pump. It will be appreciated, however, that this is for convenience of example only, and that it is intended to be equally applicable to a compressor.

Referring now to Figs. lA and IB, there is seen a pump, referenced generally 10, constructed and operative in accordance with a preferred embodiment of the present invention. Pump 10 is connected to a motor 11, which has an output shaft 12, which is connected to a drive assembly of the pump 10, referenced generally 14, which is generally similar to the drive assembly shown and described in US Patent No. 5,762,480, the contents of which are incorporated herein, by reference. It is seen

that drive assembly 14 includes a crank, referenced 16, which is connected to output shaft 12, and which drives a crank member, referenced 18. Crank member 18 is pivotally linked to wobble piston 20, and is operative to translate the rotary movement of crank 16 to a reciprocal movement of wobble piston 20. This movement of wobble piston 20 is indicated by double headed arrow, referenced 22.

An alignment member, referenced 24, is operative to absorb the lateral component of the force applied thereto by crank member 18, so that wobble piston 20 is subjected to a predominantly axial force.

A cylinder 26, having a first. rear end 25, and a second, forward end 27, is disposed within a housing 28. Housing 28 has a gas intake manifold, referenced generally 48; and a gas outlet manifold, referenced generally 50. Manifolds 48 and 50 are associated, as shown, with the first and second ends 25 and 27 of cylinder 26, as

described further in detail below, and are arranged such that the only gas communication between manifolds 48 and 50 is via cylinder 26, also as described below.

As seen, gas intake manifold 48 has a gas inlet port 32 for intake of a working gas, typically air, and first and second valved cylinder inlet ports, respectively referenced 28a and 28b, arranged in communicative association with first and second ends 25 and 27 of cylinder 26, respectively, for permitting an inflow of the working gas thereinto, from gas intake manifold 48 of housing 28.

Similarly, gas outlet manifold 50 has first and second valved cylinder outlet ports, respectively referenced 29a and 29b, arranged in communicative association with respective first and second ends 25 and 27 of cylinder 26, for permitting an

outflow of the working gas from cylinder 26 into gas outlet manifold 50 of housing 28. Outlet manifold 50 further has a gas outlet port 36.

A wobble piston 20 is attached to a piston rod 30 which extends outward from cylinder 26 and housing 28, via an opening 38 formed in housing 28. Piston 20 is driven by drive assembly 14, which is operative to impart thereto a reciprocating motion, so as to cause alternate forward and rearward working strokes thereof between first and second ends 25 and 27, of cylinder 26.

There is also provided a sealing arrangement, referenced generally 54, between piston rod 30 and cylinder housing 28, thereby to effectively prevent passage of the working gas therebetween.

It will be appreciated that piston 20 is operative to divide cylinder 26 into mutually sealed, upstream and downstream, first and second, working chambers, respectively referenced 70 and 72, the respective magnitudes of which vary in relation to the stroke position of piston 20.

Intake and outlet manifolds 48 and 50 effectively channel outputs from both inlet and both outlet sides of the cylinder in parallel, into single inlet and outlet channels, so as to effectively provide a dual pump. The swing of alignment member 24 causes piston rod 30 to undergo a slight angular movement with respect to a pivot 52 and the longitudinal axis of cylinder 26, as mentioned above.

In order to provide an effective dual pump, therefore, working chamber 72 must be sealed. Sealing arrangement 34 is thus provided, in accordance with a preferred embodiment of the present invention, as a flexible,'floating"seal, which is operative to seal between second working chamber 72 and piston rod 30. In order to accommodate the lateral translation of piston rod 30, floating seal 54 is flexible and shaped as nipple which extends away from the wall of housing 28, and so is able to "follow'the angular translation of piston rod 30. While, in the illustrated example, the protruding portion of seal 54 extends outwardly from housing 28, this is by way of example only, and, in accordance with an alternative embodiment of the invention, it may be arranged so as to extend inwardly.

Floating seal 54 is formed of a generally cylindrical, rigid inner seal portion 56, which is shaped for insertion in and retention by a flexible retainer 58, which is typically mounted as shown, between a recessed portion 60 of cylinder housing 28, and a wall portion 62 of a casing 64 in which the piston rod 30 and drive assembly 14 are located. Flexible retainer 58 is formed of a sufficiently flexible rubber type

material, such as silicone rubber, thereby to allow inner seal portion 56 to follow the angular travel of piston rod 30.

It will be appreciated by persons skilled in the art, that the described floating seal, taken in conjunction with the reciprocating mechanism, which allows for only slight angular movement of the wobble piston rod 30, allows use of a pump which, while having only a single cylinder, operates as a dual pump. This occurs due to the fact that both forward and rearward strokes of the piston 20 are"working"strokes ; every stroke-regardless of its direction-being operative to cause an inflow of working gas though inlet port 32, and outflow of the working gas through outlet port 36. effectively doubling the pump's capacity.

Referring now briefly to Fig. 2, there is seen a wobble piston 20, which is arranged for reciprocating motion within a cylinder and cylinder housing similar to those of Fig. 1, and which are thus not specifically described again herein, in detail.

In Fig. 2, while piston 20'is shown as a wobble piston, it need not be connected to a drive such as assembly 14 shown and described above in Fig. 1.

Rather it may be connected to any suitable drive. Preferably, the angular travel of the presently illustrated wobble piston should be small, typically no more than 2 , as in the embodiment of Figs. lA-JD. However, any type of wobble piston having even a much larger angular travel may be used, in combination with an appropriately formed floating seal 54'.

Referring now once again to Figs. lA and 1B, it will be appreciated that, as an increase in the vacuum at inlet port 32 occurs, flow through the pump decreases, until such time as the inlet vacuum level reaches the peak vacuum level obtainable within working chambers 70 and 72, when piston 20 is at the extremities of its translation, such that pumping ceases and the working gas is no longer driven out of the working chambers 70 and 72, to the outlet port 36.

In order to continue pumping, so as to further increase the vacuum level, at inlet port 32, the outlet port 36 may be connected to a vacuum source (not shown), such as an additional pump. This serves to raise the vacuum level at which the piston will draw the working gas through the inlet port 32; this'series'connection with another pump, allows much higher vacuum levels to be attained. This effect may be referred to as the"boost effect." The above described application is generally in accordance with prior art methods employed in single cylinder dual pumps or serially connected vacuum pumps.

In accordance with the present invention, however, there is further provided means by which the"boost efHect"is obtained without connecting two pumps in series.

Accordingly, and as illustrated in Figs. lAw), a pressure equalizing notch 66 is provided in the interior face 68 of cylinder 26, at a forward portion thereof Pressure equalizing notch 66 is configured so as to allow communication between front and rear working chambers 70 and 72, generally at or close to the extremity of the forward travel of wobble piston 20. It will be noted that working chambers 70 and 72 are also referred to herein as upstream and downstream volumes, respectively.

It will be understood that, during most of the duration of the travel of piston 20, front and rear chambers 70 and 72 are sealed from each other, according to a preferred embodiment of the invention, by virtue of a circumferential seal 74, provided on piston 20. Preferably, this is a suitable type of ring, as illustrated, although it may alternatively be a suitable sealing fluid. When seal 74 slides over notch 66, however, as seen in Fig. IB, due to the increased radius of cylinder 26

threat, and the consequent lack of contact between seal 74 and the interior face 68 of cylinder 26 thereat, working chambers 70 and 72 are no longer sealed; rather, they become interconnected.

It will be appreciated that, while communication between the front and rear chambers 70 and 72 is provided preferably by means of notch 66, any other suitable means for this purpose may also be employed.

Accordingly, during forward travel of wobble piston 20 (leftward, as represented in the drawing) a vacuum is created in rear chamber 72, which peaks when the piston 20 and seal 74 reach a location just prior to notch 66. As seal 74 is brought into alignment with notch 66 (Figs. lB and 1C), chambers 70 and 72 become connected to each other via notch 66. This connection permits equalization of the pressure in chambers 70 and 72, such that they generally share this peak vacuum level. Subsequently, when piston 20 starts its rearward travel (rightward, as

represented in the drawing), and seal 74 passes over notch 66, thereby once again sealing between chambers 70 and 72, the vacuum in forward chamber 70 rises to a level which is substantially higher than the peak vacuum level previously attained in rearward chamber 72. In short, starting the piston stroke at a high vacuum allows it to reach a substantially higher vacuum, so as to achieve the above-described"boost effect', but without having to connect two pumps or two pump stages, in series.

In the present invention, unlike the case of either serially connected pumps or pumps connected in parallel, the connection between the two chambers is unchanged.

Rather, the nature of this connection-via the notch 66-is such that the parallel effect is dominant when the vacuum level is low, and such that the series effect is dominant when the vacuum level is high, providing the desired function of both types of pump connections, without their inherent shortcomings.

The equalization of pressures and vacuum levels between chambers 70 and 72, at the time they are connected through notch 66, is particularly facilitated-although

this is not essential-by the long dwell of the wobble piston at its Top Dead Center, as

described and claimed in US Patent No. 5, 762, 480, the contents of which are incorporated herein, by reference.

Referring now briefly to Fig. 3, there is seen a dual pump which has a double-acting linear reciprocating piston 20", and in which the cylinder and cylinder housing, while similar to those of Fig. 1, have been modified so as to impart the boost effect to the illustrated pump. It will be appreciated that, while the illustrated pump does not employ a wobble piston, there remains a significant advantage in the present embodiment, in which a single cylinder provides for a dual pump opening both in series and in parallel.

It will be noted that, in the present embodiment, in which piston 20"is not a wobble piston, there may be provided any suitable sealing means, referenced generally 154, between the illustrated piston rod 30"and the housing 28".

It will be appreciated by persons skilled in the art, that the scope of the present invention is not limited by what had been shown and described hereinabove, merely by way of example. Rather, the scope of the invention is limited solely by the claims, which follow.

Claims

CLAIMS 1. A double action pump which includes ; a) a cylinder having a first, rear end and a second, forward end; b) a housing for said cylinder, which has : a gas intake manifold associated with said first and second ends of said cylinder ; and a gas outlet manifold associated with said first and second ends of said cylinder, wherein said gas intake manifold of said housing includes' i. a gas inlet port for intake of a working gas into said gas intake manifold, and ii. first and second valved cylinder inlet ports arranged in communicative association with said first and second ends of said cylinder, respectively, for permitting an inflow of the working gas into said cylinder, from said gas intake manifold, and wherein said gas outlet manifold of said housing includes: iii. first and second valved cylinder outlet ports arrange din communicative association with said first and second ends of said cylinder, respectively, for permitting an outflow of the working gas from said cylinder into said gas outlet manifold of said housing, and iv. a gas outlet port for permitting outflow of working gas from said gas outlet manifold of said housing ; c) a piston ananged for reciprocating motion within said cylinder, between said first and second ends thereof ; d) a piston rod attached to said piston and extending externally of said cylinder and of said housing ; d) a drive assembly for imparting a reciprocating motion to said piston, so as to cause alternate forward and rearward working strokes of said piston between said first and second ends of said cylinder, and e) a sealing arrangement provided between said piston rod and said cylinder housing, thereby to effectively prevent passage of the working gas between said piston rod and said cylinder housing, such that each forward stroke and each rearward stroke of said piston causes a pressure drop on an upstream side thereof and a pressure increase on a downstream side thereof thereby also to cause intake of the working gas into said cylinder via a predetermined single one of said valved inlet ports, and outflow of the working gas from said cylinder via a predetermined single one of said valved outlet ports, thereby to provide a double action ; wherein said piston is operative to divide said cylinder into mutually sealed, upstream and downstream working chambers, the respective magnitudes of which vary in relation to the stroke position of said piston, so as to impart to said pump a double action, parallel characteristic, and wherein the improvement is characterized by: pressure equalization means operative, when said piston is located in the vicinity of the extremity of its travel within said cylinder, to permit gas communication between said upstream and downstream volumes, thereby to cause a reduction in a pressure gradient therebetween, such that, when the pressure differential across said piston is at least a predetermined magnitude, a double action, series characteristic is imparted to said pump, so as to boost the performance thereof. 2. A double action pump according to claim 1, and wherein said piston is a wobble piston, such as to undergo both axial and angular translation ; and said sealing arrangement includes a floating seal having formed therein an opening through which said piston rod extends, said floating seal including a first portion operative to seal against said piston rod, thereby to substantially prevent escape of working gas therepast, and : further having a. second, flexible portion operative to undergo angular translation together with said piston rod, and so as to support said first portion in sealing relation with said piston rod. 3. A double action pump according to claim 2, wherein said sealing arrangement has a generally elongate, nipple configuration, extending along a portion of said piston rod. 4. A double action pump which includes : a) a cylinder having a first, rear end and a second, forward end; b) a housing for said cylinder, which has : I iL, a gas intake manifold associated with said first and second ends of said cylinder ; and a gas outlet manifold associated with said first and second ends of said cylinder, wherein said gas intake manifold of said housing includes: i. a gas inlet port for intake of a working gas into said gas intake manifold, and ii. first and second valved cylinder inlet ports arranged in communicative association with said first and second ends of said cylinder, respectively, for permitting an inflow of the working gas into said cylinder, from said gas intake manifold of said housing, and wherein said gas outlet manifold of said housing includes : iii. first and second valved cylinder outlet ports arranged in communicative association with said first and second ends of said cylinder, respectively, for permittig an outflow of the working gas from said cylinder into said gas outlet manifold of said housing, and iv. a gas outlet port for pernnttmg outQow of working gas from said gas outlet manifold of said housing; c) a piston arranged for reciprocating motion within said cylinder, between said first and second ends thereof d) a piston rod attached to said piston and extending externally of said cylinder and of said housing ; e) a drive assembly for imparting a reciprocating motion to said piston, so as to cause alternate forward and rearward working strokes of said piston between said first and second ends of said cylinder ; and f) a sealing arrangement provided between said piston rod and said cylinder housing, thereby to effectively prevent passage of the working gas between said piston rod and said cylinder housing, such that each forward stroke and each rearward stroke of said piston causes a pressure drop on an upstream side thereof and a pressure increase on a downstream side thereof thereby also to cause intake of the working gas into said cylinder via a predetermined single one of said valved inlet ports, and outflow of the working gas from said cylinder via a predetermined single one of said valved outlet ports, thereby to provide a double action ; wherein the improvement is characterized by: said piston being a wobble piston, such as to undergo both axial and angular translation ; and said sealing arrangement including a floating seal having formed therein an opening through which said piston rod extends, said floating seal including a first portion operative to seal against said piston rod, thereby to substantially prevent escape of working gas therepast, and further having a second, flexible portion operative to undergo angular translation together with said piston rod, and so as to support said first portion in sealing relation with said piston rod. 5. A double action pump according to claim 4, wherein said sealing arrangement has a generally elongate, nipple configuration, extending along a portion of said piston rod. 6. A double action pump according to claim 5, and wherein said cylinder is divided into mutually sealed, upstream and downstream volumes, the respective magnitudes of which vary in relation to the stroke position of said piston, so as to impart to said pump a double action, parallel characteristic, and wherein said pump also includes: pressure equalization means operative, when said piston is located in the vicinity of the extremity of its travel within said cylinder, to permit gas communication between said upstream and downstream volumes, thereby to cause a reduction in a pressure gradient therebetween, such that, when the pressure differential across said pump is at at least a predetermined magnitude, a double action, series characteristic is imparted to said pump, so as to boost the performance thereof 7. A double action pump according to any of claims 1-6, and substantially as shown and described hereinabove, in conjunction with any of Figs. 1A-3. 8. A double action pump according to any of claims 1-6, and substantially as shown in any of Figs. 1A-3. Amendments to the claims have been filed as follows CLAIMS

1. A double action pump which includes: a) a cylinder having a first, rear end and a second, forward end; b) a housing for said cylinder, which has: a gas intake manifold associated with said first and second ends of said cylinder; and a gas outlet manifold associated with said first and second ends of said cylinder, wherein said gas intake manifold of said housing includes: i. a gas inlet port for intake of a working gas into said gas intake manifold, and ii. first and second valved cylinder inlet ports arranged in communicative association with said first and second ends of said cylinder, respectively, for permitting an inflow of the working gas into said cylinder, from said gas intake manifold, and wherein said gas outlet manifold of said housing includes: iii. first and second valved cylinder outlet ports arranged in communicative association with said first and second ends of said cylinder, respectively, for permitting an outflow of the working gas from said cylinder into said gas outlet manifold of said housing, and iv. a gas outlet port for permitting outflow of working gas from said gas outlet manifold of said housing; c) a piston arranged for reciprocating motion within said cylinder, between said first and second ends thereof; d) a piston rod attached to said piston and extending externally of said cylinder and of said housing; e) a drive assembly for imparting a reciprocating motion to said piston, so as to cause alternate forward and rearward working strokes of said piston between said first and second ends of said cylinder; and

f) a sealing arrangement provided between said piston rod and said cylinder housing thereby to effectively prevent passage of the working gas between said piston rod and said cylinder housing, such that each forward stroke and each rearward stroke of said piston causes a pressure drop on one side thereof and a pressure increase on the other side thereof, thereby also to cause intake of the working gas into said cylinder via a predetermined single one of said valved inlet ports, and outflow of the working gas from said cylinder via a predetermined single one of said valved outlet ports, thereby to provide a double action; wherein said piston is operative to divide said cylinder into mutually sealed working chambers, the respective volumes of which vary in relation to the stroke position of said piston, so as to impart to said pump a double action, parallel characteristic, and wherein the improvement is characterised by: pressure equalisation means operative in dependence upon the position of the piston to permit gas communication between said working chambers thereby to cause a reduction in a pressure gradient across said piston only when said piston is located in the vicinity of the extremity of its travel within said cylinder, such that, when the pressure differential across said piston is at least a predetermined magnitude, a series characteristic is imparted to said pump, so as to boost the performance thereof.

2. A double action pump according to claim 1, and wherein said piston is a wobble piston, such as to undergo both axial and angular translation ; and said sealing arrangement includes a floating seal having formed therein an opening through which said piston rod extends, said floating seal including a first portion operative to seal against said piston rod, thereby to substantially prevent escape of working gas therepast, and further having a second ;,. flexible portion operative to undergo angular translation together with said piston rod, and so as to support said first portion in sealing relation with said piston rod.

3. A double action pump according to claim 2, wherein said sealing arrangement has a generally elongate, nipple configuration, extending along a portion of said piston rod.

4. A double action pump according to any of claims 1-3, wherein said pressure equalisation means is formed integrally with said cylinder.

5. A double action pump according to any of claims 1-4, wherein said pressure equalisation means is formed adjacent to a predetermined end of the stroke of said piston.

6. A double action pump according to any of claims 1-6, wherein said pressure equalisation means comprises a notch formed in an inward-facing surface of said cylinder adjacent to a predetermined end thereof, so as to facilitate gas communication between said working chambers when said piston is positioned in registration with said notch.

7. A double action pump substantially as described herein with reference to the accompanying drawings.