WO2009127959A1

WO2009127959A1 - Bi-stable pilot valve

Info

Publication number: WO2009127959A1
Application number: PCT/IB2009/005273
Authority: WO
Inventors: Rudolph Buhrmann; Rudolph Teodor Buhrmann
Original assignee: Iptree Trust (Trust Number 503/2009)
Priority date: 2008-04-18
Filing date: 2009-04-16
Publication date: 2009-10-22
Also published as: EP2283260A1; ZA201007080B

Abstract

A valve, in particular a pilot valve includes a valve body having a cylindrical openended passageway therein and having annular valve seats at each open end of the passageway and a cylindrical valve closing means slidingly received within the passageway of the valve body and having a pair of spaced circumferentially arranged elastomeric seals, the valve closing means being operable selectively to close one or the other of the open ends of passageway of the valve body by seating one of the elastomeric seals on a respective one of the valve seats of the valve body.

Description

Bl-STABLE PILOT VALVE

FIELD OF THE INVENTION

This invention relates to a bi-stable device in the form of a valve and to a valve seat. More particularly, the invention relates to a valve known as a pilot valve for use in a pneumatic device, such as, for example, a pneumatically operated pump.

BACKGROUND TO THE INVENTION

Pilot valves are generally balanced control valves to control the supply of a fluid to a larger system. In the field of pneumatics, such valves must often operate under conditions of low pressures. To operate efficiently, a pilot valve should have a positive operation, opening or closing fully and quickly in appropriate conditions. A problem with some pilot valves is a tendency to flutter rather than opening or closing completely. It would therefore be an advantage to provide a pilot or control valve able to operate as a pneumatic valve and to operate in a positive manner without flutter. OBJECT OF THE INVENTION

It is an object of the invention to provide a pilot or control valve and a valve closing means comprising a seal and a seat, which will have the advantages and, at least partially, address the problems set out above.

SUMMARY OF THE INVENTION

According to the invention there is provided a valve including a valve body having a cylindrical open-ended passageway therein and having annular valve seats at each open end of the passageway; and a cylindrical valve closing means slidingly received within the passageway of the valve body and having a pair of spaced circumferentially arranged elastomeric seals, the valve closing means being operable selectively to close one or the other of the open ends of passageway of the valve body by seating one of the elastomeric seals on a respective one of the valve seats of the valve body.

The cylindrical valve closing means may have a pair of spaced circumferential annular recesses defined in its side wall, an elastomeric seal being received in each such recess. Preferably, the elastomeric seals are toroidal in shape.

In a preferred embodiment of the invention, at least one of the annular valve seats comprises a plurality of annular regions, a first of which being spaced inwardly of an outer edge of the mouth of the cavity and each following region being progressively closer the edge of the mouth and having a greater diameter than the preceding region; and a respective one of the valve seals is operable to seat against the innermost annular region to close the valve and, upon opening of the valve, to unseat from the innermost annular region and to progressively seal against, and unseat from, each following annular region in order that an annular region of contact between the seal and the seat moves progressively axially outward with respect to the passageway and has a progressively increased diameter.

Then, the said at least one valve seat may comprise three coaxial regions, being a first annular region in the form of a conic section angled acutely with respect to the central cone axis defined by the conic section; a second annular region in the form of a conic section of greater diameter than that of the first region and angled more acutely with respect to the cone axis than the first region; and a third annular region in the form of a conic section of greater diameter than that of the second region and angled more acutely with respect to the cone axis than the second region.

Instead, the said at least one valve seat may comprise three coaxial regions, being a first annular region normal to the common central axis of the three regions of the valve seat; a second annular region in the form of a conic section of greater diameter than that of the first region and angled acutely with respect to the common central axis of the three regions; and a third cylindrical region of greater diameter than that of the second region having a cylinder axis collinear with the common central axis of the three regions.

Then, the third region may have defined therein a series of circumferentially spaced axially orientated recesses across the width of the said annular region to provide a fluid flow path between the valve seal and the third annular region of the valve seat when the seal is in contact with this region.

In a preferred embodiment of the invention, the valve closing means and the valve body are shaped and configured so that an end of the cylindrical valve closing means is able to be inserted at one end of the passageway of the valve body and to traverse entirely through the passageway to emerge from the other end of the passageway, the annular elastomeric seal proximate the said end of the cylindrical closing means being fitted to the closing means after its location within the passageway, to facilitate assembly of the valve.

In a further embodiment of the invention, the cylindrical open-ended passageway opens at each end into a respective one of first and second bores; the valve closing means is operable to be selectively axially displaced between a first position in which it closes a first open end of passageway of the valve body and a second position in which it closes a second of the open ends of the passageway, the valve closing means further including a piston attached to an end thereof proximate the second end of the passageway and being sealingly received within the second bore to divide the said second bore into a first chamber on a side of the piston proximate the passageway and a second chamber on a distal side of the piston; a supply inlet for air at greater than atmospheric pressure leads into the first bore; a communication passageway is defined in the cylindrical valve closing means extending generally axially from the face of the piston through the valve closing means and opening into a side wall of the cylindrical valve closing means intermediate the elastomeric seals; an outlet passageway is defined in the valve body exiting the annular passageway intermediate the opening of the communication passageway in the side wall of the valve closing means and the second end of the cylindrical open- ended passageway, and the dimensions of the communication passageway are selected so that when the cylindrical element is intermediate its first and second positions air flow from the supply inlet is greater through the passageways leading to the second chamber than through the passageways leading to the first chamber, thereby urging the piston towards the second end of the cylindrical passageway and closing the second end thereof.

The communication passageway may extend axially through the valve closing means and intersect a further passageway defined in the valve closing means and at right angles to the communication passageway, the further passageway opening into a side wall of the cylindrical valve closing means intermediate the elastomeric seals and the dimensions of the communication passageway and the further passageway being selected so that when the cylindrical element is intermediate its first and second positions air flow from the supply inlet is greater through the passageways leading to the second chamber than through the passageways leading to the first chamber, thereby urging the piston towards the second end of the cylindrical passageway and closing the second end thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying diagrammatic drawing, in which:

Figure 1 shows a sectional side view of a pneumatic pump, incorporating a pilot valve in accordance with an aspect of the invention;

Figure 2 shows a further enlarged sectional side view of the pilot valve right hand seat and seat in a first condition;

Figure 3 shows a sectional side view of the pilot valve right hand seat and seat in a second condition;

Figure 4 shows a sectional side view of the pilot valve right hand seat and seal in a third condition; Figure 5 shows a partly sectional end view of a portion of the seat and seal of Figure 4; and

Figure 6 shows an enlarged sectional side view of the pilot valve of Figure 1.

DETAILED DESCRIPTION OF THE DRAWINGS

In the drawings, reference numeral 24 generally refers to a pilot valve in accordance with the invention. It must be stressed that although the pilot valve 24 is, for explanatory purposes, described with reference to its operation in an exemplary pneumatic pump 100. The invention is not restricted to the use in the device of the drawings and may have application in a wide variety of devices and environments.

The pump 100 is a pneumatically activated pump for the pumping of a fluid. The pump 100 comprises a reciprocating piston 12 sealingly received to reciprocate in a cylindrical bore 14 having an inlet 16 and outlet 18, each controlled by one-way valves 20 and 22, respectively. The inlet 16 is connected to a fluid reservoir (not shown) containing the fluid to be pumped and the outlet 18 may be connected conveniently to a conduit (no shown) leading to any device or reservoir to which the fluid is intended to be pumped.

A control system 200 of the pump 100 is connected to the pumping piston 12 and bore 14. The control system 200 comprises three devices: namely, the remotely activated pneumatic trigger mechanism 10; a pilot valve 24; and an operating piston 26 which is sealingly received in its own cylindrical bore 28 and connected by means of a shaft 30 to the pumping piston 12 of the pump 100. For ease of description, the terms left and right will be used in order to describe features of the components of the pump 100, as they are depicted in Figure 1. Obviously, it is to be understood that these terms have reference to the device only as it is depicted in the drawing.

The pneumatic trigger mechanism 10 comprises a first bore 32 defined in a casing 34 of the pump 100, the bore 32 having a first cylindrical chamber 36 and a second cylindrical chamber 38, comprising a section of the bore 32 of greater diameter than that of the first chamber 36. The second chamber 38 is in communication with a second bore 40 defined in the casing 34 of the pump 100 via a cylindrical passageway 42, a throat 44 of which opens into the second bore 40 and widens outwardly into the second bore 40 to provide an annular valve seat 46, which is angled with respect to the common cylinder axis of the chambers 36,38. An inlet passageway 48 leads into the second chamber 38 from a pneumatic supply 50 and an outlet passageway 52 leads from the second chamber 38 to a hose 54 terminating in an open/closed actuator valve 56, which is manually operated by an operator of the pump 100 to actuate the pump 100. A further passageway 58 leads from the first chamber 36, via an inhibit valve 60, to a vent 62 to atmosphere. A piston 64 is received in the first bore 32, a piston rod 66 extending through the passageway 42 between the first bore 32 and the second bore 40 and being connected at a first end thereof to the piston 64, which is received within the first chamber 36, its proximal end 67 being aligned with an annular shoulder 68 defined between the first and second chambers 36,38. A disc diaphragm 70 overlies the proximal end 67 of the piston 64 and seals against the shoulder 68. A helical spring 72 extends between the diaphragm 70 and an opposed end 74 of the first chamber 36, holding the disc diaphragm 70 against the proximal end 67 of the piston 64. It will be appreciated that the disc diaphragm 70 thereby forms a one-way valve, allowing air at higher pressure in the second chamber 38 to leak though a passageway 76 comprising an annular space between a side wall 78 of the piston 64 and a side wall 80 of the second chamber 38 into the first chamber 36, but preventing air at higher pressure in the first chamber 36 from escaping via the same route into the second chamber 38. A shoulder 82 is defined between a distal end 84 of a piston 64 opposed to the proximal end 67 thereof and the piston rod 66 and a biasing means, in the form of a second helical spring 86, extends between that shoulder 82 and an opposed end 88 of the second chamber 38, biasing the piston 64 to its right and towards the opposed end 74 of the first chamber 36. It will be appreciated then that the piston 64 and its associated disc diaphragm 70 thereby define a second passageway 76 between the first and second chambers 36,38 and provide one-way pneumatic communication between the chambers 36,38. Proximate the free end 90 of the piston rod 66 an annular circumferential recess 92 is defined in the rod 66 and an elastomeric sealing O-ring 94 is received in the recess 92. The O-ring 94 seals against the annular seat 46 of the second bore 40 when the piston 64 and piston rod 66 of the trigger mechanism 10 moves to its right. It will be appreciated that the disc diaphragm 70 may be replaced by any other suitable form of seal, for example, an O-ring seal between the side wall 78 of the piston 64 and the side wall 80 of the second chamber 38, in which case a small axial passageway (not shown) will be defined through the piston 64 and will be controlled by a one-way valve (not shown).

The pilot valve 24 of the control system 200 comprises the second bore 40 and a third bore 102 defined in the casing 34 of the pump 100, the second and third bores 40,102 communicating by means of a passageway 104. The passageway 104 opens into the third bore 102 by way of a throat 106 comprising an outwardly widening end of the passageway 104 to provide an annular seat 108, angled with respect to the common cylinder axis of the bores 40,102 of the control system 200. The opposed end of the passageway 104 opens into the second bore 40 by way of a throat 110 comprising an outwardly widening annular portion of the passageway 104 to provide a shoulder 112, an intermediate angled outwardly widening portion 114 of the passageway 104, and an end portion 116 of the passageway 104 of increased diameter. The throat 110 of this end of the passageway 104 provides a complex three-position seat 118 for the pilot valve 24, which will be described in more detail below. A piston 120 is sealingly received for reciprocation within the second bore 40 and has an annular diaphragm 122 received within a circumferential recess 124 in its side wall 126. An outer edge 128 of the annular diaphragm 122 is received in a recess 130 in a side wall 132 of the second bore 40. A cylindrical piston rod 134 is connected to an end 136 of the piston 120 and is slidingly received within the passageway 104 communicating between the second and third bores 40,102. Two spaced circumferential recesses 138 are defined in the piston rod 134, and elastomeric O-rings 140 are received within these recesses 138, a left hand O-ring 140 communicating with the seat 108 of the third bore 102 and a right hand O-ring 140 proximate the piston 120 communicating with the seat 118 of the second bore 40. The piston 120 divides the second bore 40 into third and fourth chambers 142 and 144, respectively. The third chamber 144 is vented to atmosphere via a passageway 146. A communication passageway 148 is defined in the piston 120 extending axially from the face 150 of the piston 120 through the piston rod 134 and terminating in a further passageway 152 at right angles to the communication passageway 148 and opening into a side wall 154 of the piston rod 134 intermediate the spaced recesses 138 on the piston rod 134. The third bore 102 is in communication via a passageway 156 with the pneumatic supply inlet 50 to the pump 100. An axial shaft 158 extends from an end 160 of the piston rod 134 remote from the distal face 162 of the piston 120 through a passageway 164 defined in the casing 34 of the control system 200 into a fourth bore 166, the shaft 158 terminating at its free end in an end portion 168 of increased diameter.

A piston 170 is sealingly received within a wider portion 172 of the fourth bore 166 for reciprocating movement within the fourth bore 166, the piston 170 dividing the fourth bore 166 into fifth and sixth chambers 174 and 176 respectively. An O-ring 178 received in a circumferential recess 180 defined in the piston wall 182 provides a seal between the piston 170 and the wider portion 172 of the fourth bore 166. A free end 184 of a piston rod 186 of the piston 170 has a cavity 188 into which the wider end portion 168 of the shaft 158 is received, the cavity 188 terminating in a mouth 190 of reduced diameter so that wider end portion 168 of the shaft 158 is slidingly received within the cavity 188, but is held captive therein. The fifth chamber 174 is in communication via a passageway 192 with the pneumatic supply 50 and is in communication via the inhibit valve 60 with both the vent 62 to atmosphere and the passageway 58 from the first chamber of the trigger mechanism 10. The inhibit valve 60 comprises a rod 194, an end of which protrudes into the fifth chamber 174 and is impacted on by a rear face 196 of the piston 170 opposed to its operating face 198 when the piston 170 is at its rightmost position of travel. The sixth chamber 176 is in communication with the passageway 104 joining the second and third bores 38,102 via a passageway 202.

The operation of the pump 100 will now be described in order to elucidate the operation of the components thereof, in particular the pilot valve 24.

In its waiting state, the operative piston 170 is at its rightmost extreme position (indicated by broken lines in Figure 1) and abuts the projecting rod 194 of the inhibit valve 60, closing the inhibit valve 60. The pilot valve piston 120 is at its leftmost extreme seating against the right seat 118 (as shown in Figure 1). The trigger mechanism piston 64 is at its rightmost extreme position (indicated by broken lines in Figure 1) so that it seats against the seat 46. All of the chambers in the system are at supply pressure with the exception of those chambers shown as being vented to atmosphere.

The sequence on triggering the pump 100 by opening the actuating valve 56 is as follows: The pressure in the second chamber 38 drops. The pressure in the first chamber 36 remains at the higher supply pressure and the piston 64 is pushed to its left, opening the passageway 42. This has the effect of venting the second bore 40 through the actuating valve 56. The supply passageway 48 has a bleed orifice 49 that causes a restriction in the supply line 48, thereby ensuring that the supply pressure through the supply line 48 via the bleed orifice 49 is not sufficiently high to increase the pressure in the second cavity 38 and to offset the decrease in pressure occasioned by the opening of the actuating valve 56. The supply pressure in the third bore 102 therefore acts on the piston rod 134 pushing it to the right and seating the left O-ring 140 on its seat 108 and freeing the right O-ring 140 from its seat 118. As soon as this takes place, the third chamber 142 can vent through the passageway 148/152 in the piston 120 via the open right hand seal 140/118 and out of the vent 146 to atmosphere. As a result of this venting action the pilot valve 24 is maintained in its right hand position regardless of the position of the trigger mechanism 10 thereafter. The sixth chamber 176 also vents via the passageway 202 through the right seal 140/118 of the piston 120 and out via the vent 146. The fifth chamber 174 on the right hand side of the operating piston 170 is at supply pressure. Since the sixth chamber 176 has been vented, the piston 170 begins to move to the left. The effective areas on the rear and front ends of the piston 170 are calculated to allow it to move under supply pressure in this manner. As soon as the piston 170 starts to move to the left the inhibit valve 60 is opened, thereby venting the first chamber 36 of the trigger mechanism 10 via the vent 62. This action moves the piston 64 to the right seating the O-ring 94 and closing the trigger mechanism 10. When the piston 170 reaches its leftmost extreme position of travel, it engages with the wider portion 168 of the shaft 158 of piston 170 thereby drawing the piston 120 and piston rod 134 to the left and opening the pilot valve 24 at 140/108. The left hand seating arrangement 140/108 on the pilot valve 24 opens and the right hand seating arrangement 140/118 closes, as was the case at the start-up position. The supply air flow from supply 50 is then allowed to flow past the piston rod 134 through the passageway 148 charging up the third chamber 142 to supply pressure. This ensures that the pilot valve piston 120 remains in its leftmost position as at the waiting stage. This also allows the charging-up of chamber 176 to supply pressure via the passageway 202. The piston 170 then moves to the right effectively reaching its rightmost position and closing the inhibit valve 60. We turn now to a more detailed consideration of the construction of the valve seat 118 and its operation in association with the O-ring 140 of the piston rod 134. As shown in Figures 2 to 5, the valve seat 118 comprises three annular regions: a first region 118A normal to the cylinder axis of the cylindrical passageway 104; a second region 118B angled with respect to the cylinder axis of the passageway 104 and defining a conic section, the diameter of this angular section being greater than that of the first region 118A; and a third region 118C parallel to the cylinder axis of the passageway 104 and forming a short cylinder having its axis collinear with that of the passageway 104. It will be appreciated that region 118A could also be angled with respect to the cylinder axis, but that its angle should not be more acute with respect to the axis than that of region 118B. It will be appreciated that region 118C could also be angled with respect to the cylinder axis, but that its angle should be less acute with respect to the axis than that of region 118B. Defined in the region 118C are a series of circumferentially spaced axially oriented recesses 210. As shown in Figure 1 , the piston rod 134 is in its left-most position and the O-ring 140 is seated on the seat 118, closing off the proximal end of the passageway 104. In this position, the O-ring is seating at regions 118A and 118B. As conditions in the pump 100 are appropriate for the pilot valve 24 to change its position, the piston rod 104 begins to move to the right, as described above, and air under pressure from the pneumatic supply 50 passes the left O-ring and moves along an annular passageway 212 defined between the side wall 154 of the piston rod 134 and the wall 214 of the passageway 104. At this point the O-ring 140 begins to move to the right and seats on the second region 118B. It will be appreciated that this immediately has the effect of increasing the area against which air under pressure from the pneumatic supply 50 acts and therefore increases the opening force on the right valve combination 140/118. The effect is therefore to pop open the pilot valve 24 in a positive manner. However, it has been found that the force of the airstream may distort the O-ring 140, causing it to lift out of its recess 138 and to seat against there upper the third region 118C or, in other configurations of similar pilot valves, against the valve seat of that valve. For this reason, the recesses 210 are defined in the third region 118C, providing a series of passageways that cannot be blocked by the O-ring 140 even if it is distorted or pushed out of position. In practice, the materials and the configurations of the O- ring 140, casing 34 and piston rod 134 are selected so that the frictional forces between the O-ring and the seat 118 and between the O-ring 140 and the recess 138 are such as to that as the valve combination 140/118 closes the O-ring 140 moves down the ramp comprising the second region 118B of the valve seat 118.

It has been found that in operation of the pilot valve 24 certain of the dimensions of the orifices and passageways in and around the valve 24 are vital to the stability of the operation of the valve 24 itself and prevent fluttering of the valve 24 while in operation. Starting with the position where the pilot valve 24 is in its rightmost orientation, the left hand O-ring 140 of the valve 24 is seated. The piston 170 on moving to the left begins to crack the valve and opens the left hand seal slightly. If the conditions are such that air from the pneumatic supply 50 entering the third bore 102 is immediately diverted via the passageway 202 before sufficient pressure builds up via the passageway 148 into the third chamber 142, then the valve 24 is likely to be unstable since there is insufficient pressure in the third chamber 142 to maintain the valve 24 in its leftmost closed position with the right O-ring 140 seated on the seat 118. However, in the present configuration of the pilot valve 24 of the invention, the opening to the passageway 152 is to the left of, ie upstream of, the opening of the passageway 202, which allows passageways 152 and 148 to be pressurized before passageway 202. Further, the aperture size of the annular passageway 212 defined between the wall 154 of the piston rod 134 and the wall 214 of the passageway 104 is selected such that the pressure drop in the passageway 212 between the openings of the passageways 152 and 202 is substantially greater than the pressure drop between the opening of passageway 202 and the fourth chamber 144. This effectively creates a choke ensuring that the pressure in the passageway 148 rises more rapidly than the pressure in the passageway 202. Preferably the pressure drop between the third bore 102 and the opening to 152 should be as low as possible, also to ensure that the charge up rate of the third chamber 142 is as rapid as possible.

A further advantage of the design of the pilot valve 24 is that the piston rod 134 and piston 120 can be constructed as a single component as a result of the use of the elastomeric O-rings 140 as seals. The piston rod 134 can be slid through the cylindrical passageway 104 of the valve body one end and the O-rings can be seated on the piston rod 134 once it is in place. This is not possible using other seating methods where a valve seal seats against a valve body.

Claims

1. A valve including a valve body having a cylindrical open-ended passageway therein and having annular valve seats at each open end of the passageway; and a cylindrical valve closing means slidingly received within the passageway of the valve body and having a pair of spaced circumferentially arranged elastomeric seals, the valve closing means being operable selectively to close one or the other of the open ends of passageway of the valve body by seating one of the elastomeric seals on a respective one of the valve seats of the valve body.

2. The valve as claimed in claim 1 in which the cylindrical valve closing means has a pair of spaced circumferential annular recesses defined in its side wall, an elastomeric seal being received in each such recess.

3. The valve as claimed in claim 1 or claim 2 in which the elastomeric seals are toroidal in shape.

4. The valve as claimed in any one of the preceding claims in which at least one of the annular valve seats comprises a plurality of annular regions, a first of which being spaced inwardly of an outer edge of the mouth of the cavity and each following region being progressively closer the edge of the mouth and having a greater diameter than the preceding region; and a respective one of the valve seals is operable to seat against the innermost annular region to close the valve and, upon opening of the valve, to unseat from the innermost annular region and to progressively seal against, and unseat from, each following annular region in order that an annular region of contact between the seal and the seat moves progressively axially outward with respect to the passageway and has a progressively increased diameter.

5. The valve as claimed in claim 4 in which the said at least one valve seat comprises three coaxial regions, being a first annular region in the form of a conic section angled acutely with respect to the central cone axis defined by the conic section; a second annular region in the form of a conic section of greater diameter than that of the first region and angled more acutely with respect to the cone axis than the first region; and a third annular region in the form of a conic section of greater diameter than that of the second region and angled more acutely with respect to the cone axis than the second region.

6. The valve as claimed in claim 4 in which the said at least one valve seat comprises three coaxial regions, being a first annular region normal to the common central axis of the three regions of the valve seat; a second annular region in the form of a conic section of greater diameter than that of the first region and angled acutely with respect to the common central axis of the three regions; and a third cylindrical region of greater diameter than that of the second region having a cylinder axis collinear with the common central axis of the three regions.

7. The valve as claimed in claim 6 in which the third region has defined therein a series of circumferentially spaced axially orientated recesses across the width of the said annular region to provide a fluid flow path between the valve seal and the third annular region of the valve seat when the seal is in contact with this region.

8. The valve as claimed in any one of the preceding claims in which the valve closing means and the valve body are shaped and configured so that an end of the cylindrical valve closing means is able to be inserted at one end of the passageway of the valve body and to traverse entirely through the passageway to emerge from the other end of the passageway, the annular elastomeric seal proximate the said end of the cylindrical closing means being fitted to the closing means after its location within the passageway, to facilitate assembly of the valve.

9. The valve as claimed in any one of the preceding claims in which the cylindrical open-ended passageway opens at each end into a respective one of first and second bores; the valve closing means is operable to be selectively axially displaced between a first position in which it closes a first open end of passageway of the valve body and a second position in which it closes a second of the open ends of the passageway, the valve closing means further including a piston attached to an end thereof proximate the second end of the passageway and being sealingly received within the second bore to divide the said second bore into a first chamber on a side of the piston proximate the passageway and a second chamber on a distal side of the piston, wherein a supply inlet for air at greater than atmospheric pressure leads into the first bore; a communication passageway is defined in the cylindrical valve closing means extending generally axially from the face of the piston through the valve closing means and opening into a side wall of the cylindrical valve closing means intermediate the elastomeric seals; an outlet passageway is defined in the valve body exiting the annular passageway intermediate the opening of the communication passageway in the side wall of the valve closing means and the second end of the cylindrical open-ended passageway, and wherein the dimensions of the communication passageway are selected so that when the cylindrical element is intermediate its first and second positions air flow from the supply inlet is greater through the passageways leading to the second chamber than through the passageways leading to the first chamber, thereby urging the piston towards the second end of the cylindrical passageway and closing the second end thereof.

10. The valve as claimed in claim 9 in which the communication passageway extends axially through the valve closing means and intersects a further passageway defined in the valve closing means and at right angles to the communication passageway, the further passageway opening into a side wall of the cylindrical valve closing means intermediate the elastomeric seals and the dimensions of the communication passageway and the further passageway being selected so that when the cylindrical element is intermediate its first and second positions air flow from the supply inlet is greater through the passageways leading to the second chamber than through the passageways leading to the first chamber, thereby urging the piston towards the second end of the cylindrical passageway and closing the second end thereof.