WO2001078934A1

WO2001078934A1 - Control valves and systems for pneumatic cylinders

Info

Publication number: WO2001078934A1
Application number: PCT/GB2001/001700
Authority: WO
Inventors: Peter Helliker; Allan Ward; Olivier Tiberghien
Original assignee: Savair & Aro Limited
Priority date: 2000-04-13
Filing date: 2001-04-12
Publication date: 2001-10-25
Also published as: EP1322443A1; AU9338001A

Abstract

A selector valve for use with a resistance-welding pneumatic cylinder, the selector valve comprising a valve body (76), a valve member (74) moveable in the valve body, a first port (68) connected to a second port (70) by a first flow path defined by the valve member when said member is in a first position and connected by a second flow path defined by the valve member to a third port (72) when the valve member is in a second position, means (80, 86, 90) biasing the valve member towards the second position, the presence of pressure in the first flow path causing the valve member to move to the first position.

Description

Control valves and systems for pneumatic cylinders

This invention relates to control valves and systems for pneumatic cylinders, particularly but not exclusively for resistance welding apparatus usually known as "guns". Such guns are used in large numbers in motor vehicle manufacturing, and in the manufacture of white goods (washing machines etc.). Because of the high production volume it is important that the cyclic welding time is minimised. The preferred embodiments of this invention are directed to providing a welding gun capable of low cyclic times. The word pneumatic is used herein to mean operable by a compressible fluid, usually but not necessarily compressed air.

In one aspect the invention provides a selector valve for use with a resistance- welding pneumatic cylinder, the selector valve comprising a valve body, a valve member moveable in the valve body, a first port connected to a second port by a first flow path defined by the valve member when said member is in a first position and connected by a second flow path defined by the valve member to a third port when the valve member is in a second position, means biasing the valve member towards the second position, the presence of pressure in the first flow path causing the valve member to move to the first position.

There may be means applying a pressure present in the first flow path to a pressure surface of the valve member when the valve member is in the second position to move it to the first position.

The pressure-applying means may be a conduit through the valve member.

The biasing means is preferably arranged to bias the valve member towards the second position substantially permanently, more preferably permanently.

The biasing means may comprise a resilient element.

Alternatively or in addition the biasing means may be a pressure surface of the valve member which is exposed to a pressure present in the second flow path.

The resilient element and the last-mentioned pressure surface may act together to bias the valve member towards the second position.

A pocket containing the resilient element may act as a pressure fluid flow path through the valve member.

The pressure surface of the biasing means may bound a cylinder within the valve member.

There may be means restricting flow through the second flow path and means for bypassing the restricting means when the pressure in the second flow path falls below a predetermined value.

There may be further bypassing means for bypassing the first-mentioned bypassing means.

The further bypassing means may be a non-return valve permitting flow to the third port.

Thus the further bypassing means may comprise a valve held closed by pressure at the third port.

In another aspect the invention provides a control system for a pneumatic cylinder comprising a selector valve as set forth above, a first control valve for selectively connecting either of a pressure fluid source or an exhaust to a first chamber of the cylinder or to the second port of the selector valve, and a second control valve for selectively connecting either of a pressure fluid source or an exhaust to a second chamber of the cylinder or to the third port of the selector valve.

In a further aspect the invention provides a control system for a pneumatic cylinder comprising a selector valve as set forth above, a first control valve for selectively connecting either of a pressure fluid source or an exhaust to a first chamber of the cylinder or to the third port of the selector valve, and a second control valve for selectively connecting either of a pressure fluid source or an exhaust to a second chamber of the cylinder or to the second port of the selector valve.

The second control valve may be connected via the restricting means and the bypassing means to the said third port.

In a further aspect the invention provides a double-piston pneumatic cylinder provided with a control system as set forth above, the cylinder having a said first chamber bounded by a face of a first piston, a said second chamber bounded by a first face of a second piston, pressure fluid supplied to the first or second chambers moving the pistons to effect a working stroke of the cylinder, a third chamber of the cylinder being bounded by a second face of the second piston oppositely directed to the first face thereof such that pressure fluid supplied to the third chamber effects a return stroke of the cylinder, the third chamber being connected to the first port of the selector valve.

In a yet further aspect the invention provides resistance welding apparatus incorporating a pneumatic ram as set forth above. By pneumatic ram we mean a ram operated by compressed air or other compressible fluid.

The invention will now be described, merely by way of example, with reference to the accompanying drawings wherein:

Figure 1 is a section through a known resistance welding apparatus including a pneumatic cylinder.

Figure 2 shows a control system according to the invention, connected to the cylinder of figure 1.

Figures 3 and 4 show (with figure 2) successive stages in the operation of the pneumatic cylinder and the control system.

Figures 5 to 10 show successive stages in the operation of the pneumatic cylinder of figure 1 , in combination with a control system according to another embodiment of the invention.

Figures 11, 12 and 13 are sectional views of parts of the control system of figure 5.

Figure 14 shows the apparatus of which figures 11, 12 and 13 are sectional views.

Figures 15 to 17 show stages in the operation of a modified form of the system of figures 2 to 4.

Figure 18 shows an alternative form of a selector valve of the control system, and

Figure 19 shows a part of the valve of figure 18.

A known resistance welding apparatus is shown in a simplified section in figure 1. A workpiece 10, here shown as two sheets of metal eg. forming part of a vehicle body are squeezed between two electrodes 12, 14 and a heavy current is passed through the electrodes, heating the metal to effect the weld. The electrode 12 is moveable back and forth relative to the electrode 14 as shown by arrow 16 by a pneumatic cylinder or ram 18. It will be appreciated that if necessary the apparatus can be configured so that the electrode 14 moves relative to electrode 12, or that both move to predetermined welding positions. However the illustrated arrangement is preferred.

The electrode 12 can be withdrawn so that a relatively large gap is presented between the electrodes so that they may pass over intervening structure of the workpiece to access the weld site. Closure of the electrodes is effected by a first portion of the working stroke in which the major part of the gap between the electrodes is closed followed by a second or welding portion in which the electrodes are applied to the workpiece. In order to reduce the welding cycle time and maximise production, it is important that at least the first part of the stroke is accomplished as quickly as possible.

The pneumatic cylinder 18 is of the three-port double-stroke type. Within the cylinder is a first pressure chamber 20 bounded by a face 22 of a first piston 24. Pressure fluid is supplied via a port 26, hereafter the "forward port". The piston is slidable on a hollow guide rod 28, its rightward movement being limited by a flange 30. A second chamber 32 is bounded by the other face 34 of the piston 24, and a face 36 of a second piston 38, and is supplied with pressure fluid via a port 40 hereafter the "weld port", and the interior of the guide rod 28. The piston 38 is integral with a hollow piston rod 42, the interior of which accommodates the guide rod 28, when the piston 38 moves to the left. The internal end 44 of the hollow piston rod 42 effectively forms part of the piston face 36, the faces 22 and 36 having only a small difference in area due to the bore in the piston accommodating the guide rod 28.

An oppositely directed face 48 of the piston 38 bounds a third or return chamber 47 which is supplied with pressure fluid via a return port (hereafter the "return port") 50. The pressure face 48 is smaller in area than the face 36 by an amount equal to the cross-sectional area of the piston rod 42, and smaller in area than face 22 by an amount equal to the difference in areas of guide rod 28 and piston rod 42.

The electrode 12 is carried by the piston rod 42, and the electrode 14 either is supported from the casing of the cylinder 18, or both the casing and the electrode are supported from common structure so that they are fixed relative to each other.

In operation, at the start of the welding cycle both pistons 24, 38 are fully retracted into the cylinder with their faces 34, 36 in contact. Pressurised air is supplied to the forward port 26 and the return port 50. Due to the difference in the areas of faces 22 and 48 the pistons 24, 38 move forward together relatively slowly until piston 24 reaches its stop at flange 30 on the guide 28. Pressurised air is then supplied to the weld port 40 and removed from the return port 50 to move piston 38 forward and close the electrodes 12, 14 together. Upon completion of the weld the chambers 20, 32 are depressurised and the pressure in chamber 47 returns both pistons to their starting position, or alternatively pressure in chamber 20 is maintained and piston 38 returns to the mid position. The latter strategy is adopted if it is not required fully to open the electrode gap, for example if (as is often the case) a number of spot welds are required side-by- side along a joint between two sheet metal panels. Then only a small gap is required between the electrodes, sufficient for the welding gun to be moved laterally a few millimetres or tens of millimetres to the next weld site.

Figure 2 shows the cylinder of figure 1 with a control system according to the invention.

Compressed air from a source 52 is supplied selectively to ports 26 and 40 by solenoid- operated control valves 54, 56, hereafter the "forward valve" and the "weld valve" respectively. These valves are known per se and will not be described in detail. Each comprises a valve spool 57 which, depending on its position, will respectively connect one of two ports 58, 60 to the source 52 and the other of the two ports to exhaust. Port 58 of forward valve 54 is connected to the forward port 26 of cylinder 18 and port 58 of weld valve 56 is connected to port 40 of cylinder 18. The valve spool 57 is biased into one position by a spring 62 and moved into its other position by energising a solenoid valve 64 which applies air pressure from the source 52 to an end face of the spool 57.

A selector valve 66 has a first port 68 connected to the return port 50 of cylinder 18, a second port 70 connected to port 60 of forward valve 54 and a third port 72 connected to port 60 of weld valve 56. The selector valve has a valve body 76 and a spool 74 located with a central land 78, the ports 70, 72 being located in the valve body so that when the spool is in a first position (at the top of the valve body as seen in the drawing) the spool establishes a first flow path between ports 68 and 70. When in a second position (at the bottom of the valve body) the spool establishes a second flow path between ports 68 and 72. A spring 80 biases the spool towards the second position and a conduit 82 through the spool applies the pressure in the first flow path to a chamber 84 bounded by the end of the spool to move the spool upwards against the spring when pressure is present in the first flow path. The terms top, bottom, up, down etc. are used for convenience having regard to the orientation of the valve in the drawings: the valve may of course be operated in any convenient attitude.

The operational sequence for effecting welding will now be described with reference to figures 1, 3 and 4.

In figure 2, both pistons 24 and 38 are fully retracted, at the start of the welding sequence. Both valves 54 and 56 are de-energised, the selector valve spool 74 being held in its upper (first) position by air pressure applied to port 70 from source 52 through port 60 of forward valve 54.

In figure 3, valve 54 is energised, moving its spool, and pressure is applied via port 58 of forward valve 54 to forward port 26. Port 70 is now connected to exhaust via port 60 of valve 54. Pistons 24, 38 move rapidly rightward together, the residual pressure in chamber 47 due to the movement of piston 38 against the flow resistance presented by the valve 54 being sufficient to maintain the selector valve spool 74 in its upper position.

In figure 4 the pistons 24, 38 have reached a mid position at the end of the first part of the welding closure stroke, the piston 24 being against its stop at flange 30. Because piston 38 is no longer moving, the residual pressure in chamber 47 decays, and the spool 74 of selector valve 66 moves to its lower position under the action of the spring 80. This results in the pressure source 52 being connected via weld valve 56 and ports as disclosed hereafter. Thus the cylinder and electrodes are firmly held in the mid position enabling the welding gun to be manoeuvred to the welding position if not already aligned to it.

The weld control valve 56 is then energised (not illustrated), applying pressure to chamber 32 and chamber 47 to exhaust. The forward control valve 54 remains energised, so pressure continues to be applied to chamber 20. As a result, piston 24 remains stationary and piston 38 moves rightward to close the electrodes 12, 14 on to the work-piece. After the weld has been effected, either both valves 54 and 56 are de- energised or only valve 56, depending on whether the pistons are to be returned to the fully-open position or only piston 38 is to be returned to the mid position. If the latter, chamber 32 is connected to exhaust, and chamber 47 is re-pressurised via port 72. The cycle then repeats from the mid position shown in figure 4. To return both pistons to the fully-open position, valve 54 also is de-energised, exhausting chamber 20. The land 78 of selector valve spool 74 is located relative to port 70 such that the conduit 82 receives pressure from port 70 even when port 68 is shut-off from that port. Hence the valve spool 74 moves to its first position, establishing the flow path between ports 70 and 68. The pistons 24, 38 thus return to the positions shown in figure 2, and the full cycle can be repeated.

Figures 5 to 14 show a further embodiment of the invention. In this embodiment the selector valve 66 has its biasing spring 80 replaced by an air spring 81. This device (shown in more detail in the sectional view of the selector valve in figure 11) comprises a free piston 86 received in a cylinder 90 within the spool 74, so as to form an air spring. The lower part of cylinder 90 is pressurised from weld valve 56 through port 60 and 72 via a conduit 92 through the valve spool, so as to apply a downward force to the end 91 of the cylinder 90. This arrangement (which may be incorporated in the embodiment of figures 2 to 4) enables the selector valve to operate earlier when changing from its first (upper) position to its second (lower) position at the end of the forward stroke of piston 24.

Referring to figure 11 and also to already-described figure 1, the selector valve body 76 contains a fixed sleeve 94 provided with galleries which form respectively the first port 68, the second port 70 and the third port 72 of the valve. These ports are shown out of their true plane for clarity in figure 11. Grooves 96, 97 in the valve spool 74 respectively provide first and second flow paths through the valve between ports 68 and 72 and 68 and 70, depending on the position of the spool. When the valve is in its lower second position as illustrated, connection is between ports 68 and 72. When it is in its upper position, connection is between ports 68 and 70. Conduit 82 is formed by intersecting drillings through the spool 74 from port 70 to the end face 100 of the spool bounding chamber 84. The conduit 92 for pressurising the chamber 90 communicates with groove 96 and third port 72.

A port 98 provides a pressure supply to the top face of piston 74, from port 58 of weld valve 56, so as to move the piston 74 downwards. This is required for certain modes of operation as described hereafter. The end wall 88, and the corresponding other end wall 105 are fixed to the valve body by socket screws 106, enabling easy assembly and disassembly of the valve. O-ring seals 108 are of course provided where necessary in accordance with conventional design practice.

The figure 5 system also includes a low impact valve 110 developed from that described in our earlier application EP 0962662A but having an external fixed restrictor 112 instead of the integral restrictor of that application.

These components are connected in parallel between port 72 of the selector valve and port 60 of the solenoid valve 56. The fixed restrictor is shown in section in figure 12. The restrictor has a valve body 113 and two ports 114, 116. The flow through the device is reversible. Communication between the ports is restricted by a pin 118 the diameter of which is such as to occlude a passage between the ports apart from a small gap 120.

By substituting a pin of different diameter, the degree of flow restriction may be varied, or alternatively a tapered pin or needle may be used to provide a variable restrictor as described in our earlier application.

The low impact valve 110 comprises a valve body 122 (figure 13) containing a spool 124 having a shoulder which forms a valve 126 with a complementary conical seating surface of the body. The valve 126 controls flow between a gallery 130 connected to the return port 50 of cylinder 18 and a gallery 132 connected to port 60 of weld valve 56, which port is connected by valve 56 either to the pressure source 52 or to exhaust. The restrictor 112 is connected across galleries 130, 132 so as to be in parallel with and thus to bypass valve 126.

A pilot conduit 134 supplies pressure from port 58 of weld valve 56 to move the spool 124 upwards and thus open the valve 126. A bypass valve 135 is provided within the spool 124. It consists of a small piston or plunger 136 which cooperates with a valve seat 138. This valve controls a conduit 141 through the spool 124 which extends from the gallery 130 to the gallery 132. Thus this conduit also bypasses the valve 126. A flow path shown as line 133 in figure 5, applies the pressure in gallery 130 also to the upper end face 137 of spool 124 and to the end face of plunger 136, thereby biasing both valve 126 and valve 135 shut. This flow path is achieved by providing clearance between spool 124 and plunger 136.

Referring now to figures 5 to 10, operation of this embodiment of the system will be described. During the first two stages of the welding cycle, corresponding to figures 2 and 3, the low impact valve 110 and the restrictor play no part because although pressure is present at the valve 110 from port 60 of the weld valve 56, there is no flow because the selector valve second flow path (port 72 to 68) is closed, the valve spool being in its upper position. The sequence thus proceeds as already described.

At the end of the first part of the closure stroke (figure 5, corresponding to figure 4 of the first embodiment) the selector valve spool 74 moves to its lower or second position, due to the removal of pressure from its bottom surface 85 , aided by the pressure present in the air spring chamber 90 from weld valve 56 via the valve 110 (valves 126 and 135 being open) or fixed restrictor 112. Naive 56 is then energised to apply air pressure to chamber 32.

In figure 6, the piston 38 is moving rightwards to close the electrodes 12, 14, both valves 54 and 56 being energised. The flow from the chamber 47 is restricted by the fixed restrictor 112, the valves 126 and 135 being held closed by the back pressure in chamber 47 being applied to the upper end faces of spool 124 and plunger 136 via conduit 133, overcoming the opposing force on the other end of the spool from the pressure applied via conduit 134. Full pressure in line 134 is not achieved due to the forward movement of piston 38. Also the area of the lower end of spool 124 to which that pressure is applied is somewhat less than the upper end exposed to the pressure in line 133. Due to the restrictor 112, the speed of the piston is limited, reducing the impact and thus noise when the electrodes contact the workpiece. In figure 7, at the end of the welding stroke, the piston 38 has stopped moving, and the electrodes are closed on the workpiece. The pressure in chamber 47 decays via the restrictor 112, and when it reaches a predetermined fraction of the pressure applied to face 101 of spool 124 via conduit 134, the spool moves upwards, opening valve 126 and permitting rapid exhaust of chamber 47. The closure force on the electrodes thus is rapidly increased.

Figure 8 shows the return stroke of the piston 38, withdrawing the electrodes from the now- welded workpiece. Pressure is applied via de-energised weld valve 56 to the low impact valve, opening non-return valve 135, thus applying pressure fluid to chamber 47 via the selector valve ports 72 and 68. The flow of air through valve 135 maintains it open. The forward valve 54 remains energised, so piston 24 remains stationary, chamber 32 exhausting via port 58 of weld valve 56. The pistons then stop at their mid position, and the welding part of the cycle can be repeated.

If full opening of the electrodes is required, forward valve 54 is de-energised, exhausting chamber 20 and allowing both pistons to move leftwards (figure 9) to regain the starting position equivalent to that shown in figure 2. Naive 126 closes due to pressure having been removed from line 134, but the non-return valve 135 remains open, due to the flow through it. When valve 54 is de-energised pressure from port 60 to valve port 70 causes valve spool 74 to move to its upper position. The system is now ready for the next fast forward operation.

The port 98 (figure 11) of the selector valve 66 is used to permit a different mode of operation. The port provides pilot pressure via a line 140 from port 58 of the weld valve 56 when it is energised. In normal operation, as so far described, the piston 24 is moved before piston 38. Some saving in cycle time can be achieved if the movement of piston 38 is initiated whilst piston 24 is still moving towards it; indeed it can be initiated at the same time as the piston 24 commencing its stroke. To ensure a low impact is achieved the port 98 is used to apply pressure to the top of selector valve spool 74 when the weld valve 56 is energised, to ensure that it immediately moves to its lower position, and directs the air expelled from chamber 47 through the low impact valve. Figure 14 illustrates an engineered prototype of the valves described above, in which they are integrated into a single valve block.

Control valves 54, 56 are provided with operating solenoids 64, and connect directly with valve block 144 which contains the selector valve 66, the restrictor 112 and the low impact valve 110 side by side. Figures 11, 12 and 13 are respectively sections on lines A- A, B-B and C-C of figure 14; the compactness of the design is evident.

Figures 15 to 17 shows a modification of the system of figures 2 to 4, corresponding parts carrying the same reference numerals. In this embodiment the biasing of the valve spool 74 is reversed. The spring 80 is now positioned at the other end of the valve body 76 so that the spool is urged towards a position in which ports 68 and 70 normally are placed in communication. Thus, in terms of claim 1, port 72 is now the "second port" and the path between ports 68 and 72 in the "first flow path" shown diagrammatically at 150. Port 70 is the third port and the path between ports 68 and 70 is the "second flow path", 152. This flow path still is connected via conduit 82 to chamber 84 to apply pressure to the lower face of the spool 74. The flow path 150 is connected by a conduit 154 to a chamber 156 (figure 17) bounded by the top face 158 of the valve spool to apply thereto the pressure in the flow path 150.

In operation, starting with the state shown in figure 15, which corresponds to the state of figure 2, both pistons 24 and 38 are fully retracted, at the start of the welding sequence. Both valves 54 and 56 are de-energised, and line pressure from source 52 is applied via valves 54 and 56 to both ports 70 and 72. The selector valve spool 74 is held in its upper position by the spring 80 assisted by air pressure applied to port 70 and thence to chamber 84. Together the spring and the pressure on the lower face 85 of the spool 74 overcome the pressure applied to the upper face 158 of the spool via port 72 and conduit 154.

In figure 16, valve 54 is energised, moving its spool, and pressure is applied via port 58 of forward valve 54 to forward port 26. Port 70 is now connected to exhaust via port 60 of valve 54. Pistons 24, 38 move rapidly rightward together. The spring 80 plus the residual pressure in chamber 47 due to the movement of piston 38 against the flow resistance presented by the valve 54 and the port 70 (which may be made smaller for this purpose) are sufficient to maintain the selector valve spool 74 in its upper position. In figure 17 the pistons 24, 38 have reached a mid position at the end of the first part of the welding closure stroke, the piston 24 being against its stop at flange 30. Because piston 38 is no longer moving, the residual pressure in chamber 47 decays, and the spool 74 of selector valve 66 moves to its lower position under the action of the pressure in chamber 156 which overcomes the spring 80. This results in the pressure source 52 being connected via weld valve 56 and ports 72, 68 of the selector valve to chamber 47, to recharge the latter for control purposes as described for the figure 2 embodiment. The cycle then proceeds as described for figure 2. As in that embodiment, the spool 74 is such that conduit 82 communicates with port 70 even when port 68 does not, and likewise conduit 154 communicates with port 72 at all times regardless of whether path 150 between ports 68 and 72 is open.

Compared to the figure 2 embodiment, this modification gives a more consistent timing of the change of position of the spool valve between figures 16 and 17.

Figures 18 and 19 shows another form of the selector valve 66. The same reference numerals are used for corresponding parts also appearing in other figures. This valve is configured for use in the system of figures 15 to 17.

The selector valve body 76 contains a bore housing spool 74, which here is made of a plastics material such as acetal. This component can be manufactured at much less cost than the metal spool of figure 11 , and also enables the sleeve 94 of figure 11 to be omitted. A substantial saving can result.

The spool has at its right-hand end a shoulder which acts as end face 158 (figure 17). Chamber 156 bounded by that face is defined by the bore in which the spool 74 moves and an end plug 162 having a smaller bore 166 in which a reduced-diameter portion or tail-rod 168 of the spool is guided. The bore 166 is vented to atmosphere at 170. Port 72 is in permanent communication with the chamber 156, and this chamber is selectively connected to port 68 by axial 172 and radial 174 drillings in the spool, and a gallery 176. The spool is illustrated in its position corresponding to figure 17 in which it has moved against the spring 80 to connect ports 68 and 72.

The left-hand face 85 of the spool comprises a spring pocket 178 wherein the spring 80 is disposed, and defines chamber 84 with the end wall of the bore in body 76. Drillings 180 through the spool connect the chamber 84 to port 70 via a gallery 182. When the pressure in chamber 156 is reduced and the spool 74 is moved rightward a further set of drillings 184 connect the gallery 182 to port 68 via the spring pocket volume 178 and gallery 176.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

Statements in this specification of the "objects of the invention" relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims.

Reference numerals appearing in the claims are for illustration only, and shall have no limiting effect on the scope of the claims.

Claims

C AIMS

1. A selector valve for use with a resistance-welding pneumatic cylinder, the selector valve comprising a valve body (76), a valve member (74) moveable in the valve body, a first port (68) connected to a second port (70) by a first flow path defined by the valve member when said member is in a first position and connected by a second flow path defined by the valve member to a third port (72) when the valve member is in a second position, means (80, 86, 90) biasing the valve member towards the second position, the presence of pressure in the first flow path causing the valve member to move to the first position.

2. A valve as claimed in claim 1, comprising means (82) applying a pressure present in the first flow path to a pressure surface (1 0) of the valve member when the valve member is in the second position to move it to the first position.

3. A valve as claimed in claim 2 wherein the pressure-applying means is a conduit (82) through the valve member.

4. A valve as claimed in any preceding claim, wherein the biasing means is arranged to bias the valve member permanently towards the second position.

5. A valve as claimed in any preceding claim wherein the biasing means comprises a resilient element (80).

6. A valve as claimed in any preceding claim wherein the biasing means comprises a pressure surface (91) of the valve member (74) which is exposed to a pressure present in the second flow path.

7. A valve as claimed in claim 6 as dependent on claim 5 wherein the resilient element (80) and the last-mentioned pressure surface (85, figure 15) act together to bias the valve member towards the second position.

8. A valve as claimed in claim 7 wherein a pocket containing the resilient element acts as a pressure fluid path through the valve member.

9. A valve as claimed in claim 6 wherein the pressure surface bounds a cylinder (90) within the valve member (74).

10. A valve as claimed in any of the preceding claims comprising means (112) restricting flow through the second flow path and means (126) for bypassing the restricting means when the pressure in the second flow path falls below a predetermined value.

11. A valve as claimed in claim 10 comprising further bypassing means (135) for bypassing the first-mentioned bypassing means.

12. A valve as claimed in claim 11 wherein the further bypassing means is a nonreturn valve permitting flow to the third port.

13. A valve as claimed in claim 11 or 12 wherein the further bypassing means comprises a valve (135) held closed by pressure at the third port (72).

14. A control system for a pneumatic cylinder (18) comprising a selector valve (66) as claimed in any one of the preceding claims, a first control valve (54) for selectively connecting either of a pressure fluid source (52) or an exhaust to a first chamber (20) of the cylinder or to the second port (70) of the selector valve, and a second control valve (56) for selectively connecting either of a pressure fluid source (52) or an exhaust to a second chamber (32) of the cylinder or to the third port (72) of the selector valve.

15. A control system for a pneumatic cylinder (18) comprising a selector valve (66) as claimed in claim 7, a first control valve (54) for selectively connecting either of a pressure fluid source (52) or an exhaust to a first chamber (20) of the cylinder or to the third port (70) of the selector valve, and a second control valve (56) for selectively connecting either of a pressure fluid source (52) or an exhaust to a second chamber (32) of the cylinder or to the second port (72) of the selector valve.

16. A control system as claimed in claim 15 wherein the selector valve (66) is as claimed in claim 9 or 10 and the second control valve (56) is connected via the restricting means (112) and the bypassing means (126) to the said third port

(72).

17. A double-piston pneumatic cylinder (18) provided with a control system as claimed in claims 15 and 16, the cylinder having a said first chamber (20) bounded by a face (22) of a first piston (24), a said second chamber (32) bounded by a first face (36) of a second piston (38), pressure fluid supplied to the first or second chambers moving the pistons to effect a working stroke of the cylinder, a third chamber (47) of the cylinder being bounded by a second face (48) of the second piston oppositely directed to the first face thereof such that pressure fluid supplied to the third chamber effects a return stroke of the cylinder, the third chamber being connected to the first port (68) of the selector valve (66).

18. Resistance welding apparatus including a pneumatic ram as claimed in claim

17.

19. A selector valve, control system, pneumatic cylinder or resistance welding apparatus substantially as herein described with reference to any of figures 2 to 19 of the accompanying drawings.