DESCRIPTION OF THE PRIOR ART There already exist various types of pneumatic cylinders with either internal or external flow controls and cushioning devices.

For some types of heavy duty production operations these typical designs do not meet specific production needs. This is especially true of pneumatic cylinders used in resistance spot welding and some piercing or clamping operations.

For example, in resistance spot welding an air cylinder with an electrode and associated equipment attached to the piston rod is typically expected to stroke forward, contact the component and reach full welding force in 0.2 seconds. It must precisely hold this force for the welding and cooling period of typically 0.4 seconds and during this period rapidly follow up at constant force the collapse or indentation of the components during welding.

To achieve this fast and then stable force condition it is usual to ensure that air can be supplied to and exhausted from the power cylinder very quickly.

In addition the end of the forward stroke is determined not by the cylinder but by the contact point of the electrode which changes during its production life as the electrode is consumed, worn, re-profiled or replaced.

This combination of rapid force build up and variable end of stroke positioning make traditional cushioning and speed control devices difficult to apply, and without them the shock loads, noise, wear and vibration of tooling and equipment in use is considerable.

Control valves which address this problem are disclosed in EP0633402.

These valves operate in combination with a bleed which limits the rate at which

pressure fluid can be exhausted from a return chamber of the cylinder during a forward stroke of the piston. The speed of the forward stroke thus is controlled to reduce the impact of the electrode on the workpiece.

When the electrode has contacted the workpiece the pressure fluid in the return chamber must be dumped (vented to atmosphere) rapidly, to enable the full welding force to be developed at the electrode. In'402 this is achieved by a venting valve which is held shut by the pressure in the return chamber. When that pressure decays via the bleed, the valve opens and the chamber is vented.

For the valve to open without delay it must be responsive to only a small fall in pressure in the return chamber. In the'402 valve, this can be difficult to achieve without the valve becoming unstable at other times during the welding cycle. The present invention, at least in its preferred embodiments, is directed to providing a control valve which can be more readily designed for compatibility with a range of different cylinders.

SUMMARY OF THE INVENTION According to the invention there is provided a control valve for a pneumatic cylinder having a piston, a first chamber to be pressurised to effect a forward stroke of the piston and a second oppositely acting chamber to be pressurised to effect a return stroke of the piston, the control valve comprising: a valve block connectable to the second chamber, and having a valve spool moveable axially therein; a first conduit through the block and in fluid communication with the second chamber of the cylinder when attached thereto, the first conduit including an axial passage through the spool and being controlled by valve surfaces of the valve block and spool ; a pressure surface of the spool through which surface the axial passage passes being exposed to pressure fluid in the first conduit such that pressure applied to the pressure surface biasses the valve spool in a first axial direction to maintain the first conduit closed ; means for returning the spool in an opposite axial direction to open the first conduit; and

a restricted bleed conduit by-passing the first conduit.

By providing the valve spool with an axial passage it is possible easily to tune (adjust) the area of the pressure surface of the spool for a particular cylinder by suitably selecting the size of the axial passage: for a given valve spool, the larger the passage, the smaller the area of the pressure surface, and thereby the return pressure at which the valve opens can be selected.

Although the control valve can be built into the structure of the pneumatic cylinder (and the invention includes a pneumatic cylinder comprising a control valve as set forth above) there are advantages in making the control valve as a separate unit, for example configured for attachment to the return port of the cylinder. Then the control valve can be adapted for a variety of different cylinders by the simple expedient of selecting an appropriate valve spool from a range of spools having different sizes of axial passage.

Preferably, the axial passage is a circular section bore and the surface area of the pressure surface is predeterminable depending on the pressure required to be applied to the pressure surface by selection of the diameter of the axial passage.

Preferably, the means for returning the spool includes a pilot fluid conduit in the valve block for applying pressure fluid to a face of the spool oppositely- directed to the said pressure surface.

Preferabiy, the outer surface of the spool includes a collar having said face to which the pilot fluid is to be applied.

Preferably, the block includes a circumferential channel for seating the collar of the spool, to restrict movement of the spool within the block.

Preferably, the restricted bleed valve is provided in a detachable element of the control valve.

Preferably, the restricted bleed conduit is provided in a detachable connector to which in operation working fluid is applied to and exhausted from the cylinder.

Preferably, the valve block has a stepped bore wherein the valve spool is dispersed coaxially with the connector, the valve being disassembled by withdrawing the spool through the bore after removing the connector.

Preferably, the channel is defined in part by a sleeve which also defines in part the first conduit.

The invention also includes a cylinder provided with a control valve as set forth above. The cylinder may comprise means for depressurising the first chamber if the piston fails to complete or is delayed in completing its forward stroke. Alternatively or in addition it may have means for providing pilot fluid to the pilot fluid conduit only when the piston is at or adjacent the end of its forward stroke.

The invention also provides as separate aspects kits of parts comprising a control valve as set forth above packaged with at least one further valve spool and/or at least one further said detachable element respectively having an axial passage or bleed conduit of different size to the one installed in the valve.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings where like numerals denote like parts, in which: Figures 1 to 3 are part-sectioned side views of a pneumatic cylinder employing a control valve according to the present invention, where each figure illustrates components of the pneumatic cylinder and control valve in various positions; Figure 4 is a part-sectioned detail side view of the control valve illustrated in Figures 1 to 3; Figure 5a, 5b and 5c are end, side and end views respectively of a valve spool for use with the control valve illustrated in Figure 4; Figure 6 is a part-sectioned side view of a dual piston cylinder : and Figure 7 shows another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figures 1 to 3, a preferred embodiment of the present invention is a control valve for use with a single piston pneumatic cylinder 12 as illustrated in Figures 1 to 3. The pneumatic cylinder 12 of Figures 1 to 3 is

typically used in resistance spot welding. However, as wit) be apparent to the person skilled in the art, alternative embodiments of the control valve 10 may be used with cylinders for use in piercing machines, powered jigs or clamps for example. Furthermore, the valve 10 may also be adapted for use with hand-held pneumatic equipment, such as portable welding guns, for example.

The pneumatic cylinder 12 includes a cylinder block 14 having a main internal chamber 16. A piston 18 divides the main internal chamber 16 into a first chamber 20 and a second chamber 22. Fluid, typically air from a pressure- regulated compressed air source, is supplied to the first chamber 20 via forward port 24 to move the piston 18, and its associated piston rod 26, in a forward stroke direction of arrow 28. As best illustrated in Figure 2, at the end of the forward stroke, the rod 26 is in contact with a contact surface 30. In practice, of course, the piston rod 26 carries a welding electrode or other tool and surface 30 is a workpiece. However, these do not form part of the present invention and for the purpose of simplicity are not shown.

The second chamber 22 is pressurised to effect a return stroke of the piston in the direction of arrow 32, as illustrated in Figure 3. This is achieved by supplying air from a compressed air source to the return port 34 of the cylinder 12 via a control valve return port 35 on the control valve 10.

The control valve 10 is employed to control the forward and return strokes of the piston 18 and piston rod 26 during use of the cylinder 12, to aid in alleviating impact forces and to facilitate variable end of stroke positioning of pneumatic cylinders, as described above in reference to the prior art. The control valve 10 includes a valve block 36 which is attachable to the return port 34 of the second chamber 22, usually by a standard male and female threaded arrangement. In alternative embodiments of the invention, the control valve 10 can be incorporated within the cylinder block 14, or connected to the cylinder via an intermediate device.

As illustrated in Figure 4, the valve block 36 consists of three sections: a main body 36a; a sleeve 36b and an end cap 36c. The end cap 36c threadedly engages the main body 36a, trapping the sleeve 36b therebetween. The valve block 36 has a first conduit in the form of a main bore 38 passing therethrough

and in fluid communication with the second chamber 22. The main bore 38 is defined by a series of portions: a neck 38a, which is immediately adjacent the pneumatic cylinder 12 when the control valve 10 is connected to the cylinder 12; a main portion 38b which is adjacent to and of stepped larger diameter than the neck 38a; an enlarged portion 38c of the main portion 38b; a bore 38d through sleeve 36b, which forms the largest diameter portion of the main bore 38; and radially-extending holes 38e (only one shown) through the end cap 36c, connecting bore portion 38d to control valve return port 35. The portions 38a-c and 35 of the main bore 38 share an axis, the holes 38e being distributed around that axis.

A restricted bleed conduit 40 passes through end cap 36c between the main portion 38b and control valve return port 35 of the main bore 38. The restricted bleed conduit also shares the axis of the portions 38a-c and 35 of the main bore 38, and has a smaller diameter than all portions of the main bore 38.

It is the diameter of the restricted bleed conduit 40 which determines, during the forward stroke, the exhaust rate of fluid from the second chamber 22, and therefore the back pressure upon the piston 18 applied from fluid in the second chamber 22.

A valve spool 48 is located in the main bore 38 and includes a collar 50 for seating the spool 48 in the channel 38c and which defines a pressure surface 53. A bore 52 through the spool 48 is in axial alignment with the main bore 38.

The valve block 36 further includes a means for returning the spool 48 in the form of a pilot air inlet 49 for the supply of pressurised pilot air to the channel 38c, to act on pressure surface 53. In this embodiment, the pilot air inlet 49 is supplied with air from the line supplying pressurised air to the first forward port 24.

The axial length of the portion 38c is greater than the axial length of the collar 50 to allow the spool 48 to move axially, though in a restricted fashion, within the block 36. That is to say, when the spool 48 moves into a first position at one end of its travel (illustrated in Figure 1) toward the end cap 36c, a first spool end 54 abuts the end cap 36c, in turn closing one end 42 of the bore portion 38d. The stability of the spool 48 when in contact with the end cap 36c

depends on the bore portion 38d remaining at a lower pressure than the pressure in spool bore 52, even when some leakage across the first spool end 54 occurs.

This is achieved by ensuring that the holes 38e together have ample flow area.

When the spool 48 moves into a second position (for example, illustrated in Figure 2) toward the neck 38a, the collar 50 abuts a shoulder portion 55 of the channel 38c. A vent hole 56 is provided to vent the fluid pressure in the portion 38c, between the collar 50 and the shoulder portion 55, to atmosphere so that movement of the spool 48 is not impeded.

The valve spool 48 includes an annular pressure surface in the form of a second spool end 57 through which the spool bore 52 passes.

The dimensions of the valve spool 48, collar 50 and bore portion 38c are such that when the spool 48 is in the first position, the collar 50 does not close the pilot air inlet 49. That is to say, as illustrated in Figure 4, air from the pilot air inlet 49 is always able to contact collar surface 53. Further, when the spool 48 is in the second position, the second spool end 57 cannot abut a shoulder 58 of the main portion 38b. That is also to say, exhaust air from the second chamber 22 is always able to act on pressure surface 57.

Leakage between the pilot air duct 49 around the valve spool 48 is prevented by o-ring or similar seals 59. Whereas these seals are illustrated as seated in channels on the valve block 36, in an alternative arrangement any or all of the seals instead may be seated in channels on the valve spool 48. The control valve is assembled by inserting the valve spool 52 via the large (end cap) end of the bore 38, followed by the sleeve 36b (which defines one end of the annular channel portion 38c) and then fitting the end cap 36c.

The use of the control valve 10 will now be described in with respect to the three stages of movement of the piston 18 in the pneumatic cylinder 12: forward stroke, end of forward stroke and return stroke.

Forward stroke At the start of the forward stroke, the return air pressure applied to second chamber 22 to return the piston to its starting position at the end of the previous stroke is removed. Forward air pressure is supplied to the first chamber

20 via forward port 24 and return air-pressure is released from chamber 22 via valve 10. The pressure in first chamber 20 increases and in second chamber 22 decreases to give a pressure difference sufficient to drive the piston 18 forward in the direction of arrow 28.

The flow of exhausting air through the control valve 10 acts upon the spool 48 and in particular upon the second spool end 57. Furthermore, the force acting upon the end 57 of the spool is greater than the force acting upon the collar 50 from the air introduced via the pilot air inlet, giving a net force and resultant movement of the spool 48 in the direction of the exhausting air, to a position where the spool closes the end 42 of the bore portion 38d. Then, the flow of exhaust air through the control valve 10 is solely via the restricted bleed conduit 40, resulting in a higher (or back) pressure in the second chamber 22.

After the start of movement, the pressures in chambers 20 and 22 are maintained at approximately constant values until the movement stops. The speed of the piston 18 is determined by the size of the conduit 40 and the difference in pressures in the chambers 20, 22. Thus the forward stroke of the piston 18 is relatively slowed and is achieved with a relatively low net force on the piston; the impact of the electrode on the workpiece thereby is controlled.

End of forward stroke At the end of the forward stroke when the piston rod (electrode) 26 contacts the surface (workpiece) 30 as illustrated in Figure 2, the piston 18 ceases to move whereas the air from the second chamber 22 continues to exhaust via the control valve 10. Therefore, the pressure within the second chamber 22 and control valve 10 starts to reduce. Once the force acting upon the second spool end 57 falls below the force of the pilot air acting on the pressure surface 53, the net force acting upon the spool 48 moves the spool 48 toward the cylinder 12. This movement of the spool 48 opens the end 42 of the bore portion 38d to enable it to return to fluid communication with the main bore 38. This in turn allows a rapidly increased volume of air to be exhausted from the second chamber 22, thereby dumping the air from the second chamber 22. The force applied by the piston to the workpiece thereby rapidly increases to its

maximum value.

Return stroke To enable the return stroke of the piston 18 in the direction of return arrow 32, the supply of pressurised air to the forward port 24 and pilot air inlet 49 ceases and air is supplied into the second chamber 22 via the control valve 10 and return port 34. During the return stroke the spool 48 remains in the end of stroke position, as illustrated for example in Figure 2. This allows an unrestricted airflow through valve spool end 54 via both the restricted bleed conduit 40 and bore portion 38d, and consequently a fast piston 18 return speed is achieved.

The pressure is greater at spool end 54 than second spool end 57 and there is some residual pressure maintained on pressure face 53 due to the return movement of piston 18. These factors result in the spool remaining in its end of stroke position. At the commencement of the next forward stroke its position is stabilised by', the flow of exhaust air from the second chamber 22 as already described.

As will be apparent to the skilled person, increasing the surface area of pressure surface 57, eg by decreasing the diameter of the spool bore 52, would result in an increased force acting upon the spool 48 during the forward stroke of the piston 18. That is to say, changing the surface area of pressure surface 57 relative to that of pressure surface 53 will for a given pressure of pilot air change the back pressure in return chamber 22 at which the spool 52 moves and opens bore portion 38d. This controls the timing of the application of full welding force after the electrode has contacted the workpiece.

Furthermore, changing the diameter of the bleed conduit 40 enables the speed of the forward stroke, and hence the impact force on the workpiece, to be selected.

Thus it is very easy to produce a range of control valves matched to different pneumatic cylinders or operating requirements from common parts. The only difference from valve to valve is the diameter of bore 52 and the diameter of bleed conduit 40. Indeed it is possible to supply each control valve with a range

of differently-bored spools 48 and a range of end caps 36c with different size bleed conduits, so that an installed welding machine or other apparatus can be reconfigured in situ to have a different performance.

The control valve may also be used with other types of cylinders such as a dual tandem piston cylinder as illustrated in Figure 6. In this example, the control valve 10 is attached to the return port 34 and functions in the same manner as described above with respect to the single piston cylinder.

In an alternative embodiment of the invention, the sleeve valve may be biassed towards its open portion by a spring instead of by a supply of pilot air.

For example a helical compression spring could act between a shoulder provided in the bore 52 and the inner end surface of the end cap 36c, although in this application the valve operates with actual pressures instead of relative pressures and any adjustement to the equipment supply pressure, may, depending on the application, require a corresponding adjustment in spring compression. In another variation, an adjustable bleed valve eg as shown in Figure 9 of EP0633402 may be used instead of the fixed bleed conduit 40. However these adjustable valves can be difficult to set accurately, and also may be subject to unauthorised adjustment. when in use. Thus the fixed bleed conduit is generally to be preferred.

The configuration of the valve 10 has the advantage that exhaust airflow is directed through, rather than around, the spool 48. This benefit of in-line exhaust airflow through the spool 48 improves the spool's stability in use. It also allows for a more compact valve assembly.

Other advantages relate to improved safety in cylinder operation when using the valve 10. For example, there are no exposed adjustment screws or circlip retainers which may blow our under pressure. Also, the valve aids in reducing operating noise of its associated pneumatic equipment.

Also, in an alternative embodiment, a small electric solenoid valve 60 (Figure 7) may be configured to control the pilot air supply from main supply 61 to pilot air inlet 49. This valve could be triggered open by an electronic sensing switch 62 only towards the end of the forward piston stroke, which provides a signal to a controller 64. If the piston rod then stops due to unwanted contact

witn something before reaching the sensing switch and a squeeze time of say 0.2 seconds is exceeded, a main pneumatic directional control valve 66 for the first chamber 20 can be de-energised, or a dump valve on the first chamber 20 set to dump, and the forward air in chamber 20 exhausted. The pressurised air then remaining in second chamber 22 through to first spool end 54 would cause the piston rod 26 to return. Alternatively a touch-sensing switch, mounted so as to close simultaneously with contact of the rod 26 with surface 30, could operate in a similar manner to apply pilot air to inlet 49 but with a faster response.

In another alternative embodiment, the portion of the spool 48 between first spool end 54 and the collar 50 is of slightly smaller outer diameter that the outer diameter of the portion of the spool 48 between second spool end 57 and the collar 50. This allows for an increased bias of the spool toward the closed position, and compensates for friction of the seals.

Although the invention has been described with reference to particular examples, it'will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

While the present invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made to the invention without departing from its scope as defined by the appended claims.

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.

The text of the abstract filed herewith is repeated here as part of the specification.

The present invention includes a control valve 10 for a pneumatic cylinder 12 having a piston 18, a first chamber 20 to be pressurised to effect a forward stroke of the piston and a second opposite acting chamber 22 to be pressurised to effect a return stroke of the piston 18. The control valve 10 comprises a valve block 36 connectable to the second chamber 22, and has a valve spool 48 moveable axially therein. The control valve 10 also comprises a first conduit 38 through the block 36 and in fluid communication with the second

chamber 22 of the cylinder 12 when attached thereto, where the first conduit 38 includes an axial passage through the spool 48 and is controlled by valve surfaces of the valve block 36 and spool 48. A pressure surface 56 of the spool 48 through which the axial passage passes is exposed to pressure fluid in the first conduit 38 such that pressure applied to the pressure surface 56 biasses the valve spool 48 in a first axial direction to maintain the first conduit 38 closed. The control valve 10 also comprises means for returning the spool in an opposite axial direction to open the first conduit 38, and a restricted bleed conduit 40 by- passing the first conduit 38. By providing the valve spool 48 with an axial passage it is possible easily to tune (adjust) the area of the pressure surface 56 of, the spool 48 for a particular cylinder by suitably selecting the size of the axial passage: for a given valve spool, the. larger the passage, the smaller the area of the pressure surface, and thereby the return pressure at which the valve opens can be selected.