DESCRIPTION

TECHNICAL FIELD The invention relates to fluid actuators, more particulary, but not exclusively, fluid actuators for gates, doors and the like.

BACKGROUND ART

Fluid actuators are complex in structure and difficult and expensive to manufacture and maintain.

Again, such actuators often do not include simple and efficient timers for controlling the flow of fluid and thus controlling operation of the actuators.

Furthermore, the known actuators do not include means for preventing their operation in the case where overpressures occur in a main driving ram included within them.

Objects of the invention include the provision of actuators which alleviate or overcome the above noted difficulties, and of gates, doors and the like which are drivabie to open and close by such actuators. DISCLOSURE OF THE INVENTION

In a first aspect the invention provides a fluid actuator comprising a main fluid ram including piston means drivabie in first and second directions to move a piston rod between first and second positions, a control valve operable to cause the piston means to be driven in said first and second directions and driver means actuated directly or indirectly in response to movement of the piston means in one direction to cause operation of the control valve to drive the piston means in said other direction.

The driver means may be actuated by movement of the piston rod, or by movement of the piston rod to a particular position.

An operating lever for the actuator may be coupled to the piston rod.

The piston means may comprise a single piston movable in a cylinder and said first and second directions of movement of the main piston ram may be in a sense to extend or retract the piston rod or vice versa. Alternatively said piston means may comprise a pair of pistons coupled to opposite ends of said piston rod.

In this arrangement each said piston is preferably moveable in a respective one of a pair of horizontally opposed cylinders.

The coupling of the piston rod to the operating lever with advantage comprises a mechanism allowing both sliding and pivoting movement of the lever relative to the piston rod.

Preferably the control valve comprises a body member moveable between first and second positions in which fluid is applied to said piston means to drive the piston means in said first and second directions respectively.

The driver means may comprise a valve having a body member movable between first and second positions under the influence of a cam which moves in response to movement of the piston rod.

The cam is preferably arranged to carry the driver valve body member to a first position in which fluid is passed via a flow control valve to one side of a timer piston in a sense to drive the timer piston to move the control valve body member to a position to cause the piston means to move in one direction.

The flow control valve advantageously offers resistance to fluid flow therethrough in only one sense.

The driver valve body member, when in said second position, is preferably operable to couple the fluid supply, under the control of a pressure relief valve, to a second side of the timer piston.

The control valve may include detent means for controlling movement of the valve body between said first and second positions.

The detent means may comprise a pair of O-ringε mounted in the body and a bore in which the body moves respectively, the O-rings being arranged to inter-engage to restrict movement of the body between its first and second positions.

The control valve may be provided with a spring surrounding the rod coupling the timer piston to the control valve body member, the spring being compressed when the body member is in one said position and acting in a sense to urge the piston to carry the control valve body to said second position. The driver valve body member when in its second position is preferably operable to couple the fluid supply under the control of the pressure actuated flow control valve to a second side of the timer piston.

The pressure actuated fluid control valve may responsive to the fluid pressure level in the or one of said cylinders of said main fluid ram.

The pressure actuated fluid control valve desirably includes a body having an aperture through which fluid may flow from the fluid supply and a closure member drivabie in response to the pressure in the or one of said cylinders of the main fluid ram to close said aperture.

The driver means and control valve means control

may be operable to the flow of air supplied under pressure to a pair of reservoirs containing a liquid, operation of the driver and control valve means being such that liquid is driven from and to said reservoirs to drive said piston means between said first and second positions.

Advantageously the reservoirs are formed to surround a horizontally opposed pair of cylinders each having one of a pair of pistons coupled to opposite ends of said piston rod and in which said pistons of the main fluid ram move.

In a second aspect the invention provides a gate or door fitted with an actuator as defined above.

The above aspects, features and advantages of the invention will become more apparent from the following description of a fluid actuator for a gate now made with reference to the accompanying drawings in whic :- BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically illustrates a first fluid actuator embodying the invention, Figures 2 and 3 schematically illustrate the actuator of Figure 1 in different parts of its operational cycle,

Figure 4 shows at A, B and C respectively plan views of a second actuator embodying the invention in three operational positions,

Figures 5, 6, 7 and 8 illustrate different operational modes of the actuator shown in Figure 4.

Figure 9 illustrates a pressure actuated flow control valve used in the arrangement described with reference to Figures 5 to 8 ,

Figure 10 illustrates a timer valve body member in two of its operational positions.

Figure 11 schematically illustrates a further actuator embodying the invention, and

Figures 12, 13, 14 and 15 illustrate various stages in the cycle of operation of the actuator of Figure 11.

MODES OF CARRYING OUT THE INVENTION

Figures 1 to 3 of the drawings show a first fluid actuator embodying the invention to comprise a main fluid ram 10 including a piston 12 and having ports 14 and 16 enabling fluid to be fed to and taken from either side of piston 12.

When fluid is fed to port 16 (and taken from port 14) piston 12 is driven to move in the direction of arrow A. When fluid is fed to port 14 (and taken from port 16) piston 12 is driven to move in the direction of arrow B.

Piston 12 is coupled to a piston rod 18 the free end of which carries a clevis or coupling 20 pivotally linked to a lever 22 in turn pivotally coupled to a gate

24. Adjacent the coupling 20 the piston rod 18 carries a cam 26 shaped as shown.

When piston 12 moves in the direction of arrow A gate 24 is driven to open, and when it moves in the

direction of arrow B gate 24 is driven to close.

Fluid is fed to ports 14 and 16 via a spool valve 30 acting as a control valve and having two inlet ports 32 and 34 coupled to a pressurised fluid (e.g. water) supply 36 and an exhaust port 38. Outlet ports 40 and 42 of spool valve 30 are coupled respectively to ports 16 and 14 of the main fluid ram 10.

Valve 30 includes a body member 44 moveable from a first position (Figure 2) in which port 34 is coupled to port 42 and port 38 is coupled to port 40 allowing fluid to be passed to the port 14 and taken from the port 16 of main ram 10.

In the second position of the body member 44 (Figure 3) port 32 is coupled to port 40 and port 38 is coupled to port 42 allowing fluid to be passed to port 16 and taken from port 14 of the main ram 10.

The position of the control valve body member 44 is controlled by a timer or driver 41 comprising a housing 43 with ports 45 and 47 opening into the housing on either side of a piston 48 having a rod 50 coupled to the control valve body member 36. Fluid is fed to and passed from the ports 45 and 47 under the control of a second spool valve 52.

Spool valve 52 has two inlet ports 54 and 56 coupled to the pressurised fluid supply 36 and an exhaust port 58. Port 56 is coupled to the fluid supply line by an adjustable pressure relief valve 60. Outlet ports 62 and

64 of valve 52 are coupled respectively to ports 45 and 47 of driver 41. The coupling of port 62 to port 45 includes a one way flow constrictor valve 66 - a valve acting to constrict flow of fluid from port 62 to port 45 but offering little or no resistance to fluid flow from port 45 to port 62.

Valve 52 includes a body member 68 normally biased (e.g. by a spring) to a first position (shown in Figure 2) in which port 56 is coupled to port 64 and port 58 is coupled to port 62 allowing (if the pressure relief valve 60 is open) fluid to pass from the supply 36 to port 47 and to be taken from port 45 of driver 41.

Body member 68 carries a cam follower 70 which when the piston 12 of main ram 10 is fully retracted is engaged by cam 26 moving body member 68 to its second position (see Figure 3) in which port 54 is coupled to port 62 and port 58 is coupled to port 64 allowing fluid to be passed to port 45 (via the flow constrictor 66) and taken from port 47 of driver 41. An automatic mechanical gate latch (not shown) is operable to hold the gate closed.

Operation of the actuator is as follows. Assuming the gate is closed the resultant overpressure in the supply 36 causes pressure relief valve 60 to open passing fluid from supply 36 via ports 56 and 64 of timer valve 52 to port 47 of driver ram 41. Thus piston 48 has moved leftwards as viewed in Figures 2 and 3 and the control

valve body member 44 is in the position shown in Figure 3. Fluid from supply 36 is coupled via ports 32 and 40 of control valve 30 to port 16 of the main ram. The gate is held closed, however, by the latch. When a user wishes to pass through the gateway he unlatches the gate and it will be seen that piston 12 begins to be driven to move in the direction of the arrow A pulling the gate open. As fluid is fed to port 16 to one side of piston 12 fluid is driven from the other side of piston 12 via port 14 and ports 42 and 38 of control valve 30 to exhaust. As the piston 12 in main ram 10 is driven in the direction of arrow A piston rod 18 will start to be retracted. In time the gate 22 will be fully open and cam 26 carried on piston rod 18 will engage cam follower 70 moving body member 68 to a position in which ports 54 and 58 of valve 52 are respectively coupled to its ports 62 and 64.

The fluid supply 36 is now connected to port 45 of driver 41, via flow constrictor 66, and piston 48 begins to be driven towards the control valve 30. After a period determined by the setting of flow constrictor 66, valve body member 44 is moved to a position in which ports 34 and 38 of control valve 30 are respectively coupled to ports 42 and 40. Thus fluid will passed from the supply 36 to port 14 and driven from port 16 of the main fluid ram 10 causing piston 12 to be driven to move in the direction of arrow B closing the gate. Cam follower 70 will ride down cam 26

allowing the timer valve body member 68 to move again to the position in which ports 56 and 58 are coupled to ports 62 and 64 respectively.

When the gate has fully closed it bears on its gatepost and the automatic latch operates to hold it shut.

The overpressure in the supply causes pressure relief valve

60 to open driving piston 48 of driver 41 and control valve body member 44 to their initial positions.

Should the gate meet an obstruction before it fully closes (for example a vehicle) there will be resistance to its continued movement and an overpressure in supply 36 will be generated. This has the effect of opening valve 60 coupling supply 36 to port 47 of driver 41 moving piston 48 and control valve body member 44 to their initial positions. As the gate is not latched at this time the gate opening and timed closure sequence described above will be restarted - allowing the obstruction to be removed.

Both the pressure relief valve 60 and the flow constrictor 66 may be fully adjustable. The pressure relief valve to take account of, for example, windy conditions which might be met in use and the flow constrictor 66 to allow a user to determine the period for which the gate is held open by the actuator before it begins to close.

The end plate of the main ram 10 carries on its outer surface an apertured lug 80 for receiving a bolt (not shown) by means of which the actuator may be anchored to the ground.

It will be appreciated that variations may be made to the described arrangements without departing from the invention. Some of these variations will now be described with reference to the remaining Figures of the drawings.

Figure 4 schematically illustrates the main fluid ram of a second actuator embodying the invention. It will be seen that the main fluid ram comprises a pair of pistons 110, 112 mounted in respective ones of a pair of horizontally opposed cylinders 114, 116. The two pistons 110, 112 are coupled together by a piston rod 118. Approximately at the mid-point of rod 118 there is a sliding/pivotal connector 120 coupling rod 118 to a drive pin 122. Drive pin 122 is linked at 124 to a main drive shaft 126 to be driven to rotate by the actuator. Main drive shaft 126 is carried on a crank case 128 and may be linked, by means not shown, to drive a gate between open and closed positions.

Figure 4 shows at A, B and C respectively the main fluid ram of the actuator in three operation positions - in Figure 4A with the pistons 110, 112 in their rightmost (as viewed) positions within cylinders 114, 116, in Figure 4B with the pistons 10, 12 substantially centred within their cylinders 114, 116 and in Figure 4C with the pistons 110, 112 at their leftmost positions within the cylinders

114, 116.

It will be appreciated that in moving from the

position shown in Figure 4A to the position shown in Figure 4C fluid will be passed to cylinder 116 to drive piston 112 leftwards (as viewed in the Figure) and abstracted from cylinder 114 to permit motion of piston 110 in the same sense.

In moving from the position shown in Figure 4A sliding/pivotal connector 120 coupling rod 118 to pin 122 permits pin 122 to move through the position shown in Figure 4B to the position shown in Figure 4C. In doing this pin 122 will cause ths main drive shaft 126 to rotate carrying with it any gate to which that shaft is coupled.

It will further be appreciated that although pistons 110, 112 are driven to move in their cylinders 114, 116 at substantially constant speed the rate at which main drive shaft 126 is driven to rotate is variable as the ram moves between the position shown in Figure 4A and Figure 4C.

The speed at which main drive shaft 126 is driven to rotate is, it will be appreciated, inversely proportional to the length of drive pin 122 between the sliding/pivot coupling 120 and the link 124. Thus the speed at which the main drive shaft 126 will be at a minimum when the ram is in or near the positions shown in Figures 4A and 4C and at a maximum when it is in the position shown in Figure 4B. This arrangement is advantageous in that any gate being driven to open or close by the actuator will begin and end its movement at

relatively low speed with an acceleration to a maximum and subsequent deceleration from a maximum at a position in which it is half fully open (or fully closed) .

Finally, it will be noted from Figure 4 that fluid is fed to and abstracted from the cylinders 114, 116 via ports 130, 132 associated with those cylinders respectively and that the main drive shaft 126 carries a cam 134.

Figures 5, 6, 7 and 8 illustrate various stages in the operation of an actuator having the main fluid ram as shown in Figure 4.

Fluid is fed to the ports 130, 132 via a spool valve 140 acting as a control valve and having an inlet port 142 coupled to a pressurised fluid supply 146 and two exhaust ports 148, 150. The outlet ports 152, 154 of valve 140 are coupled respectively to the ports 130 and 132 of the main fluid ram.

Control valve 140 includes a body member 156 movable from a first position (Figures 5 and 6) in which port 142 is coupled to port 154 and port 148 is coupled to port 152 allowing fluid to be passed to port 132 and taken from port 130 of the main ram.

In the second position of the body member 156 (Figures 7 and 8) port 142 is coupled to port 152 and port 154 is coupled to port 150 allowing fluid to pass to port 130 and from port 132 of the main fluid ram.

The position of the control valve body member 156

is controlled by timer or driver 160 comprising a housing 162 with ports 164 and 166 opening to the housing on either side of a piston 168 coupled to a piston rod 170 in turn coupled to the control valve body member 156. Fluid is passed to and fed from ports 164 and 166 under the control of a second, driver spool valve 172.

Valve 172 has two inlet ports 174 and 176 coupled to the pressurized fluid (e.g. water) supply 146 and an exhaust port 178. Port 176 is coupled to the fluid supply line by a pressure actuated flow control valve 180. Outlet ports 182 and 184 of valve 172 are coupled respectively to the ports 166 and 164 of the driver 160.

The coupling of port 182 to port 166 includes a one-way flow constrictor 186 - a device acting to restrict the flow of fluid from port 182 to port 166 but offering little or no resistance to fluid flow in the reverse direction (from port 166 to port 182) .

Valve 172 includes a body member 188 normally biased (e.g. by a spring) to a first position (shown in Figure 5) allowing (if the pressure actuated flow control valve 180 is open) fluid to be passed from the supply 146 via ports 176 and 184 to port 164 and of driver 160 and to be taken from port 166 of driver 160 via flow constrictor 186 and ports 182 and 178 to exhaust. Body member 188 carries a cam follower 190 which when the pistons 110, 112 of the main ram are in the position shown in Figure 4C is engaged by cam 134 moving

the timer valve body member 188 to its second position (see Figure 7) in which port 174 is coupled to port 182 and port 178 is coupled to port 184 allowing fluid to be passed to the port 166 (via the flow constrictor 186) and taken from the port 164 of driver 160.

An automatic gate latch (not shown) is operable to hold any gate closed against the action of the actuator.

Figure 9 illustrates in more detail the pressure actuated flow control valve 180 and shows it to comprise a main body 200 having an inlet 202 and an outlet 204.

The body 200 is provided with an axial through bore 206 which interconnects the inlet 202 and outlet 202 of the valve. Within the bore 206 is located a hollow metering jet 208. The upper, open, end of jet 208 is adjacent outlet 204 of the valve. Jet 204 is provided with an aperture 210 at its lower end adjacent inlet 202 and coupling the inlet to its hollow interior. The flow of the main fluid supply 146 through the valve is from the inlet 202, via aperture 210 along the hollow body of jet 208 to the outlet 204. Fluid cannot pass outside the body of the jet 208 due to sealing rings 212 which are provided.

Bore 206 receives at its upper end a rod 214 coupled to a piston 216. The piston 216 is covered, as can be seen, by a flexible diaphragm 218 above which is a fluid tight chamber 220 communicating with a control inlet 222 for the valve. The lower end of the rod 214 carries a rubber seal 224 which, when the piston 216 is driven

downwardly by a positive pressure at the control inlet 222 rests on the top of the jet 208 cutting off the flow of fluid therethrough.

It will be appreciated that if the force of the fluid pressure at inlet 202 (from the fluid supply 146) exceeds the force of the fluid pressure at the control inlet 222 (from port 130 of the main ram) the rubber seal 224 will be lifted upwardly (carrying with it the piston 216) allowing fluid to flow from the supply 146 to the outlet 204 and then to the port 176 of spool valve 172. The relative sizes of the hollow bore in jet 208 and the piston 216 are selected such that valve 208 is held closed for all pressures at the control inlet 222 likely to be met in service of the actuator when a gate driven by it is actually moving.

Figure 10 shows in more detail the control valve 140 with its driver 160.

As can be seen from Figure 10 the main body member 156 of the valve is provided with a rubber O-ring 240. Again it will be seen that the bore in which the body 156 moves is provided with a further rubber O-ring 242. When fluid is supplied to the port 166 of the driver these two rubber O-rings engage one another and act as a detent or delay. Again to assist operation of the control valve body the piston rod 170 provided in driver 160 for the control valve 140 is surrounded by a compression spring 250

as shown .

Operation of the actuator is similar to that already described with reference to the arrangement of Figures 1 to 3 and is as follows. Starting from a position at which the gate is closed (Figure 5) , valve body member 156 is in its rightmost position (as viewed) and the fluid supply 146 is coupled via ports 142 and 152 of valve 140 to pass fluid to port 130 of the main fluid ram. At the same time port 132 of the main fluid ram is coupled via ports 154 and 150 to exhaust.

As port 130 is connected to supply 146 the pressure actuated flow control valve 180 is held closed.

At this time body 188 of valve 172 is in its rightmost position (as viewed) and thus port 176 is coupled to port 184 and port 178 to port 182. In this condition port 166 of driver 160 is coupled via the flow constrictor 186 and port 182 to exhaust. Port 164 at this time has no fluid fed to it as valve 180 is held closed. It will be appreciated that in this position the actuator will urge pistons 110, 112 to move from the position shown in Figure 4C to the position shown in Figure 4A to drive the gate coupled to the drive shaft 126 open. However, the gate will be held closed by the latch. When a user wishes to pass through the gate he unlatches it and pistons 110, 112 begin to be driven to move from the position shown in Figure 4C to the position

shown in Figure 4A causing the drive shaft 126 to rotate and open the gate. As fluid is fed to port 130 piston 110 is driven leftwards (as viewed) in its cylinder 114 and fluid is driven by the piston 112 from the cylinder 116 via port 132 and the ports 154 and 150 of control valve 140 to exhaust.

As pistons 110 and 112 of the main fluid ram are driven from the position shown in Figure 4C to the position shown in Figure 4A, piston rod 118 moves and begins to rotate drive shaft 126 in a sense to open the gate. In time the gate 126 to which the drive shaft is coupled will be fully opened and cam 134 will be carried by the drive shaft 126 to engage cam follower 190 (Figure 8) moving the valve body member 188 to a position in which its ports 174 and 178 are respectively connected to ports 182 and 184.

The fluid supply 146 is now connected to port 166 of driver 160 for the control valve 140, via flow constrictor 186. After a time delay caused by flow constrictor 186 piston 168 of driver 160 will carry body 156 rightwards (as viewed) connecting port 130 of the main ram to the supply 146 and port 132 to exhaust.

The detent O-rings 240, 242 will act further to allow the pressure to build up behind piston 168 before it begins to move and that increase in pressure taken with the action of compression spring 250 provides that there is sufficient energy in the system to carry the body 156 fully across to its leftmost (as viewed) position.

After a predetermined period (determined by the flow constrictor 186) the control valve body member 156 is carried by the piston 168 (to which it is connected by the rod 170) to a position in which the ports 142 and 148 of control valve 140 are connected respectively to the ports 154 and 152 (Figure 5) . At this time fluid will pass from the fluid supply 146 to port 132 of the main ram and be driven from port 130 of the main ram, via port 152, to exhaust. This allows the pistons 110, 112 to move back from the position shown in Figure 4C to the position shown in Figure 4A. As the pistons move and the drive shaft 126 rotates cam follower 190 will ride down the cam 134 allowing the timer valve body 188 to move to the position in which its ports 176 and 178 are coupled to its ports 184 and 182 respectively. As the gate is being driven closed there will be fluid pressure at port 130 (from which fluid is being driven) sufficient to keep valve 180 closed.

When the gate is fully closed it bears on its gate post and the automatic mechanical latch operates to hold it shut.

At this time no fluid is being driven from cylinder 110 and the pressure at port 130 falls to zero. Thus valve 180 is driven to open driving piston 168 of driver 160 and the control valve body member 156 back to their initial positions. This will cause the pressure at port 130 of cylinder 110 to rise closing valve 180 once

again (Figure 6) . As noted above the gate is held closed by the latch mechanism.

If in closing the gate should meet an obstruction before it is fully closed (for example a vehicle) there will be a resistance to its continued movement and the pressure at the port 130 will fall to a level below that holding valve 180 closed. Thus valve 180 will open coupling the supply 146 to port 164 of the timer ram 160 driving the piston 168 and the control valve body member 156 to their initial positions. As the gate is not latched at this time the opening and timed closure sequence described above will be re-started allowing the obstruction to be removed.

Both the pressure actuated flow control valve 180 and the flow constrictor 186 are fully adjustable by a user - the flow control valve 186 being adjustable to allow a user to determine the period for which the gate is held open by the actuator before it begins to close and the pressure actuated flow control valve 180 to take account, for example, of particularly windy weather conditions which might be met in use.

When the flow of fluid to ports 164 and 166 is reversed pressure will build up on the rightmost side of piston 168. The valve body 156, however, will be held in its rightmost position by the inter-engaging O-rings 240, 242. When the pressure behind the piston 168 has increased to a degree allowing the piston body to move that movement will be accelerated by the spring 250 thus ensuring that

the control valve body 156 moves quickly between its two operative positions. The provision of the O-rings 240 and 242 and the compression spring 250 further overcomes any problem that might otherwise arise when the body 156 is half way in moving from its rightmost to its leftmost positions (as viewed) caused by the port 130 of the main ram being connected to exhaust with the result that the pressure actuated valve 180 is prematurely closed.

Each of the above described actuators described may be operated by any suitable fluid, for example, water, air or oil under pressure or a combination of fluids if the user desires.

The remaining Figures of drawings illustrate a hybrid system in which movement of the driver and control valves is effective to direct compressed air to alternate ones with a pair of fluid reservoirs coupled to respective ports of the main ram and in this way drive the piston rod in different directions.

Figure 11 illustrates schematically a main ram in accordance with this embodiment of the invention. Parts of this ram similar to those already described with reference to Figure 4 are given the same reference numerals but are distinguished by the suffix A.

It will be seen from Figure 11 that liquid (e.g. oil) passes to and from the cylinders 114A, 116A from fluid reservoirs 300, 302 surrounding the cylinders 114A, 116A respectively. The inlets to the cylinders 114A, 116A from

their respective reservoirs 300, 302 are shown at 130A and 132A. These inlets 130A and 132A provide a restriction to flow of liquid from the reservoir to its associated cylinder but offer no resistance to liquid flow from each cylinder to its associated reservoir.

Liquid is drivabie from each reservoir to its associated cylinder by the application of compressed air to inlets 304, 306 to the reservoirs as indicated.

The mechanical operation of the actuator is as described above with reference to Figures 4 to 10.

Operation of the hybrid actuator shown in Figure 11 will now be described with reference to Figures 12 to 15. The inlets 304, 306 of the reservoirs 300, 302 are coupled to respective ports 310, 312 of a spool valve 314 acting as a control valve. Control valve 314 has a valve body 316 movable between a position in which an inlet port 318 is coupled to either of two ports 310, 312. When port 310 is coupled to port 318, port 312 is coupled to an exhaust port 320 and when port 312 is coupled to port 318 port 310 is coupled to an exhaust port 322. Port 318 is coupled to a compressed gas (e.g. air) supply 324.

Movement of body 316 between its two positions is effected making use of a timer piston 326 in a cylinder 328 coupled to receive the liquid in reservoir 302 via a flow control valve 330. An air port to cylinder 328 is shown at 332 and is coupled to one port 334 of a driver spool valve 336. Port 334 of valve 336 may be coupled to a port 338 in

turn coupled to the compressed air supply 324 or to an exhaust port 340 depending upon the position of a valve body 342 of the valve 336. A second port 344 of valve 336 is coupled to a port 346 of the valve 314. A port 348 of valve 336 is coupled via a pressure control flow 350 valve to the compressed gas supply 324. Pressure control valve 350 is controlled in response to the pressure of fluid in the reservoir 300.

Valve body 342 is normally biased leftwards (as viewed in Figures 12 to 15) , for example by a spring, such that it rests on the surface of the cam 134A coupled to the shaft 126A of the actuator.

Port 346 of valve 314 opens into a chamber 352 housing a reversing piston 354. In the relative positions of the actuator parts shown in Figure 12 it is assumed the actuator has driven the gate which it is controlling fully closed and the gate is being held in that position by the automatic mechanical latch. The pressure in the reservoir 300 is relatively high with the result that the pressure control flow valve 350 cuts off the supply of compressed air 324 to port 348 of valve 336. It will be seen that in this position the body 342 of the valve is in its rightmost position (as viewed) with the result that the port 334 of valve 336 is connected to exhaust. Timer piston 326 is also in its rightmost position (as viewed) and liquid from reservoir

302 fills the cylinder 328 to the left (as viewed) of the piston 326.

In this position port 318 is coupled to the compressed air supply 324 applying pressure to the reservoir 300.

When a user releases the latch the pressure in reservoir 300 will drive the fluid therefrom in cylinder 114A causing the piston 110A of the main ram to move in a sense to drive the gate open. This movement will increase the pressure in cylinder 316A such that the fluid in that cylinder is driven back into the reservoir 302 via the port 132A. Air is driven out of the reservoir 302 via the ports 312 and 320 of valve 314 to exhaust.

Movement of the pistons 110A, 112A causes rotation of the shaft 126A until the gate is fully open. This position is illustrated in Figure 13 where it will be seen that the body 342 has risen on cam 134A such that port 338 is coupled to port 334 and port 340 is coupled to the port 334 of valve 336. It will be appreciated that in this position the compressed air supply 324 will be coupled to port 332 driving timer piston 326 leftwards (as viewed) . Thus liquid in cylinder 328 will be driven out of it via the flow control valve 330, to reservoir 332. Movement of the timer piston 326 will continue until it abuts the ends of valve body 316 and eventually shifts that body to the left (as viewed) carrying with it

the reversing piston 354. In this position port 322 is coupled to port 310 and port 318 to port 312 of valve 314. Thus compressed air from the supply 324 will be coupled to drive fluid from the reservoir 302 which will begin to move the piston 112 leftwards (as viewed in Figure 11) and the timer piston 326 rightwards (as viewed in Figure 14) .

The increase in pressure caused by movement of the piston 110A in cylinder 114A will drive the liquid therein via the port 130A back into reservoir 300. It is to be noted that in this condition the pressure control valve 350 is held closed by the pressure of liquid in reservoir 300. Thus movement of the gate will continue until it reaches the gate post and the automatic latch acts to hold it closed. The various components of the actuator will then adopt the position shown in Figure 15. In this position the pressure in the reservoir 300 is reduced and falls below that at which pressure control vale 350 is operative and thus port 348 of valve 336 couples the compressed air supply to port 334 which leads to an increase in the gas pressure in the cylinder 352. This pressure causes the reversing piston 334 to move leftwards (as viewed in Figure 15) until it contacts the leftmost end of the valve body 316 driving the body leftwards, into the cylinder 328. In time the components of the actuator illustrated in Figures 13, 14 and 15 will re-adopt the positions shown in Figure 12. The gate will be held in position by the automatic

mechanical latch.

It will further be appreciated, however, that should the gate meet an obstruction whilst it is closing the pressure in reservoir 300 will immediately reduce allowing pressure control valve 350 to open coupling port 348 to the compressed air supply 324. This will have the effect of immediately beginning to drive reversing piston 354 righttwards until it moves body 316 to a position in which port 318 (which is coupled to the compressed air supply 324) communication with port 310 applying pressure to reservoir 300 and beginning the opening cycle described above again.

The actuators may be operated by simply coupling them to a gate and ensuring the gate is automatically, mechanically, latched when it bears against the gate post or by using any other suitable means.

The flow constrictors described in this Specification may be replaced by bleed valves.

INDUSTRIAL APPLICABILITY It will be seen from the foregoing description that the actuators now presented offer advantages over the known actuators, in particular it is thought that the variation in gate speed when it is moving from an open to a closed position (or vice versa) is aesthetically desirable to users of the apparatus. It is also thought the reversal of operation should a gate be obstructed when closing is particularly advantageous.

Again it is believed that the use of the pressure actuated flow control valve now described enhances operation of the actuator - as does the use of the particular form of the detent O-rings and spring for controlling valve body movement.