Hydraulic control system

Abstract

 An hydraulic control system, particularly for controlling the movements about two axes of the nozzle of a fire fighting monitor. The supplies of working fluid, preferably water, from a source 18 to two double-acting actuators 58, 9 provided for. this purpose are controlled by four pilot operated control valves 13-16 fed with hydraulic pressures signals, transmitted through a different fluid, from a remote hand-operated pressure generator23. The transmission of signal pressures to the control valves may be effected via long lengths of flexible plastics tubing 27A-27D filled with the signal fluid and in order to avoid problems with the propagation of pressure changes through such tubing due to its inherent elasticity, the range of signal pressures to which each control valve is adapted to respond is selected to be at a relatively high level as compared with the pressure of the working fluid.

Description

[0001] The present invention relates to hydraulic control systems.

[0002] Particularly, though not exclusively, the invention is concerned with an hydraulic system for the remote control of the movements of fire fighting devices of the type commonly referred to as cannons, monitors or turrets, (hereinafter collectively referred to as "monitors), which are used for directing jets of water or foam at a fire. Typically, a fire fighting monitor comprises a nozzle borne by a mechanism which permits the orientation of the nozzle to be adjusted by pivotal movements about two orthogonal axes - a generally vertical axis about which the nozzle can be pivoted to traverse its jet from side to side, and a generally horizontal axis about which the nozzle can be pivoted to adjust its angular elevation or depression. Such devices may be embodied as portable free-standing units to be set up as required at the scene of afire, or may be mounted on trailers or self-propelled fire fighting vehicles, or may be used in fixed installations at tanker jetties, oil refineries or other fire risk areas. Similar devices also find application in certain forms of mining and industrial washing processes.

[0003] It will be appreciated that in use of a monitor as described above, it is frequently desirable for the device to be positioned as close to the fire or other target area as practicable, while the operating pesonnel need to be stationed at a safe distance away from the target area. In order to make the best use of the monitor in such circumstances, therefore, there is the need for a system whereby its nozzle-orienting mechanism can be controlled remotely. An electrical control system is a possibility, but is not preferred owing to the danger associated with the sparking of electrical equipment in the inflammable atmospheres within which the monitor may be required to operate. In addition, problems of insulation and continuity of supply may be significant in many fire fighting situations. A safer alternative which has been used is a pneumatically controlled system. A drawback of pneumatic control, however, is that by virtue of the inherent compressibility of the medium involved there is an inevitable lag, and subsequent over-reaction, in the response of the pneumatically operated actuators employed, which increases as the distance between the pressure source and responding mechanism increases, and makes precise control over the orientation of the nozzle difficult to achieve. Accordingly we consider the most favourable solution to be an hydraulic control system, such a system having the advantage that both power and control signals can be transmitted by a medium which is inherently safe and relatively incompressible. In other respects, too, an hydraulic system is eminently suited to operation under the harsh conditions to be expected in fire fighting.

[0004] .In any situation where fire fighting monitors are put into operation there will naturally be supplies of pressurised water readily available, and one of the aims of the invention is to provide a control system which can employ pressurised water as a working fluid to power actuators for adjusting the orientation of the nozzle of a monitor. The water pumps commonly available on fire appliances typically operate in the region of 80 or 100 psig (5-7 bar), which is adequate for the task of driving mechanical actuators for the nozzle-orienting mechanism of a monitor.

[0005] Another aim of the invention is to provide a control system which can employ relatively long lengths of small-bore flexible plastics tubing to transmit hydraulic pressure signals from a remote operator's pontrol station to pilot-operate control valves associated with the device to be controlled. In the embodiment of the invention to be more particularly described hereinafter a tubing length of 50 metres is envisaged. The use of such tubing has many advantages, particularly in relation to portable fire fighting monitors intended to be carried as part of the equipment of a fire appliance or other vehicle and to be set up as required at the scene of a fire - the tubing can be reeled for storage between use and readily deployed in use to link the monitor to a safe, remote control station, the tubing being run around corners or over obstacles as required to permit optimum siting of both the monitor and the control station. In one sense, however, the inherent elasticity of such tubing poses problems. That is, it has been found that the bore of such tubing expands and contracts somewhat in response to changes in the fluid pressure within it, which can have a significant delaying effect on the propagation of pressure changes from one end of the tubing to the other, this effect increasing as the length of the tubing employed is increased.

[0006] To illustrate this effect, curve Cl in Figure 1 of the accompanying drawings indicates a typical form of pressure characteristic which is exhibited at one end of an initially unpressurised, fluid-filled length of flexible plastics tubing when a guage pressure of ^p₁ is applied to the other end (substantially instantaneously) at time T_o. It is assumed that the compressibility of the fluid itself is negligible and that the elastic limit of the tubing is not exceeded. By way of example, with a 50 metre length of nylon tubing of the type indicated in the ensuing particular description, upon the application of a pressure P_I of 800 psig (55 bar) to the other end the time (T₁ - T ) taken for the pressure at the said one end to rise to this value is in the region of a second. Curve C2 indicates the corresponding pressure characteristic at one end of the tubing when initially pressurised to P₁ and then, at time T_2' the gauge pressure at the other end is (substantially instantaneously) relieved to zero. In the same example as quoted above in relation to curve Cl, the time (T₃ - T₂) taken for the pressure at the said one end to decay fully from 800 psig (55 bar) to zero is also in the region of a second (although the two periods (T₁ - T₀) and (^T₃ - ^T₂) are not necessarily equal in all cases).

[0007] It should be noted at this point that for a given type and length of tubing the curve Cl is only one of a family of curves which would be required to illustrate the pressure response of the tubing for all applied pressure rises. For example, if the pressure applied to the other end was the value P₂ shown on Figure 1, the characteristic at the said one end would be somewhat as indicated by curve C3.

[0008] Generally, however, the curve C2 (or the relevant part of it) is applicable to the decay of pressure whatever its initial value. Thus if the pressure P₂ at the opposite end of the tubing was relieved to zero at time T₅ the characteristic at the said one end would be represented by that part of the curve C₂ between T₅ and T₃. In the terms of the example quoted above, P₂ represents a pressure of 100 psig (7 bar) ,and it will be seen that whereas P₂ is only one eighth of P₁, the time (T₃ - T_S) taken for this pressure to decay at the said one end is just over one half of the time (T₃ - T₂) taken for the pressure to decay from P₁. In any remote control system it is, of course, a requirement that the device to be controlled has an adequately rapid response to transmitted control signals if precise control is to be achieved and problems of over- and under- control avoided. In practice, dealing for example with a system for controlling the movements of a fire fighting monitor nozzle, some delay and possibly over-reaction in the initiation of a movement can be tolerated provided, however, that once initiated the rate of movement can be brought rapidly under control and, most importantly, that movement can be terminated rapidly when the nozzle reaches a desired position. In the case where hydraulic pressure signals are transmitted to pilot-operated control valves by tubing having the characteristics indicated in Figure 1, therefore, it is desirable for the valves to have an. operational pressure or range (i.e. the signal pressure or range of signal pressures required to move the valves from one limiting condition to another) which lies on a part of the curve C2 where there is a steep gradient - for example between pressures P3 and P₄. In the case of the example quoted above this represents an operational range of 400 to 450 psig (27-31 bar). Clearly, if the operational range lay somewhere between zero and P₂ the valve response would be unacceptably slow and it follows that, in the case of the quoted example, if pressurised water at 80 or 100 psig (5-7 bar) is used as the working fluid for mechanical actuators controlled by pilot valves in response to pressure signals transmitted by such tubing the conventional practice of tapping off the signal fluid from the main supply of working fluid cannot be followed if an adequate valve response is to be achieved.

[0009] Accordingly, in one aspect the present invention resides in an hydraulic control system for a mechanical device, comprising: at least one actuator adapted to perform a function in relation to the mechanical device under the action of a pressurised hydraulic working fluid; at least one pilot-operated control valve associated with the mechanical device for controlling the supply of said working fluid to said actuator in a manner determined by the pressure of an hydraulic signal fluid transmitted thereto; means, intended to be located remote from the device, for generating signal pressures in said signal fluid in response to an operator's control; and conduit means for transmitting signal pressures from said generating means to said control valve; the arrangement being such that, in use, the signal pressure or range of signal pressures to which said control valve responds is or are of a greater value than the working pressure at which said working fluid is supplied to that valve.

[0010] In the light of the foregoing, such a system is particularly applicable to the control of a fire fighting or the like monitor where the working fluid is water and said conduit means comprise a length of small-bore flexible plastics tubing. The bore of such tubing may be no more than, say 5 mm,preferably less than 2.5mm. The invention can, however, equally by applied to the remote control of other devices, e.g. cranes and the like article-handling equipment, and with other working fluids if desired. Neither is it essential that the conduit means by which pressure signals are transmitted comprise flexible plastics tubing. In particular the invention may also be found useful where pressure signals are transmitted through conventional steel pipework or other conduit means which can normally be regarded as 'rigid' (i.e. inexpansible by the transmitted pressures) and by hydraulic fluids which can normally be regarded as incompressible, where the length of conduit is so great, however, that at conventional signal pressure levels the elasticity of the conduit and/or compressibility of the fluid would have a delaying effect on the propagation of pressure changes in effectively the same way as described above for flexible plastics tubing. Thus the invention makes it possible to employ 'rigid' conduit lengths e.g. in the order of hundreds of metres long to transmit signal pressures and still obtain an adequate valve response, the signal pressures being e.g. in the order of hundreds of bar in such a case. Of course a system according to the invention can still be used, if desired, where the conduit type and length is such that elasticity or compressibility effects are not significant.

[0011] In a preferred embodiment of the Control System especially adapted for use with a fire fighting monitor, there are a pair of double-acting actuators for controlling the elevation/depression and traverse positions of the monitor nozzle, respectively, and four pilot-operated control valves each one for controlling the supply of working fluid to a respective side of a respective actuator, there being a correspondong number of conduits for transmitting signal pressures from the generating means to the valves. Each such valve is biased into a position in which it can communicate the source of working fluid with the respective side of the respective actuator, and each valve is operable independently,against its bias, in response to the transmission thereto of a selected signal pressure or range of signal pressures, to restrict the communcation of the source of working fluid with the respective side of the respective actuator and to communicate that side of that actuator with a relatively unpressurised reservoir, drain or the like region whereby a pressure differential is set up across the actuator under which it performs the corresponding function in relation to the monitor.

[0012] A preferred form of pilot-operated control valve usable in a system as defined above comprises: means defining a chamber; first, second and third ports opening to said chamber; first and second valve seats respectively surrounding said first and second ports; first and second valve elements disposed in said chamber for co-operation respectively with said first and second seats to control fluid flow through said first and second ports; first and second spring means for biasing respectively said first and second valve elements against said first and second seats; means for unseating said first valve element against the bias of said first spring means in response to a selected signal pressure or range of signal pressures; and means for establishing an operative connection between said first and second valve elements when said second valve element moves away from said second seat against the bias of said second spring means by a certain distance, whereby when such connection is established the operation of said unseating means is effective both to move said first valve element away from said first seat and to move said second valve element back towards said second seat.

[0013] When such a valve is used in a control system as defined above said first port is connected to the aforesaid drain or other relatively unpressurised region, said second port is connected to the source of pressurised working fluid and said third port is connected to the respective side of an actuator. Normally the second valve element is seated under the bias of its spring means so as to isolate the third port and actuator from the relatively unpressurised region. The second valve element can, however, be unseated by the working fluid against the bias of its spring means in the manner of a non-return valve, the working fluid thus being supplied through the chamber and third port to the actuator.

[0014] Upon operation of the unseating means (which in this case may comprise, e.g. a slidable piston to one side of which the signal pressure is applied) the first valve element is unseated to communicate the actuator with the relatively unpressurised region while the second valve element, through the aforesaid operative connection with the first valve element, is moved towards its seat to restrict the communication of the source of working fluid with the actuator.

[0015] The invention will now be further described, by way of example, with reference to Figures 2 to 4 of the accompanying drawings, in which:

Figure 2 illustrates a fire fighting monitor equipped with an hydraulic control system according to the invention;

Figure 3 is a schematic diagram of the control system; and

Figure 4 illustrates schematically the structure of a pair of control valves suitable for use in the system of Figure 3.

[0016] Referring to Figure 2, there is shown a portable, ground- standing fire fighting monitor 1 which in the illustrated example is of the spherical head type and which in particular may be constructed as described in our copending United Kingdom patent application no. 8107483. Briefly, this monitor comprises a head 2 which carries a nozzle via an outlet fitting 3 and which is borne in a housing 4 for pivotal movement about a horizontal axis so as to adjust the angular elevation or depression of_the nozzle. The assembly of the head 2 and housing 4 is also rotatable as a whole about a vertical axis so as to traverse the nozzle from side to side, to this end the assembly being fast on the upper end of a hollow vertical axle (not shown) which is borne rotatably in the lower body 5 of the monitor and which also serves to lead water to the head 2 from main water inlets 6 in each side of the body 5. In the illustrated device these elevation/depression and traverse adjustments can be effected either manually (for which purpose a handle-bar 7 is provided), or remotely by means of the hydraulic control system to be described below. The hydraulic control system includes a pair of water- driven double-acting, rotary-output actuators 8 and 9 mounted to the monitor, one each for controlling the horizontal axis and vertical axis movements of the monitor nozzle. The output member of the actuator 8 drives an extension of the axle (not shown) by which the head 2 is pivoted in the housing 4, while the output member of the actuator 9 drives a ring gear (not shown) within the body 5 which is keyed to the vertical axle which bears the housing 4/head 2 assembly. These actuators are connected to a valve block 10 at the rear of the monitor by means of suitable pipework, elements of which are indicated at 11 in Figure 2, and a water supply to the valve block is provided by a pipe 12 fed from the inlets 6. Turning to Figure 3, each actuator 8,9 has a pair of ports 8A, 8B and 9A, 9B opening to chambers either side of a movable piston 8C, 9C whereby pressure differences between the ports in each pair result in movement of the respective piston which drives a sector gear (not shown) to give a rotary output to move the monitor nozzle in a corresponding sense. The valve block 10 is also seen to include four identical three-port pilot-operated control valves 13, 14, 15 and 16. One port of each control valve, designated by the suffix A, incorporates a respective check valve element 17 and is connected to a common inlet 18. In use this inlet is supplied from the pipe 12 of Figure 2 with pressurised water at, say, 80 or 100 psig (5-7 bar), which is provided to the monitor from a suitable appliance-mounted pump. A second port of each control valve, designated by the suffix B, is connected to a common drain 19 from the valve block. The third port of each control valve, designated by the suffix C, is connected to a respective port 8A or 8_B, 9A or 9B of a respective actuator. Also depicted in Figure 3 is a five-port manually operable valve 20 (the control member of which is seen at 21 in Figure 2), the purpose of which will be described hereinafter. It will be seen from the Figure that each actuator port 8A, 8B, 9A and 9B is connected to a respective port 20A, 20B, 20D or 20E of the valve 20. However, when the monitor is being operated under remote control each port of the valve 20 is disconnected from all of the others and in this condition, therefore, the existence of the valve 20 has no effect upon the rest of the hydraulic system.

[0017] As depicted schematically in Figure 3, each control valve 13 - 16 comprises a movable valve member biased into one limiting position by a spring. In this position the valve is effective to communicate its 'A' port with its 'C' port but to blank its 'C' port from its 'B' port. While each valve remains in this position, it follows that water from inlet 18 is supplied past the check valve elements 17 to each of the actuator ports 8A, 8B and 9A, 9B, all at the same working pressure, while the drain 19 is isolated from each of these ports. Consequently, each actuator piston 8C, 9C is hydraulically locked in position and, in the event that an unbalanced, external load is applied to the monitor, tending to displace the piston of either actuator from its set position, and thereby to displace water from one side of the actuator back through the 'C' and 'A' ports of the corresponding control valve, the check valve element 17 of that valve will close to prevent such displacement. In order to effect movement of the piston in the respective actuator 8 or 9 and thereby adjust the orientation of the monitor nozzle about the corresponding axis, one of the valves in the pair 13, 14 or 15, 16 mustbe displaced from its illustrated position, and this can be achieved by the application of a signal fluid pressure in opposition to the bias of the respective valve spring.

[0018] For this purpose there is provided a pilot system filled with hydraulic oil. This system includes a control unit generally indicated at 22 in Figure 3, which in use is positioned at a location remote from the monitor, so that the monitor can be positioned close to the fire while the operating personnel are permitted to be stationed a safe distance away. The control unit includes a manually operable signal generator 23, which may comprise e.g. a hand lever working a piston in a cylinder filled with the hydraulic oil. Connected to the generator is a manual selector valve 24 whereby the generated pressure signals can be transmitted to a selected one of four separate hydraulic outlets 25A - 25D in the control unit. A preferred form of combined signal generator and selector valve for use as the unit 22 is described in our co-pending United Kingdom patent application no. 8107484. Corresponding to the outlets 25A - 25D the block 10 includes four separate hydraulic lines 26A - 26D which lead respectively to the control valves 13 - 16, and the respective outlets 25 and lines 26 are connected together by four individual small-bore thermoplastics hoses 27A - 27D made up into a flexible, 50 metre long loom 28. With reference to Figure 2, the loom 28 is normally stored on a reel 29 which houses also the control unit 22, and which is carried together with the monitor 1 to the scene of the fire. After setting up the monitor and connecting its water supply, the operator detaches the reel 29 and retreats with it to a safe distance while the loom 28 unwinds; the control unit 22 is withdrawn from the reel at the selected remote location, and the operator is now ready to assume control of the movements of the monitor nozzle.

[0019] By way of example, the hoses 27 may each comprise a length of plain 3.175 mm nominal outside diameter 2.375 mm nominal inside diameter tubing made from nylon 12, e.g. TECALAN (Trade Mark) TTR-L tubing as manufactured by the Plastics Division of Tecalemit (Engineering) Limited.

[0020] Returning to Figure 3, the selector valve 24 is shown as connecting the signal generator 23 with outlet 25C. Consequently in this condition pressure signals generated by an operator manipulating the lever of generator 23 are transmitted to outlet 25C and then through hose 27C and line 26C to the movable valve member of control valve 16. When a given pressure is generated, the bias of the spring acting on the valve member will be overcome and the valve member will be displaced from its illustrated position. As indicated schematically in Figure 3, in the limiting position of the valve member opposite to that in which it is shown it will communicate port 16C with port 16B while port 16A is blanked off from port 16C. In intermediate positions the communication between ports 16A and 16C progressively decreases while the communication between ports 16C and 16B progressively increases. It will be appreciated, therefore, that in any such position a passage is opened up between the actuator port 9B and the drain 19 while at the same time the pressure of water which can be supplied through the valve to port 9B is reduced. As a result a pressure difference will exist between the ports 9A and 9B of the actuator 9 under which the piston 9C moves so that the monitor nozzle is traversed, say, from right to left, in so doing water draining from the actuator through port 9B and drain 19 while the volume is made up on the other side of the piston by water supplied from inlet 18 to port 9A via valve 15. The rate at which the piston moves depends upon the relative sizes of the passages between the ports of valve 16 or in other words upon the relative position of the valve member, and this in turn is determined by the applied signal pressure. When the operator wishes to terminate movement of the nozzle he simply releases the lever of the signal generator, allowing the signal pressure to decay so that the valve member is returned by its spring to the position illustrated in Figure 3, whereupon the actuator piston 9C once more becomes hydraulically locked, in the new position into which it has been moved.

[0021] As will be appreciated, if the operator wishes to traverse the nozzle from left to right instead of from right to left, he selects outlet 25D at the selector valve 24 and thereby applies a signal pressure to control valve 15. This will react in the same manner as described above in relation to valve 16, on this occasion the water pressure at port 9A being reduced so that the actuator piston now moves in the opposite sense. Similarly, if the operator wishes to elevate or depress the nozzle he will move the selector valve so as to apply a signal pressure to control valve 13 or 14 as appropriate.

[0022] In order for the operator to exert precise control over the positioning of the monitor nozzle it is most desirable, as previously explained, for the control valves 13 - 16 to react rapidly to the operator's manipulations of the signal generator 23, especially in the sense of termination of the movement of the nozzle. To an extent, also as previously explained, this requirement conflicts with the desirability of using long lengths of flexible plastics hose in the transmission of the pressure signals from generator 23 to the control valves because such hose exhibits certain elasticity effects as its internal pressure is varied. In accordance with the invention, however, this conflict can be satisfactorily resolved by the judicious selection of the range of signal pressures to which the control valves are adapted to respond, the selected range being at a relatively high level as compared with the working pressure of the water which drives the actuators 8 and 9. For the hoses 27 exemplified above, the pressure which is chosen to initiate displacement of the control valve members is in the region of 400 psig (27 bar), while the pressure required for maximum .displacement of the control valve members is in the region of 450 psig (31 bar).

[0023] Various forms of pilot-operated control valve suitable for use as the valves 13 - 16 are possible, it being appreciated that the desired pressure response of the valve will depend upon the characteristics of the valve biasing spring and the effective area of the movable valve member which is exposed to the signal pressure. However, a preferred form of valve is shown in Figure 4 and will now be more fully described with reference to that Figure.

[0024] Figure 4 shows a pair of control valves, say valves 13 and 14 of the system described above, incorporated in the valve block 10. The valve pair 15 and 16 will be similar. Each valve has a compound valve member comprising a piston 30 slidably sealed in a bore 31; a first ball 32 in a chamber 33 and urged by a spring 34 against a seat 35, to control communication through the respective valve port 13B, 14B between the chamber 33 and bore 31; and a second ball 36 urged by a spring 37 against a seat 38, to control communication through the respective valve port 13A, 14A between the chamber 33 and a bore 40. The spring 37 acts between the ball 36 and the ball 32 via a spacer 41, and is lighter than the main valve biasing spring 34. Opening from each chamber 33 between the seats 35 and 38 are the respective valve ports 13C and 14C which connect through respective bores 42 to the ports 8A, 8B of the actuator 8. A bore 43 leads to the bore 40 between the two seats 38, and is connected to the source of working fluid, i.e. to the inlet 18. A respective bore 44 leads from the bore 31 of each valve between the seat 35 and piston 30, this constituting the exhaust connection of the respective valve and leading to the drain 19. Finally, the respective signal pressure lines 26A and 26B lead to the bores 31 of the valves on the side of the respective piston 30 opposite to the seat 35.

[0025] Figure 4 illustrates the valves in the condition which pertains when there is no pressurised working fluid connected to the bore 43 and no signal pressures applied to the lines 26A and 26B, i.e. both balls 32, 36 of each valve are on their respective seats 35, 38. In normal operation, however, when the pressurised water supply is connected to bore 43, and thence to bore 40, each ball 36 can be displaced from its seat 38 against the biasing action of its spring 37 to seat instead against the spacer 41, thereby opening the chambers 33 to the bore 40 through the respective port 13A, 14A. In the absence of a signal pressure in lines 26A, 26B the balls 32 remain on their seats 35 under the action of the springs 34, so the full water pressure supplied to the chambers 33 from the bore 40 is transmitted via_respective ports 13C, 14C and bores 42 to opposite sides of the actuator 8, to hydraulically lock its piston 8C.

[0026] When a signal pressure of adequate strength is now applied to, say, the line 26A, the piston 30 of valve 13 is moved to the right (in the sense of Figure 4) and by way of its integral piston rod 45 unseats the ball 32 against the biasing action of its spring 34. More particularly the entire assembly of piston 30/ball 32/spacer 41/ball 36 behaves in effect as a single valve element moved to the right (in the sense of Figure 4) under the action of the signal pressure in line 26A. The effect of this movement is to open the chamber 33 of valve 13 through port 13B to its bore 21 and thence bore 44 by virtue of the unseating of ball 32, and at the same time to restrict the communication through port 13A of the bore 40 with its chamber 33 by virtue of the movement of spacer 41 and ball 36 back towards the seat 38. As will be appreciated the size of the opening between the bore 40 and chamber 33 progressively decreases as the size of the opening between the chamber 33 and bore 31 progressively increases, the displaced position of the compound valve member 30/32/41/36 being determined by the strength of the applied signal pressure opposing the spring 34. As will be appreciated, in any such position there will be a corresponding reduction in the water pressure within the chamber 33 of valve 13 and hence in the pressure applied to port 8A of the associated actuator. Throughout, however, the valve 14 is unaffected and the full water pressure remains applied from its chamber 33 to port 8B of the actuator. The actuator piston 8C therefore moves to the left (in the sense of Figure 4) under the differential pressure at ports 8A and 8B, water draining from the actuator through port 8A and passing through the bore 42, port 13C,chamber 33, port 13B and bore 31 (of valve 13), to the respective bore 44 and drain 19. As will also be appreciated, when the signal pressure is released from line 26A the ball 32 of valve 13 is allowed to re-seat under the action of its spring 34, so that the actuator piston 8C is relocked in its new set position.

[0027] The balls 36 have a non-return function in addition to their function of controlling the water pressure in the respective chamber 33 when a signal pressure is applied to the respective valve, this non-return function being equivalent to that of the check valve elements 17 of Figure 3. That is to say in the event that an unbalanced, external load is applied to the monitor tending to displace the actuator piston 8C from a position in which it has been set, and thereby to displace water from one side of the actuator back through the respective chamber 33 to the bore 40, the corresponding ball 36 will close against its seat 38 to prevent such displacement. In fact, by virtue of the biasing action of its spring 37 each ball 36 will tend to close against its seat 38 whenever the pressure within the corresponding chamber 33 is in balance with the supply pressure in the bore 40.

[0028] Returning to Figure 3, it has been indicated that the effect of the pressurised water connections to each actuator 8 and 9 is to hydraulically lock the actuator pistons in place, except when one of the control valves 13 - 16 is being operated to effect a controlled adjustment of the orientation of the monitor nozzle. There may be occasions, however, when it is desired to adjust the orientation of the monitor nozzle by shifting it manually even while the hydraulic control system is set up, and to do this it is necessary to override the hydraulic locking of the actuator pistons. This can be achieved by manipulating the valve 20 to connect its ports 20A and 20B together and to connect its ports 20D and 20E together, while port 20C remains isolated. With these connections made it will be seen from Figure 3 that the two ports 8A, 8B and 9A, 9B of each actuator are now connected together through respective loops of pipework, by-passing the control valves 13 and 14 or 15 and 16, and it is therefore now possible to move the nozzle and its connected actuator pistons by hand, in so doing water being displaced from one side of a piston to the other via the respective loop. Once an adjustment has been made in this fashion and the valve 20 is returned to its original state, the actuator pistons will once more be hydraulically locked in their new positions.

[0029] Valve 20 has a further function, which is that it can also connect all of the ports 20A - 20E together to enable the water within the block 10 and actuators 8 and 9 to drain away through drain 19 (the water source being disconnected at this time). Draining the water from the system after use is a useful precaution against corrosion and deposition (particularly bearing in mind that contaminated or seawater, for example, may have been used) and against freezing in cold weather. To permit this operation the actuators 8 and 9 have automatic "snifter" valves, (not shown), which admit air to the system when it is being drained and which also bleed air from the system when it is being filled. Normally, the pilot system will not be drained as the same problems of corrosion, deposition or freezing do not arise with its hydraulic fluid. Therefore, when the loom 28 of hoses 27 is reeled between uses it will remain connected to the rest of the pilot.system as indicated in Figure 2, or alternatively valved connectors can be used which retain the hydraulic fluid if the hoses are to be disconnected.

Claims

1. An hydraulic control system for a mechanical device (1), comprising: at least one actuator (8;9) adapted to perform a function in relation to the mechanical device under the action of a pressurised hydraulic working fluid; at least one pilot-operated control valve (13;14;15;16) associated with the mechanical device for controlling the supply of said working fluid to said actuator (8;9) in a manner determined by the pressure of an hydraulic signal fluid transmitted thereto; means (23), intended to be located remote from the device, for generating signal pressures in said signal fluid in response to an operator's control; and conduit means (27A;27B;27C;27D) for transmitting signal pressures from said generating means (23) to said control valve (13;14;15;16) characterised in that, in use, the signal pressure or range of signal pressures to which said control valve (13;14;15;16) responds is or are of a greater value than the working pressure at which said working fluid is supplied to that valve.

2. A control system according to claim 1 characterised in that working fluid is provided from a source (18) of pressurised water.

3. A control system according to claim 1 or claim 2 characterised in that said conduit means (27A;27B;27C;27_D) comprise a length of flexible plastics tubing.

4. A control system according to claim 3 characterised in that the bore of said tubing is no greater than approximately 5mm.

5. A control system according to claim 4 characterised in that the bore of said tubing is no greater than approximately 2.5mm.

6. A control system according to any preceding claim characterised in that there are two said actuators (8,9) being double-acting actuators adapted to perform respective functions in relation to the mechanical device (1); thereare four said pilot-operated control valves (13-16) each one for controlling the supply of said working fluid to a respective side of a respective said actuator (8,9); and there are respective said conduit means (27A-27D) for transmitting signal pressures from said generating means (23) to said control valves (13-16); each said valve (13-16) being biased into a position in which it can communicate a source (18) of said working fluid with the respective side (8A; 8B; 9A; 9B) of the respective actuator; and each said valve (13-16) being operable independently, against its bias, in response to the transmission thereto of a selected signal pressure or range of signal pressures, to restrict the communication of the source (18) of working fluid with the respective side (8A;8B; 9A;9B) of the respective actuator and to communicate that side of that actuator with a relatively unpressurised region (19) whereby a pressure differential is set up across the actuator (8,9) under which it performs the corresponding function in relation to the mechanical device (1).

7. A control system according to claim 6 characterised in that each said control valve (13-16) comprises: means defining a chamber (33); first (13B-16B), second (13A-16A) and third (13C-16C) ports opening to said chamber; first (35) and second (38) valve seats respectively surrounding said first and second ports; first (32) and second (36) valve elements disposed in said chamber (33) for cooperation respectively with said first (35) and second (38) seats to control fluid flow through said first and second ports; first (34) and second (37) spring means for biasing respectively said first (32) and second (36) valve elements against said first (35) and second (38) seats; means (30) for unseating said first valve element (32) against the bias of said first spring means (34) in response to a selected signal pressure or range of signal pressures; and means (41) for establishing an operative connection between said first (32) and second (36) valve elements when said second valve element (36) moves away from said second seat (38) against the bias of said second spring means (37) by a certain distance, whereby when such connection is established the operation of said unseating means (30) is effective both to move said first valve element (32) away from said first seat (35) and to move said second valve element back towards said second seat (38); in use the first port (13B-16B) of each said valve being connected to said relatively unpressurised region (19), the second port (13A-16A) of each said valve being connected to the source (18) of working fluid, and the third port (13C-16C) of each said valve being connected to the respective side (8A,8B,9A,9B) of the respective actuator.

8. A control system according to claim 7 characterised in that, in each said control valve (13-16): said first (13B-16B) and second (13A-16B) ports are located at opposite ends of said chamber (33) with the third port (13C-16C) at an intermediate location; said first spring means (34) act effectively between said first valve element (32) and a fixed abutment while said second spring means (37) act effectively between said second valve element (36) and said first valve element (32); and said means for establishing an operative connection comprise a rigid element (41) which is seated at one end against the first valve element (32) and which is engaged at its other end by the second valve element (36) when the latter (36) moves away from its seat (38) by the aforesaid distance.

9. A control system according to any one of claims 6 to 8 characterised by further valve means (20) which are selectively operable to connect together the two sides (8A,8B,9A,9B) of each respective actuator, to permit the displacement of working fluid out of one side of an actuator and into the other side of the same independently of said pilot-operated control valves (13-16).

10. A control system according to any one of claims 6 to 9 characterised by further valve means (20) which are selectively operable to connect both sides (8A,8B,9A,9B) . of both actuators simultaneously to said relatively unpressurised region (19).

11. A control system according to any preceding claim characterised in that said generating means (23) comprise means selectively operable to pressurise said signal fluid by the exercise of manual effort.

12. A control system according to any one of claims 6 to 10 characterised in that said generating means (23) comprise means selectively operable to pressurise said signal fluid by the exercise of manual effort, and there is associated with said means (23) a manually- operable valve (24) for communicating the pressure generated thereby to a selected one at a time of said respective conduit means (27A-27D).

13. A fire fighting or the like liquid-projecting monitor (1) characterised in that it is equipped with a control system according to any one of claims 6 to 10 or claim 12, a first (8) of said actuators controlling the angular elevation or depression of the nozzle of the monitor (1) and the second ( 9) of said actuators controlling the angular traverse position of such nozzle.

14. A monitor according to claim 13 characterised by means (6) for receiving a pressurised supply of liquid for projection by the monitor (1), and means (12) for passing a proportion of said liquid to the control system of the monitor to constitute said working fluid.

15. An hydraulic control system for a mechanical device (1), comprising; a pair of double-acting actuators (8,9) adapted-to perform respective functions in relation to the mechanical device under the action of a pressurised hydraulic working fluid; and four pilot-operated control valves (13-16) each one for controlling the supply of said working fluid to a respective side (8A,8B,9A,9B) of a respective actuator; characterised in that each such valve (13-16) is biased into a position in which it can communicate a source (18) of said working fluid with the respective side (8A,8B,9A,9B) of the respective actuator; and each such valve (13-16) is operable independently, against its bias, in response to the transmission thereto of a selected signal pressure or range of signal pressures, to restrict the communication of the source (18) of working fluid with the respective side (8A,8B,9A,9B) of the respective actuator and to communicate that side of that actuator with a relatively unpressurised region (19) whereby a pressure differential is set up across the actuator (8,9) under which it performs the corresponding function in relation to the mechanical device (1).

16. A fluid control valve (13-16) characterised by: means defining a chamber (33); first (13B-16B), second (13A-16A) and third (13C-16C) ports opening to said chamber; first (35) and second (38) valve seats respectively surrounding said first and second ports; first (32) and second (36) valve elements disposed in said chamber (33) for cooperation respectively with said first (35) and second (38) seats to control fluid flow through said first and second ports; first (34) and second (37) spring means for biasing respectively said first (32) and second (36) valve elements against said first (35) and second (38) seats; means (30) for unseating said first valve element (32) against the bias of said first spring means (34) in response to a control action; and means (41) for establishing an operative connection between said first (32) and second (36) valve elements when said second valve element (36) moves away from said second seat (38) against the bias of said second spring means (37) by a certain distance, whereby when such connection is established the operation of said unseating means (30) is effective both to move said first valve element (32) away from said first seat (35) and to move said second valve element (36) back towards said second seat (38).

Drawing