Abstract: The throttle-part of the valve comprises a two-way seat valve with a follow-up throttle valve (8), which is driven by a differential guide piston (12) and an edge control system (20). The guide piston is pressurized via a pressure-spring (16) in a pressure-spring compartment in the direction of the closed position of the edge control system. The drive is effected via a pilot line A which pressurizes or depressurizes the pressure-spring compartment (18) via a proportional directional control valve (6), while the position of the guide piston (12) is determined by an inductive displacement transducer.

Abstract: A fluid-operated valve includes a control spool, actuated by a source of pilot pressure to control a pilot flow of fluid from a loading pressure inlet port to a drain line. The pilot flow passes a throttle which reduces the fluid pressure acting in one direction, thereby performing stroke compensation of the actuation of a valve poppet. A differential pressure compensation valve is moved to partially block fluid flow to a rate control orifice group in response to a difference in pressure on the two sides of the rate control orifice group to control further the actuation of the valve poppet, whereby differential pressure is compensated.

Abstract: There is provided a closed loop system for measuring an amount of liquid under pressure in a container. The system comprises a container holding a liquid under pressure, a novel differential valve with check valves which are connected to the container and to a differential gauge, and a means for activating the valve.

Abstract: A drive for a feed valve having an actuating line and a device for controlling the pressure in the actuating line. The device comprises three valves connected to one another to form a hydraulic auctioneering circuit. The drive is suitable for a comparatively high oil pressure, and the drive is provided with a test system which monitors the operativeness of the drive. The test system is pressurized by the device, and a pressure drop can be sensed in the test system by a sensor.

Abstract: An actuator for a control valve with a control circuit has a main piston which slides in a main cylinder. On one side of the main piston, there is arranged a drive volume, which can be pressurized with oil in a controlled manner, on the other side of which there is arranged an oil-filled buffer volume. The actuator despite having a greater capacity, has a comparatively high dynamic performance which has positive effects particularly when closing the control valve. This is achieved in that the drive volume and the buffer volume can be connected through a connecting line which can be sealed off by a valve. This valve is activated directly hydraulically via a second valve.

Abstract: In a fuel management system, shutoff means in a fuel supply line (12) closes when the pressure at a port (28) on the shutoff means exceeds some predetermined level. A most selector valve (42) attached to the port (28) connects to two electrically and mechanically independent shutdown valves (50, 52). Actuating either valve (50, 52) pressurizes the port (28), closing the shutoff means and stopping all flow through the supply line (12).

Abstract: A gas venting device serves to automatically closing and opening vent holes of a foam mold so that air and reaction product gases are sufficiently exhausted and a foamed product is effectively prevented from extending through the vent holes. The gas venting device includes valves for closing and opening the vent holes, valve driving cylinders for driving the valves, a pilot operated two-position change-over valve, pressure tanks connected through the pilot operated two-position change-over valve to the valve driving cylinders, and mechanical change-over valves connected to the pilot operated two-position change-over valve for changing over it by dogs provided on a passage of the foam mold. The device further includes an air timer for delaying closing the vent holes by the valves.

Abstract: A tank pressure control valve is provided for controlling the discharge of fuel vapor from a fuel tank. In one embodiment, the tank pressure control valve is configured to use pressurized fuel vapor to control a flexible diaphragm operable between a closed position blocking discharge of fuel vapor from the fuel tank and an opened position allowing such discharge. In another embodiment, the tank pressure control valve is configured to use pressurized fuel vapor to move a flexible diaphragm to move a rigid valve member between a closed position and an opened position.

Abstract: Universal binding element for binding loose documents in a file, characterized in that it principally consists of an inner cover formed of a first part (2) that is intended for forming a front sheet; a second part (3) that is intended for forming a back sheet; and a third part (4) that is intended for forming a spine, which includes adhesive means (5) which are formed of glue (9, 10, 23) meltable under the influence of heat and which, when heated and subsequently cooled can provide for an adhesive connection on both the inside (6) and the outside (7) of the formed spine.

Abstract: An on demand, non-contact sensor controlled flush valve actuation system includes a flush valve between a water supply and a plumbing fixture. An actuator for the flush valve includes a moveable piston in fluid communication with a control water inlet, with a control water conduit carrying water from the flush valve through a solenoid operated control valve to the actuator assembly. A radiation sensing/detecting unit generates a first control signal in response to the return reflection of infrared radiation. A timer receives the first control signal and, in response thereto, generates a second control signal of a predetermined duration which activates the solenoid. When the solenoid is activated, a portion of the pressurized supply water flows into the actuator assembly and against a piston therein, with the piston moving to operate a trip mechanism for the flush valve.

Abstract: A main poppet body of a logic valve receives inlet pressure at first and second ends. A flow-restricting orifice between the first and second ends produces a differential pressure between the two ends when inlet fluid is permitted to flow from the second end. The main poppet body is normally resiliently seated against a seat. A pilot poppet body, movable in the main poppet body, seats against a seat in the main poppet body. The pilot poppet body, when sealed to its seat, seals the fluid at the second end of the main poppet body from flowing. Thus, equal pressures exist at opposed ends of the main poppet body, and the main poppet body remains in its sealing position. An external source of controllable pilot pressure acts on the pilot poppet body to unseal it from its seat. This permits a flow of inlet fluid from the second end of the main poppet body, and produces a pressure reduction at the second end.

Abstract: A control circuit includes one or more pressure activated fluid flow valves and one or more moisture and pressure sensitive controllers. Each controller includes a body with three chambers separated by movable diaphragm assemblies, one of which moves in response to changes in pressure exerted by the amount of moisture or fluid in the environment. Responsively, a passageway through the other diaphragm allows the passage of fluid and thus venting of the flow valves when a low moisture or fluid condition is detected.

Abstract: Electrical contacts (10) and (12) are bridged by surface (22) when a fluid flow rate from fluid inlet (30) to exit (34) is sufficient to displace poppet (26) against the bias of bellows (28). Bellows (28) may be sealed at opposite ends to provide an environment for contacts (10) and (12) that is isolated from the fluid.

Abstract: A proportional priority flow regulator valve structure comprising a cartridge-type modulated orifice valve element (MOS) in a composite multiple element valve assembly. The MOS valve operates in an hydraulic circuit to receive pressurized fluid from a source of supply and responds to variations in fluid pressure and fluid flow rates acting hydraulically on a pair of inter-dependent spring-biased spools of the valve to provide a modulated orifice for delivery of a controlled fluid flow.

Abstract: An air actuated valve system for shutting off fluid flow in a fluid conduit includes a seismic valve disposed in a seismic air circuit which is coupled to a primary air circuit. The primary air circuit delivers air at a high pressure to an air driven actuator which maintains a shut off valve in a fluid conduit in an open position. The high air pressure flows through an air switch which is operationally open and coupled to the seismic air circuit such that when the seismic valve is closed a pilot air pressure is prevented from flowing to the air switch. In the absence of the pilot air pressure, air pressure is bled from the air switch, causing the air switch to move from an open to a closed position, thus preventing air from flowing to the air actuator.

Abstract: A rapid acting valve capable of delivering a high pressure impulse of compressible fluid has a pilot section, an intermediate section and an output section which are pressurized to approximately equilibrium with the inlet pressure during the load portion of the operating cycle and which cooperate to provide extremely rapid full opening of the output section to release fluid uner pressure from an accumulator through an output section piston seat. The valve is particularly suitable for use in combination with pneumatic impulse separation systems such as those employed for deicing aircraft surfaces.

Abstract: An operator for remotely opening and closing the valve on a gas cylinder includes a metal frame having an enclosed upper portion, a support plate extending across the upper portion and a mounting wall extending downwardly from a side of the upper portion. On the mounting wall is a C-shaped clamp which is adapted to extend around and be secured to a non-cylindrical portion of the cylinder valve to secure the frame to the cylinder. On the mounting wall below the clamp is an additional clamping means which extends around a cylindrical portion of the cylinder to further secure the frame to the cylinder. A pneumatic activator is mounted on the support plate and has a shaft extending through an opening in the support plate in alignment with the valve stem of the cylinder valve. A coupling member is on the shaft and connects the shaft to the valve stem.

Abstract: An air actuated valve system for shutting off fluid flow in a fluid conduit includes a seismic valve disposed in a seismic air circuit which is coupled to a primary air circuit. The primary air circuit delivers air at a high pressure to an air driven actuator which maintains a shut off valve in a fluid conduit in an open position. The high air pressure flows through an air switch which is operationally open and coupled to the seismic air circuit such that when the seismic valve is closed a pilot air pressure is prevented from flowing to the air switch. In the absence of the pilot air pressure, air pressure is bled from the air switch, causing the air switch to move from an open to a closed position, thus preventing air from flowing to the air actuator.

Abstract: A flow-control valve has a valve housing defining a chamber formed with ally offset source, sump, and user ports. A valve body in the chamber has first and second coaxial but axially independently movable parts, the first part having a mainly axial throughgoing passage having one end exposed continuously to one of the source and sump ports and an opposite axially open end axially sealingly engageable with the second valve part. A central seat between the source and sump ports is axially sealingly engageable with one of the valve parts to isolate one of the source and sump ports from the other of the source and sump ports.

Abstract: The invention relates to a valve with at least one diaphragm valve element, two servo valve elements and an inverter valve element. The valve according to the invention is distinguished by impulse actuation.

Abstract: A pilot valve for connecting a source of fluid under pressure to an actuator where the pilot valve may be connected to the actuator in any of 360.degree. of rotation so that the fluid connections may be made from any desired direction. Further disclosed is a coupling adapter joining the pilot valve to the actuator with passages so arranged in the adapter that 180.degree. rotation of the coupling adapter reverses the fluid flow connections between the pilot valve and the actuator simultaneously exposing indicia indicating the condition. Still further disclosed is a spring return manual override that is slotted to enable operation by a coin and has a mating slot in the pilot valve body allowing locking in the override position by insertion of a coin or similar device. A ball valve actuator is described as an actuator device.

Abstract: A pneumatic control system for a reversible stepping actuator has an analog pneumatic control signal and an analog signal indicative of actual position of the actuator. Imbalance in these signals opens one or the other of a pair of comparator valves having inlets connected to a source of pneumatic pressure. Opening of either of the comparator valves enables a pulse timer which converts the pneumatic pressure from either of the comparator valves to pulses of pneumatic pressure applied to one or the other of the operators of the reversible stepping actuator. An oscillating pneumatic timer may be used to produce the pressure pulses, or an electric timer can be used to "chop" one of the pneumatic pressures into periodic pulses.

Abstract: The pressure responsive, pilot actuated, modulating valve comprises a main valve assembly which defines with the housing a first chamber and controls flow of fluid from an inlet port to an outlet port. A pressure responsive piston is provided which defines with the housing a second chamber. A vent passageway is provided in the piston for communicating the second chamber with the outlet port. A pilot valve assembly is provided to control flow through the vent passageway. A means is provided for selectively actuating the pilot valve assembly. A discharge orifice is comprised in the main valve assembly for communicating the vent passageway with the outlet port. A vent valve assembly is disposed to control flow of fluid between the first chamber and the discharge orifice.

Abstract: A control device for determining what one, in a system of monitored functions, first became inoperative.

Abstract: An apparatus for the monitoring of physical quantities on systems for example for driving a quick-closing valve in turbines, wherein the monitoring itself is performed by monitoring channels which are interconnected in a majority-determining switching logic, primarily 2 of 3 system. The monitoring channels (I, II, III) are formed by a system-dependent number of identical solenoid valves (1, 2, 3).

Abstract: A control arrangement for a gas-fired system. Three diaphragm-operated valves are in series providing a stage operation. The valves are pilot controlled. A booster pump is provided for cooperation with the pilot valves to boost the line pressure to the first of the valves, the boosted pressure being sufficient to operate the diaphragm valves, and the boosted pressure being controlled by the pilot valves. The booster pump operates only when needed. Diaphragm-operated pilot valves means are provided which operate two separate pilot valves which control the pressure on the opposite sides of the diaphragm of the diaphragm valves. The system provides stage operation, the stages including a first stage which is closing of a vent valve connected between diaphragm control valves, opening of a first diaphragm valve, and a third stage which is a controlled slow opening of the second diaphragm control valve. The first diaphragm valve is both a control valve and a pressure regulator.

Abstract: An actuator control system is disclosed using fluid dispensing means to selectively add or remove fluid from a closed actuator system, whereby the movement of the actuator is volume rather than pressure responsive.

Abstract: A hydraulic piloted pressure reducing and regulating valve which is fail safe for holding the valve at the last selected operating pressure in the event of the loss of control signals. A pressure regulating valve having a body and a control piston therein is provided with a pilot piston in the body which is adapted to be connected to a hydraulic pilot pressure. A spring is provided between the pilot piston and the control piston whereby the control piston can move in response to flow demands in the valve. A rod is connected to the pilot piston with a locking sleeve engaging the rod and releasably locking the rod and pilot piston against movement. A hydraulic pressure port is in communication with the space between the rod and the sleeve for releasing the rod upon application of pressure to the port. The port and the pilot piston are separately connected to a hydraulic fluid supply and a control system whereby upon failure the valve will maintain the last selected operating pressure.

Abstract: The present invention is directed to an apparatus (10) for providing a gaseous pressure pulse. Apparatus (10) includes a container (12) having an exhaust path through ports (30) and (26) of a first valve (20) when a space (32) above a ball closure element (44) is relieved of overambient pressure through ports (82) and (46) of a second valve (22) when space (84) is relieved of overambient pressure by a solenoid valve (72). The first and second valves (20,22) are structured to relieve the overambient pressure on the unsealed sides of the closure elements (44,74) rapidly thereby rapidly opening the exhaust path to create a high volume, low pressure pulse.

Abstract: A valve device, especially for die casting machines. The device comprises a control drive and a control member which is arranged between the control drive and the valve cone, and which converts the rotary movement of the control drive into an axial stroke movement. The object of the invention is to provide a quick acting valve device with which the adjustable limited opening of the valve cone can, with the aid of small manual or motor forces, be effected with structurally simple means in a space saving device housing in the direct drive. For this purpose, the control member which serves to adjust the opening limitation of the valve cone is a threaded sleeve which engages in a thread of the valve housing. Every time the working surfaces of the valve cone are acted upon by pressure, different hydraulic pressures are exerted upon the oppositely located end faces of the threaded sleeve for releasing the sides of the threads of the sleeve from the sides of the threads of the housing.

Abstract: The present invention is directed to a dust collector (100). Dust collector (100) has a housing (102) separated by partition (130) into inlet chamber (126) and clean air chamber (128). A clean air drum (140) rotatably supports columns of horizontally extending, filter elements (142). A low pressure, high volume pulse apparatus (10) is operable through blowpipe (148) to sequentially provide a low pressure reverse pulse to a column of filter elements (142) thereby separating any dust cake from them and allowing it to fall into conical portion (106) for removal from collector (100) by air lock (108). The motor (158) which rotates blowpipe (148) also disengageably rotates clean air drum (140) and attached filter elements (142) to align a column of filter elements (142) with door assembly (114) for maintenance.

Abstract: A solenoid-operated control valve has a main orifice and a valve member for controlling fluid flow therethrough. A leakage orifice extends through the valve member. A check valve seals the leakage orifice when the main orifice is blocked and moves to open the leakage orifice before the main orifice is opened.

Abstract: A valve body is disclosed located in the hydraulic system of a machine which valve serves to direct hydraulic fluid under pressure to hydraulic components or divert the fluid to a system reservoir to prevent component operation. A check valve in the valve body is opened or closed by control means with the check valve indirectly determining the position of a pressure responsive spool for either normal fluid passage to hydraulic components or fluid diversion back to a reservoir. Remote actuation of the control means is by a hand operated air, lockable pump to enable an authorized equipment operator to remotely actuate said control means to effect locking or unlocking of the hydraulic system by positioning the check valve. A modified check valve control dispenses with the air pump and utilizes a lock plunger to bias the check valve to a closed position for normal hydraulic system operation or, conversely, to open the check valve to divert pressurized fluid back to a system reservoir.

Abstract: A pressure regulating device for gaseous and liquid fluids contains a pressure regulator with an inlet and an outlet and a pressure space between them. The pressure in that space is regulated by a shiftable main valve. An actual pressure value chamber is connected with the pressure space. A set value or desired value chamber communicates through a diaphragm with the actual value chamber. The diaphragm is movable to move the valve body for adjusting the actual pressure in the pressure space to the desired pressure in the desired value chamber. An electromagnetic set-point adjuster adjusts the pressure in the desired value chamber. The adjuster includes a set-value transducer, with an actual value sensor which is exposed to the pressure in the pressure space, and the transducer produces an electric signal corresponding to that pressure.

Abstract: An overflow preventing valve apparatus comprises a valve main body provided at a terminal end of a fluid supplying system passage, a main valve fixed to a separating membrane stretched within the valve main body to separate an upper chamber and a lower chamber, when the main valve is displaced upwards and downwards accompanied by deformation of the separating membrane and closing a fluid supplying passage upon moving downwards, a pilot valve for opening and closing the main valve in response to the opening and closing of the pilot valve itself, where the pilot valve is opened by a fluid supply starting operation, a lock mechanism for locking the pilot valve in an open state, a first lock releasing mechanism responsive to the detection of a dangerous fluid level which could lead to an overflow, for releasing the locking operation by the lock mechanism to close the pilot valve, and a second lock releasing mechanism responsive to the closing operation of the main valve at the time when the supply of fluid is sto

Abstract: A mud pulser for mud flow in a conduit. A fluid reservoir operates when positioned in the flow of mud for imparting energy from the mud to separate fluid in the reservoir. A hydraulically operated valve is inserted in the flow of mud displaced from the fluid reservoir and includes an actuation chamber for the separate fluid. A fluid operated control valve is coupled between the fluid reservoir and the actuation chamber. An electrically controllable valve is coupled between the fluid reservoir and a control port for the control valve. The electrically controllable valve is operative in response to a first electrical signal applied thereto for providing communication in the separate fluid between the fluid reservoir and the control port to thereby allow an energy transfer in the fluid between the mud flow and the control port. The electrically controllable valve is also operative in response to a second electrical signal for blocking fluid communication between the fluid reservoir and the control port.

Abstract: In one form of the invention, the control apparatus includes two stages: a poppet first stage and a piston second stage. The poppet first stage includes an altitude sensitive evacuated bellows coupled to a poppet valve. The second stage piston includes a pair of opposing head portions separated by a rod portion. The apparatus includes an output port which is coupled to a low pressure pneumatic signal or to a high pressure pneumatic signal, depending upon the position of the piston. For altitudes less than a predetermined altitude, e.g., 20,000 feet, the first stage develops a pneumatic servo signal which causes the piston to be in a first position which couples the output port of the apparatus to ambient air pressure. For altitudes greater than the predetermined altitude, the first stage develops a pneumatic servo signal which causes the piston to be in a second position which couples the output port of the apparatus to a high pressure pneumatic signal.

Abstract: Apparatus for individually isolating the hydraulic actuators of steam valves for a steam turbine while the turbine remains in operation and while all other steam valves associated with the turbine remain functional. Isolation is achieved by providing a manual isolation valve for blocking the flow of hydraulic fluid from a main supply; a flow restrictor to limit the inflow of hydraulic fluid; and a flow direction control means to limit hydraulic fluid to flow only from the actuator toward an emergency trip hydraulic fluid supply.

Abstract: A system for regulating the flow of a fluid in dependence of an input signal includes a valve housing, a valve between an inlet and an outlet of the valve housing, a cylinder substantially closed off from the cartridge valve housing, and a piston in driving connection with the valve, and movable within the cylinder in opposite directions. A resilient device urges the piston to drive the cartridge valves so as to close the outlet , and a fluid control device communicates with a main fluid conduit and with first and second openings of the cylinder disposed on respective opposite sides of the piston. The valve housing inlet and a source of fluid are connected to the first and second openings of the cylinder, respectively, in a first active station of the fluid switch, but this connection is reversed in a second active station thereof; the first and second openings, and the source of fluid are blocked in a neutral station of the fluid switch.

Abstract: Apparatus including a manually operated electrical switch operably connected through electrical and pneumatic systems to a hydraulic system for remote manual control of hydraulically actuated devices, such as hydraulically actuated throttling valves. The apparatus is particularly useful as a remote manual override for malfunctioning automatic hydraulic controllers.

Abstract: A controller for at least one damper maintains a constant volume rate of flow of air through a duct into a controlled space for a given condition in the controlled space. The dual reset controller resets the volume rate of flow as functions of two sensed conditions.

Abstract: A servo-operated shut-off valve arrangement has a main valve responsive to a servo pressure and fluid pressure operated pilot valve for controlling the servo pressure. The pilot valve has two stable operating positions in which the main valve servo pressure is respectively vented and maintained. The fluid pressure which operates the pilot valve is controlled by a valve device movable in opposite directions from a center-stable position to effect movement of the pilot valve. The pilot valve co-operates with the center-stable valve device so that the pilot valve remains in the position in which it was last urged by movement of the valve device away from its stable central position.

Abstract: An electromagnetic pilot type valve having at least one auxiliary pilot valve disposed and operatively connected between the pilot valve and the main valve. The auxiliary valve is operated in response to the pilot valve to operate the main valve. With this construction a main valve body of a relatively large diameter can be operated with a small electromagnetic pilot valve, so that the whole construction of the valve can be compactly made.

Abstract: A valve arrangement having a housing with an inlet for pressurized fluid and an outlet. A chamber is provided in the housing having a flexible diaphragm therein dividing the chamber into a control chamber and a pressure chamber. The pressure chamber and the control chamber are each connected in fluid circuit with the pressurized fluid supplied to the inlet through a constant cross section throttle opening. An analogue pressure regulator is provided for maintaining the pressure of the pressurized fluid in the control chamber constant independent of pressure variations in the pressure of the pressurized fluid. The analogue pressure regulator includes a control opening providing fluid communication from the control chamber to the outlet and a pivotally supported armature movable into and out of blocking relation with the control opening.

Abstract: A gas-oil drive for forward and reverse mechanical adjusting movements. Gas, which is available for every direction of movement of an adjusting drive at a pressure greater than atmospheric pressure, conveys its pressure in a pressure tank to control oil which flows to an adjusting device for carrying out a control operation. For both oppositely directed adjusting movements of the gas-oil drive, a single pressure tank for delivering control oil under pressure to the respective corresponding position of the adjusting device, and a single unpressurized container for receiving expelled, unpressurized control oil are provided. The unpressurized container is located above the level of the pressure tank, and a bore on the unpressurized container is connected with a bore on the pressure tank by means of a control oil line in which is located a check valve which closes in a direction of flow toward the unpressurized container.

Abstract: In a safety system for controlling the closing of a primary valve in response to an indication of an undesirable condition from a monitoring device, the system including a valve arrangement which supplies operating fluid to the primary valve and an operator connected between the monitoring device and the valve arrangement for controlling the operation of the latter in response to the indication produced by the monitoring device, the speed and reliability of the system is improved by providing a direct fluid connection between the output of the monitoring device and the operator, whereby the system is directly controlled by the fluid pressure variations at the output of the monitoring device. Preferably, the system includes an hydraulic subsystem and a pneumatic subsystem.

Abstract: Variable differential pressure control apparatus is used in a hydraulic system including a pump, an accumulator for receiving hydraulic fluid under pressure from the pump and for discharging hydraulic fluid to a work device, and an unloading valve in the circuit between the pump and the accumulator for regulating pressure of the fluid to the accumulator. The control apparatus is connected with the unloading valve for opening and closing the latter and is adjustable (1) for selectively setting the maximum fluid pressure to the accumulator and (2) for selectively setting the variation between the maximum pressure to the accumulator, at which pressure the unloading valve will open, and the minimum pressure to the accumulator at which pressure the unloading valve will close for recharging the accumulator.

Abstract: An instantaneous signal applied to the inlet of the timing device is passed through a restrictor into a capacitance and the resultant rising pressure is compared with an adjustable-level reference pressure signal which defines the timing period. When the capacitance pressure exceeds the reference signal value, a comparison signal initiates the release of an obturator. The reference pressure propagates to the outlet of the device and produces the time-delay signal.

Abstract: In a safety system for controlling the closing of a primary valve in response to an indication of an undesirable condition from a monitoring device, the system including a valve arrangement which supplies operating fluid to the primary valve and an operator connected between the monitoring device and the valve arrangement for controlling the operation of the latter in response to the indication produced by the monitoring device, the speed and reliability of the system is improved by providing a direct fluid connection between the output of the monitoring device and the operator, whereby the system is directly controlled by the fluid pressure variations at the output of the monitoring device. Preferably, the system includes an hydraulic subsystem and a pneumatic subsystem.

Abstract: A rapid opening valve assembly has a housing in which there is positioned a cylinder body slidably receiving, in a cylinder chamber, a valve piston which maintains the valve assembly in an open or closed state. The valve piston has an axial extension which constitutes a spindle and which is received in a spindle chamber formed in the cylinder body. The valve piston is urged into the closed position by a spring, the force of which is opposed by the pressure in the high pressure side of the valve assembly, seeking to move the valve piston away from its seat. In the valve piston there is provided a servochamber which receives a slidable servopiston cooperating with a servovalve seat also carried by the valve piston. The servopiston is urged against its seat by a servospring, the force of which is opposed by the cylinder chamber pressure admitted to an end face of the servopiston through a channel provided in the valve piston.