Abstract: A hydraulic valve system particularly adapted for controlling a hydraulically powered elevator. The system involves essentially a bypass valve which selectively bypasses hydraulic fluid from the pump directly back to the tank during the up mode operation, thereby controlling the up speed of the elevator; and a down valve which selectively controls the escape of fluid from the elevator jack back to the tank, thereby controlling the down speed of the elevator. The down valve also serves as a check valve for retaining fluid pressure in the jack. Both the bypass valve and the down/check valve are each controlled by closely integrated cylinder-and-piston arrangement, with pressure to the cylinder being controlled by a control valve structurally interlinked to the respective piston. The bypass valve also serves as a check valve between the jack and the tank during the down mode of operation. The cylinder-and-piston control for the bypass valve makes possible an initial limit opening of the bypass valve, i.e.

Abstract: A redundant swashplate control system includes five or more actuators coupled to the swashplate. A feedback network independently controls each actuator to track a commanded position and a reference ram differential pressure that it related to the ram differential pressures sensed from all of the actuators.

Abstract: A high-speed switch machine for actuating railroad switch points. The machine has an electric motor which drives a hydraulic pump. The pump supplies pressure fluid to a hydraulic cylinder adapted to move the switch points from an initial to a second limit position. A four-way, two-position hydraulic control valve directs fluid to the cylinder such that the cylinder automatically operates in an extension and retraction cycle. The uni-directional motor and two-position control valve minimize response times of the actuating components so that the switch points are thrown in a short time period.

Abstract: A fluid flow controlling device includes a flow control that is interposed between the valve spool of a directional control valve and the work port channel of the directional control valve, and that is effective to control fluid flow to and from one motor port of a fluid motor in proportion to respective sizing of first and second fluid flow paths by the valve spool. The fluid flow controlling device is preferably used in load responsive hydraulic systems.

Abstract: A hydraulic control system comprising a hydraulic actuator having opposed openings adapted to alternately function as inlets and outlets for moving the element of the actuator in opposite directions, a pump for supplying fluid to said actuator, pilot operated meter-in valve means to which the fluid from the pump is supplied for controlling the direction of movement of the actuator, pilot operated meter-out valve means associated with each opening of the actuator for controlling the flow out of said actuator. The pressure of fluid being supplied to the actuator by the meter-in valve means is sensed and caused to produce a force opposing the movement of the meter-in valve means by the pilot pressure resulting in a smooth and accurate control of the movement of the actuator.

Abstract: In an apparatus for forming leak-proof joints between tubes and a tube sheet by the internal application of hydraulic swaging pressure, the flow of pressurized fluid from an adjustable pressure reduction valve is permitted or interrupted by a control valve. An actuator moves the control valve between its flow permitting and interrupting positions in response to a control signal. In the sensor, which is connected to the input and output sides of the control valve, a piston is movable in response to the difference in the input and output pressures to produce the control signal when a predetermined comparative pressure relationship exists.

Abstract: A hydraulic control system for use in machine tools, construction machines, hoists and the like, with a driven unit the hydraulic circuit of which is controlled by one or more electromagnetically positioned slide valves, and with an arrangement for automatically regulating the velocity of the hydraulic drive unit regardless of fluctuations in the pressure of the oil supply and similar disturbances. The regulating arrangement includes a potentiometer mechanically controlled by a flow meter interposed in the hydraulic circuit; and an electric comparator the inputs of which are connected to the potentiometer and to a desired-flow-value selector, respectively, and the output of which controls the valve electromagnet(s). In this manner the difference signal developed by the comparator is used to change the setting of the valve in a correcting sense. Several implementations for this regulating circuit and for the design of the flow meter and the valves are disclosed.

Abstract: The distributor is designed to control the supply of a fluid under pressure into a chamber of a hydraulic actuator and the outlet of fluid from another chamber of said actuator, and comprises a first chamber and a second chamber, which are designed to be communicated with one of said chambers of the actuator, and a slide valve movable in the direction of its axis in a corresponding seat of the distributor; the distributor comprises means which respond to the pressure prevailing inside said first and second chambers of the distributor and means for choking the cross-section of the outlet for the fluid from said first and second chambers of the distributor, said pressure-responsive means being connected to said choking means so that, as a result of a movement of the movable member of said actuator at a velocity higher than a predetermined velocity, said choking means bring about a reduction in the velocity of said movable body.

Abstract: A control mechanism (48) is provided to controllably maintain a constant force on an actuator (18) and provide fluid flow to a downstream control valve (24) by controlling a pilot differential pressure between first and second ends (38,40) of a pilot operated control valve (14). A two position valve (56,66) acting in response to a predetermined pressure level in the actuator (18) modulatably controls the flow of pressurized pilot fluid to the pilot operated control valve (14) to position the pilot operated control valve (14). This maintains the pressure in the actuator (18) at the predetermined pressure level and passes the excess flow of fluid from a source (12) to the downstream control valve (24).

Abstract: A fluidic servovalve is disclosed comprising a pilot stage valve having a pilot output for supplying a control fluid dependent upon a movable element, a boost valve responsive to the control fluid for supplying an output fluid, and a fluidic mechanical feedback connected from the output of the boost valve to the pilot stage valve, the fluid mechanical feedback having a ball-type motor responsive to the output fluid acting against it for positioning the movable element.

Abstract: The flow of fluid to a hydraulic load (18) is controlled by controlling a bleed-off path (31, 32) by means of a main valve (20) responsively to the difference between a first input force produced by a force motor (52) and a second (hydraulic feedback) force produced by a flow sensor (74) in the passage (33) leading to the load (18). The pressure difference across the flow sensor is applied by lines (75, 76 and 71, 70) to opposed feedback chambers (56, 55) to act on the pilot spool (40) upon which the force motor (52) acts.

Abstract: A linearization circuit for use with servovalves for controlling reciprocating actuators. The circuit will maintain a substantially linear flow as a function of spool position from the servovalve regardless of the differential pressure across the servovalve orifices, for example the differential pressures which result from changing direction of movement of the controlled actuator under a high load. A differential pressure signal from the actuator, which correlates to differential pressure on the servovalve orifices is processed to provide a compensation signal combined with the normal control signal to achieve the desired output.

Abstract: An electromagnetically controlled hydraulic servo-control valve includes a hydraulic slide activated by an electromagnet with a plunger. The distributed pressure hydraulically pushes the slide and the plunger toward each other. Constrictions in the hydraulic conduits attenuate oscillations. The plunger and the slide have slightly different cross-sections to provide a control loop for the distributed pressure, affecting the position of the side. Use of a polarized electromagnetic supplied by a current that ranges from a negative value to a positive value doubles the regulating capacity. To prevent hysteresis of the pressure curves, as a function of the current, the electro-magnet can be supplied by a pulsed current or an alternating current with a continuous component. The servo-control valve might be used in connection with an automatic gear box control.

Abstract: An electronic compensation circuit for a pneumatic load bearing bellows system closed on position, in which a servo valve provides pressure to the bellows to actuate the top of the bellows to move to a predetermined position in response to a control signal. The compensation circuit (42) cancels at least one load-variable system characteristic which, if not cancelled would cause system instability at system gain levels which are necessary for desired system response. The compensation circuit electronically provides a replacement system characteristic which permits the system to be operated at a gain level substantially higher than the gain level at which the uncompensated system goes unstable.

Abstract: A fluid flow regulator valve to regulate flow to a fluid driven prime mover wherein a selectively shaped, spring biased spool is positioned within a cylinder and has a sensor orifice therein to establish a pressure drop thereacross in response to controlled fluid flow rate and hence cause the spool to move within the cylinder to control both the metering orifice to and the return orifice from the flow conduit which connects the cylinder to the prime mover.

Abstract: A rock drill feed system moves a drill toward and away from a wall with a reciprocating hydraulic motor powered by pressurized fluid. The pressurized fluid is controlled by a movable control valve that has a first position obtained manually to move the drill inwardly to drill, a second position obtained automatically when the drill has completed drilling, and a third position obtained automatically when the drill has been completely withdrawn. The sensing of the location of the drill to position the control valve is accomplished by measuring the pressure and flow rate of the hydraulic fluid delivered to the motor with pressure responsive switches. When the pressure exceeds a preselected level and the flow rate drops below a preselected level, the two pressure switches close to produce a signal that moves the valve to another position.

Abstract: A valving system for regulating the pressure of the fluid within a hydraulic actuator of the type having a hydraulic cylinder and movable piston therein that separates the cylinder into a first and second chamber comprises sensing apparatus in fluid communication with the first chamber of the hydraulic actuator cylinder. The sensing apparatus senses the differences in pressure between the fluid within the first chamber and the fluid in the second chamber. The valving system further includes a pressure regulator for regulating the pressure within the first chamber in response to the sensed pressure difference. Apparatus is also included for communicating the magnitude of the sensed pressure difference to the pressure regulator.

Abstract: A control system for a machine tool, such as a gear shaper, having a hydro-mechanically stroked cutter spindle, controls the stroking speed of the spindle, the feed of a workpiece relative to a cutter or other tool carried by the spindle, and the pressure of the fluid supplied to the hydraulic portion of the spindle stroking mechanism in such a way as to improve the efficiency of the machine both with regard to the time required to complete a given job and with regard to power consumption. The work force exerted by the spindle during its work stroke may be maintained within a selectable maximum limit and the cutter spindle stroking position is remotely adjustable to bring it into proper working relationship with a workpiece or to raise it to an elevated position to bring it out of working relationship with the workpiece. Various different stroking speeds of the spindle and various different feed rates of the workpiece relative to the spindle are available to suit different job requirements.

Abstract: A servo-steering mechanism with a steering nut threadably movably connected with a steering worm and being supported with respect to the working piston of a servomotor to be rotatable and also axially movable by way of compression springs in directions of the steering worm axis. The steering nut is additionally supported in the directions of the steering worm axis with respect to the working piston by way of at least one hydraulic support connected with a hydraulic system of the servomotor of the servo-steering mechanism.

Abstract: A conveyor arrangement wherein difficulties associated with breakage of a conveyor chain are overcome by providing hydraulic rams mounted between conveyor parts which are urged together in operation. The pressure in the hydraulic rams is monitored and when the chain breaks, the change in pressure or hydraulic ram extension is detected by switches which switch off the conveyor motor. A computer means is notified of the monitored pressure of the hydraulic ram.

Abstract: An arrangement for a hydraulic brake booster through which a pump-circulated fluid is throttled for supplying fluid to the brake booster includes a throttle element which is actuated by the booster pressure through a control spring. The control spring and a piston associated therewith limits the maximum amount of pressure that can be throttled so that a separate pressure-limiting valve, for instance, is not necessary.

Abstract: A hydraulic brake pressure control installation with a master brake cylinder actuatable by a plunger of the pedal. The plunger acts on the master brake cylinder by way of a piston rod of an adjusting cylinder which is arranged ahead of the master brake cylinder and which includes a connecting nipple on the side of the adjusting cylinder piston facing the brake pedal, with which a feed line leading to an electro-hydraulic two-stage control valve is connected. The control valve is adjusted by a control signal, and is operable to connect the feed line with either with a supply pressure line or with a pressureless return line.

Abstract: Two circuit breakers or circuit breaker modules are actuated by hydraulic jacks fed by supply and drain valves operated by a single closing-operation control unit. A differential pressure detector is subjected to the pressure of the two jacks. In the event of a relative variation in the jack-operating times, the free piston of the detector actuates a switching device which connects the supply and drain valves to the drain tank, thus preventing the complete performance of any non-simultaneous operation of the circuit breakers.

Abstract: A fluid operated linear actuator control system for drive arrangements of the type wherein two fluid operated linear actuators act upon a shaft at an angle with respect to each other for causing rotation thereof is disclosed. The control system includes a fluid pressure source including a reservoir means, fluid passages connecting the pressure source with each of the fluid operated linear actuators, and control means for communicating the first of the linear actuators with fluid from the pressure source and fluid from the other with the reservoir means to displace the drive arrangement through a first arc sector, and for changing-over the communication to communicate the reservoir with fluid from the one actuator and the other actuator with fluid from the pressure source causing displacement of the drive arrangement through a second arc sector, wherein one of the actuators is in a driving mode while the other actuator is in a floating mode.

Abstract: A paper winder assembly of the type in which a rider roll is positioned to bear against a roll on which the traveling paper web is being wound. The rider roll position is controlled by hydraulically actuated lifting means which vary the pressure at which the rider roll bears against the paper roll. In accordance with the present invention, there is included a pressure modulating valve which converts air pressure signals to hydraulic signals which operate the hydraulically actuated lifting means. The valve includes a pair of flexible diaphragms therein with means to introduce air under pressure on opposite sides of the two diaphragms depending upon whether the rider roll is to be raised or lowered. The valve has a spool actuator therein which is movable in response to the movement of either of the diaphragms. A spring means biases the spool downward and establishes fluid communication between an inlet port and an outlet port for transmitting hydraulic fluid to the hydraulically actuated lifting means.

Abstract: Pressure regulating equipment for an air cushion, which has a check valve, an outlet of which is connected with a large capacity air cushion such as a balancer, diecushion, etc. of a press.

Abstract: A valved conversion element for connection to a source of compressed fluid such as air or vapor and to a cylinder in an internal combustion engine. A sensor is provided which communicates with the pressure conditions in the cylinder served so that repetitive power impulses drive the piston in the cylinder as in normal operation of the internal combustion engine. The disclosure contemplates one valved element per cylinder in the engine and that the conversion be achieved by the manifolding of gas, vapor or fluid power to all cylinders through the conversion elements. Each of the conversion units is connected to the cylinders by means replacing the spark plugs of existing engines. Cyclic rate and power are adjustable by valve means controlling the quality and quantity of input fluid, such as air.

Abstract: A hydraulic control system for use in machine tools, construction machines, hoists and the like, with a driven unit the hydraulic circuit of which is controlled by one or more electromagnetically positioned slide valves, and with an arrangement for automatically regulating the velocity of the hydraulic drive unit regardless of fluctuations in the pressure of the oil supply and similar disturbances. The regulating arrangement includes a potentiometer mechanically controlled by a flow meter interposed in the hydraulic circuit; and an electric comparator the inputs of which are connected to the potentiometer and to a desired-flow-value selector, respectively, and the output of which controls the valve electromagnet(s). In this manner the difference signal developed by the comparator is used to change the setting of the valve in a correcting sense. Several implementations for this regulating circuit and for the design of the flow meter and the valves are disclosed.

Abstract: The pulsator for hydraulic systems controlling actuating mechanisms has a housing accommodating a pusher made in the form of a piston with a rod, a delivery valve, a pneumatic piston displacing in the cavity connected to the pneumatic control system. Above the pusher piston there is an upper cavity communicating with a high-pressure main, and under the piston there is a lower cavity communicating with the actuating mechanism via a non-return valve. A stem is provided between the rod and the piston. Also made above the pusher piston is an auxiliary cavity which is an extension of the upper cavity, defined by a leading edge and connected to the actuating mechanism. Above the lower cavity of the pusher piston there is an annular cavity disposed coaxially with said cavity, separated from said cavity by the leading edge, connected to the valve controlling resetting of the pusher.

Abstract: A lock located within a door has a bolt that is extended and retracted by a double acting air cylinder. A single air line extends from the door frame and into the door where it is connected with a valve having two discharge lines that lead to opposite ends of the cylinder. To energize the cylinder and move the lock bolt, a charge of pressurized air is directed to the valve which in turn directs it to one end of the cylinder. The charge of air further causes the valve to reset itself so that the next charge will be directed to the opposite end of the cylinder. Consequently, with successive charges of air the piston rod of the cylinder and the lock bolt move in opposite directions.

Abstract: Disclosed is a hydraulic servomechanism including a control loop consisting of an input device, an adder-amplifier, a servovalve, a cylinder or actuator and a load operatively connected in the order named for controlling the position of the load, a position feedback loop and an acceleration feedback loop. In one embodiment of the invention, the hydraulic servomechanism further includes a feedback loop wherein an output from an accelerometer or pressure sensor for detecting a differential pressure across a piston in the cylinder and an error signal from the adder-amplifier are arithmetically processed and fed back to the adder-amplifier.

Abstract: A fluid control system includes a directional control valve assembly adapted for use in either an open center or a closed center configuration. The assembly is capable of incorporating one or more control sections, each with a manual control valve having its own variable pressure compensated flow controlling mechanism. Motor port lock-out valves are associated with each manual control valve, and a pressure responsive logic circuit automatically actuates the lock-out valves to hold them open when their associated manual control valve is in the power or float positions. When any manual control valve is in the float position, any other manual control valve may be operated in the power positions.

Abstract: A remotely adjustable back pressure relieving hydraulic regulator system particularly suitable for use on the work roll bending and balancing systems of a rolling mill. A normally open air operated valve and a normally closed air operated valve are modified and interconnected to provide the disclosed remotely adjustable pressure regulator. The normally closed valve is connected to a pressurized hydraulic supply. The normally closed regulating valve opens in response to a control air pressure signal applied thereto and is modified to close when the hydraulic pressure at its outlet port exceeds a desired or selected pressure. The pressure at the outlet of the pressure regulating valve required to move it to the closed position depends on the pressure of the applied air control signal. Thus, as the pressure of the air control varies, the hydraulic pressure required to close the valve also varies.

Abstract: A two-stage regulator valve, particularly for regulating the operation of single acting hydraulic lift motors in lift trucks, which embodies a spool valve controlling first and second ports in the valve body and responsive at one end to lift motor fluid pressure. At the opposite end the valve is responsive first to the force of a first spring which closes the spool on one port when lift motor fluid is below a predetermined pressure, and secondly, following predetermined compression of the first spring during which both ports are opened to exhaust pressure fluid from the lift motor, to the added combined force of a lift motor pressure responsive piston and a second spring. The piston forms with the spool valve a differential area response to said fluid pressure and together with the second spring regulates the position of the spool valve so as to variably restrict the other port to control the exhaust from the lift motor inversely as a function of the fluid pressure therein.

Abstract: A pilot-operated, spring-centered, directional control valve includes a valve body having a bore, chambers at opposite ends of the bore, a valve spool slidably disposed within the bore, a hollow piston slidably disposed within each of the chambers and biased by springs inwardly toward the spool, and a secondary piston slidably disposed within one of the primary pistons so that pilot pressure directed into the outward ends of the chambers acts on the end of the spool or the end of the secondary piston to selectively operate the valve. The various components of the valve have differing effective cross-sectional areas upon which pilot pressure acts. The control valve is advantageously employed in a hydraulic system controlling front wheel drive in a vehicle.

Abstract: In the cyclic pitch stick controls of a helicopter of the type in which feel is provided to the pilot by inducing a force which is proportional but opposite to the motion of the cyclic stick from an adjustable trim position, null offset errors which may be due, inter alia, to temperature variation of the hydraulic fluid in the system, are periodically overcome by means of sensing the pressure differential across the force augmentation hydraulic servo actuator at times when the differential pressure thereacross should be nearly zero, during trim release with small stick motion, and providing a countermanding bias in response thereto; the pressure differential is measured by a differential pressure transducer having a full scale which is only sufficient to measure the expected null errors, which may be about an order of magnitude lower than the full scale, operative pressure differentials which may occur.

Abstract: System and method to ensure that the pressure in a hydraulic pressure system is at a pre-determined level and is maintained for a pre-determined length of time.

Abstract: A main valve controls the flow to a load in a first or second direction depending upon the signal applied to a pilot valve. A spool of the main valve is biased to a central position blocking flow by springs. A throttle valve having an adjustable restrictor area is connected in one line between the main valve and the load. The restrictor area is adjusted in correspondence to an input signal. The spool is moved to a first or second extreme position by the pilot valve. The pressure difference developed across the throttle valve is applied to the spool in a direction to force the spool towards the central position. The spool is balanced at a predetermined pressure difference across the throttle valve.

Abstract: The system includes a way valve of the type provided with a housing, fluid inflow and consumer ports in the housing, and at least one valve member movable in the valve housing for establishing connections between ports. A positioning arrangement receives an electrical input signal and positions the valve member in dependence thereon. A desired-value selector generates a desired-value signal indicative of the desired volumetric flow rate for the flow of fluid into the fluid inflow port and out one of the consumer ports. Transducers sense the position of the valve member and the pressures at the inflow and consumer ports and generate corresponding electrical feedback signals.

Abstract: The invention relates to a hydraulic control apparatus of the type used in vehicle steering systems. In all such systems there is a reversible type hydraulic motor connected to the steering linkage. The motor has complementary expansible chambers. With the use of control means pressurized fluid is selectively delivered to one chamber which is caused to expand concomitantly with the collapsing of the other chamber from which fluid is returned to a sump. There is provided in the control and power circuit throttle valves having respective opening positions which correspond to the pressure difference on both sides of the valves. Between the passages which extend to the two control circuit throttle valves is a pressure comparator which is operable to vary the pressure upstream of the power circuit throttle valve in proportion to the pressure upstream of the control circuit throttle valve.

Abstract: Valve-positioning apparatus especially adapted for use with fluid-operated spool valves for positioning the spool member of such a valve in a predetermined position whenever the valve is not operating such as upon installation or during shutdown. In the preferred embodiment, the positioning apparatus includes a piston member normally held out of contact with the spool by fluid pressure but urged into contact with the spool to position the spool when fluid pressure fails or is absent. The positioning apparatus may be used on either or both ends of the spool valve.

Abstract: A pilot operated directional control valve is provided with pressure controlling means responsive to motor line pressure to control the shifting of the control valve to maintain a precisely controlled variable torque in a motor controlled thereby. The pressure controlling means includes constant force centering means for the valve and a piston at each end of the control valve with means communicating motor line pressure to the piston to operate in opposition to a piston subject to pilot control pressure for shifting the valve.

Abstract: A steam-driven tow line winch includes a pressure control valve in the live steam line of the winch, and is characterized by the fact that the pressure control valve is equipped with a sleeve valve, the sleeve position of which is controlled by the working pressure in the steam winch.

Abstract: The specification describes a three-point regulator, for example of the pneumatic type, in the case of which with a drop and a rise in the measurement value of the regulation quantity after overcoming an operating distance a respective switch is operated. One switch is moved or turned by a servo-member (motor) in one direction and the other switch is moved or turned by the servo-member in the other direction. A feedback is provided which acts on both switches in such a manner that the operating distance is increased and accordingly the switched-on switch is switched off.

Abstract: A control system for controlling the operation of a valve actuator. The system includes a control valve for conveying operating fluid to the actuator in response to an actuator control signal. A single conduit operably connected to the actuator control valve both supplies operating fluid to the control valve and conveys an actuator control signal to the control valve.

Abstract: A device for controlling a hydraulic actuator comprises two servo loops having in common a fluid pressure operated main valve and an electrically operated pilot valve for controlling the main valve. One of the servo loops is for flow control and contains a flow transducer for producing an electrical feedback signal responsively to fluid flow to the actuator and a comparator for comparing the flow feedback signal with a first electrical input signal. The other servo loop is for force control and contains pressure transducing means for producing an electrical feedback signal responsively to the pressure drop across the actuator and a second comparator for comparing the force feedback signal with a second electrical input signal.

Abstract: A multiple control valve device for connection in parallel between a single fixed displacement pump and one or more oil actuators to control operation of all or a number of the actuators simultaneously. The pump can supply sufficient oil at an optimum pressure which is the highest loading pressure required among the working actuators. At the same time the desired controlled oil flow rate can be held to the desired value for each working actuator by this device. Also this valve device has stepless speed changing control, provides a self-depressing function of pump pressure loading, and can also have the function that a pressure rise or a pressure variation in the actuators can be treated automatically.

Abstract: A motor vehicle fluid power circuit is provided with a constant displacement pump, a control valve, an open-center steering valve, and a hydraulic ratio type brake booster. The control valve includes a control spool which controls fluid flow between the pump and the brake booster and between the pump and the steering valve. A control spool modifying means modifies the operation of the control spool during initial actuation of the brake pedal to produce a large increase in brake booster pressure.

Abstract: The invention relates to a gerotor type of rotary piston machine which is operable as either a pump or a motor. In this type of device an externally toothed rotor member is eccentrically offset relative to an internally toothed stator member and in operation these members mesh to form expanding and contracting cells on opposite sides of the line of eccentricity. There is shaft means for driving the rotor and a distributing valve for feeding and exhausting the cells is attached to the shaft means. In order to compensate for torsion stresses in the shaft means which would cause the distributing valve to be slightly out of phase relative to the rotor the connection between the valve and the shaft means is made adjustable in response to sensing means which sense the pressure conditions in the gerotor when the rotor is symmetrical relative to the line of eccentricity.

Abstract: A servo-steering system for vehicles, especially for motor vehicles, in which the steering lever is displaced by means of a steering nut movable on a steering spindle, and in which adjusting devices are coordinated to the steering nut which are acted upon by a pressure medium whose pressure is controllable by means of control elements as a function of the deflection of the steering nut; a high pressure accumulator thereby serves as pressure medium source whereby the inflow of the pressure medium to the spaces containing the working pressure at the control element is completely interrupted in the normal rest position of the steering nut.