Abstract: A recirculating-ball steering system for transmitting a steering movement to a steering arm, includes: a housing having a steering piston arranged inside the housing, wherein the steering piston includes a ball screw drive and is displaceable along its longitudinal axis, and an electrical actuator for assisting the movement of the steering piston. Also described is a vehicle having the recirculating-ball steering system.

Abstract: A hydraulic control system is disclosed. The hydraulic control system may have a pump configured to pressurize a fluid, an actuator, a position sensor associated with the actuator and configured to generate a first signal indicative of a position of the actuator, and a circuit fluidly connecting the pump to the actuator. The hydraulic control system may also have a valve associated with the circuit and configured to move from a first position, at which fluid relief from the circuit is inhibited, toward a second position, at which fluid is relieved from the circuit through the valve, when a pressure of fluid within the circuit exceeds a threshold setting of the valve. The hydraulic control system may additionally have a controller in communication with the position sensor and the valve. The controller may be configured to selectively cause adjust the threshold setting of the valve based on the first signal.

Abstract: A holding control valve of the present invention is configured such that: a spool is configured to perform strokes by a piston configured to operate by introduction of pilot pressure and have a larger diameter than the spool; the piston is divided into a pilot piston configured to receive the pilot pressure and a relief operation piston disposed adjacent to the spool to receive pressure of relief oil discharged when a relief valve operates; and the relief oil is introduced to between the pilot piston and the relief operation piston.

Abstract: Device for the emergency lowering of a load carrying means for a stacker truck, in particular a reach-type forklift truck, with a valve arrangement accommodated in a valve block, via which the inflow and outflow of a hydraulic means is regulated to and from a lift cylinder for the load carrying means, with a bypass between the lift cylinder and a tank, with a barrier element in the bypass. The barrier element is screwed into a threaded bore of the valve block and, in the fully screwed-in position, closes off the bypass branch. One end of a cable transmitting a torque is rotationally fixedly connected to the barrier element and the other end of the cable has a handle for turning the cable.

Abstract: An improved hydraulic arrangement is provided for a spring support system. The arrangement comprises a hydraulic cylinder, a hydraulic reservoir, a hydraulic fluid feeder, a hydraulic accumulator, a first control valve between the hydraulic cylinder and the accumulator, a second control valve between the hydraulic cylinder and the hydraulic reservoir as well as an automatic shut-off valve and a controller with a lifting position, a lowering position and a neutral position for the hydraulic cylinder. In order to prevent different pressure loads between the accumulator and the hydraulic cylinder, it is suggested to equalize the pressure between the accumulator and the hydraulic cylinder, without impairing the spring support system or the automatic shut-off valve. Accordingly, the accumulator is suitably connected to a non-return valve and to a flow restrictor or orifice.

Abstract: A hydraulic drive control device which is capable of translating the effect of oil pressure loss reduction into the effect of fuel consumption reduction that is very real to the user. The hydraulic drive control device has (i) a driving hydraulic circuit for driving a hydraulic actuator by supplying pressure oil to or draining it from the hydraulic actuator through control valves, the pressure oil being discharged from a hydraulic pump driven by an engine and (ii) a quick return circuit for directly flowing part of hydraulic oil discharged from the hydraulic actuator back to a tank, while the hydraulic actuator being driven. The hydraulic drive control device also includes an engine controller for controlling the output of the engine such that the output of the engine is restricted when the quick return circuit is opened.

Abstract: An energy cylinder for operating a valve of a turbine, in particular of a steam turbine, is disclosed, wherein a cylinder housing and a control block are combined into a common block. The latter has two diametrically arranged lateral surfaces on which a control valve and a logic valve for controlling the pressure medium flows are arranged.

Abstract: A controller for controlling the operation of gripping devices of a loader harvester bulk handling device. In the controller, the coupling of a valve with the operating device of the loader harvester's cross-cutting device, and with a tilting mechanism of a tilting frame, is arranged to operate in conjunction with the cutting device and felling so that the gripping devices are normally in the closed position. The gripping devices are arranged, at least in the felling position, to always open when the guide bar or equivalent cutting device is operating or when the harvester head turns to the delimbing position, and to close immediately after the operation in question.

Abstract: The invention provides a hydraulic control system capable of preventing an objective from crashing even if a load supporting passage is fractured or the like during a crane operation for moving down the objective with a hydraulic shovel. When a pilot pressure for switching a control valve 2 to a downward position b is less than a predetermined value, only first selector means switches, and when the pilot pressure exceeds the predetermined value, second selector means as well as the first selector means switches.

Abstract: An electro-hydraulic control device (10, 60) for a hydraulic servo motor for controlling a volume flow is proposed, which is embodied as a 4/2 valve module. A blocking valve in accordance with seat valve technology between a motor connection (B) and a return flow (R) form a lowering element (11), while the associated unblocking member (44) is designed as a longitudinal slide (45), which controls the connection between an inflow connection (P) and a motor connection (A) and is actuated by a proportional magnet (16). After unblocking the blocking valve, its seat valve body (23) is mechanically taken along by the longitudinal slide (45), and the two volume flows are proportionally controlled via the lowering element (11) and the lifting element (12), so that a large switching capacity is achieved along with a construction with few leaks.

Abstract: In a counter balance valve, a valve body is provided with a valve bore having first and second pump-side ports, first and second motor-side ports and an auxiliary port. A spool is slidably fitted in the valve bore in a longitudinal direction thereof to establish and block communication between the respective ports. Left and right springs maintain the spool at a neutral position in which the respective ports are closed. A left pressure receiving chamber shifts the spool by a pressure oil supplied therein to a first travelling position at which the first pump-side port and the auxiliary port are communicated with each other and the second pump-side port and the second motor-side port are also communicated with each other.

Abstract: A pneumatically operated mechanism for a glass machine comprises a piston and cylinder device, first and second air lines leading to opposite sides of the piston, and first and second control means for controlling the passage of air through the air lines, which control means each comprise an operating valve and a speed control valve. The valves are solenoid operated and by control of the timing of operation of the valves the speed of movement of the piston and the degree of cushioning at the ends of its stroke may be adjusted. In a modified mechanism pressure sensors which sense the pressures in the piston and cylinder device are connected to a processor which is adapted in response to data sensed by the sensors to operate the valves to obtain a desired movement of the piston.

Abstract: An apparatus for felling and bunching trees is disclosed. The apparatus includes a front frame and a front axle supported by the front frame. The apparatus further includes a number of front wheels mounted on the front axle and a work tool supported by the front frame. The work tool has a blade for felling trees and a number of work arms which are movable to secure the trees within the work tool. The apparatus further includes a front hydraulic motor operable to rotate the front axle. The front hydraulic motor is supported by the front frame. The apparatus yet further includes a first hydraulic pump operable to supply pressurized hydraulic fluid to the front motor. The apparatus still further includes a rear frame which is pivotably coupled to the front frame, a rear axle supported by the rear frame, and a number of rear wheels mounted on the rear axle. The apparatus still further includes a rear hydraulic motor operable to rotate the rear axle. The rear hydraulic motor being supported by the rear frame.

Abstract: A hydraulic system for working cylinders of construction equipment is disclosed. The system allows oil to be smoothly drained from a chamber of a working cylinder into an oil tank through a first passage or/and a second passage without being resisted during an operation of the cylinder. The system thus effectively and smoothly moves the working cylinder while improving operational efficiency and operational speed of the cylinder. When the system is used with a boom cylinder of a power excavator, the system effectively performs a boom-up or boom-down action without losing pressure.

Abstract: The present quick drop valve control provides both quick drop and float functions by moving a quick drop valve to a position interconnecting the drop and lift sides of a pair of lift cylinders when a directional control valve reaches a predefined position. The quick drop function valve is established by simultaneously energizing a first solenoid valve to communicate a shift end of the quick drop valve with a resolved pressure port of a resolver connected to the drop and lift sides and a first solenoid of a second solenoid valve to communicate a spring end of the quick drop valve with the drop sides so that the quick drop valve moves to the interconnecting position combining fluid expelled from the lift sides with fluid going to the drop side from a pump.

Abstract: A pneumatic circuit for use in causing rapid, upward movement of a piston and cylinder device in a glass machine incorporates a supplementary valve which connects an upper part of the cylinder to exhaust for part of the upstroke, and through a restrictor valve to exhaust towards the end of the upstroke.

Abstract: To influence the periodic stroke movement of the closing element of a valve, a control cylinder working on the closing element in the stroke direction is provided, which can be acted upon or released by a control element periodically with pressure medium. The control element is connected in the supply or discharge conduit of the pressure medium and can variably accelerate or slow its pressure formation or release and with it also the stroke movement of the closing element, at last gradually. Thus a partially or completely automatic control of compressor valves, for example, can be made possible in a simple manner.

Abstract: A fluid control system (10) for a vehicle power steering mechanism having a manually driven steering member and a fluid driven driving member (16) operatively connected to steering means of the vehicle by which steerage of the vehicle is achieved, the control system (10) including a first valve (12) for metering pressurised fluid flow to the fluid operated driving member (16) in response to the force required to move the steering means when operating the manually driven steering member, the first valve (12) including at least two independent metering valve means (170, 171) each of which are simultaneously operated by the steering member to meter fluid flow to and/or from the fluid operated driving member (16) at a predefined rate, at least one of the metering valve means (170, 171) communicating independently to a drain via a second valve (14), the second valve (14) being arranged to selectively connect and disconnect said at least one of said valve means (170, 171) to the drain in response to vehicle speed.

Abstract: A hydraulic control system has a closed center control valve disposed to control flow of pressurized fluid from a pump to an actuator and a bypass valve disposed to control fluid flow through a bypass line connecting the pump to a reservoir. A controller connected to the control valve and the bypass valve is operative to controllably move the control valve toward an open, flow communicating position and the bypass valve toward a closed flow blocking position. A pressure compensating valve is disposed in the bypass line to maintain a predetermined pressure differential across the bypass valve when the pressure upstream of the bypass valve exceeds a predetermined level so that the flow through a area opening in the bypass valve is substantially commensurate with the opening at pressures above the predetermined level.

Abstract: Tandem arranged cylinders having a common wall therebetween, with a common piston rod extending through said common wall, said rod carrying a piston for reciprocation within each said cylinder, and a valve member for fluid communication between said cylinders, permitting pressure equalization therebetween.

Abstract: The disclosure involves a hydraulic cylinder having an elongate interior chamber, a piston movable in the chamber, and a first fluid passage generally parallel to the chamber. In the improvement, the cylinder includes a second fluid passage generally parallel to the chamber and both passages terminate at faces at either end of the cylinder housing. Air bleed fittings and hydraulic fluid fittings are at the faces and because of such location, jammed and broken fittings and damage to the boat transom and other structure are substantially avoided. And since each passage has both bleed and fluid fittings, it is not necessary to "crack" a fluid fitting to bleed the cylinder during installation.

Abstract: A motion control system for hydraulically operated lifting devices. The system provides for the dynamic weighing of a loaded container by controllably directing hydraulic flow through a bypass chamber of a two-way valve. An analog output signal is generated by a hydraulic pressure transducer fluidly communicating with the two-way valve or by a tensiometer mounted on the arms of the lifting device. The two-way valve further provides a second bypass chamber which receives fluid under gravity from the lift circuit line of the hydraulic cylinder for controllably lowering the lifting device. The pressure transducer or tensiometer generates an analog signal proportionate to the weight being lowered and whereby the hydraulic fluid is returned to the reservoir of the hydraulic system independently and downstream of the main operating valve.

Abstract: A hydraulic positioning means is provided which includes means for selectively allowing or prohibiting the flow of the first hydraulic fluid under the operating pressure. It also includes means in the hydraulic positioning means for selectively positioning the workpiece to a plurality of predetermined locations for performing work operations thereon in response to allowing or prohibiting the flow of the first pressurized hydraulic fluid under the operating pressure. Means are also furnished for supplying the first pressurized fluid under the operating pressure to the hydraulic positioning means for such selective positioning of the workpiece. First fluid conduit means transfer the first pressurized fluid at said operating pressure from the pressurized fluid supply means to the hydraulic positioning means. A hydraulic impact load cushioning means is also bifurcatedly provided.

Abstract: A poppet valve device according to the present invention is adapted to sequentially operate a plurality of poppet valves the switching operation of a pilot valve, for example, an electromagnetic solenoid or the like which operates in response to an external input signal. The above-mentioned poppet valve device includes first and second poppet valves (A, B) each of which is constructed so that a poppet (32) for communicating or intercepting an inlet port (30) and an outlet port (31) with or from each other may be moved towards a communicating position by an inlet port side pressure acting upon its pressure receiving portion (32a). The poppet (32) may be moved towards an intercepting position by a pressure acting upon its back pressure chamber (38a, 38b).

Abstract: In normal operation, a control or regulating device controls vehicle rear wheel steering in accordance with freely-selectable specifications. If a malfunction of the control or regulating device is detected, the latter becomes inoperative immediately. A piston-cylinder unit drivingly connected to the front wheel steering and a piston-cylinder unit drivingly connected to the rear wheel steering are simultaneously positively coupled to one another, with the result that the rear wheel steering is actuated positively via the front wheel steering.

Abstract: A system for hydraulically detecting the seizure of a control valve in a hydraulic servo valve system wherein a hydraulic signal generated responsive to the control valve seizure is utilized to translate a bypass valve from its operational position to its bypass position and simultaneously to provide a hydraulic signal for maintaining the bypass valve in its bypassed position.

Abstract: Automatic device for economizing compressed air comprises a means for ensuring the passage of driving fluid under pressure during most of the stroke of the moving element of a user apparatus. The device consists of a single-unit body having a first circuit connecting a fluid supply pipe to one chamber of the cylinder actuator. The first circuit is controlled by a valve moving with a piston subjected to the action of an elastic means and slidably mounted in the body. The piston is actuated by the presence or absence of a pressure signal, the body having a second circuit by-passing the first circuit and connecting a fluid exhaust pipe to the cylinder chamber.

Abstract: An emergency drain device for pneumatic cylinders which is integrated in an orientatable connection fixed directly to the cylinder body and comprises a valve which closes an orifice of the bolt of the connection, going to the atmosphere, when a control pressure is applied to the head of the bolt.This device is advantageously used in installations where a general isolating switch, controlled by the same control pressure, connects the general distribution line to the atmosphere, for example in the case of an emergency, when said pressure disappears.

Abstract: The servo device for controlling flow (10) situated in a branch hydraulic line (6), incorporates in a body (11) an inlet passage (12) and an outlet passage (9) connected by an intermediate passage (13) forming a seat (14) for a valve component (15) which is pushed against its seat by a spring (26) and by an electromagnetic actuator (16) via a push-rod (23) which is slidingly sealingly mounted in a bore (22) between the intermediate passage (13) and an internal chamber (19) which is connected to the inlet passage (12).

Abstract: Apparatus for cutting out blocks from a silo fodder bin has an hydraulically-driven, blade-carrying travelling gear travelling to and fro across a bridge and a conduit between hydraulic feed and return lines thereto such that the fluid flow through said conduit varies the speed of movement of the travelling gear.

Abstract: Electro-hydraulic valve arrangements are useful in controlling a displacement control member of a variable displacement pump or motor. However, previous digital electro-hydraulic valves having brief pulse widths required special high speed solenoids to speed up the response time of the valve and resulted in increased cost of the valve. The subject digital electro-hydraulic valve arrangement includes a pair of on/off solenoid actuated valves arranged in series so that a flow path is initiated by a first of the valves and is disrupted by the second valve. The response times of the valves are equal such that directing an electrical signal to the second valve during the response time of the first valve results in extremely brief pulse widths of the valve arrangement. Using two solenoid actuated valves in this manner permits the use of less costly solenoid valves to provide very brief pulse widths through the total valve arrangement.

Abstract: A brake booster having a constant pressure chamber and a variable pressure chamber formed respectively at both front and rear sides of a power piston within a housing, characterized in that a hollow expansion body expansible axially of the power piston is disposed within the constant pressure chamber and one end of the hollow expansion body is secured fixedly to the housing while the other end of the expansion body is connected to a push rod so that the force magnifying ratio of the brake booster is varied by adjusting the internal pressure in the hollow expansion body.

Abstract: A brake booster can switch operating mode from a positive-pressure mode using a signal-controlled fluid pressure supplier to a vacuum-pressure mode and vice versa. The booster includes a booster converter holding both an air valve and a vacuum valve of the booster closed in the positive-pressure mode or holding the booster air valve in its closed and open positions while holding the booster vacuum valve in its open and closed positions respectively when the brake pedal is released and depressed in the vacuum mode. The fluid pressure supplier is enabled while both booster valves are in the closed position in the positive-pressure mode.

Abstract: The hydraulic system of this invention is for operating a tree processing implement on a work vehicle so that during a first part of an operating cycle the implement is supplied by two hydraulic pumps jointly, while during another part of the operating cycle the implement is supplied by only one of the pumps. The result is that during the first part of the cycle the implement moves rapidly but during the other part it moves slowly.

Abstract: In a vehicle hydraulic system of the open center kind, hydraulic fluid for operating vehicle services is circulated continuously, normally by an engine-driven pump. A closed center booster which requires a substantially constant pressure differential to operate it, is included in the open center system, the pressure differential being provided by a fixed restriction. The fixed restriction may be operative when the pump is operative, or only when the booster is operative. The booster is arranged so that in operation, high pressure fluid entering a boost chamber displaces an equal volume of fluid from a forward chamber into the system. The booster may also include a one-way valve allowing flow of fluid from the forward chamber to the boost chamber.

Abstract: A control system for a fluid pressure operated actuator includes an arming valve having open and closed positions and a pilot operated selector valve having a neutral position and an extend position. The arming valve is connected to a source of fluid and the selector valve is connected to the arming valve so that two control inputs are necessary to connect the source to an operating chamber of the actuator. A solenoid actuated pilot valve is connected to the source and to a sump and has an on position in which the flow of fluid is unblocked and in an off position in which the flow of fluid is blocked. A pilot passage connects the pilot valve to a pilot chamber of the selector valve and a branch passage connects the pilot passage and the operating chamber of the actuator so that the operating chamber communicates with the sump whenever the pilot valve is off regardless of the position of the selector valve.

Abstract: Disclosed is a representative agricultural implement towed by a tractor equipped by a hydraulic power source connected to a lift mechanism for raising and lowering the implement frame so as to vary the ground-working depth of the working tools. The frame carries a sensor responsive to changes in implement depth above and below a preselected norm. This sensor serves as the input for the inventive control valve and system so as to control the implement lift system automatically. The valve is designed as a four-position valve, with one of the positions serving dual modes so that the system produces fast and slow raising, fast and slow lowering and a neutral or balanced position. The valve has a pressure port connected to the pressure side of the tractor source via a manual valve to afford three of the four active positions with one setting of the manual valve. The fourth active position (fast raise) is achieved by changing the manual valve to reverse the pressure and return ports in the depth control valve.

Abstract: A hydraulic sectional control valve includes separate pump pressure and function control spools. The spools may be actuated independently and sequentially to provide raising, holding and power down functions for earthmoving and construction vehicles. Additional function control spools may be added in the valve structure to provide multiple function control.

Abstract: A control arrangement for a hydraulic motor has an adjustable pressure medium supply device with a container, an unblockable check valve with a control connection, a plurality of chambers including a supply chamber, a motor chamber, first and second return chambers, and a control chamber, a control slider displaceable relative to the chambers, wherein a control conduit extends from the supply device and leads to the control chamber, a first pressure control member is arranged on the control slider so that in a lowered position of the latter the first pressure controlling member is located between the control chamber and the second return chamber and the control slider in its raised position blocks the control chamber, and a second pressure controlling member located between the control chamber and the motor chamber and opening into the latter.

Abstract: A first pressure-intensifying apparatus includes a working cylinder having a working piston mounted within it and reciprocally movable by the application of working fluid alternately to opposite sides of the working piston. The working piston in turn drives a high-pressure piston movably mounted within a high-pressure cylinder. The flow of pressurized working fluid to the working cylinder is controlled by a control valve. The control valve receives pressurized fluid from a source of working fluid and directs the fluid to the working cylinder and then back to a working fluid reservoir or tank. The control valve includes a valve body and has a spool movably mounted therein that is movable between a first position in which fluid is directed to one side of the working piston and a second position in which fluid is directed to the other side of the working piston.

Abstract: Fluid control systems frequently eliminate use of fluid operated jack protecting relief valves because additional visibility blocking and damage prone conduits are required to connect the relief valves to the reservoir. A fluid operated system having a controlling valve, a selecting valve, a fluid operated jack, a relief valve, a reservoir and a bypass valve is provided to controllably connect the relief valve to other portions of the system in response to the position of controlling valve and selecting valve. Therefore, no additional conduits are required to connect the relief valve to the reservoir and thus the problems of reduced visibility, premature conduit leakage and conduit damage are eliminated. The fluid control system is particularly suited for use in a work vehicle such as a lift truck having a lift mast.

Abstract: In an electro-hydraulic control assembly for turbine valves, including a shaft for the turbine valve, a valve housing, electrical control means, an electro-hydraulic converter connected to the electrical control means, a hydraulic control cylinder being connected to the electro-hydraulic converter, a control piston being movable in the cylinder in a valve opening and a valve closing direction and being connected to the valve shaft, a force storing spring loading the piston in the valve closing direction, and a hydraulic system including a hydraulic fluid reservoir, a fluid pump being fed from the reservoir and having a pressure side, an electric motor for driving the pump and a hydraulic pressure accumulator connected to the pressure side of the pump, the improvement including a support structure disposed in the housing including two mutually parallel support plates, the spring being disposed between the support plates, the electro-hydraulic converter, the cylinder and the supply system being fastened to the

Abstract: An improved power steering control apparatus includes a power steering motor having a piston rod which extends through one end of the motor cylinder. This results in the rod end of the piston having a smaller working area than the head end of the piston. In order to equalize the fluid pressure forces on opposite sides of the piston when the motor is in an inactive condition, a power steering control valve vents both sides of the piston to atmospheric pressure when the control valve is in a neutral or unactuated condition.

Abstract: In a vehicular power steering system, the hydraulic fluid supply to a hydraulic power actuator is controlled in accordance with the vehicle speed and the angular velocity of the steering wheel. Preferably, the fluid supply is reduced so that the steering effort becomes heavier as the vehicle speed increases while the fluid supply is increased when the angular velocity of the steering wheel is high. To control the fluid supply, there is provided, in a power steering system, a bypass valve means for draining a portion of the fluid flow to bypass the power actuator, and control means for producing a control signal to control the bypass valve means in accordance with the vehicle speed and the angular velocity of the steering wheel.

Abstract: An oil well pump driving unit with a horizontally disposed hydraulic cylinder having a cylinder rod coupled to a drive rope extending into a pumping tee-stuffing box arrangement for driving the sucker rod string leading to a conventional oil well reciprocating pump. The drive rope extends over a first rotating sheave mounted near the wellhead and passes over a second rotating sheave mounted on a carriage which traverses a carriage channel in a draw works on which the hydraulic cylinder is mounted. A hydraulic drive/control system utilizing limit switches on the draw works provides control over the stroke position, the stroke length, and the stroke rate.

Abstract: In a hydraulic elevator, in which fluid is pumped to and from a tank into a piston to raise and lower the car, the flow of fluid is controlled by an apparatus which includes an electric motor that opens and closes a single valve to control the motion profile of the elevator car.

Abstract: An hydraulic tilt system and method for its use with a cab pivotally mounted on a motor vehicle frame. An hydraulic piston-cylinder assembly pivotally connects between the cab and the frame. A remote control station has an hydraulic pump, a reservoir, and a control for operating the pump to tilt the cab. Hydraulic lines connect the control station with the piston-cylinder assembly. Check valves in the hydraulic lines stop the tilting motion of the cab in case of loss of hydraulic pressure. A controlled return free-fall valve is also disposed in the hydraulic lines to selectively restrict the flow of hydraulic fluid.

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 self adjusting actuator system (32/32') has an actuator housing (40), a telescoping piston (52) defining therewith head and rod end chambers (58,60), and a compression spring (66) disposed for biasing the piston (52) in a preselected direction against an external force. A control system supplies fluid selectively to either of the chambers (58,60) and includes control device (33/33') for selectively permitting egress of fluid from the rod end chamber (60) to permit the piston (52) to move in the preselected direction in response to a change in the external force. Such system is particularly useful in a log grapple (10).

Abstract: A pressure relief valve for the working pressure of a servo-steering mechanism of a vehicle, with a pressure medium connection between a high pressure connection adapted to be placed under a working pressure and a zero connection adapted to be placed under a lower pressure, whose valve-adjusting member is retained in a normal rest position closing off the pressure medium connection during normal conditions by a control device operating as a function of an operating magnitude of the servo-steering mechanism and which upon exceeding a critical value of the operating magnitude is adapted to be displaced into a by-pass position more or less opening up the pressure medium connection; the control device thereby operates in dependence on the load of the servo-steering mechanism while the by-pass position is independent of the angle of rotation of the steering shaft of the servo-steering mechanism.