Abstract: A method for controlling a cylinder includes providing a cylinder having a piston, a servo valve and a measuring device having at least one first position sensor and one second position sensor, measurements are taken of the position of the piston in the cylinder body simultaneously with the first position sensor and the second position sensor, at least one first displacement speed of the piston is determined on the basis of the piston position measurements obtained with the first position sensor, at least one second displacement speed of the piston is determined on the basis of the piston position measurements obtained with the second position sensor, and each of the first and second determined displacement speeds of the piston are compared with a modelled or predetermined displacement speed of the piston, in such a way as to identify the most reliable position sensor.

Abstract: A hydraulic valve assembly including a main control valve having a valve body housing a valve member that is movable between a first valve position and a second valve position. The hydraulic valve assembly further includes a mechanical interface operatively coupled to the movable valve member to manually move the movable valve member between the first valve position and the second valve position and one or more electro-hydraulic actuators operatively coupled to the movable valve member to automatically move the movable valve member between the first valve position and the second valve position in response to receiving a control signal from a controller.

Abstract: A pneumatic cylinder with a piston rod movable to fully extended, intermediate and fully retracted positions. The cylinder may have a first head engaging one end of a first tube with a first piston slidably received therein, an intermediate connector engaging the other end of the first tube and one end of a second tube with a second piston slidably received therein, and a second head engaging the other end of the second tube. The piston rod may be attached to the second piston for movement therewith and extend through the intermediate connector, first piston, first tube, and the first head to the exterior thereof.

Abstract: An actuator includes a cylinder, a piston, a rod, a rod-side chamber and a piston-side chamber, a tank, a first pump capable of supplying liquid to the rod-side chamber, a second pump capable of supplying the liquid to the piston-side chamber, a first control passage allowing communication between the rod-side chamber and the tank, a second control passage allowing communication between the piston-side chamber and the tank, a first variable relief valve capable of changing a valve opening pressure for permitting a flow of the liquid by being opened when a pressure in the rod-side chamber reaches the valve opening pressure, a second variable relief valve capable of changing a valve opening pressure for permitting a flow of the liquid by being opened when a pressure in the piston-side chamber reaches the valve opening pressure, and a center passage allowing communication between the tank and the interior of the cylinder.

Abstract: The present invention relates to a board inverter and a boarding inverting system. The board inverter includes a support, a first board picking device, an second board picking device and a controller. Each of the first board picking device and the second board picking device includes a driving mechanism and a first board picking device. The driving mechanism comprises a first linear driving means mounted on the support, a second linear driving means mechanically coupled to the first linear driving means, and a rotary driving means attached to the second linear driving means. The controller is connected to all the power supply members. The controller controls the motion of the first board picking device and the second board picking device such that the first board picking device and the second board picking device cooperatively inverting workpieces transmitted on a production line. The board inverter can be installed on a production line without altering the arrangement of existing production line.

Abstract: A reciprocating linear motor powered by one directional fluid flow is provided, having small external diameter, optimized for length, with few moving parts and effective seals, low mass and thus low inertia in direction changes. The motor provides reciprocating linear powered motion for use by attached equipment such as a pump, chisel, hammer, valve, or other machine requiring such power.

Abstract: The present invention generally relates to an air control module unit for a portable pneumatic tool, more particularly, to an air control module unit that actuates a valve stem inside an air inlet block of the air inlet module by a correspondent air bleed valve in the air actuation module. The air control module unit for a portable pneumatic tool comprising: an air inlet module, which has at least one valve stem inside an air inlet block, the air inlet block having plug screws on both ends towards the valve stem and one plug screw on the intersectional direction of the valve stem with at least one air inlet port opening for hooking up the compressed air; an air actuation module having a bleed valve inside the valve block; a plurality of flexible air hoses with a plurality of dimensions and being prepared for connecting the air inlet module and air actuation module; and a plurality of hose connectors and rubber seats with a plurality of types for connecting the air inlet module and the air operation module.

Abstract: Hydraulic actuator as an element of a system for controlling an engine valve is presented. The actuator includes a fluidic actuation chamber, defined by an actuation piston that increases chamber volume as the actuation piston is urged away from a neutral. Chamber includes a high pressure fluid inlet and a fluid outlet. A fluidic actuator control chamber, defined by a control piston connected to the actuation piston that increases chamber volume as the actuation piston is urged away from neutral. A control fluid outlet is connected to a fluid control spool valve. The fluid outlet of the actuation chamber is fluidly connected to an inlet of the control valve. There is a valve actuation plunger operably connected to the actuation piston, and a drain outlet to maintain a fluidic backpressure to the actuator control chamber.

Abstract: An actuator having a piston member slideable within a cylinder, the piston member defining with the cylinder a first chamber and a second chamber, the effective cross-sectional of the piston member exposed to the fluid pressure within the first chamber being greater than that exposed to the fluid pressure within the second chamber. The actuator further has first and second ports through which fluid can be supplied to the first and second chambers, respectively, and a third port located intermediate the first and second ports. The piston member and the third port are cooperable to throttle the rate at which fluid is able to escape from the first and second chambers through the third port, in use.

Abstract: A hydrostatic transmission has a pair of overcenter variable displacement pumps, each having a flow control element controlled by a double acting actuator to control direction and flow rate of the pump. Fluid pressure in first and second chambers of each actuator are equal at a neutral position of the transmission. A forward control valve controllably communicates the first chambers of both actuators to a tank through a first orifice to reduce the pressure in the first chambers to simultaneously move the flow control elements in a first direction for forward travel. A reverse control valve controllably communicates the second chambers of both actuators to the tank through a second orifice to reduce the pressure in the second chambers to simultaneously move the flow control elements in a second direction for reverse travel.

Abstract: A venting piston ring is provided with ports extending partly through the ring outer surface which cooperate with the end edge of a cylinder wall to vent the cylinder volume while intermediate surface lands between ports maintain ring to cylinder contact and alignment.

Abstract: A hydraulic drive system for positioning a part such as the electrode of an electrical discharge machine includes a hydraulic cylinder and a piston slidably mounted in the cylinder and separating it into opposite first and second chambers on opposite sides of the piston. A piston rod extends from the piston through one end of the cylinder for connection to a part to be positioned. A reservoir containing hydraulic fluid is mounted on the cylinder, and a pump has an inlet communicating with the reservoir and an outlet connected via passageways to the first and second chambers. Return passageways are provided for returning fluid from the chambers to the reservoir, and valves control fluid flow from the pump to the chambers and back from the chambers to the reservoir in order to drive the piston in opposite directions. The reservoir, pump, valves, and at least part of each connecting passageway are located within a single outer housing.

Abstract: The hydraulic setting device (10) has a differential cylinder (11) whose pressure spaces (14, 18) can each be acted upon by a pump (17). The pressure conduits (15) and (19) between the pump and the two pressure spaces (14) and (18) are connected to a control valve (31) by means of control conduits (29) and (33). A part pressure can be set in the pressure spaces (14) and (18) by spilling pressure medium by corresponding activation of the control valve.

Abstract: A hydraulic cylinder assembly for use in a variable external pressure envnment is provided. A hydraulic cylinder exposed to the variable external pressure environment has two chambers separated from one another by a piston. Hydraulic fluid fills each of the two chambers. In an "at rest" mode, the hydraulic fluid in the two chambers is pressurized to the variable external pressure. In an "at work" mode, pressure of the hydraulic fluid is selectively varied from the variable external pressure in one of the two chambers such that the piston moves. The entire assembly can be installed between the pressure and outer hulls of a submarine and utilize seawater to equalize the pressure in a balanced power stroke circuit.

Abstract: An axial piston-type servomotor uses control connections on the working cylinder and corresponding control edges on the working piston, together with selector valve apparatus for controlling the working piston into a central stroke position.

Abstract: A fluid cylinder mechanism couplable to a vehicle and having a piston slidable within the cylinder, a rod couplable to an implement and shiftable within an opening in the piston, the rod being adapted to shift vertically within the piston a an implement rides over ground undulations, and an abutment mechanism coupled to the rod for abutment with the piston when the piston shifts upwardly due to selective forcing of fluid into the rod end of the cylinder by the operator to transfer the weight of the implement to the vehicle or to lift the implement for transport.

Abstract: Disclosed is an arrangement that includes a piston cylinder unit having pressure chamber in which fluid acts in opposition to a spring piston combination, a pressure generator and a fluid conduit extending between a tank and the tank. A first and a second valve are connected in series between the pressure generator and tank with the pressure chamber being in fluid communication with the connection between the valves. In known arrangements of the above type there is dead play wherein not every control signal corresponds to a defined position of the piston in the cylinder. However as disclosed herein the setting accuracy is improved by a throttle in parallel with valve that is fluidly connected between the pressure chamber and the piston cylinder unit. Also third and fourth valves may be connected in series between the pressure generator and tank and have a fluid connection between the third and fourth valves to the unit pressure chamber that is opposite the first chamber to provide a bridging circuit.

Abstract: A circuit assembly device (200) comprises a cylinder (202) containing a piston (204) coupled to an end-effector for facilitating an assembly of a circuit carrying substrate. The piston (204) defines a first chamber (212) and a second chamber (214) in the cylinder (202) for maintaining substantially the same pressure such that a difference in areas of the first and second chambers (212, 214) establishes a compliant force on the end-effector.

Abstract: A hydraulic actuator system, for use in an aircraft, and in which the servo fluid supply to the system is used to provide for the operation and cooling of the system. The system comprises a hydraulically operated piston mechanism including at least one hydraulically operated piston, a servo vluid input to the system, which is connected in a ring main to a first side of the hydraulically operated piston, a servo fluid output from the system, which is connected in a ring main to a second side of the hydraulically operated piston, a variable restrictor arrangement which provides an interface between the servo fluid input ring main and the servo fluid output ring main, and a restrictor arrangement positioned in the servo fluid output side of the system. The position of the piston of the hydraulic piston and the adjustment thereof, are controlled by the restriction applied on the system by the variable restrictor arrangement.

Abstract: The present invention relates to a hot press to manufacture a substrate in which a printed circuit is provided.The hot press comprising at least one of vertically movable heating plates, a freely elevating movable platen which elevates the heating plates to press a plate to be treated between the heating plates, and a detecting means for detecting the movement of the uppermost heating plate when said uppermost heating plate starts to be elevated, in which the elevating speed of the movable platen is changed to a lower speed by a signal generated from the detecting device.This low speed control is carried out as follows.

Abstract: A flow control passage fluidly interconnects an output flow passage and a return flow passage to bypass a control valve. A control valve operation modifying variable orifice is disposed in and partly defines the flow control passage. The control valve operation modifying variable orifice is variable in flow area in response to at least one of vehicle control factors other than steering torque. A fixed flow area orifice is disposed in and partly defines the flow control passasge at a location downstream of the control valve operation modifying variable orifice. The control valve has a plurality of steering torque responsive variable orifices and bypass passage means for bypassing at least one of the steering torque responsive variable orifices. A differential pressure passage provides communication between the upstream and downstream sides of the fixed flow area orifice. An actuator is disposed in and partly defines the differential pressure passage.

Abstract: An electrically controlled, fluid driven actuator valve, in which an electrical signal drives a solenoid, which at any one time operates one of two flexible valve elements which, when opened causes a fluid pressure differential in a piston like hydraulic actuator chamber, to thereby drive an actuator rod, which then acts on any device desired to be controlled. The valve is of simple design with a minimum of moving parts, and is especially useful in missile and aircraft applications.

Abstract: Apparatus for controlling operation of an aerial lift having a boom. A hydraulic fluid pump is connected to a hydraulic cylinder, and the control arrangement controls the supply of hydraulic fluid from the hydraulic fluid pump to the cylinder and controls the supply of hydraulic fluid pressure to a first pilot fluid port and to a second pilot fluid port of a pilot operated control valve to control operation of the pilot operated control valve. A first hydrulic fluid conduit operably connects the hydraulic fluid pump to a first pilot fluid port, and a second hydraulic fluid conduit operably connects the hydraulic fluid pump to a second pilot fluid port of said control valve. A control handle selectively vents hydraulic fluid from the first fluid conduit and the second fluid conduit to control fluid pressure at the pilot ports.

Abstract: A submersible unit comprises a portable, open or closed bottomed pneumatic chamber mounted on flexible drive tracks controlled by personnel from the chamber for operation along the bottom of a body of water, on the surface, at intermediate levels, in the air and on land. A safety chamber, connected to the submersible unit by an extendable linkage, can limit the depth of submergence of the submersible unit, stores cargo and carries power. Flooded compartments in the submersible unit and the safety chamber are supplied with compressed air to control the supported load and depth of submergence. Various accessories carried by the underwater unit enable the performance of a number of different on and under the water and land based tasks. A special pneumatic circuit enables equipment and propulsion to be controlled by touch control panels. A majority of operations are contained within the machine to minimize contamination of surrounding areas.

Abstract: A hydraulic well pump for operating a pump sucker rod string including a hydraulic cylinder assembly for raising and lowering the sucker rod string and a fluid counterbalancing system for offsetting the combined weight of the reciprocating service equipment, the fluid column above the pump plunger, and the sucker rod string. One counterbalancing embodiment includes an air supported piston on the movable cylinder exposed to an air chamber below the air piston functioning independently of the hydraulic and power and logic system of the pump. Another counterbalancing system embodiment uses a hydraulic fluid accumulator connected into the system fluid power and logic for supercharging the hydraulic power pumps during the lift stroke and providing opposing force during the downstroke.

Abstract: A pneumatic servo valve comprising a piston in a cylinder. Both controlling surfaces of the piston are exposed to controlling air. The piston is provided with an axial drilling extending through it, which communicates via a radial groove with an inlet of controlling air. Both openings of the drilling are normally closed by spring actuated mushroom valves. Controlling chambers in the cylinder communicate through calibrated conduits in the piston with the radial groove, and can be put into communication with the surrounding atmosphere via outlets. The outlets are controlled by electromagnet valves. A microcomputer receives signals from correct and actual value indicators respectively, and administers opening and closing of the valves. When one of these is opened the connected controlling chamber is decompressed and the piston moves in the direction of the empty chamber. One of the mushroom valves follows the movement of the piston, while the other mushroom valve is prevented from moving by a valve seat.

Abstract: The distributor, typically one having a star-shaped rotor in a disc-shaped stator, incorporates two components, primary and secondary, defining two parallel distribution circuits (10, 10') controlling an assistance actuator (V) and incorporating, in each circuit, three variable restrictions (1-3 or 1'-3'), together with, advantageously, two other parallel reaction circuits (20, 20') with two restrictions in series (4, 5 and 4', 5'). The chamber (V.sub.1, V.sub.2) of the assistance actuator (V) are joined to the distribution circuits (10, 10'), respectively, between the second (2, 2') and third (3, 3') variable restrictions which create the differential assistance pressure, the central point of the distributor being determined by the first (1, 1') and second (2, 2') variable restrictions.

Abstract: The servo-motor system is provided with an internal control line which communicates one control valve with a cylinder chamber which is connected to a control valve connected with an external control line. The internal control line permits the second valve to be actuated via the externally controlled control valve.

Abstract: An active, mechanical hydraulic control device for vehicles including watercraft and aircraft, comprises an acceleration pick-up having a relatively small mass. Movements of the mass are sensed by a nozzle and deflection plate system which controls as a preamplifier the displacement of a power drive member provided with one or more control apertures, the flow cross-sectional areas of which depend on the instantaneous position of a piston to which the power drive member is connected. The movements of the mass vary the gap width between the nozzle or nozzles and the deflection plate forming part of the movable mass. The change in the gap width controls the adjustment or displacement of the power drive member through a follower amplifier formed by the piston. The central or balanced position of the piston is assured by the control apertures forming respective piston position responsive valves.

Abstract: Two fluid passages are connected through flow restrictors to a fluid supply. Downstream of the restrictors the fluid supply has a drooping pressure-load characteristic. Venting means for the fluid passages comprises vent openings therein downstream of the restrictors and variable position obstructor means cooperating with vent openings to vary venting and thereby vary fluid pressures in the fluid passages.Fluid conduits connect these pressure outputs to a double acting piston moving in a cylinder whose opposite ends are connected to the fluid conduits. The piston and cylinder form the responder of the system.A receiver is formed by a piston and cylinder or other fluidic motor remotely fluidically connected to the responder, the motor driving a load.A feedback means controlled by the position of the responder piston and/or the load varies pressure in the fluid conduits by variably venting same.Both rotary and linear types of transmitters and load feedback means and receivers may be used.

Abstract: A pilot stage for use in pilot pressure control applications includes a magnetic torque motor having an output torque in response to an input current, a flapper operated by the magnetic torque motor and responsive to the output torque, the magnetic torque motor being arranged to apply a resultant spring rate to the flapper, and first and second nozzles responsive to the output torque applied to the flapper for providing a differential output pressure proportional to input current, the first and second nozzles having a size sufficiently large to swamp out the resultant spring rate applied to the flapper so that the first and second nozzles apply a feedback force to the flapper sufficient by itself to substantially balance motor torque.

Abstract: A fluidic Repeater, includes a rotary transmitter, a responder, and a rotary feedback.Various constructional features useful in conjunction with the system are disclosed.

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: Two fluid passages are connected through flow restrictors to a fluid supply. Downstream of the restrictors the fluid supply has a drooping pressure-load characteristic. Venting means for the fluid passages comprises vent openings therein downstream of the restrictors and variable position obstructor means cooperating with vent openings to vary venting and thereby vary fluid pressures in the fluid passages.Fluid conduits connect these pressure outputs to a fluid to mechanical translator comprising a double acting piston moving in a cylinder whose opposite ends are connected to the fluid conduits.A receiver is formed by a piston and cylinder or other fluidic motor remotely fluidically connected to the responder, the motor driving a load. Alternatively the remote motor may be omitted, the responder itself constituting a receiver and its piston connected, mechanically or fluidically, to an output display or load for indication or proportional control.

Abstract: A power steering device includes a housing which journals coaxial input and output shafts. Drivingly connected to the input shaft is an eccentric which rotatably carries a gear wheel. The gear wheel is prevented from rotating in the housing so that it gyrates when the input shaft rotates. An internally toothed ring meshes with the gear wheel and is drivingly connected to the output shaft. The internally toothed ring defines a greater number of gear teeth than does the gear wheel so that gyration of the gear wheel incident to the rotation of the input shaft drives the output shaft via the internally toothed ring. A valve device is responsive to rotation of the input shaft relative to the output shaft to control fluid flow to and from a fluid motor. The fluid motor is coaxial with and drivingly connected to the output shaft so as to increase the torque derivable from the output shaft incident to rotation of the input shaft.

Abstract: A fluid repeater system may be balanced so that displacement of the load has a desired correspondence to displacement of the transmitter. This may be done by making the fluid resistance offerd to fluid flowing along a path from the source to the reservoir and through the feedback controlled directly by the load, exclusive of the resistance offered by such load feedback, approximately equal to that offered to fluid flowing along a path from the source to the reservoir and through the transmitter, exclusive of the resistance offered by such transmitter.

Abstract: A hydraulic servo-actuator has a casing embracing therein a hydraulic fluid chamber which is divided into two hydraulic drive chambers by a vane projecting from a first shaft. To each of the divided chambers are connected an oil feeding passage provided with a variable restrictor and intended for the feeding of pressurized oil and an oil discharge passage intended for the discharging of the pressurized oil. A second shaft is coaxially disposed relative to the first shaft, and the first and second shafts are correlated with each other in such a way that the relative rotary displacement occurring between the two shafts controls the aperture of the oil discharge passages communicating with the divided chambers. The difference between the static pressures which are consequently produced within the divided chambers because of the aperture of the oil discharge passages causes the vane to swing, making it possible to control the torque even when the vane is in the neighborhood of its neutral position.

Abstract: A circulating flow hydraulic pilot system supplies pressurized fluid from a source to a sump through first and second parallel disposed conduits. Selective pressurization of each conduit is achieved by a pilot valve and a fluid restrictor that are series interposed into each conduit. A fluid actuator includes two ports that communicate with respective ones of the conduits intermediate of respective ones of the pilot valves and fluid restrictors. Other pilot valves are provided for manually overriding the pilot system.

Abstract: A power assisted gear assembly in which a piston 3 forms with its cylinder opposed chambers 4, 5. Displacement of the piston provides a steering output and such displacement is effected by rotation of an input component 6 which screw threadedly engages the piston 3. Two fluid systems are provided with respective inlets 18, 20 and with the chambers 4, 5 being located one in each fluid system. A valve 10, 15, 16 adjusted by the input component 6 when displacing the piston 3 controls fluid flow through the two fluid systems to an outlet 29 and such valve has a neutral condition to which it is biased by springs 25, 26 or by fluid pressure in the fluid systems in which both fluid systems communicate with the outlet 29 for an open center system.

Abstract: A flow divider valve cooperates with a housing to control flow between an input port, a first output port and a second output port. The valve defines a first land and a second land and is movable within the housing to a first position wherein flow is decreased to the first output port and increased to the second output port, and to a second position wherein flow is decreased to the second output port and increased to the first output port. Movement of the valve to the first and second positions is controlled by pressure within the first and second output ports. A passage on the valve communicates flow to both sides of the valve.

Abstract: A fluid amplifier comprising a housing having a cavity therein and a diaphragm in the housing dividing the cavity into first and second chambers. First and second supply passages lead to the first and second chambers, respectively, and terminate in first and second supply ports. The diaphragm is movable in the cavity to selectively substantially seal off the first and second supply ports. First and second control passages extend between the first and second supply passages and the first and second chambers. First and second signal passages are provided in the housing and terminate in the first and second chambers, respectively. The first and second control passages are in communication with the first and second signal passages through the first and second chambers regardless of which of the supply ports is sealed off by the diaphragm. First and second output passages terminate in the first and second supply passages, respectively.

Abstract: Two fluid passages are connected through flow restrictors to a fluid supply. Downstream of the restrictors the fluid supply has a drooping pressure-load characteristic. Venting means for the fluid passages comprises vent openings therein downstream of the restrictors and variable position obstructor means cooperating with vent openings to vary the venting and thereby vary fluid pressures in the fluid passages.Fluid conduits connect these pressure outputs to a piston moving in a cylinder whose opposite ends are connected to the fluid conduits. The piston and cylinder form the responder of the system. A feed back means controlled by the position of the responder piston and/or the load varies pressure in the fluid conduits by variably venting same.The feedback means includes variable cross-section surface pasages, e.g. sloping grooves, in the responder piston or fluidic load piston or both. Alternatively or additionally a separate feedback piston may be connected to the receiver or load piston.

Abstract: Two fluid passages are connected through flow restrictors to a fluid supply. Downstream of the restrictors the fluid supply has a drooping pressureload characteristic. Venting means for the fluid passages comprises vent openings therein downstream of the restrictors and variable position obstructor means cooperating with vent openings to vary the venting and thereby vary fluid pressures in the fluid passages. This forms a mechanical to fluidic translator. Pressures at the translator's outputs are a function of the obstructor's position. This translator forms the transmitter of a servo or remote control or indicator system.Fluid conduits connect these pressure outputs to a fluid to mechanical translator comprising a double acting piston moving in a cylinder whose opposite ends are connected to the fluid conduits. The piston and cylinder form the responder of the system, which may be adjacent the mechanical to fluidic translator and form part of the transmitter.

Abstract: An orbiting, floating mechanism which reciprocates while rotating and is capable of varying, stopping, or reversing a flow of fluid therethrough at any one revolution or any combination of revolutions or any part thereof.

Abstract: An electrohydraulic control unit which regulates the flow of hydraulic fluid to and from one or more consumers by way of a directional control valve has a cylinder for a piston which is mechanically connected with or constitutes the spool of the directional control valve and divides the interior of the cylinder into two plenum chambers connected to the outlet of the pump, either directly or by way of a pressure reducing valve. The chambers are further connected with the tank and each of the fluid supplying and evacuating conduits for each of the two chambers contains a flow restrictor. Th flow restrictors in conduits for admission of fluid to the chambers are mounted upstream of check valves and the flow restrictors in conduits for evacuation of fluid from the chambers are mounted upstream of or constitute integral parts of two solenoid-operated auxiliary valves. The four flow restrictors together constitute a hydraulic bridge circuit.

Abstract: There is disclosed a control device for an air cylinder unit which functions to automatically orient the unit to retract if all air is shut off to the unit during the extension operation thereof. The device includes a control piston having a first passage connecting supply pressure to only one side thereof and a second restricted passage intercommunicating opposite sides thereof.

Abstract: A servo device for steering gears for vehicles and ships is adapted to be coupled in a mechanical connection between said gear or the like and the steering rod or the like operated thereby. The device comprises a pair of opposed cylinder-and-piston units each having a working chamber, defined by the respective piston and adapted to be supplied pressurized fluid from a suitable source during control by valve means responsive to a movement of the output member of said gear and the resistance thereagainst of the steering rod operated thereby so as to amplify the power for overcoming said resistance by means of the pressure fluid in one of said pair of working chambers.