A discharge pressure scale (30) includes a valve housing (41) having a functional connector (A), a return flow connector (T) and a user connector (28). A valve piston (52) is guided such that it moves longitudinally against the effect of an energy accumulator (42), moving from a respective opening or regulating position, against a valve seat (94), into a closed position. The user connector (28) and return flow connectors (T) are separated from one another. The fluid pressure present at the user connector (28) can be guided onto a pressure-active surface (A.sub.1*) of the valve piston (52) by a pressure compensation device (70) in such a way that it moves into its respective opening or regulating position in a pressure-compensated manner due to the force of the energy accumulator (42).

A valve device having an inlet port (ZA) of an inlet side for supplying a hydraulic consumer with hydraulic fluid, wherein said hydraulic consumer can be connected to the inlet port (ZA), having an outlet port (AA) of an outlet side for discharging pressurized fluid from the connectable consumer, wherein, depending on the direction of actuation of this consumer, the inlet side becomes the outlet side and the outlet side becomes the inlet side, having a pressure supply port (P), and having a return port (T), is characterized in that a pressure regulating device is acting on the respective inlet side and a volume flow regulating device is acting on the respective outlet side.

To reduce the cost by reducing the number of parts and simplify the control of regeneration in the hydraulic actuator even though the meter-in control and the meter-out control can be performed separately while the supply and the discharge of hydraulic fluid is controlled. A meter-in valve that controls supply flow from a hydraulic pump into a hydraulic cylinder is provided, and meter-out switching valve that switches the direction of supply and discharge of the hydraulic oil into the hydraulic cylinder and controls the discharge flow from the hydraulic cylinder to the oil tank is installed on the down stream side of the meter-in valve, and further, a regeneration control valve is installed on the down stream side of the meter-out switching valve.

The invention discloses systems for improving performance and operational efficiency of wireline tractors. A cooling system improves power capabilities of electrical motors mounted in wheels, which are mounted on a linkage pivotably connected to an arm extended from a tractor tool body, in an in-line arm configuration. The linkage and arm mechanism significantly improve the tractor's ability to traverse wellbore obstructions. The hydraulic system of the tractor ensures adequate wellbore centralization

A hydraulic control circuit includes a first pressure regulating valve for regulating a pump discharge pressure, a second pressure regulating valve for adjusting a pilot pressure to a torque converter pressure, and a first oil passage that connects an output port of the first pressure regulating valve and a torque-converter input-side oil passage, the second pressure regulating valve having an input port connected to the first oil passage through a second oil passage, a drive port connected to the first oil passage through a third oil passage, a spool axially displaced according to a drive pressure inputted into the drive port for regulating the torque converter pressure, and an output port. A throttle is provided between the connection section of the first and third oil passages and the connection section of the first and second oil passages, for adjusting the torque converter pressure to an appropriate pressure.

The invention relates to a discharge pressure scale (30) consisting of at least one valve housing (41) having at least three fluid connection points in the form of a functional connector (A), a return flow connector (T) and a control connector (28), wherein a valve piston (52) is guided such that it moves longitudinally against the effect of an energy accumulator (42), moving from a respective opening or regulating position, against a valve seat (94), into a closed position, wherein the control (28) and return flow connectors (T) are separated from one another, characterised in that the fluid pressure present at the control connector (28) can be guided onto a pressure-active surface (A.sub.1*) of the valve piston (52) by means of a pressure compensation device (70) in such a way that it moves into its respective opening or regulating position in a pressure-compensated manner due to the force of the energy accumulator (42).

A hydraulic control device of a forklift truck includes a first hydraulic cylinder, a first instructing member, a second hydraulic cylinder, a second instructing member, a hydraulic pump, an electric motor, a first oil passage, a lowering control valve, a second oil passage that is branched from the first oil passage at a junction, a flow control valve, a first pilot passage through which pressure in the first oil passage at a position between the first hydraulic cylinder and the lowering control valve is applied to a first accommodating chamber of the flow control valve as the pressure upstream of the lowering control valve, and a second pilot passage through which pressure in the first oil passage at a position between the lowering control valve and the hydraulic pump is applied to a second accommodating chamber of the flow control valve as the pressure downstream of the lowering control valve.

A fan control system includes a tank, a pump, a hydraulic motor, a fan, and a control valve. The control valve is adapted to selectively direct a flow of hydraulic fluid from the pump through the control valve into a pressure control chamber of the hydraulic motor or from the pressure control chamber to the tank to vary the control pressure therein to move the hydraulic motor's swashplate between forward and reverse positions. The control valve is adapted to selectively direct hydraulic fluid from the pump into the pressure control chamber such that the control pressure therein is pressurized to an idle pressure to move the swashplate to an intermediate position between the forward and reverse positions such that the output shaft rotates at an idle rate which is less than the rotational speed of the output shaft when in the forward mode.

A hydraulic drive system includes control valve and operating devices, a variable displacement pump, and a flow regulator. When an operating lever inclination angle becomes a value, a control valve opening area becomes a reference. When the operating lever inclination angle maximizes, the opening area maximizes. The flow regulator: until the operating lever inclination angle becomes the value, increases the pump discharge flow rate with the inclination angle, so a differential pressure between pump discharge and actuator load pressures is constant; when the operating lever inclination angle becomes the value, controls the pump discharge flow rate, so a control valve passing flow rate is an actuator maximum flow rate when the differential pressure is constant; and when the operating lever inclination angle is between the value and the maximum, defines a maximum pump discharge flow rate, so the pump discharge flow rate is kept to the actuator maximum flow rate.

To keep operability of a hydraulic actuator excellent even in a state pressure has been sufficiently accumulated in a pressure accumulator. In a hydraulic driving device of a work machine including a hydraulic actuator, a tank, a flow control valve, and a pressure accumulator, there are further provided with a first pressure compensation valve that is for controlling difference between front and back pressures of the flow control valve constant and a second pressure compensation valve that is arranged between the pressure accumulator and the tank and is for controlling difference between front and back pressures of the flow control valve and the first pressure compensation valve constant.