Abstract: When a prime mover (1) is driven, a controller (25) outputs an open-drive signal for bringing an opening/shutting valve (23) into an open position and, upon a control lever (68, etc.) being operated, a hydraulic fluid is supplied from an auxiliary hydraulic pump (7) through pressure reducing valves to driving sectors of directional control valves (8 to 16) for shifting them. A flow rate and a direction of flow of the hydraulic fluid from main hydraulic pumps (3, 4) are thereby adjusted so that actuators (57A, 57B, 57C, 56A, 54) can be each operated at a speed depending on the amount by which the control lever is operated. When one of the actuators (57A, 57B, 57C, 56A, 54) is to be driven when the prime mover (1) is not driven, an open command is selected in a selection command device (24), whereupon the controller (25) outputs the open-drive signal, causing the opening/shutting valve (23) to open a line interconnecting the accumulator (21) and the pressure reducing valve (17), etc.

Abstract: A hydraulic drive system for a construction machine comprising a hydraulic pump (1) of variable displacement type, a pump regulator (2) for controlling a delivery rate of the hydraulic pump, a plurality of hydraulic actuators (7) driven by a hydraulic fluid supplied from the hydraulic pump, a plurality of directional control valves (4A-4D) for controlling respective flows of the hydraulic fluid supplied from the hydraulic pump to the plural hydraulic actuators, a low-pressure circuit (22), a center bypass line (23) for connecting in series center bypasses of the plural directional control valves (4A-4D) to the low-pressure circuit (22), a plurality of bleeding-off restrictors (26) in the center bypass line and having their openings variable in accordance with respectively associated directional control valves, a fixed restrictor (5) disposed in the center bypass line for producing a control pressure (PZ), and a pressure sensor (8) for detecting the control pressure and outputting a corresponding electric sign

Abstract: A hydraulic drive system for construction machines comprising a valve group (52) including a plurality of directional control valves (9-13) of center bypass type, a center bypass line (2a) for connecting in series center bypasses of the plural directional control valves to a low-pressure circuit (29), a plurality of bleeding-off variable restrictor means (56) respectively disposed in the center bypasses of the plural directional control valves, a pressure compensating valve (20) provided in the center bypass line, and first and second differential pressure detecting lines (22, 24) connected to the center bypass line for transmitting a differential pressure to the pressure compensating valve.

Abstract: A construction machine has a flow control device (2, 8, 10 ) for controlling a flow rate of the hydraulic fluid supplied to an actuator, and a pump control device (2, 40, 42) for controlling a delivery rate of the hydraulic pump such that the pump delivery rate is reduced as a load of the actuator increases, and is increased as the load of the actuator decreases. A magnitude of the load exerted on the actuator is detected (6), and an abrupt reduction in the load of the actuator based on the detected load is monitored. The hydraulic fluid supplied to the actuator is limited when it is determined that the actuator has reached a predetermined condition related to an abrupt reduction in the load, for the purpose of preventing an abrupt increase in the actuator speed incidental to an abrupt drop of the load not occurring during ordinary work.

Abstract: A hydraulic system for a construction machine comprising plural hydraulic actuators (23-28) driven by fluid supplied from a pump (22), first and second flow control valves (29-34) for controlling flows of fluid supplied to the first and second actuators, and first and second distribution compensating valves (35-40) for controlling first differential pressure (.DELTA.P v1-.DELTA.P v6) produced between inlets and outlets of the first and second flow control valves. The first and second distribution compensating valves have respective drives (45-50, 35c-40c) for applying control forces (Fc1-Fc2) in accordance with the second differential pressure to the associated distribution compensating valves, to thereby set target values of the first differential pressures. The hydraulic system includes first means (59) for detecting the second differential pressure (.DELTA.

Abstract: A hydraulic drive controlling apparatus for a construction machine has at least one variable displacement type hydraulic pump, a plurality of actuators driven with a hydraulic fluid from the hydraulic pump, directional control valves driven in accordance with amounts of manipulation of operation means for controlling the plurality of actuators, means for detecting a delivery pressure of the hydraulic pump, means for selecting maximum one of load pressures of the plurality of actuators, and first control means for controlling displacement of the hydraulic pump to bring a differential pressure between the delivery pressure and the maximum load pressure to a specified value.

Abstract: A hydraulic drive system for a construction machine in which a hydraulic circuit has at least one hydraulic pump, at least first and second hydraulic actuators driven by a hydraulic fluid discharged from the pump, and at least first and second directional control valves connected to the pump in parallel with each other for controlling flows of hydraulic fluid supplied from the pump to the first and second actuators, respectively, and a control unit is responsive to first and second operation signals for driving the first and second actuators, respectively, to produce first and second control signals for actuating the first and second valves and deliver such control signals thereto, respectively, each of the first and second valves having a degree of opening changed in accordance with a level of the corresponding one of the first and second control signals for controlling a flow rate of hydraulic fluid supplied to the corresponding one of the first and second actuators.

Abstract: A load weight calculating system for a load moving machine, such as a hydraulic excavator, including a support, and a front attachment pivotably supported by the support. The front attachment includes a first linkage element pivotably supported by the support, a second linkage element pivotably supported by the first linkage element, a loading section supported by the second linkage element, and first and second hydraulic actuators for moving the first and second linkage elements, respectively. The load weight calculating system includes a first displacement detector and a second displacement detector for obtaining displacements .alpha..sub.1 and .beta..sub.1 of at least the first and second linkage elements, respectively, a pressing force sensor for obtaining a pressing force K.sub.1 exerted by at least one of the first and second hydraulic actuators, a weight calculating unit for calculating the weight of a load carried by the loading section based on the values .alpha..sub.1, .beta..sub.1 and K.sub.

Abstract: A method of and system for controlling a hydraulic power system comprising an internal combustion engine receiving a supply of fuel from a fuel injection pump, and at least one variable displacement hydraulic pump driven by the internal combustion engine. An engine speed deviation is obtained from the difference between a target speed set by an accelerator operation of the internal combustion engine and an output speed thereof. Then a fuel injection target value in predetermined functional relation to the engine speed deviation, is obtained based on at least the engine speed deviation, and a fuel injection rate of the fuel injection pump is controlled based on the fuel injection target value.

Abstract: A control system for a hydraulic circuit having a variable displacement hydraulic pump, and an actuator driven by the pump, in which a pump control controls a displacement volume varying member of the pump based on an operating signal giving a command with respect to the position thereof and a detector signal indicative of the actual position thereof while restricting the operating speed of the member to a level below a predetermined maximum speed.

Abstract: A control system for a hydraulic circuit including at least first and second variable displacement volume type hydraulic pumps, at least first and second hydraulic actuators driven by the pumps, and valves for controlling hydraulic connections between the pumps and the actuators. The control system includes a device for deciding the order of priority of hydraulic connections between the first actuator and the first and second hydraulic pumps and the order of priority of hydraulic connections between the second pump and the first and second actuators. A first sensor is provided for sensing maximization of the displacement volume of the first pump, sensing that the displacement volume of the second pump has become substantially zero. A device decides target displacement volumes of the first and second pumps based on information supplied at least by the priority order deciding device and first and second sensor.

Abstract: A hydraulic power system having a variable displacement hydraulic pump, a hydraulic motor driven by the hydraulic pump for actuating a load, with the pump and motor being connected to constitute a closed hydraulic circuit. A hydraulic servo controls the displacement volume of the pump, and a motor stop arrangement prevents, in its operative position, the motor from rotating. The system includes a control valve responsive to a main line pressure in the closed hydraulic circuit for controlling a hydraulic fluid supply to the hydraulic servo to prevent the main line pressure from substantially exceeding a predetermined level, and a control is responsive to the operation of the hydraulic servo for releasing the motor stop arrangement from the operative position when the hydraulic servo is operating.

Abstract: A control system for a hydraulic circuit having at least first and second hydraulic pumps of the variable displacement type, a first hydraulic actuator arranged for hydraulic connection with the first pump through first valve to be driven thereby, and a second hydraulic actuator arranged for selective hydraulic connection with said first and second pumps through second and third valve respectively to be driven thereby.

Abstract: A hydraulic drive system including at least one hydraulic actuator, two variable displacement hydraulic pumps each connected to the hydraulic actuator to form a closed circuit therewith and connected in parallel with each other, two prime movers each driving one of the hydraulic pumps, and directional control valves each arranged between the hydraulic actuators and one of the hydraulic pumps. The directional control valves are each operative to shut, when one of the prime movers associated therewith is shut down, the hydraulic pump connected to the shutdown prime mover off the other hydraulic pump and the hydraulic actuator, to thereby prevent the hydraulic pump from being rotated by the pressure fluid delivered by the other hydraulic pump.

Abstract: A flushing valve system suitable for use in a closed circuit hydrostatic transmission including a variable displacement hydraulic pump, a hydraulic actuator and two main lines. The flushing valve system includes poppet valves mounted in passages for respectively communicating the main lines with a fluid tank, passages mounting throttles for causing the pressure in the main lines to act on pressure chambers to keep poppets of the poppet valves from moving to an open position, and a change-over valve responsive to the pressure differential between the main lines to communicate the pressure chamber of the poppet valve connected to the main line on the lower pressure side with the fluid tank. The poppet valve connected to the main line on the lower pressure side opens when a predetermined pressure level is exceeded by the pressure in the main line, to thereby allow working fluid in the main line to be drained to the fluid tank.

Abstract: A control system for a hydrostatic power transmission having a variable displacement hydraulic pump (2) and a hydraulic actuator (3) connected in a closed or semi-closed circuit, wherein the displacement of the pump or the delivery thereby is adjustable in accordance with a manipulated variable of an operating lever (13). A control unit of the system (16) is operative to calculate a hydraulic pump displacement command (X) on the basis of a manipulated variable signal (X.sub.

Abstract: A hydraulic drive system including a main line, a variable displacement hydraulic pump and another main line connected to a single rod cylinder in a closed hydraulic circuit, and a flushing valve having a first switching position in which one main line is communicated with a low pressure line and a second switching position in which the other main line is communicated with the low pressure line. The flushing valve is operative to maintain at least one of the two main lines in communication with the low pressure line at all times while moving from one switching position to the other switching position, to keep a lock-up phenomenon from taking place in the closed hydraulic circuit. The system further includes a pressure generating device for generating between the two main lines a pressure differential necessary for effecting switching of the flushing valve.

Abstract: A circuit pressure control system for a hydrostatic power transmission having a variable-displacement hydraulic pump driven by a prime mover, a hydraulic actuator for actuating a load and a displacement adjusting mechanism for the hydraulic pump. The hydraulic pump and actuator are connected together in closed or semi-closed circuit, and the displacement adjusting mechanism is actuated by a signal indicative of the operating lever manipulated variable and a signal indicative of the actual displacement of the hydraulic pump to control the speed of the hydraulic actuator.

Abstract: A hydraulic power system having a variable displacement hydraulic pump, a hydraulic motor driven by the hydraulic pump for actuating a load, the pump and motor being connected to constitute a closed hydraulic circuit, hydraulic servo for controlling the displacement volume of the pump, and motor stopping device including a brake arrangement adapted to release the motor from a braked condition when a control pressure from a hydraulic fluid source is received.

Abstract: A control method and a control system for a hydrostatic drive system including a variable displacement hydraulic pump and a hydraulic actuator connected together to form a closed hydraulic fluid circuit. When the flow rate of the hydraulic pump is increased, the rate of change in the tilt of the hydraulic pump is controlled so that the discharge pressure does not exceed a first set of preset values with respect to the flow rate of the pump, meanwhile when the flow rate of the pump is reduced, the rate of change in the tilt of the hydraulic pump is controlled so that the suction pressure does not exceed a second set of preset values with respect to the flow rate of the pump.