Hitachi Construction Machinery Co., Ltd.

Takashi Kanai

Yasutaka Tsuruga

Junya Kawamoto

Masami Ochiai

Morio Oshina

Yasuharu Okazaki

Satoshi Hamamoto

Kenichiro Nakatani

Yukiaki Nagao

Mitsuo Sonoda

Kiwamu Takahashi

Ken Ichiryu

Koji Nozaki

Hirohisa Tanaka

Nobuhiko Ichiki

Hideyo Kato

Atsushi Tanaka

Yuusaku Nozawa

A differential pressure between a delivery pressure of a fixed displacement hydraulic pump and a maximum load pressure among a plurality of actuators to is maintained at a target differential pressure by an unloading valve . A setting pressure of the unloading valve is changed depending on an engine revolution speed by introducing a differential pressure &Dgr;Pp across a throttle , which is disposed in a delivery line of a fixed pump , to a pressure bearing sector of the unloading valve. With such an arrangement, in a hydraulic drive system including an LS system, it is possible to ensure fine operability based on setting of the engine revolution speed, to perform flow rate control at a good response, and to realize superior operability.

Hydraulic driving device

Differential pressures across flow control valves and are controlled by pressure compensating valves and to be held at the same value, i.e., a differential pressure &Dgr;PLS, and the differential pressure &Dgr;PLS is maintained at a target differential pressure &Dgr;PLSref by a pump displacement control unit . For changing the target differential pressure depending on change in revolution speed of an engine , a flow detecting valve is disposed in a delivery line of a fixed displacement hydraulic pump , and a differential pressure &Dgr;Pp across a variable throttle portion of the flow detecting valve is introduced to a setting controller . A selector valve operable to shift between a fully closed position and a throttle position is disposed in parallel to the flow detecting valve and is shifted by a control lever

Hydraulic drive device

In a hydraulic drive system in which a target compensated differential pressure for each of pressure compensating valves is set in accordance with a differential pressure between a pump delivery pressure and a maximum load pressure, and a target LS differential pressure is set as a variable value depending on a revolution speed of an engine a fixed throttle and a signal pressure variable relief valve are disposed in a maximum load pressure line A relief setting pressure P of the signal pressure variable relief valve is set so as to satisfy P &equals;P &minus;P &plus;a (where a is a value smaller than P ) with respect to a target LS differential pressure P and a setting pressure P of the main relief valve

Hydraulic drive system

An actuator lock switching valve is provided which communicates a drain line and a pilot line with each other when the valve is in a position C, and which communicates pilot lines with each other when it is shifted to a position D. The pilot line is connected to a delivery line of a hydraulic pump , and the pilot line is connected to pressure receiving sections provided at ends of the pressure compensating valves on the side acting in the closing direction. The actuator lock switching valve has a pressure receiving section connected to the output side of a pilot lock switching valve , and is switched over in interlock with shifting of the switching valve . In a hydraulic drive system including pressure compensating valves controlled by an LS system, an actuator can be locked with a simple construction and can be prevented from malfunctioning in an inoperative condition while an engine is being driven, even when the system includes a mechanically shifted directional control valve, or even when a mechanically shifted directional control valve is retrofitted to the system.

A hydraulic drive system having a swing control system, includes a pump control unit for controlling a pump delivery rate such that a pump delivery pressure is held a predetermined value higher than a maximum load pressure among a plurality of actuators. Pressure compensating valves are each constructed to set, as a target compensation differential pressure, a differential pressure between a delivery pressure of a hydraulic pump and a maximum load pressure among the actuators. A pressure compensating valve for a swing section is given such a load dependent characteristic that when the load pressure rises, the target compensation differential pressure is reduced, the load dependent characteristic being set so as to provide a flow rate characteristic simulating constant-horsepower control of a swing motor.

A hydraulic drive system includes a pump control unit for controlling a pump delivery rate such that a pump delivery pressure is held at a predetermined value higher than a maximum load pressure among actuators . Pressure compensating valves are each constructed to set, as a target compensation differential pressure, a differential pressure between the delivery pressure of a hydraulic pump and the maximum load pressure among the actuators . The pressure compensating valve is given such a load dependent characteristic that the target compensation differential pressure is reduced when a load pressure rises. A lower limit setting spring for limiting the target compensation differential pressure from becoming smaller than a predetermined value is provided in the pressure compensating valve for the swing section.

Hydraulic driving unit

A differential pressure &Dgr;PLS between a delivery pressure of a hydraulic pump,and a maximum load pressure among a plurality of actuators,is maintained at a target differential pressure &Dgr;PLSref by pump displacement control means,The target differential pressure &Dgr;PLSref is modified depending on an engine rotational speed by introducing a differential pressure &Dgr;Pp across a throttle,disposed in a delivery line of a fixed pump,An unloading valve,has first and second auxiliary control pressure chambers,to which the differential pressure p across the throttle,is introduced, and a target differential pressure &Dgr;Pun of the unloading valve is also modified in match with change in the target differential pressure &Dgr;PLSref modified by the operation driver,Stable load sensing control is thereby achieved without being affected by the engine rotational speed.

Hydraulic drive apparatus

An engine speed control system for a construction machine is arranged in association with its hydraulic circuit, which has main pumps, a communication line through which input ports of travel-controlling directional control valves are connected with each other, and a travel-controlling communication valve arranged in the communication line. The engine speed control system is provided with pilot valves arranged in a pilot line to control traveling, pilot valves arranged in a pilot line to control booms and the like, pilot valves arranged in a pilot line to control revolving, earth moving and the like, a pressure sensor for detecting pressures in the pilot lines, another pressure sensor for detecting a pressure in the pilot line, an engine controller for performing automatic idling control or its cancellation in accordance with signals from the pressure sensors, and a signal line for changing over the travel-controlling communication line into a communicating position in response to a pressure in the pilot valve.

Engine speed control system for construction machine

Hydraulic drive system and valve apparatus

A first hydraulic circuit comprises a variable displacement hydraulic pump driven by a prime mover, a variable displacement hydraulic motor connected in a closed circuit to the variable displacement hydraulic pump, and a displacement control means for controlling a displacement of the variable displacement hydraulic motor. A second hydraulic circuit comprises a hydraulic pump driven by the prime mover, and drives a loader attachment. A displacement limitation means limits the maximum value of the displacement of the variable displacement hydraulic motor in accordance with a load pressure of the second hydraulic circuit. For example, the displacement limitation means reduces the maximum value of the displacement of the variable displacement hydraulic motor in proportion to the pressure in the second hydraulic circuit. The motor torque is controlled by directly adjusting the displacement of the motor, and a load of the hydraulic motor can thus be stably controlled.

Hydraulic driving circuit with motor displacement limitation control

A variable displacement hydraulic pump and a hydraulic motor are connected in a closed circuit. A pressure oil supply means includes the variable displacement hydraulic pump and a tilting controlling means for controlling a tilt angle of the hydraulic pump and supplies an amount of pressure oil corresponding to the tilt angle of the hydraulic pump to the hydraulic motor. When rotation of a prime mover is detected by a rotation detecting means, an opening and closing valve is changed over to a first position in which the pressure oil supply means is permitted to supply pressure oil. When stopping of the prime mover is detected by the rotation detecting means, the opening and closing valve is changed to a second position in which the pressure oil supply means is inhibited from supplying pressure oil. Upon starting of the prime mover, at least the opening and closing valve remains at the second position, and the tilting controlling means increases its tilt angle from its neutral position so that the pressure oil supply means supplies pressure oil. Consequently, upon re-starting of the prime mover, pressure oil is prevented from being supplied inadvertently to the hydraulic motor, and the safety upon starting is assured.

Safety device for hydraulic closed circuit

A hydraulic drive system is equipped with an engine, a plurality of hydraulic pumps adapted to be driven by the engine and including a variable displacement hydraulic pump provided with a displacement varying mechanism, a hydraulic motor connected to the variable displacement pump, a hydraulic actuator for actuating the displacement varying mechanism of the variable displacement hydraulic pump, a working fluid feeding means for feeding the hydraulic actuator with a hydraulic fluid of a pressure according to the degree of control of an accelerator lever which is adapted to instruct a target revolution number of the engine, and a low-pressure circuit for allowing the hydraulic fluid to be discharged therethrough from the hydraulic actuator. The system is provided with a maximum pressure limiting means for guiding the hydraulic fluid of the hydraulic fluid feeding means to the low-pressure circuit depending on the degree of control of the accelerator lever, thereby limiting the maximum pressure of the hydraulic fluid and a means for selectively actuating the maximum pressure limiting means.

A solenoid operated valve apparatus in which a solenoid device includes an inner yoke, a plunger guide disposed in the opposed relationship with the inner yoke, a plunger movably accommodated in s space formed between the inner yoke and the plunger guide, a bobbin frame disposed to surround the inner yoke and said plunger guide, a coil wound around the bobbin frame, and an outer yoke disposed to surround the coil. The bobbin frame is made of a polymeric material, and is adhered to the inner yoke and the plunger guide in a unitary structure.

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Kashiwa, Japan

Takashi Kanai