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 control valve assembly having a plurality of control valves, each including a pair of logic valves, with each of the logic valves having a pump port, a work port and a pilot port. The control valve assembly has a housing, and a pair of hydraulic fluid passageways arranged in the housing in side-by-side relationship. Each pair of logic valves, constituting one of the control valves, are arranged in the housing in such a manner that they are located in opposed relationship to each other in a direction substantially perpendicular to a direction in which the hydraulic fluid passageways extend. The pump ports of the logic valves are in communication with the associated hydraulic fluid passageways. The work ports of the logic valves are oriented in a direction substantially perpendicular to both the direction in which the hydraulic fluid passgeways extend and the direction in which the pair of logic valves are arranged.

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.