Abstract: An oil hydraulic circuit for a hydraulic shovel with which a composite operation can be performed readily and smoothly without special skill in operation of the hydraulic shovel. The oil hydraulic circuit comprises a first actuator which can operate with pressure oil from a predetermined one or both of a pair of hydraulic directional control valve sets, a second actuator which operates only with pressure oil from the other hydraulic directional control valve set, a proportional changing over device interposed intermediately in a pipe line communicating the other hydraulic directional control valve set with a first actuator for adjusting the maximum meter-in opening value to the first actuator in response to a magnitude of a signal received at a signal receiving portion of the proportional changing over device, and a controller for arbitrarily adjusting the signal to be supplied to the signal receiving portion of the proportional changing over device.

Abstract: A control system having a flow-dividing selector valve mechanism (52) for changing the channels for causing a pressure fluid to flow from two main pumps (40), (50) into two control valve groups (60), (70) therethrough, and confluent selector valve mechanisms (80), (90) for causing the pressure fluid flowing into each group to flow into a working device control valve of the other group. The confluent selector valve mechanism (80) of the group wherein the working device control valve (62) has a need to compensate for the flow of fluid thereinto cancels the confluence function. The system has a mechanism for restricting the stroke of the spool of a working device control valve (160) to be subjected to varying loads to cause the fluid discharged from the actuator connected thereto to flow into the actuator again when the load is small. A sequence valve (170) operates the spool stroke restricting mechanism (190) by a pilot primary pressure.

Abstract: A hydraulic actuator control circuit capable of increasing the flow rate of the working fluid supplied to hydraulic actuators when the load on the hydraulic actuators increases beyond predetermined values includes a first valve group having a plurality of pilot controlled selector valves connected, respectively, to hydraulic actuators, a second valve group having a plurality of pilot controlled selector valves connected, respectively, to lines connecting the hydraulic actuators to the pilot controlled selector valves of the first valve group, two hydraulic pumps connected, respectively, to the first and second valve groups, and a tank for containing working fluid.

Abstract: A control circuit for a hydraulic actuator which allows the hydraulic actuator to be operated readily without much skill and at a suitable speed in accordance with a load applied thereto. The control circuit includes a change-over valve provided intermediately of a pilot pipe line communicating with a pilot oil chamber of a hydraulic change-over valve which belongs to one of a pair of hydraulic change-over valve sets of a hydraulic operating circuit for the hydraulic actuator. The change-over valve is normally urged to a position wherein the pilot oil chamber of the hydraulic change-over valve is communicated with a tank, but is changed over, when a hydraulic pressure higher than a predetermined value acts upon a pilot oil chamber of the change-over valve, to another position in which the pilot oil chamber of the hydraulic change-over valve is communicated with the pilot pipe line.