The present invention relates to a system preventing the pressured oil leakage in a cylinder line enabling very low leakage rates without using the valve or the system, or without reducing the diametrical space between the housing and spool.

A hydraulic drive system (1) including a meter-in compensator (37) and a bleed-off compensator (42) comprises a plurality of sensors (64 to 68), a controller (62), and an outlet pressure switching valve (61). The controller (62) determines whether or not the state of a wheel loader (2) which is detected based on the signals output from the sensors (64 to 68) meets a predetermined steering limiting condition. When the controller (62) determines that the state of the wheel loader (2) meets the steering limiting condition, it outputs a command signal to the outlet pressure switching valve (61). The outlet pressure switching valve (61) reduces the flow rate of the hydraulic oil flowing to steering cylinders (18L, 18R), in response to the command signal in such a manner that the flow rate becomes lower than that corresponding to the operation amount of a handle of a steering device (35).

An excavator includes a hydraulic actuator driven by hydraulic oil discharged from a main pump to move a work element; a first control valve disposed in a center bypass pipeline; a second control valve disposed in a parallel pipeline; and a control device for controlling the movement of the second control valve. The first control valve and the second control valve are formed in a valve block of control valves, and the second control valve is disposed upstream of the first control valve.

A fluid power system comprises a pump with multiple independently variable outlets (11, 12, 13, 14), each of which is capable of delivering fluid in individually controllable volume units and a plurality of hydraulic loads (15, 16, 18, 20). A system of switching valves is configured to create fluid connections between the pump outlets and the loads. A control system commands both the pump and the switching valves, so as to create valve state combinations to satisfy load conditions as demanded by an operator. The number of pump outlets (11, 12, 13, 14) connected to one or more of the loads (15, 16, 18, 20) is changeable to satisfy the flow required of the load due to the operator demand, each pump outlet being commanded to produce a flow depending on the status of other outlets connected a load to which the outlet is connected and the operator demand for that load.

The present disclosure relates to a load-sense system such as a load-sense steering system that operates in a static load-sense mode for low flows and operates in a dynamic load-sense mode for high flows.

In a hydraulic driving system for construction machines, when track motors 3f and 3g are operated and the delivery pressure of a main pump 2 increases to a second value PS2 of the set pressure of a main relief valve 14, the set pressure of a signal pressure relief valve 16 increases from a third value PA1 to a fourth value PA2, which is smaller than the second value PS2 of the set pressure of the main relief valve 14, the difference between the second value PS2 and the fourth value PA2 being smaller than the target LS differential pressure. With such a structure, even if one of actuators reaches the stroke end and the delivery pressure of the hydraulic pump rises to the set pressure of the main relief valve, the other actuators do not stop, and further when the main relief valve is configured to increase the set pressure during operation of a specific actuator, the load pressure of the specific actuator does not increase to the increased set pressure of the main relief valve.

A hydraulic system for a work machine including a first and a second hydraulic actuator, and a first and a second hydraulic machine for providing hydraulic fluid to at least one of the first and the second actuator via a first and a second priority valve, respectively. The first and second priority valves are controlled by a pressure drop P over the inlet valve of the first actuator such that when P is lower than a first threshold value T1, a flow of hydraulic fluid is allowed only to the first actuator. The second priority valve allows a flow of hydraulic fluid to both the first and second actuator for a pressure drop higher than T1. The first priority valve is further configured to allow a flow of hydraulic fluid to both of the first and second actuator for a pressure drop higher than a second threshold value T2 which is higher than T1.

A dual pressure margin priority circuit and method for controlling flow from a pump to steering valve and low priority inlets. A steering pressure valve controls flow from pump to steering valve inlets, and provides a steering valve load sense pressure. A priority valve controls flow from pump to low priority inlets. A load sense cutoff valve has a first inlet receiving the steering valve load sense pressure. The load sense cutoff valve controls flow through the priority valve based on steering valve load sense pressure at the first cutoff valve inlet. The cutoff valve can include a second inlet coupled to tank, and a load sense input coupled to the steering valve load sense pressure. The cutoff valve can be a pressure limiter valve. The priority and steering pressure valves can be 2-way proportional flow spool valves with bias springs, and contributing and opposing load sense inputs.

A control system for controlling a work machine including a working unit including elements and actuators that drives the elements includes: a first hydraulic pump and a second hydraulic pump each of which supplies operating oil to at least one of the actuators; and a control device that calculates a distribution flow rate of operating oil distributed to each of the actuators based on an operating state of the working unit and switches, based on the calculated distribution flow rate, between a first state in which the operating oil supplied from both the first hydraulic pump and the second hydraulic pump is supplied to the actuators and a second state in which the actuator to which the operating oil is supplied from the first hydraulic pump is different from the actuator to which the operating oil is supplied from the second hydraulic pump.

A method of controlling a hydraulic system having a hydraulic lift function and an auxiliary function includes disabling the hydraulic lift function and the auxiliary function by routing pump flow to tank and opening the lift function and auxiliary function to tank with a single valve; enabling the lift function by closing pump flow to the auxiliary function and routing pump flow to the lift function with the single valve; and enabling the auxiliary function by closing pump flow to the lift function and routing pump flow to the auxiliary function with the single valve.