Hydraulic Circuits

1. Valves Used in Hydraulic Circuits

Valves are used in hydraulic circuits in order to control hydraulic pressure, direction of flow and amount of flow.
4) Check Valves This is a value that allows oil to flow freely in one direction, but stops it from flowing in the opposite direction  (directional control valve). A check valve that has the function of disengaging the return flow prevention is called a pilot check valve. These are installed in the jack cylinder of the outrigger.

When oil flows from left to right, the spring is contracted, the poppet opens and the oil flows freely. The oil cannot flow from right to left.
	[Pilot Check Valve] This valve can be opened and closed as necessary. When there is no pilot pressure, it works just like a regular check valve, allowing flow only in one direction, from the intake to the outlet. When pilot pressure is applied, the spool is raised, the poppet opens, and oil can then flow from the outlet to the intake.
If there is no pilot check valve... The hydraulic oil in the outrigger would flow out and the outrigger would retract.
When a pilot check valve is installed.. The check function keeps the hydraulic oil from flowing out. The outrigger remains extended as it is. Even if there is a leak in the circuit, the crane will be prevented from lowering itself due to its own weight.

5) Counterbalance Valve
The counterbalance valve is a flow control valve that controls the fluid returning from an actuator such as a cylinder, so that the actuator does not make, due to the load, any undesired motions exceeding what corresponds to the oil flow from the directional control valve.
Just like the pilot check valve, this valve prevents the crane from lowering itself due to its own weight. In order to stress this function, this valve is also sometimes called a "holding valve".
If there is no counterbalance valve, when lowering a load, the load will accelerate due to its weight, and the speed cannot be controlled.		The counterbalance valves are also used in circuits for boom elevation, boom telescoping and winch operation.

6) Solenoid Valve
When an electrical signal is input to a solenoid valve(magnetic changeover valve), the drawing force of the solenoid moves the spool, changing the direction of flow. Because the electrical signal is switched at the valve, remote control and automatic control are simple.