Pressure control valves used to control force in a hydraulic system.
For accurate control of force, six types of pressure control valves have been develop.
I will begin presenting information about one of the common pressure control valves namely pressure relief valve.It is one of the most used hydraulic valves.
A relief valve is a pressure operated valve which by-passes pump delivery to the tank, limiting system pressure to a predetermined maximum value.
The main purpose of a pressure relief valve is to protect the hydraulic system from excessive pressure.
The valve should not be used to direct excess pressure to the tank, as this may cause the system to overheat.
This valve limits the maximum system pressure.
It is a normally closed valve whose function is to limit the pressure to a specified maximum value by diverting the pump flow back to the tank.
When no flow is available in the hydraulic system due to safety reason, the pressure relief valve let go of the fluid back to tank to maintain the desired system pressure in the hydraulic circuit.
It provides protection against any actuators overloads in the hydraulic system.
One important function of a pressure relief valve is limiting the force or torque produced by the hydraulic cylinders and motors.
Pressure Reducing Valves
Pressure reducing valves are normally open pressure valves and their functions are to reduce pressure in one or more legs of a hydraulic circuit.
Reduced pressure results in a reduced force being generated.
Pressure reducing valves are the only pressure control valves that are normally open.
A normally open pressure control valve has primary and secondary passages connected. Pressure at the bottom of the spool is sensed from the pilot line which is connected to the secondary port.
The reducing valves will limit the pressure to the motor, therefore limiting the output torque of the motor.
Keep in mind that the pressure reducing valves are normally open.
A counterbalance valve is a normally closed pressure valve and is used in cylinders action to counter a weight or overrunning load. In a simple circuit of a cylinder application, without a counterbalance valve the load would fall uncontrolled or overrun, and pump flow would not be able to keep up.
To avoid the uncontrolled operation, we place a counterbalance valve just after the cylinder.
Counterbalance valves are used to aid a cylinder in lowering a load at a controlled rate.
Counterbalance valves control the gradual buildup of pressure in a hydraulic system.
Brake valve is a normally closed pressure control valve with internal and remote pilot passages which are connected simultaneously for its operation.
During running of the hydraulic system remote piloting is using system pressure to kept valve open. Any back pressure on the hydraulic motor that might arise because of downstream resistance and subsequent load on the hydraulic motor is eliminated.
Assume the spring of the valve is set at 950psi/65bar that keep the valve spool seated. When the pressure exceeds 950psi/65 bar the valve actuates and the spool moving off his seat. The spool area is usually 8 times the area of the piston. With the remote pilot connected to the opposite hydraulic motor line a pressure of 120psi/8 bars is required to open the valve.
The brake valve being a normally closed pressure valve does not allow fluid to pass through in the reverse direction. Therefore a check valve is used to give this reverse flow and bypass the valve.
A sequence valve is a normally closed pressure control valve used for ensuring a sequential operation in a hydraulic circuit, based on pressure. In other words allows one operation to take place before another.
When the pressure set is reached the valve opens.
If properly adjusted the sequence valve assures that the cylinder will extend before motor starts.
The sequence valve has a drain line from the spring chamber as the outlet is not connected to the tank because is under pressure.
Unloading valves are remotely piloted, normally closed pressure control valves, used to direct flow to the tank when pressure at a particular location in a hydraulic circuit reaches a predetermined value.
Unlike the simple spool type relief valve the internal passage that connected the main line to the spool is now externally operated.
When the system pressure reaches the unloading valve setting, the valve opens diverting flow back to the tank at minimum pressure.
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