When open- or tandem-center valves are used in circuits requiring pilot pressure to shift the valves, there must be a means of maintaining pressure when the valves are in neutral. One method is to install a backpressure check valve in the tank line. The check valve maintains a backpressure of, say, 50 psi.
Energizing the solenoid extends the cylinder at a maximum pressure corresponding to the main system relief valve setting. De-energizing the solenoid valve retracts the cylinder and holds it retracted at the reduced pressure setting of the pilot relief valve. The check valve prevents the pilot relief valve from operating during cylinder extension.
Two relief valves
Using two relief valves in this circuit gives two working pressures. On the up stroke of the cylinder, the low-pressure relief valve limits system pressure. During the down stroke, the high-pressure valve limits maximum press tonnage for doing work. Using the low-pressure relief on the up stroke saves power by supporting the cylinder with low-pressure fluid.
In a system with only one pump, reduced pressure for one branch of the circuit can be obtained with a pressure-reducing valve. This circuit is typical for a welder, which requires high clamping force to be set by the relief valve and reduced force on the welding gun to be set by the pressure-reducing valve. Placing the check valve in parallel with the pressure-reducing valve allows free return flow when the weld cylinder retracts.
Regulating pump pressure from a remote station can be accomplished by using small, pilot relief valves connected to the systems main pilot-operated relief valve. With the 3-way solenoid valve de-energized, system pressure is limited to 1,500 psi in this circuit. Energizing the 3-way solenoid valve permits venting the relief valve to either 1,000 psi or 500 psi, depending on the position of the 4-way valve, which is determined by the pilot signal it receives.
Pilot-operated relief valves provide two pressures for the mold-close cylinder and the injection cylinder of this plastic molding machine circuit. With the manual valve in neutral, the air-operated valves are actuated to extend the mold-close cylinder at a maximum pressure of 2,000 psi. Operating the manual valve to extend the injection cylinder vents the 400-psi relief to tank and vents the 2,000-psi relief at 400 psi. High-pressure fluid is held by the check valve.