Selection of System Operating Pressure
The circuit shown in Figures 28.2-28.4 allow selection of operating pressure limits in a hydraulic system from three options, namely, two maximum pressures, plus venting. First note the components, namely, A: Reservoir with Filter, B: Hydraulic Pump, C, E: Pilot Relief Valve, D: Solenoid activated Four-way Directional valve.
In Figure 28.2, both solenoids a and b of the directional valve D are de-energized. The open-center spool is centered by the valve springs, and the vent port on the relief valve is opened to tank. Therefore, the pump flow opens to tank at a very low pressure.
Intermediate Maximum Operating Pressure
In Figure 28.3, the left-hand solenoid a of the directional valve is energized. The valve spool is shifted to the leftmost position and connects the relief valve vent port to the remote control valve. Pump flow is now diverted to tank when the pressure setting of the remote valve E is reached.
High Maximum Pressure
In Figure 28.4, the right solenoid b of the directional valve is energized. The spool now shifts right to connect the relief valve vent port to a plugged port in the directional valve. The relief valve C now functions at the setting of its integral pilot stage.
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