Two-pressure circuit with a pressure-reducing valve
Always connect the drain line of a pressure-reducing valve to a free-flow tank line. Backpressure in the drain line adds to the spring setting, thus raising the set pressure. A constant backpressure can be offset by a lower spring setting, avoiding a problem. With intermittent and/or fluctuating backpressure, the reduced outlet pressure changes when the backpressure changes.
Some circuits require a reduced pressure to position a part, then full pressure to do the work. A reducing valve easily gives two pressures by opening or blocking the drain line. Figures 16-8 through 16-11 show a simple way to get two pressures using a reducing valve and a normally open 2-way directional valve.
Figure 16-8 shows a normally open 2-way directional control valve piped in the drain line. There is no leakage from the drain port in the at-rest condition.
Figure 16-9 shows the directional valve on CYL2 shifted to advance the cylinder to the work at low pressure. During this part of the cycle the reducing valve stays open.
Figure 16-10 shows the cylinder contacting the work with pressure at the reducing valve setting. The low pressure continues as long as required. During this time the operator can check part alignment or other details. If a problem is detected, the operator simply reverses the cylinder to realign any out of place or problem components.
After determining all is well, the operator energizes the solenoid on the 2-way directional valve as shown in Figure 16-11. This blocks drain flow from the reducing valve. Blocking drain flow at the reducing valve causes it to open fully. Backpressure in the blocked drain line, plus the internal valve spring, pushes and holds the spool open. When the reducing valve opens, full system pressure goes to the cylinder to generate high force. This action poses no problem to the reducing valve. This circuit is a reliable way to get two pressures for an actuator.