In many applications, it is necessary to perform operations in a definite order. Following is one of several such circuits. The components of the system are as follows. A : Reservoir and Filter ; B : Hydraulic Pump ; C : ; Relief valve : D ; F1, F2, G : Relief valve with integral check valve ; H, J : Cylinders ; I : Check Valve
The sequence of operation realized by the circuit shown in Figure 28.11-28.14 is:
Step A – Extend Cylinder H
Step B – Extend Cylinder J while holding pressure on Cylinder H
Step C – Retract Cylinder J
Step D – Retract Cylinder H
Pressing a pushbutton would start the cycle and shift the directional valve E to the position shown in Fig 28.11. At first the fluid flows through the integral check valve in G into the cap end of H and returns freely through the check valve in F2. The pump pressure is low during this period, only to the extent of pushing the load on H.
Once H reaches its rod end, the pressure builds up and now the flow develops through F1 into the cap end of J and out through the rod end to go back directly to tank through F2, E and C. Note that a pressure equal to the setting of the valve F1 is maintained on H. When J is fully extended, pressure increases further and is limited by the setting of D, providing overload protection to B.
Similarly, when the other solenoid of E is energized, the directional valve shifts to the other position, as shown in Fig. 28.13. Now, pump delivery is directed through D, E and F2, into the rod end of J. As before, the flow out of the cap end of J flows to tank through F1, E and C. Step C is illustrated in Fig. 28.13.
On completion of Step C, the pressure increases again, and the flow is directed through F2 to the rod end of H and out through the cap end to flow into the tank through the valve G at its pressure setting and then freely to tank through F1, E and C. Note that F2 maintains a pressure equal to the setting of H at the rod end of J during the retraction of H. Note further that, while H is retracting, a back pressure is provided to it by G, to prevent rapid falling of the load during lowering, under gravity.
In the above circuit, sequencing is achieved by grading the pressure settings of the relief valves. Note that sequencing can also be achieved electronically by PLC control of the solenoids of separate directional valves driving H and J.
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