Hydraulic Sequence Valves
Sequence valves control the operating sequence between two branches of a circuit. The valves are commonly used to regulate an operating sequence of two separate work cylinders so that one cylinder begins stroking when the other completes stroking. Sequence valves used in this manner ensure that there is minimum pressure equal to its setting on the first cylinder during the subsequent operations at a lower pressure.
Figure 5-7, diagram A, shows how to obtain the operation of a sequencing pressure by adjusting a spring’s compression, which holds piston 1 in the closed position. Liquid enters the valve at inlet port C, flows freely past piston 1, and enters the primary circuit through port D. When pressure of the liquid flowing through the valve is below the valve’s setting, the force acting upward on piston 1 is less than the downward force of the spring 2. The piston is held down and the valve is in the closed position.
When resistance in the primary circuit causes the pressure to rise so it overcomes the force of spring 2, piston 1 rises. The valve is then open (Figure 5-7, diagram B). Liquid entering the valve can now flow through port E to the secondary circuit.
Figure 5-8 shows an application of a sequence valve. The valve is set at a pressure in excess of that required to start cylinder 1 (primary cylinder). In its first operating sequence, pump flow goes through ports A and C (primary ports) to force cylinder 1 to stroke. The valve stays closed because the pressure of cylinder 1 is below the valve’s setting. When cylinder 1 finishes stroking, flow is blocked, and the system pressure instantly increases to the valve setting to open the valve. Pump flow then starts cylinder 2 (secondary cylinder).
main piston. This piston throttles flow to port B (secondary port) so that pressure equal to the valve setting is maintained on the primary circuit during movement of cylinder 2 at a lower pressure. Back pressure created by the resistance of cylinder 2 cannot interfere with the throttling action because the secondary pressure below the stem of the main piston also is applied through the drain hole to the top of the stem and thereby canceled out. When cylinder 2 is retracted, the return flow from it bypasses the sequence valve through the check valve.
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