The sequence valve is used to ensure that a certain pressure level is achieved in one branch of the circuit before a second branch is activated. Consider a machining operation where the workpiece must be clamped with a certain force before it is extended to make contact with the cutting tool. If the piece is not securely fastened, it can slip and damage both the tool and the piece.
In the circuit shown in Fig. 3.21, the sequence valve is set on 600 psi, meaning that pressure must build to 600 psi before the valve opens. This setting ensures that the clamp cylinder exerts a 600-psi clamp force before the extend cylinder moves. When the directional control valve is shifted for reverse flow, the check valve provides free flow, and there is no sequencing of the cylinders. Either one can retract before the other, depending on the pressure required for retraction. The cylinder with the lowest pressure requirement always retracts first.
Proper sizing of the cylinders will minimize energy loss in a sequence valve circuit. Suppose the maximum pressure to extend the workpiece in Fig. 3.21 is 400 psi. Pressure drop across the sequence valve is 600 ? 400 = 200 psi. If a larger clamp cylinder is used, such that the pressure required to achieve the clamp force is only 500 psi, the sequence valve can be set at 500 psi. Pressure drop across the valve is 500 ? 400 = 100 psi, and energy loss is reduced.
A functional diagram of a sequence valve with flow through to a primary circuit is shown in Fig. 3.22. When pressure at the inlet creates a hydraulic force large enough to offset the spring force, the spool shifts to open a passage to the secondary circuit and close the primary circuit.
