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We have seen working principle of pressure relief valve and the working principle of pilot operated pressure relief valve. We have also discussed the working principle of pressure reducing valve and its internal construction. Now we will look here another type of pressure control valve i.e. Pressure sequence valve. We will study here the working principle and internal construction of Pressure sequence valve.

Sequence valve is also one type of pressure control valve which is used in order to operate two actuators i.e. cylinders in a sequence. Let we have two hydraulic cylinders in a hydraulic circuit. If we want to operate the first cylinder first and when first cylinder completed the stroke then second cylinder starts to operate, we will use pressure sequence valve here to fulfill the requirement.
Pressure sequence valve

Figure shows the internal construction of pressure sequence valve. There will be one adjusting screw as displayed in figure at top position. Adjusting screw will be used for securing the desired pressure setting. Piston or spool is displayed here by 1. Inlet port is displayed here by C and outlet port is displayed here by D in figure. Outlet port for secondary circuit is displayed here by the E.

In diagram A, Pressure of fluid flowing through the valve is less than the setting pressure of valve and therefore piston will be in closed position and therefore fluid will enter the valve through the inlet port and will flow freely past the piston and hence fluid will flow to the primary circuit via the outlet port D.

As we have discussed above that pressure of fluid flowing through the valve is less than the setting pressure of valve, therefore pressure force acting on the piston in upward direction will be less than the spring thrust force acting downward and therefore piston will be remain in closed position and fluid will freely flow to primary circuit through the outlet port D.

Once the primary circuit actuator completed its stroke then pressure will be increased in primary circuit due to resistance in flow of fluid. Once the fluid pressure value in primary circuit overcomes the setting pressure of pressure sequence valve, piston will move in upward direction by overcoming the thrust force of spring and therefore fluid will flow to secondary circuit through the port E as shown in figure B.

Application of pressure sequence valve

Let us see the operation of machining of a part. Part must be clamped with the help of clamping cylinder with predefined and specified pressure force and after that workpiece or part will be extended to come in contact with cutting tool. We want the clamping of part first with predefined and specified pressure force as if part will not be clamped properly, part or workpeice may slip or damage the cutting tools.
Application of sequence valve in operation of machining of a part

Let us see the circuit diagram. There are clamp cylinder and extend cylinder, clamp cylinder will have to clamp the part and extend cylinder will have to extend the part to make contact with cutting tool. Setting pressure of sequence valve is 500 psi, it indicates that pressure sequence valve will permit the flow of fluid to extend cylinder when pressure increased up to 500 psi.

When fluid will flow through directional control valve, it will flow to sequence valve but sequence valve will be closed and check valve will also not permit the flow of fluid, therefore fluid will flow to clamp cylinder and clamp cylinder will be operated and hence clamp cylinder will clamp the workpiece or part. 

Once the clamping cylinder completed its stroke, pressure will be increased. When pressure increases to 500 psi, sequence valve will be operated and will permit the fluid flow to extend cylinder and part will come in contact with cutting tool.

During the position of reverse flow of DC valve, there will not be any sequence as fluid will flow freely through the check valve. Either extend cylinder or clamp cylinder will retract first on the basis of pressure requirement.

If we need sequence operation during retraction of cylinder also, we will use one more sequence valve for sequencing the retraction of cylinders, such as the circuit for operation of clamping and punching mechanism. Sequence valve 2, as displayed in following circuit, will restrict the retraction of clamp cylinder until the punching cylinder retracted completely.

Application of sequence valve in clamping and punching hydraulic circuit

Figure shows the hydraulic symbol of pressure sequence valve with check valve. Sequence valve will have external drain line and therefore drain port of sequence valve will be connected with the tank.

Pressure sequence valve with check valve

We will discuss Counterbalance hydraulic valve in our next post.
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Image Courtesy: Google

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