Short Answer:

A sequence valve is a type of hydraulic valve used to control the sequence of operations in a hydraulic system. It allows fluid flow to a second actuator only after the first actuator has completed its operation and reached a preset pressure. Sequence valves ensure that operations in a system occur in the correct order automatically.

In simple words, the sequence valve maintains proper timing between two or more hydraulic cylinders or motors. It opens only when the required pressure is reached, ensuring smooth and controlled movement of components. This makes it very useful in automation and hydraulic machinery.

Detailed Explanation :

Sequence Valve

A sequence valve is a pressure-operated valve used in hydraulic systems to control the order in which different actuators operate. It ensures that one operation takes place before another begins, depending on the pressure in the system. When the pressure in one line reaches a preset value, the valve opens and allows the flow of hydraulic fluid to another line or actuator. This function helps maintain the correct operational sequence in systems where two or more actuators are working together.

In a typical hydraulic system, various actuators like cylinders or motors are used to perform specific tasks. If these actuators operate randomly, it can lead to mechanical failure or improper functioning. To avoid this, a sequence valve is used so that one actuator performs its job first, and then the next one operates automatically after a specific pressure is achieved.

Construction of Sequence Valve

A sequence valve consists of a valve body, spool or poppet, spring, pilot line, and adjusting screw.

The valve body houses all the components and provides the fluid passage.
The spool or poppet acts as the main controlling element that moves according to the pressure.
The spring holds the valve in a normally closed position.
The pilot line senses the pressure in the system.
The adjusting screw is used to set the pressure at which the valve opens.

When the pressure in the line increases and overcomes the spring tension, the spool or poppet moves, allowing fluid to flow to the next actuator. Once the pressure drops, the valve closes again automatically.

Working of Sequence Valve

The operation of the sequence valve is based on pressure sensing. At the start, the valve remains closed because the pressure in the line is below the set value. When the first actuator (for example, Cylinder A) completes its stroke, the pressure in the line rises. When this pressure reaches the preset limit, it pushes the valve spool or poppet against the spring force, opening the passage to the second actuator (Cylinder B).

This way, Cylinder B operates only after Cylinder A finishes its operation. Once Cylinder B starts moving, the pressure drops slightly, causing the valve to close again when the system returns to normal pressure. This ensures smooth and automatic sequencing without the need for external control devices.

Types of Sequence Valves

Direct-acting Sequence Valve:
It operates directly by fluid pressure on the valve poppet or spool. It is suitable for low-pressure systems where response speed is not critical.
Pilot-operated Sequence Valve:
It uses a small pilot valve to control the main valve. It is used for high-pressure and high-flow systems because it provides smoother operation and better control.

Applications of Sequence Valve

Used in hydraulic presses, where one cylinder must extend before another.
In machining operations, where tool clamping must occur before the cutting operation.
In automatic machinery, to ensure the right sequence of movements.
In aircraft hydraulic systems, for example, the landing gear doors open before landing gear extension.
In injection molding machines, where specific movements must happen step by step.

Advantages of Sequence Valve

Ensures proper sequence of actuator operations.
Reduces need for manual control or electrical circuits.
Provides automatic operation based on pressure settings.
Protects system components from mechanical damage.
Simple design and easy to maintain.

Limitations of Sequence Valve

Generates heat due to fluid throttling.
Not suitable for very high-speed operations.
Pressure setting must be accurate for reliable operation.
Can cause pressure losses in the hydraulic system.

Conclusion

In conclusion, a sequence valve is an essential hydraulic control component that ensures the correct order of operation between actuators. It functions automatically by sensing pressure and allows smooth transition between sequential operations. This valve improves safety, efficiency, and automation in hydraulic machinery. Although it has some limitations, its advantages in maintaining proper system operation make it an important part of modern hydraulic systems.