What is a Spool Valve? Configuration, Construction & application

What is the spool valve? 1

What is the spool valve?

  • A spool valve is a type of directional control valve, & is the most essential component in various industrial applications.
  • A spool valve is a mechanical or electromechanical device used in hydraulic and pneumatic systems to regulate and direct fluid or air flow, often a liquid or gas, within various system components or actuators.       
  • A spool valve is a cylindrical unit with a large diameter, this valve is machined to slide within a close-fitting bore of the directional control valve body.
  • The basic design of a spool valve encompasses a cylindrical or tubular-shaped component called a Spool located within a valve body or housing.
What is the spool valve?2
  • The spool travels linearly or rotary within a housing or body, the channels permit the fluid to flow through the valve and direct to various outlets or ports when the spool is moved within the housing.
  • Based on process application, these spool valves can be actuated by an electrical signal, mechanical signal, hydraulic, or even manually through an operator.
  • Usually, the radial clearance is less than 0.02 mm.
  • When the spool valve is moved or shifted from its neutral position, it opens a certain amount of flow path to permit the flow of fluid to the desired path within the hydraulic or pneumatic system.
  • Spool valves are known for their precision and versatility in controlling the direction and rate of fluid flow. 
  • A spool valve achieves precise controlled movements in various types of machinery and equipment.
  • Various types of spool arrangements and valve types are selected based on the specific requirements of a given system.

Various configurations of Spool valves

  • The configuration of the Spool valve describes the number of ports and positions the valve can control. 
  • These spool valves are available in various configurations, such as 
  1. Two-way (2/2),      
  2. Three-way (3/2), and 
  3. Four-way (4/3) designs, 

Construction of Spool Valve

  • As said above these valves consist of a cylindrical shaft known as “SPOOL” that travels within a machined bore in the directional control valve housing. 
  • The valve housing is built with fluid inlet port and outlet port to bridge the hydraulic circuit with the directional control valve.
  • The sliding spool-type directional control valves can be designed with unlike combinations of spool and housing. 
  • The spool’s lands split the housing bore into a succession of distinct chambers.
  • The valve body’s ports lead into these chambers, while the nature of the connectivity between the ports is established by the orientation of the spool.
  • Spool-type valves are frequently used in transportable hydraulic systems since they can be easily and quickly altered to two, three, or more places for routing fluid between various inlet and outflow port configurations.
  • There are four commonly used neutral position designs in terms of flow-directing states.

Closed Centre

  • In the case of a closed centre valve, all of the ports tend to be closed. Ports are separated from one another.
  • A closed centre valve can be employed when it is essential for the actuator to remain restricted even when the pump discharge is being utilized to power an accumulator or run auxiliary actuator.   

Open Centre

  • Typically, all ports are subject to open status. It suggests that all of the valve’s ports are linked to their neutral state.
  • This spool valve is unable to support the weight of the object against gravitation at the center position. In addition, it is impossible to secure the actuator in places that are desired.

Tandem Centre

  • Attach the pump (P) to the tank port (T), to permit hydraulic pump discharge flow for quick travel into the tank.
  • Since the spool is in a neutral state and restricts both operating ports (A and B) to maintain the actuator pressure constant at both sides.

Float Centre

  • It unites both functioning ports to the reservoir port, allowing the working fluid to be returned to the tank freely.
  • Nevertheless, it blocks out the pump’s port to the container port to prevent the flow of the pump from sinking to the tank when it is in the neutral state.

Typical working of a spool valve


  • The spool is a cylindrical or tubular component located at the centre of the valve and is manufactured with metal or durable materials. 
  • It consists of various lands, grooves, or openings machined into its surface to regulate fluid flow.
What is the spool valve? 3
  • The spool is intended for linear movement within the valve body with passages or channels machined into it. These passages can adjust with the various ports in the valve body to regulate the fluid or airflow.

Valve Body: 

  • A spool valve encompasses a housing called a valve body with various ports for fluid or air connections. 
  • The spool usually consists of an inlet and outlet ports.
  • Based on the specific applications, & valve design, the arrangement & of number ports varies
What is the spool valve? 4


  • The valve body consists of definite ports for fluid or air inlet and fluid or air outlet
  • These ports are bridged to the hydraulic or pneumatic connection within the system.

Control Mechanism: 

What is the spool valve? 5
  • Spool valves can be energized using various control mechanisms. 
  • Lever or knob as an operator is used for manual spool valve operation.
  • An electric current is used to move the spool to a specific position in case of automatic or solenoid-operated valves


  • The mode of actuation of Spool valves depends on the specific application and requirements.
  • It can be actuated manually by the operator, electrically through solenoids, pneumatically by using instrument air, or hydraulically through hydraulic pressure.
What is the spool valve? 6


  • Based on spool position, various flow paths are established within the valve permitting fluid or air to flow from one port to another.
  • Spool valves are intended for various purposes like on-off control, directional control, pressure relief, and so on.

Application of Spool valve: 

  • Spool valves control the movement of hydraulic cylinders, electric motors, and other actuators in hydraulic systems.
  • Spool valves control the flow of instrument air to execute various system tasks like valve opening and closing, & regulating the direction of airflow in pneumatic systems.
  • Spool valves are employed in a huge range of industrial applications.

Hydraulic Systems: 

Spool valves are commonly used in Industrial, & mobile hydraulic machinery, & in construction equipment.

Pneumatic Systems: 

In automation processes & in manufacturing industries spool valves are employed in pneumatic systems to regulate the flow of compressed air.

Automotive Industry: 

Spool valves are employed in automatic transmission systems to regulate the flow of hydraulic fluid.

They help in shifting gears smoothly.

Process Control: 

Spool valves are utilized in industrial processes to regulate the flow of liquids or gases in various manufacturing industries, chemical plants, refineries, and so on. 


The spool valve in aircraft hydraulic systems controls various functions such as landing gear, flaps, and brakes.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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