صمامات التحكم Control valves

Types of Control Valves

Learn about the types of control valves, as controlling fluid power is crucial in any system. Control valves act as the brain of hydraulic systems. If unsuitable control valves are used for an application, the hydraulic system cannot perform its function as required. Fluid capacity is controlled using devices called control valves, which are placed between the pump and actuators. It is essential to know the function, operation, and applications of these various control valves components and their most important applications.

There are three main types of control devices in hydraulic systems:

  1. Flow Control Valves
  2. Directional Control Valves
  3. Pressure Control Valves

The Directional Control Valves

Directional control valves are devices that control the direction of fluid flow in the system and also stop it, thus determining the movement direction of actuators, whether they are cylinders or hydraulic motors.

Directional valves are named based on the number of service ports in the valve and the number of switching positions, designated by two numbers separated by a ( / ).

Example: A directional valve (4/3) is a directional valve with four service ports and three switching positions.

Classification of Directional Valves

Directional valves can be either directly operated or indirectly operated, depending on the conditions in which the valve is used, such as its size or the force required to operate it. The valve can be operated by hand, foot, mechanical movement, electric solenoid, hydraulic signal, or pneumatic signal (compressed air).

  • According to the operating mechanism
  • To the number of positions and number of service ports
  • According to the normal position of the valve

Non-Return Valves

Non-return valves are used to allow fluid to flow in one direction and prevent it from flowing in the opposite direction.

Pressure Control Valves

They regulate the pressure in the hydraulic system and its components. They can be classified according to their function as follows:

Pressure Relief Valve

This valve determines the maximum pressure the oil reaches in the hydraulic system to prevent it from exceeding a level that could damage system components such as hoses, pumps, motors, and other parts. The valve is set to open at a specific pressure called the cracking pressure. When the valve opens, it redirects the oil from the pump back to the tank, ensuring the pressure does not exceed the cracking pressure, thus protecting the system components from high pressures.

Pressure Reducing Valve:

This valve maintains low pressure in specific areas of the hydraulic system.

Sequence Valve:

This valve controls the sequence of operations in the hydraulic circuit, ensuring one actuator moves before another. An example is machinery where a part is clamped first, then operated on. The actuator holding the part moves first, and once it completes its movement, the second actuator begins its operation on the part. The valve is placed before the actuator performing the operation. It is set to open once the first actuator completes its movement. As the pressure builds up, it opens the valve, allowing oil to flow to the second actuator, which then performs the operation.

Brake Valve:

The purpose of this valve is to prevent the pressure from rising to the maximum value due to the inertia torque when the directional valve is suddenly closed.

Counterbalance Valve:

This valve is used with vertical loads to prevent the sudden drop of the load under its weight due to gravity. It holds the load suspended in the upper position until the pressure increases to the value that opens the valve, allowing oil to flow from the cylinder to the tank, thus controlling the load descent at a moderate speed.

Flow Control Valves

They control the rate of hydraulic oil flow from the pump to the actuator, thus controlling the speed of the actuator movement.

Examples of flow control valves based on their position before the actuator include:

  • Meter-in Flow Control
  • Meter-out Flow Control

Examples based on throttle area include:

Fixed Restrictors
Pressure Compensated Valve
Non-Pressure Compensated Valve
Temperature-Pressure Compensated Valve


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