Directional Control Valves, Difference between Monostable and Bistable Valves

Directional control valves are those which change the flow of compressed air that goes to the cylinders, pneumatic valve actuators, etc., By switching, opening, or closing their internal connections, they control compressed air passage. It allows control of the inlet and outlet of a cylinder, the opening or closing of a pneumatic clamp, or the rotation of a rotary actuator.

Types of directional control valves

  1. Monostable valves
  2. Bistable valves

Monostable valves

This type of valve will remain indefinitely at a stable or rest position if the control device does not act on it. The return to the rest position is usually done with a spring.

Thus, in the case of a two-position monostable valve, the stable position will be that corresponding to the spring. The stable position, by convention, is usually located on the right.

In the case of three positions monostable valve, the stable position is the central one.

A monostable valve is to be either normally open (NO) or normally closed (NC). They will normally open (NO) when in their stable position they allow the pressurized fluid to pass through them towards the consumption elements. They will be normally closed (NC) when in their stable position they do not allow the fluid to pass or they allow it to pass from the actuator elements to the exhaust.

Examples:

  1. 5/2 valve, Monostable, Pneumatic actuation, spring return
  2. Solenoid valve 3/2, Monostable, Piloted + Button, and Spring return

Bistable Valves

A bistable valve does not have a single stable resting position. Even if the signal that caused the position it is in is canceled, the valve will remain in that same position until the signal corresponding to a new position is activated.

They are named based on the number of ways (number of connections the valve has), number of positions (number of maneuvers it can perform), normal position, and activation method. In case both the signals are activated, the oldest act on.

The control device is actuated either by a mechanical, electrical signal, manual, or hydraulic. The control device will remain and the way how each valve position is activated is symbolically represented by adding the corresponding actuation symbol to the side of the square as shown (in every figure).

Bistable directional valves are also sometimes called normally open (NO) or normally closed (NC). When this happens, it will be normally open (NO) when it is in the right position, in case of having two positions, or in the central position, in case of having three positions.

The same applies to NC.

Examples:

  1. 5/2 directional control valve, Bistable, Pneumatic actuation.
  2. Solenoid valve 5/2, Bistable, Pilot operated + Push button and Spring return

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