Restrictor and Check Valves Pneumatic Controls

Types of Restrictor Valve

Unlike the directional control valves described hitherto, flow or restrictor valves affect the volume of air passing through them. There are two types of valve that accomplish this:

Fixed restrictors that restrict the flow to a constant volume
Adjustable restrictors where the degree to which the flow is impeded can be adjusted

The following graphic shows the symbols for both these types of valve (fixed restricter left, adjustable retrictor right, 






Henceforth we shall consider in detail only the adjustable restrictors. This type of valve can often be found in conjunction with so-called check valves that allow flow in only one direction, preventing it in the other (much in the way that a diode only conducts electricity in one direction).  Check valves (see below) are mainly used to bypass other valves. A combination of this kind can be termed a restrictor check valve


Adjustable Restrictor Valves

A restrictor valve is an adjustable resistance that restricts the volume rate of air flow. Reducing the air flow affects both directions of flow. Restrictor valves are mainly used to adjust the piston speed of a pneumatic cylinder.

The following graphic shows a cross section of a restrictor valve, in which the restricted area is designed as an opening barred by a movable cone at the end of a knurled screw, whose area is increased or decreased by the screw moving up and down. The flow of air (that controls the speed at which the piston of the controlled cylinder moves) is directly proportional to the cross-sectional area of the restricted opening. Rotating the screw so that the cone moves down reduces the area and the flow is reduced, while turning the screw so that the cylinder moves up allows greater rate of air flow.



A restrictor valve can be mounted on the compressed air feed or at the exhaust outlet of the cylinder or the corresponding directional control valve. All the cylinders used on this course have integrated restrictor valves (see graphic below).


Check Valves

Check valves automatically  block the flow of air in one direction or the other. The design involves a spring that blocks that flow by pushing either a ball or a membrane over the relevant opening. If a pressure is applied in the intended direction of flow that is greater than the force exerted by the spring, the sealing element (e.g. the ball) lifts off the opening and air can flow freely through it. The following graphic shows a cross-section of a check valve. The arrows indicate the direction of flow. The advantage of a check valve is that it is completely airtight in the reverse direction and thus holds even against heavy loads.


In a more advanced design the check valve can be released even in the direction that would normally be blocked so that air can also flow thrugh it this way. This is achieved by a pneumatic control signal.

The check valves included in the course are FESTO HGL-M5 types, and can be released in this way. The graphic below shows the symbol of a simple check valve (left) and the releaseable form (right).


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