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Design & Technology

Pneumatics

Single-acting cylinder with three-port valve

In a single-acting cylinder compressed air is used to push a piston out, and a spring to push it in again. When compressed air is supplied to the cylinder, the piston rod extends (called the outstroke). When the air supply is switched off, the internal spring returns the piston rod to its retracted position (the instroke). The movement of a single-acting cylinder is normally controlled by a three-port valve - a type of simple switch which governs the flow of air. In the diagram below, a unidirectional-flow control valve is used to slow the speed of the piston on the outstroke. The instroke will be full speed. Press play to see how this works.

Drawing pneumatic circuits

Pneumatic circuits are drawn using the symbols from the chart on the previous page. The diagram below shows the pneumatic circuit for the single-acting cylinder, unidirectional flow control valve and three-port valve.

3/2 push button spring return valve and single acting cylinder

3/2 push button spring return valve and single acting cylinder

Exam tip:

When drawing circuit diagrams, it is common to show the connection being made in the bottom half of the three-port valve symbol, ie the line from the cylinder should be heading towards the exhaust or the main air symbols. This is because the spring is in control when the other end of the 3/2 is not activated, so that is what is drawn.

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