PRACTICAL AUTOMATION: Understanding fluid power diagrams

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This article is dedicated to those people who have worked with a hydraulic or pneumatic circuit drawing filled with boxes, lines and squiggles they know nothing about. These drawings are plentiful in industry but explanations of what they are and how to read them are scarce.

Two-, three- and four-way valves are the three basic directional valves used in fluid power circuitry. These are complemented by accessory valves, including shuttle valves, check valves, pressure control valves and flow controlling valves.

Two-way valves

Two-way valves are, in general, shut-off valves. They have only two positions: open or closed, and two ports, the inlet and outlet. Household water taps are two-way valves–as are air line blow guns and gate or ball valves used in piping installations.

A two-way valve will not operate a single acting cylinder. “Of course it will!” you say. “Open the valve and the cylinder will extend.” But what happens when the valve is closed? The air is trapped behind the piston with no way to exhaust it. The cylinder will not retract.

Three-way valves

Three-way valves have three ports: an inlet, an outlet to the work piece and an exhaust port. They generally have two positions: the inlet open to the outlet with the exhaust blocked and the inlet blocked with the outlet connected to the exhaust port.

A three-way valve can extend a single-acting cylinder and also let it retract by directing the exhaust through its exhaust port to atmosphere.

Four-way valves

A four-way valve is actually two three-way valves operated by the same actuator. The inlet ports as well as the two exhaust ports may be joined internally to give four ports: the inlet, two outlets and an exhaust port.

Four-way valves are used to control double acting cylinders or reversible air motors where one port must be pressurized while the other is exhausted.

Graphic symbols

The majority of Graphic Symbol Systems all use the same basic principles. They differ only in the details of the actual drawings. The SIC standard industrial graphic symbol system will be used in these explanations.

The directional valves described previously generally have two or three positions. Each position or mode of operation is represented by a function square. A two-position, three-way valve (Fig. 1) would have two squares in the main body of the symbol. The lines or arrow within each box or square graphically represent the flow paths within the valve for that particular actuator or mode.

Each mode of operation is generally selected by a particular actuator such as a spring, lever or solenoid. The actuator that causes the mode of operation to occur is graphically represented next to the box showing that mode.

In Fig. 2, the spring actuator holds the valve in the exhaust position and is drawn adjacent to the square showing that flow path. The hand actuator is shown next to the square depicting air pressurizing the cylinder. That’s all very simple, but why are the inlet and outlet lines only drawn to one square?

You may not believe this, but it’s to avoid confusion.

Machinery or control circuits using valve systems generally have a starting or rest position. This doesn’t mean that every valve in the system is in its rest, relaxed or non-actuated position, but simply that the whole system is in the initial position, ready to start. The actuation squares that represent the condition of each valve in the system while in this initial position are the squares that the interconnecting lines are drawn to.

In our example (Fig. 2), the spring actuator on the valve takes precedence in the rest position, so all connections are shown attached to the appropriate positions on the left square. The confusing part of this exercise is understanding the flow path in the actuated position.

To do this, simply superimpose the square showing the actuated flow path over the square with the connections made to it and the symbol clearly shows the new actuated mode. It is recommended that this superimposing process be done in your mind and not by cutting the circuit drawing (or this magazine) to shreds.

Three-position valves require us to bend the rules a bit. The third position, when used in a three-way or four-way valve, is usually in a rest position with a flow path showing the outlet ports docked or open to exhaust or even to the inlet. There are generally only two actuators, usually solenoids or pilots, used as the prime actuators. When both actuators are relaxed, the valve spool is held in the centre position with an air or mechanical spring system (Fig. 3).

The spring system is drawn as two opposing springs, one at each end of the flow stack. Manual valves with one actuator may have two, three or more positions, with each position shown by a separate flowpath square. The actuator can be drawn at either end in conjunction with a detent symbol depicting a friction catch or detent for each square (Fig. 4).

Ted Grove is corporate training manager for Wainbee Limited, Mississauga, Ont., and an widely experienced fluid power trainer.