These diagrams cover both pneumatic and hydraulic circuits. The symbols that we shall use do not illustrate the physical make-up, construction or shape of the components. Neither are the symbols to scale or orientated in any particular position. They are only intended to show the `function' of the component they portray, the connections and the fluid flow path.
Complete symbols are made up from one or more basic symbols and from one or more functional symbols. Examples of some basic symbols are shown in and some functional symbols are shown in
Energy converters
Let us now see how we can combine some of these basic and functional symbols to produce a complete symbol representing a component. For example, let us start with a motor. The complete symbol is shown in
The large circle indicates that we have an energy conversion unit such as a motor or pump. Notice that the fluid flow is into the device and that it
Now let us analyse the symbol shown in Figure 2.75.
- The circle tells us that it is an energy conversion unit.
- The arrowheads show that the flow is from the unit so it must be a pump.
- The arrowheads are solid so it must be a hydraulic pump.
- . The arrowheads point in opposite directions so the pump can deliver the hydraulic fluid in either direction depending upon its direction of rotation.
- The arrow slanting across the pump is the variability symbol, so the pump has variable displacement.
- The double lines indicate the mechanical input to the pump from some engine or motor.
Summing up, we have a variable displacement, hydraulic pump that is bi-directional.
Test your knowledge 2.25
Draw the symbol for:
(a) a unidirectional, fixed displacement pneumatic pump (compressor)
(b) a fixed capacity hydraulic motor.
Directional control valves
The function of a directional control valve is to open or close flow lines in a system. Control valve symbols are always drawn in square boxes or groups of square boxes to form a rectangle. This is how you recognize them. Each box indicates a discrete position for the control valve. Flow paths through a valve are known as `ways'. Thus, a 4-way valve has four flow paths through the valve. This will be the same as the number of connections. We can, therefore, use a number code to describe the function of a valve. Figure 2.76 shows a 4/2 directional control valve (DVC). This valve has four flow paths, ports or connections and two positions. The two boxes indicate the two positions. The appropriate box is shunted from side to side so that, in your imagination, the internal flow paths line up with the connections. Connections are shown by the lines that extend `outside' the perimeters of the boxes.
Test your knowledge 2.26
A valve symbol is shown in
(a) State the numerical code that describes the valve.
(b) Describe the flow path drawn.
(c) Sketch and describe the flow path when the valve is in its alternative position.
As drawn, the fluid can flow into port 1 and out of port 2. Fluid can also flow into port 3 and out of port 4. In the second position, the fluid flows into port 3 and out of port 1. Fluid can also flow into port 4 and out of port 2.
Valve control methods
Before we look at other examples of directional control valves, let us see how we can control the positions of a valve. There are five basic methods of control, these are:
- Manual control of the valve position.
- Mechanical control of the valve position.
- Electromagnetic control of the valve position.
- Pressure control of the valve positions (direct and indirect).
- Combined control methods.
The methods of control are shown in With simple electrical or pressure control, it is possible only to move the valve to one, two or three discrete positions. The valve spool may be located in such positions by a spring loaded detent.
Combinations of the above control methods are possible. For example, a single solenoid with spring return for a two-position valve. Let us now look at some further DCVs.
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