Hydraulic Symbols Explained

Hydraulic Circuits Design Symbols

Hydraulic symbols provide a clear representation of the function of each hydraulic component. Laying each symbol out on the page in the same sequence the components are used in the circuit allows people to understand the complete function of the hydraulic equipment.

Hydraulic symbol standards

It would be nice to think that each type of valve had its own symbol and that all valves can be clearly defined, however, the international standard has been the focus of much discussion for many years and is something of a compromise. The problem is that in the past manufacturers have often had subtle differences in the detail of their valve design, which, they'd all like to form part of the standard symbol.

A relief valve, for example, may always have the same basic function but can be constructed in significantly different ways. Cost may vary between £10 and £1000 and performance may be equally different. As a hydraulics engineer, it's important to understand the difference between valves, even though this may not always be possible from the symbol. For example knowing whether the valve uses a sealed poppet, spool element or both can make a big difference to the circuit operation; as can pilot feed or orifice positions etc.

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Basic symbol shape meaning

Circles and semi-circles represent rotary devices, either continuous, in one or both directions, or oscillating/reciprocal (semi-circle). The size represents the type of unit with a larger circle perhaps a pump, the middle size a gauge, and smallest size a roller.

Squares and rectangles form the basis of pressure and directional control valves. A single box for pressure control and multiple boxes directional control. The valve operator box is also rectangular.

Diamond shape boxes indicate a fluid conditioning device e.g. a filter or cooler etc.

Flow control elements are shown by back to back semi-circles (viscosity dependent) and arrow head Vs (viscosity independent).

The saw tooth symbol represents a spring.

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Advanced circuit design techniques

The approach we recommend is generally to build your own larger symbols from the same basic elements whenever more detail is required. If you learn the basics you should be able to interpret all circuits provided you pay particular attention whenever the fine detail changes. Sometimes putting too much detail will confuse things so designers might leave it out. Often the manufacturers are not keen to disclose too much information on what is essentially their own unique IP that has probably taken them many years to develop.

Caution when analysing a circuit

Always be careful not to assume circuit drawings are correct. Circuits may be changed without the drawings being updated and some are just plain wrong. Use them as a master to work through the components in the actual circuit you are working on; checking the valves to the symbols as you go.