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Basic Elements of Schematic Symbols

Objective

Schematic symbols and schematic drawings are very common in the field of hydraulics.

The schematic is a simple way to show all of the components and connections in a hydraulic system.

A schematic also allows a skilled reader to understand some of the engineering and operating principles that the designers of a machine intended.

At first glance a schematic can be overwhelming!
This is because schematic symbols are a visual language. As with any language, you need to understand the words before you can understand what is being said.

First we're going to learn the basic elements that make up schematic symbols.

Then we'll get into some of the more specialized symbols you may come across.

With that in mind, let's learn to speak schematic!

Line Styles

The lines that represent hydraulic hoses, pipes, and tube assemblies are an essential part of every schematic.

A solid line represents a main path for flow. It's the pipes or hoses between components and the flow channels within components.

Short dashed lines are return/drain/tank lines. These lines are the main path for flow to return to the reservoir.

On the other hand, long dashed lines are for pilot lines. Very little, if any, flow passes through most pilot lines. Many are only used to detect pressure.

A large bulge on an otherwise normal line indicates that this line is a flexible hose. Note that many schematics don't bother to mark the flexible hoses, so don't assume that a straight schematic line is always a solid pipe.

White (or empty) arrows on a line indicate that this is a pneumatic line rather than a hydraulic line. This will only be seen on a schematic that has both pneumatic and hydraulic circuits.

So as you can probably guess, black (or filled-in) arrows on a line indicate that this is a hydraulic line. Again, this will only seen on schematics that have both pneumatic and hydraulic circuits.

A line that consists of both short and long dashes is an enclosure or manifold line. This is used to show that a group of separate components are actually all contained within a single block of metal.

Finally, an electrical line will have cool-looking lightning bolt arrows on it.

Mouse over or tap the names below the schematic to highlight all lines of that type.

When Lines Collide

Hydraulic schematics can get pretty complicated. Lines can be criss-crossing all over the place. Some of these crossing lines will be connected, some will be separate.

It is important to be able to tell the difference!

Connected Lines
3 Way Line Connection
(without connection nodes)
3 Way Line Connection
4 Way Line Connection

If a line terminates on another line, as in the first example, those lines are connected in the system. But using a connection node, as in the other two examples, is the preferred way to show that two lines are connected.

Crossing Lines

Line Crossing Over

Line Crossing Over (with hop-over)

Similarly, two lines that aren't connected in the hydraulic system can be drawn crossing straight through. But using a hop-over, like the second example, makes it more clear that the two lines are not connected.

Mouse over or tap the names below the schematic to highlight the matching symbols.

Component Shapes

In addition to the lines, the other essential part of every schematic are the symbols for system components.

The outer shape of a schematic symbol gives you a clue about the function of that system component.

A circle indicates a component that primarily rotates (e.g. pumps and motors).

Diamonds are components that affect the hydraulic oil in a system. (The symbols for oil filters, heaters, and coolers are diamond-shaped.)

A square is usually a valve. Relief valves, pressure reducing valves, and counterbalance valves are all examples of square schematic symbols. A series of squares together show the various positions of a directional control valve. More on this later.

Mouse over or tap the names below the schematic to see the shape of each component symbol.

Operators

Operator symbols are drawn beside a component symbol to show how that component is controlled.

Solenoid

Hydraulic Pilot

Mechanical Linkage

Detent

Spring

Multiple operators can be combined. The way they are combined tells you more about how the component works.

Stacking operators end to end means that both have to be activated in order actuate (operate) the component. In this example, the valve must receive both an electrical signal to the solenoid and a pilot signal to act.

Operators placed side by side mean that either can be activated in order to actuate (operate) the component. In this example, the valve will act if it receives a pilot signal, an electrical signal to the solenoid, or both at the same time.

Mouse over or tap the operator names below the schematic to highlight the matching symbols.

Extra Pieces

There are still a few basic symbols that simply do not fit nicely into any group.

An accumulator. The empty arrow means it has a gas-filled bladder.

A standard cylinder.

An orifice (or flow control).

A hydraulic reservoir (or tank). Several of these symbols will often be drawn in a schematic, even for a system that only has a single reservoir, to avoid drawing long return-to-tank lines across a schematic.

A diagonal arrow is used to indicate that a component is variable or adjustable. In this example the arrow is used to show an adjustable spring.

A curved arrow is used to show that a component rotates. They are primarily used on mechanical linkages (shafts), as in the example shown here.

A cartridge (or logic) valve. The large "T" shape in the center represents the poppet, with a spring drawn above it.

A ball valve. White (or empty) arrows mean the ball valve is normally-open, while black (or filled) arrows mean the ball valve is normally-closed.

A check valve (or non-returning valve). This valve allows flow in one direction only. In the example shown here flow is permitted to from left to right, but not the reverse.

A shuttle valve. This valve compares the pressures of the two lines coming in from the left and right sides. The line coming out the bottom will be the flow path for whichever incoming line had greater pressure, and the other incoming line will be blocked.

Mouse over or tap the component names below the schematic to highlight the matching symbols.

Review

Click and drag each symbol to the correct box.

Accumulator
Hydraulic Pilot
Drain Line
Check Valve

Select the correct name for each symbol.

OrificeManifold LineConnection Node
Check ValveValve ComponentPilot Line Ball ValveDetentElectrical Line

Conclusion

You've just taken the first step toward mastery of schematic reading.

With this knowledge, you're ready to tackle the more in-depth schematic lessons and puzzles in the Hydraulic Schematic Symbols session. Soon you will be able to quickly read and understand the schematic symbols used throughout LunchBox Sessions and the wider world of hydraulics.

If you are ever unsure of how to understand a schematic, come back to this lesson. Use it as a resource. It will serve you well.

We hope you enjoyed Basic Elements of Schematic Symbols

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