Basic Elements of Schematic Symbols

Objective

  • Interpret basic schematic elements as used in components and system drawings.

Introduction

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

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

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

At first glance a schematic can be overwhelming!

Schematic symbols are a visual language, and like any language, you need to understand the individual words before you can understand what is being said.

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

Line Styles

The basic building blocks of a schematic are the lines that make up the hoses, pipes, and tubes assemblies.


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

Short dashed lines are return/drain/tank lines. A 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 is used to tell the reader that this particular line is a flexible hose line. This would imply that any line without large bulge on it would have to be a solid pipe. Sadly, that is not an assumption we can make.

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

So it goes without saying (almost) then that a black, or filled in, arrow on a line would be a hydraulic line. Generally only seen on schematics that have both pnuematic 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, a line can have these lightning bolt arrows on it. This indicates that this is an electrical line.

When Lines Collide

Hydraulic schematics can get pretty complicated. Lines can be crisscrossing all over the place. Some of these lines will be connected, some will be passing by unconnected.

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. 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 crossing unconnected to each other can be shown as one line drawn over the second. But using a hop-over makes it explicitly clear that the two lines are not connected.

Shapes

The general shape of a hydraulic symbol provides a clue as to the function of that component.

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

Diamonds are used to represent components that affect the hydraulic oil in a system. (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.

Operators

Operators attach to components to indicate the methods by which that component is controlled. The most common examples:

Solenoid

Hydraulic Pilot

Mechanical Linkage

Detent

Spring

Multiple operators can be combined. How they are combined is another indication of 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 a pilot signal and an electrical signal to the solenoid to act.

Operators placed side by side mean that either can be activated in order actuate the component. In this example, the valve must receive either a pilot signal or an electrical signal to the solenoid to act.

Extra Pieces

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

Accumulator symbol: The empty arrow indicates that it has a gas-filled bladder.

A standard cylinder symbol

An orifice/flow control

Tank/reservoir symbol: Commonly used to mean this line returns to the tank rather than drawing a long line 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.

An curved arrow is used to show that a component rotates. They are primarily used on shafts, as in the example shown.

Cartridge/logic valve: The large "T" represents the poppet. There is also a spring above it.

Ball Valve Symbol: A white fill means that this valve is normally open while a black fill means the valve is normally closed.

Typical check/non returning valve symbol

Shuttle Valve: The horizontal line segments are the two lines whose pressures are being compared. The vertical line will carry the output of whichever line's pressure is greater.

Review

Click and drag the symbol to its matching label.

Accumulator
Hydraulic Pilot
Drain Line
Check Valve

Conclusion

This has been only the start of learning to speak schematic. You are now armed with the knowledge to tackle more in-depth schematic reading lessons!

We hope you enjoyed Basic Elements of Schematic Symbols

Vacuum Pressure
Atmospheric Pressure
Low Pressure
Medium Pressure
High Pressure
Ground/Common
Lowest Voltage
Medium Voltage
Highest Voltage
Magnetic Field
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