Which of the lines shown below represent a drain line on a fluid power schematic?

11NovNovember 11, 2020

If you are going to be designing, implementing or maintaining hydraulic systems, the ability to understand schematics is an essential skill you will need to develop

At first glance, a schematic of a hydraulic system can appear overwhelming, but schematic drawings are actually easier than they initially appear. It’s a bit like learning a new language – although this particular language has a comparatively small vocabulary!

In this blog, we’ll take a look at some of the basic vocabulary that you’ll need to know when you are getting started with hydraulic system schematics.

Learning the language of schematics

To help with our introduction to schematic symbols, LunchBox Sessions have kindly given us permission to reuse some of their training materials in this blog.

If you’ve not checked out their website before, we highly recommend you do. It’s full of fantastic online training content that is free to subscribers. Some taster sessions are accessible to all – and that’s from where the images in this article have been sourced.

Line styles in hydraulic schematics

In a hydraulic system schematic, the pipes, hose and tube assemblies are represented by lines. A number of different types of lines are used to represent different types of assemblies. As with all other hydraulic symbols, these symbols are issued and controlled by The International Standards Organization (ISO), standard ISO 1219-1:2012.

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

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

Pilot lines are represented by long dashed lines. Minimal flow, if any, passes through most pilot lines; many are only used to detect pressure.

If a normal line has a large bulge, or bump,  it shows that this is a flexible hose line. It does not, however, always mean that a line without a bulge like this is a solid pipe, unfortunately.

Lines with white or empty arrows are used to depict a pneumatic, rather than hydraulic, line. You should therefore only see these on a schematic that has both hydraulic and pneumatic circuits.

Black, filled in arrows are – you guessed it – used to represent hydraulic lines.

Manifold lines, or an enclosure, are depicted by a line consisting of both short and long dashes; a single block of metal containing a group of individual components.

Finally, ‘lightning bolts’ on a line show that this is an electrical line.

Free Symbols Chart Download

You can download a .pdf of these symbols – along with other pdfs of other useful symbol types – from within our Technical Knowledge Hub here.

Connections

In any pipe or hose assembly, you’ll have connections, ends and fixings. It is best practice to represent connections using a node (or dot) on the line where the pipes or hose meet. Where pipes or hose cross but do not connect, it is best practice to indicate this using a small hump, or “hop-over” in the line.

Connected Lines

In the example on the left, below, we see a line that finishes on another line – that means that those lines are connected. But using a connection mode (below, centre) is actually the preferred way to represent depicted lines. From left to right below we therefore show: 3-way line connection (no connection nodes) | 3-way line connection | 4-way line connection.

Crossing Lines

Two unconnected lines that cross over can be shown as one line drawn over the other. But it is even easier to see that these lines are not connected if a hop-over is used.