Every hydraulic machine comes with a circuit diagram, and the diagram is the fastest way to understand what the machine should be doing when it isn’t. The symbols look cryptic at first, but there are only a few building blocks, and they follow a logic. Pick a symbol in the animation to see the real part it stands for, then switch to “Test yourself” and find them without the names.
In simple terms
The symbols don’t show what a part looks like. They show what it does.
- Circles are things that turn. A pump or a motor. A small black triangle shows which way the oil goes: pointing out of the circle means oil is pushed out (a pump), pointing in means oil drives it round (a motor). An arrow drawn across means its size per turn can be changed.
- Squares are valves. One square for each position the valve can be in. Arrows inside show where oil can go in that position; a T shape means “blocked”. A directional valve with three squares has three positions, usually “one way”, “hold” and “the other way”.
- Long rectangles are cylinders. A line across is the piston, a line coming out is the rod. One port means oil pushes one way only; two ports means it can push both ways.
- Zigzags are springs, dashed lines are pilot lines (small control signals of oil pressure) and solid lines are the main oil lines.
- A diamond is something that treats the oil: a filter, a cooler or a heater, depending on what’s drawn inside.
Once you know these, a schematic reads like a map. Follow the line from the pump, through each valve, to the cylinder or motor, and back to the tank.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free account.
For engineers
ISO 1219-1 defines the graphic symbols; ISO 1219-2 the way circuit diagrams are laid out. A few conventions decide most of what you read:
| Mark | Meaning |
|---|---|
| Filled triangle | Hydraulic flow (an open triangle is pneumatic) |
| Arrow through a symbol at an angle | Adjustable or variable (spring setting, displacement, orifice) |
| Square, arrow offset from the line | Normally closed pressure valve (relief, sequence) |
| Square, arrow on the line | Normally open pressure valve (reducing valve) |
| Dashed line | Pilot or drain line |
| Port letters | P pump, T tank, A and B work ports, X and Y pilot, L drain |
| Envelope beside the valve squares | Actuation: lever, pedal, spring, solenoid, pilot pressure |
Directional valves are named by ports / positions: a 4/3 valve has four ports and three positions; a 4/2 has two. The square drawn with the port connections is the neutral (unactuated) position. To read another position, imagine sliding that square across under the ports. The animation in the basic circuit lesson does exactly that.
The centre square tells you the most about a circuit: all ports blocked (closed centre), P to T with A and B blocked (tandem), all connected (open), or A and B to T (float). Each suits a different pump and load, covered in lesson 9.
Pressure valves are told apart by where their pilot line comes from and where the outlet goes:
- Relief valve: pilot from its own inlet, outlet to tank, normally closed.
- Sequence valve: pilot from its inlet, outlet to another circuit, with a separate drain.
- Pressure-reducing valve: pilot from its outlet, normally open.
- Counterbalance valve: a relief-type valve with an extra pilot from the opposite line and a bypass check, drawn as a combined block (lesson 11).
Deep dive
Reading a real machine drawing. Start at the power unit: tank, suction strainer, pump, motor, relief valve. Then find each actuator and work backwards to its directional valve. Mark the load-holding valves (pilot-operated checks, counterbalance valves) next to each cylinder; they are where drift and lowering faults live. Finally, trace the return path through coolers and filters back to the tank.
Lines and boxes. A chain-dotted box around a group of symbols means they are built into one assembly, such as a valve block or a pump with its control. Ports on the edge of that box are the real connections you can put a gauge on. Test points have their own small symbol on the line, usually labelled M or MP, and they are where measurements are meant to be taken.
Older and non-standard drawings. Machines built to older standards, or by makers with their own house styles, can draw the same part differently: cut-section drawings instead of symbols, different spring or solenoid marks, or Japanese and German conventions for valve envelopes. The logic usually survives. When it doesn’t, the parts list and the nameplate on the valve are the tie-breakers.
What a schematic won’t tell you. Hose lengths and sizes, where things physically sit, and the actual settings of relief and counterbalance valves. Engineers write the settings on the drawing after commissioning; if yours has none, measure them before you adjust anything.
When you send us a drawing. A photo of the schematic, the machine’s nameplate and a note of which function misbehaves lets an engineer narrow down the likely valve before attending.
Key numbers
| Symbol | Count of basics in this lesson |
|---|---|
| Pumps and motors | 3 |
| Cylinders | 2 |
| Pressure and check valves | 3 |
| Directional, flow, storage, filtering | 4 |
| Standard that defines them | ISO 1219-1 (symbols), ISO 1219-2 (diagrams) |
