The basic hydraulic circuit & ISO symbols
The three master relations governed the physics. Now meet the hardware — seven building blocks, their standard ISO 1219 symbols, and the circuit that connects them into every hydraulic machine.
Source: Rabie, Fluid Power Engineering, Ch. 1; symbols ISO 1219.
Before you start
What you need first
- The three master relations: \(F=pA\), \(v=\tfrac{Q}{A}\), \(N=pQ\) (Topics 2–4).
What you'll be able to do
- Name the seven building blocks of a hydraulic system.
- Recognise each one's ISO 1219 symbol.
- Read the basic circuit and trace the oil's path.
Start here · the parts
The seven building blocks
Almost every hydraulic system is built from the same seven parts, in the same order from tank to load:
- Reservoir (tank) — stores and cools the oil.
- Filter — keeps the oil clean (the number-one reliability factor; full detail in Topic 31).
- Pump + electric motor — the motor drives the pump, which makes the flow.
- Relief valve — the safety valve; caps the maximum pressure.
- Directional control valve — sends oil to the right side of the actuator.
- Flow-control valve — sets the actuator's speed.
- Actuator (cylinder or motor) — turns oil power back into motion.
Meet the ISO 1219 symbols
Hydraulic circuits are drawn in a standard symbol language (ISO 1219) so any engineer, anywhere, reads the same diagram. Here are the core symbols — learn the shape and what it does.
How they connect — the basic circuit
Wire the symbols together in order and you get the circuit behind almost every hydraulic machine. Follow the oil: up from the tank, made into flow by the pump, guarded by the relief valve, steered by the directional valve, throttled for speed, into the cylinder — and back to the tank.
Power changes form twice
- The motor + pump turn electrical/mechanical power into oil pressure & flow.
- The valves & lines carry and control that hydraulic power.
- The cylinder turns it back into force & motion (\(N=pQ=Fv\)).
✏️ Try it yourself
Using the circuit above, name the component that:
- makes the flow;
- protects the system from too-high pressure;
- decides which way the cylinder moves;
- sets how fast the cylinder moves.
Recap — the whole topic on one screen
| Block | Job | Symbol clue |
|---|---|---|
| Reservoir | store & cool oil | open-top tank |
| Filter | clean the oil | diamond + dashed line |
| Motor + pump | make the flow | circle + M; circle + solid triangle out |
| Relief valve | cap the pressure | box, spring, arrow to tank |
| Directional valve | steer the oil | boxes — one per position |
| Flow-control valve | set the speed | adjustable throttle |
| Cylinder | make force & motion | barrel + piston + rod |