Parts of a Hydraulic System – Labeled Circuit Diagram

All 18 parts on the diagram

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The power unit 10

Reservoir It also lets air bubbles escape and dirt settle to the bottom. It gives off heat from the oil.
Air vent (breather) Air moves through it as the oil level goes up and down. That keeps the space above the oil at the pressure of the outside air.
Baffle It makes the returning oil go the long way round. That helps it cool before the pump draws it in again.
Strainer It catches only the large particles. That way it slows the oil less than a filter would.
Pump It makes flow, not pressure. Pressure builds only when the flow meets resistance, such as a load on a cylinder.
Electric motor The motor or engine driving the pump is called the prime mover.
Pressure line It runs from the pump's outlet to the control valve.
Relief valve It sends the pump's oil back to the reservoir. Then it closes again when the pressure drops.
Pressure gauge Most have a curved Bourdon tube inside. It straightens as the pressure rises and moves the pointer.
Filter Here it sits on the return line. That is just before the oil goes back into the reservoir. Its element has to be changed regularly.

Control and work 8

Check valve Flow pushes the ball off its seat. A spring and any flow the other way push it shut.
Accumulator Gas in the top is squeezed as oil is pumped in. It pushes the oil back out when the system needs it.
Directional control valve Moving a lever slides a spool inside. That opens one path for the oil and closes another.
Hoses Hose is used where pipe or tube cannot be. This is because it bends as the machine moves.
Cylinder Oil on one side of the piston inside drives the rod out. Oil on the other side brings it back.
Hydraulic motor It works like a pump run backward. The oil pushes it round and its shaft drives the machine.
Return line It ends below the oil level in the reservoir. Its opening is turned away from the pump's inlet.
Cooler It keeps the oil temperature within safe limits, so the oil lasts.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two of these three use it: (1) US Navy, Fluid Power, NAVEDTRA 14105 (1990), full text at https://archive.org/details/fluidpower; (2) US Army, FM 5-499 Hydraulics (1997), full text at https://archive.org/details/fm-5-499-hydraulics-1997; (3) Wikipedia, Hydraulic machinery, https://en.wikipedia.org/wiki/Hydraulic_machinery, text only. The detail lines are in our own words. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no maker's unit, nothing traced. Licence: own work

How it works

A basic hydraulic system in two drawings. The power unit stores the oil and pumps it out; the control valve sends it to a cylinder or a motor, and the return line brings it back.

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Follow the oil

Oil leaves the reservoir through the strainer. The pump, turned by the electric motor, pushes it into the pressure line, past the relief valve and the pressure gauge. It passes the check valve and the accumulator and reaches the directional control valve. A lever sends it down a hose to the cylinder or the hydraulic motor. The used oil comes back along the return line, through the cooler and the filter, into the reservoir.

A pump makes flow, not pressure. Pressure appears only when the oil meets resistance, such as a load.
The baffle keeps returning oil away from the pump's inlet, so it has time to cool.

Questions and answers

What are the five basic parts of a hydraulic system?

A reservoir to hold the oil, a pump to move it, lines to carry it, a directional control valve to steer it and an actuator, a cylinder or a motor, to do the work. Relief valves, filters and gauges are added as needed.

What does the relief valve do in a hydraulic system?

It guards against too much pressure. When the pressure reaches the valve's setting it opens and sends the pump's oil back to the reservoir; when the pressure drops it closes again.

What is the difference between a strainer and a filter?

A strainer is a coarse wire screen that catches large particles, usually on the pump's inlet inside the reservoir. A filter catches much finer dirt. On this drawing it sits on the return line.

What is a hydraulic accumulator for?

It stores oil under pressure, the way a battery stores electricity. It can keep the pressure up when the pump stops, make up for small leaks and soften shocks.