A hydraulic circuit moves oil under pressure to do heavy work. A pump sends the oil through a valve that steers it into a cylinder, and the piston pushes a load. Follow the oil from the reservoir and back again.
A hydraulic circuit is a loop of parts that uses oil under pressure to move something heavy. A pump pressurises the oil, valves control where it goes, and a cylinder converts the pressure back into a powerful push. Because oil barely compresses, the force is immediate and can be enormous for the size of the parts.
From outside, a hydraulic machine is just a tangle of hoses and a steel body. Drawing the circuit as a cutaway shows how the pump, the valve and the cylinder are connected, and makes the path the oil follows — out to the cylinder and back to the reservoir — easy to trace.
The electric motor spins a gear pump. As its two gears turn, oil is trapped between the teeth and carried from the suction side to the pressure side, so a steady stream of oil leaves the pump.
Pushing oil into a closed pipe raises its pressure. A gauge on the pressure line shows this rise, and the relief valve stands ready to spill oil back to the reservoir if the pressure gets too high.
The directional valve is the traffic controller. Sliding its spool lines the ports up so the pump's oil is sent to one side of the cylinder. Move the spool the other way and the flow swaps sides.
Oil entering one end of the cylinder pushes the piston, and the piston rod slides out to shove the load. Oil trapped on the other side of the piston is forced out through the valve and back to the reservoir.
Return oil drops into the reservoir, where it cools and any air or dirt settles out before being pumped around again. Reversing the valve sends pressure to the other side, pulling the rod back in.
It moves oil under pressure to where it is needed. A pump pressurises the oil, valves steer and control it, and a cylinder or motor turns that pressure back into force and motion. Because oil is almost incompressible, the force arrives instantly and can be very large.
Oil carries much higher pressure than air without leaking away easily, and unlike water it also lubricates the pump, valves and cylinder, so the parts last far longer.
It decides where the oil goes. Moving its spool lines the ports up in different ways, so the same pump can push the cylinder out, pull it back, or hold it still in the centre position.
Pressure rises until the relief valve opens and spills oil back to the reservoir. That keeps the pump and hoses from being over-pressurised, so the relief valve is the circuit's safety valve.
A double-acting cylinder does: oil pushes the piston out on one side while the oil on the other side is squeezed back to the reservoir. Reversing the valve swaps which side gets pressure, so the rod retracts.