At its simplest, a hydraulic system is a way of turning pressurised oil into muscle.

Instead of you lifting something with your arms, a pump pushes oil through hoses and valves into cylinders or motors, which produces the heavy lifting - pushing, pulling or spinning things like:

The key idea is this:

• Oil doesn’t really compress

• If you push on it in one place (with a pump), it can push on something else (a cylinder piston or motor), somewhere else

• Because of the way areas and pressures work, you can get a much bigger force out the other end

This all takes place inside a closed loop. The oil is reused over and over again, circulating from tank to pump to valves to actuators and back to tank again

The main components

A hydraulic system is a collection of parts that all play different roles in keeping the oil:

• Moving

• Clean

• At the right pressure

• Going where it’s meant to

Let’s walk through them in the order the oil “sees” them in a typical setup.

1. Reservoir tank - home base for the oil

The reservoir (tank) is where all the oil rests,  when it’s not being pumped around the system.

It does several quiet but important jobs:

• Storage: Holds enough fluid to feed the pump and keep the system topped up.

• Air removal: As oil circulates, tiny air bubbles can get mixed in. In the tank, those bubbles float up and escape so you’re not pumping froth.

• Contamination settling: Heavier particles and sludge settle to the bottom instead of being constantly swept through the system.

• Cooling: When oil comes back from doing work, it’s usually pretty hot. Sitting in the tank, cools it cool down before the next cycle.

You’ll usually see:

• A vented breather cap, so air can move in and out as the oil level rises and falls.

• A suction point filter, to stop big chunks of dirt and rubbish getting sucked into the pump.

2. Pump and engine – creating flow and pressure

Next up is the pump, driven by some form of power:

• Diesel or petrol engine

• Electric motor

• Manual handle on very simple systems (like a jack)

The pump:

• Draws oil from the reservoir

• Pushes it into the system, creating flow

On its own, flow is just movement. Pressure is created when that flow meets resistance – for example, when a cylinder hits a load or a valve starts restricting flow. That combination of flow and pressure is what creates movement and power.

There are different pump styles (gear, vane, piston; single‑stage, two‑stage), but they all exist to do one basic thing: get oil moving around the circuit.

3. Pressure gauges and relief valve – watching and protecting the system

Once the oil is circulating, you want to know what’s going on inside the hoses and components.

• Pressure gauges can be installed in various points in the system so you can see how much pressure is being generated. That’s handy for fault‑finding and checking that everything’s working as designed.

• A relief valve is the safety net:

  • It’s set to a maximum safe pressure

  • If the pressure climbs too high (say a cylinder reaches the end of its stroke against a solid load), the relief valve opens

  • That lets excess oil flow back to the tank instead of blowing hoses or damaging components

Without a correctly set relief valve, a simple operator mistake can turn into a very expensive failure.

4. Valve assembly - sending oil to the right places

The valve assembly is the traffic controller of the hydraulic system.

It controls:

• Direction - which way the oil flows (extend a cylinder vs retract it, forwards vs reverse on a motor)

• Pressure and flow paths - where the oil goes as it works its way around the system

• Speed - by adjusting flow, it can speed things up or slow them down

The most common type is the directional control valve:

• One port from the pump (pressure in)

• Two working ports (to a cylinder or motor)

• One port back to tank

Move the lever one way and oil goes into one side of the cylinder and the other side returns to tank - cylinder extends. Move it the other way and the flow reverses - cylinder retracts. In the centre position, oil is usually returned straight to tank or held in a neutral condition, depending on the valve design.

5. Cylinders – turning oil into straight‑line force

A hydraulic cylinder is where the magic happens.

Inside a cylinder:

• Oil flows into one side of a sealed tube

• It pushes on a piston

• The piston is connected to a rod

• The rod moves in or out, lifting, pushing or pulling something in a straight line

Because of Pascal’s law:

• Force = Pressure × Area

If you have high pressure and a decent piston area, you get serious force. That’s how a relatively compact cylinder can lift big loads – tipper bodies, loader arms, stabiliser legs, presses and so on.

6. Motors – turning oil into rotation

Hydraulic motors are like cylinders that spin instead of slide.

You’ll see hydraulic motors driving things like:

• Augers and drills

• Conveyor belts

• Winches

• Wheel drives on some machines

7. Flow control valve - sets the pace

A flow control valve is like a tap for hydraulic oil.

• It doesn’t care much about direction; its main job is to meter the amount of flow going to a cylinder or motor.

• Less flow = slower movement

• More flow = faster movement

This is how you get smooth, controllable motion instead of everything slamming open and shut.

Flow controls are often used where you need consistent speed - for example, a conveyor, a slowly lowering platform, or a function that must move gently near the end of travel.

8. Fluid cooler – keeping the oil happy

Whenever oil does work and squeezes through small spaces, it generates heat. Too much heat leads to:

• Thinner oil (lower viscosity)

• Faster wear on pumps and valves

• Damage to seals and hoses

The fluid cooler (oil cooler) is there to pull that heat back out.

• Hot oil passes through the cooler

• Air (or water) removes heat from the oil

• Cooled oil returns to the tank and is ready for another lap

Keeping temperature in the right range massively extends both component life and oil life.

Putting it all together: a simple flow story

Here’s how a typical circuit works, step by step:

Hydraulics can seem complex, however if you understand the key components and what they do, it all makes sense.