Hydraulic Systems Basics
A small excavator can lift several tonnes with an engine no bigger than a family car's, and the trick behind that feat is hydraulics. By using a confined liquid to transmit force, hydraulic systems multiply a modest input into an enormous output, which is why they power everything from car brakes to aircraft controls to factory presses.
Pascal's principle
The whole field rests on a discovery by Blaise Pascal: pressure applied to a confined fluid is transmitted equally in all directions. Because liquids are, for practical purposes, incompressible, squeezing the fluid in one place raises the pressure everywhere in the connected system by the same amount. That pressure then pushes outward on every surface it touches, and that is exactly what a hydraulic system harnesses.
Force, pressure, and area
The governing equation could not be simpler:
F = P × A,
where F is force, P is pressure, and A is the area the pressure acts on. This is the source of hydraulics' famous force multiplication. The same pressure acting on a larger piston produces a proportionally larger force, so a small pump can generate a huge push simply by acting on a big enough area. Pressure is the currency; area is the lever.
Units of pressure
Two units dominate. The bar is metric and close to atmospheric pressure — 1 bar ≈ 100,000 pascals (N/m²). The psi (pounds per square inch) is the imperial equivalent, with 1 bar ≈ 14.5 psi. Industrial hydraulics commonly run at 100 to 350 bar (roughly 1,500 to 5,000 psi), and it is this high pressure, multiplied over a piston's area, that yields such large forces from compact equipment.
The core components
- Reservoir: the tank that stores the fluid, lets it cool, and allows air and contaminants to settle out.
- Pump: driven by a motor or engine, it moves fluid and creates flow; the resistance that flow meets is what builds pressure.
- Valves: direct the fluid, limit its pressure, and regulate its flow — the system's steering and throttle.
- Actuator: the cylinder or hydraulic motor that turns fluid pressure back into mechanical force or rotation.
A key point about the pump: it creates flow, not pressure directly. Pressure only rises when that flow is resisted by a load. A pump pushing fluid into open air develops almost no pressure at all.
Bore and rod: extend versus retract
A typical cylinder is not symmetrical, and this catches people out. On the extend stroke, pressure acts on the full bore area of the piston. On the retract stroke, the piston rod occupies part of that face, so pressure acts only on the bore area minus the rod's cross-section. With less area to work on, a cylinder always pushes harder than it pulls at the same pressure. The difference can be substantial and must be accounted for whenever the pulling stroke does the real work.
Flow controls speed
If pressure and area set the force, it is the flow rate that sets the speed. The volume of fluid delivered per minute has to fill the space behind the advancing piston, so a larger flow extends the rod faster, while a bigger-bore cylinder moves more slowly for the same flow because there is more volume to fill. Force and speed thus pull in opposite directions: the large piston that gives great force also moves slowly unless you feed it more flow.
Worked example
Take a cylinder with a bore diameter of 100 mm and a rod diameter of 40 mm, running at 200 bar.
- Convert the pressure. 200 bar = 20 N/mm². (Handily, 1 bar = 0.1 N/mm².)
- Bore area. π × 50² ≈ 7854 mm².
- Extend force. 20 × 7854 ≈ 157,000 N, about 16 tonnes of push.
- Rod area. π × 20² ≈ 1257 mm², so the annulus area is 7854 − 1257 = 6597 mm².
- Retract force. 20 × 6597 ≈ 132,000 N, about 13.5 tonnes of pull.
The same cylinder at the same pressure pushes with 16 tonnes but pulls with only 13.5 — the rod has stolen roughly a sixth of the working area. That asymmetry is a direct, quantitative consequence of F = P × A.
Why hydraulics win
Nothing else packs so much controllable force into so small a package. High pressure over a modest area yields forces no electric motor of similar size could match, the fluid can be piped around corners to wherever the work is, and the same fluid carries away heat and lubricates as it goes. The trade-offs are leaks, the need for a pump and reservoir, and energy lost as heat, but where brute force in a compact form is the goal, hydraulics remain unbeaten.
Working out cylinder forces is quick once you have the bore, rod, and pressure. Try yours with the Hydraulic Cylinder Force Calculator on MechKit.