Fluid Power Basics
Pascal’s law, force and pressure, flow and speed, power, hydraulic against pneumatic and the standard symbols.
What fluid power is
Fluid power moves energy through a pressurized liquid (hydraulics, usually oil) or gas (pneumatics, usually compressed air). A pump or compressor turns the shaft power of a motor into flow and pressure; valves direct and control it; and cylinders and motors turn it back into force and motion. It is chosen where very large forces, compact actuators, easy speed control, overload protection and the ability to hold a load matter: presses, excavators, aircraft controls, injection molding machines, clamping and pick-and-place. The electric side of motion is in Motion Control and Actuators, and the actuator view in Pneumatic and Hydraulic Actuators; this guide goes deeper into the fluid and the components.
Pascal’s law, force and pressure
Pressure applied to a confined fluid is transmitted undiminished in every direction. A force F on a piston of area A makes a pressure p = F ÷ A, and that pressure pushes with force F = p A on any other piston it touches. Different areas therefore multiply the force (and divide the stroke by the same ratio):
p = F ÷ A F₂ = p A₂ = F₁ × (A₂ ÷ A₁)
Worked example: a hydraulic jack
- A 20 mm piston is pushed with 300 N: p = 300 ÷ 3.14 cm² = 9.5 bar.
- On a 100 mm piston that pressure gives F₂ = 9.5 bar × 78.5 cm² = 7.50 kN, a multiplication of 25.
- The price is stroke: the small piston must move 25 times as far as the large one, and the work in equals the work out (less losses).
Flow, speed and power
Oil flow Q sets the actuator speed: v = Q ÷ A for a cylinder and N = Q ÷ Vg for a motor of displacement Vg. The power in the fluid is the product of pressure and flow:
P = p × Q P (kW) = p (bar) × Q (L/min) ÷ 600
Oil is almost incompressible but not quite: its bulk modulus is about 1.4 GPa, so 160 bar compresses it by 1.14 percent. That tiny compressibility is what makes a hydraulic system stiff, and it also stores energy (a long hose full of high-pressure oil behaves like a spring).
Worked example: a hydraulic press
Worked example: the press of this guide
A press cylinder has a bore of 80 mm and a rod of 45 mm. It must push at 0.10 m/s at a pressure of 160 bar (2 321 psi). The same press runs through every page.
- Piston area A = π ÷ 4 × 0.08² = 50.27 cm².
- Force F = p A = 16 MPa × 50.27 cm² = 80.4 kN (about 8.2 tonnes).
- Flow Q = A v = 0.00503 × 0.10 = 30.2 L/min.
- Fluid power = 160 × 30.2 ÷ 600 = 8.04 kW; the electric motor needs more, about 9.2 kW, once the pump losses are included (see Hydraulic Pumps and Motors).
- The stroke of 400 mm takes 4.0 s out and 2.7 s back, because the return side has less area (Cylinders and Actuators).
Hydraulic or pneumatic?
| Property | Hydraulic (oil) | Pneumatic (air) |
|---|---|---|
| Working pressure | 100 to 350 bar | 5 to 10 bar |
| Force from a small actuator | very high | modest |
| Stiffness and position holding | high (oil is nearly incompressible) | soft (air compresses) |
| Speed | moderate, controllable | fast, hard to control precisely |
| Cleanliness | leaks and mess; contamination control needed | clean (exhausts to the air); food and electronics |
| Energy cost | efficient with load sensing | expensive: compressed air costs about 5 to 10 times as much per unit of work as electricity |
| Safety | injection injuries, fire, stored energy | stored energy, noise, whipping hoses |
| Typical use | presses, mobile machinery, heavy lifts | automation, clamping, sorting, packaging |
The same 80 kN press on air at 6 bar would need a piston of 413 mm diameter (area = 80 425 ÷ 600 000 = 1 340 cm²), about 13 times the area and a cylinder the size of a barrel. That is the whole argument for hydraulics at high force, and for pneumatics at light duty.
Components and symbols
Circuits are drawn with the standard symbols of ISO 1219-1 (and ANSI/NFPA): a circle for a pump or motor with a triangle showing the flow direction (solid for oil, open for air), a rectangle with arrows for a valve (one square per position), a rectangle with a piston for a cylinder, a box for a tank, and a spring or a pilot line for the actuation. Learn a handful and you can read any diagram: pump, motor, cylinder, relief valve, directional valve (the numbers of ports and positions: a 4/3 valve has four ports and three positions), check valve, flow control, filter, accumulator, tank.
| Component | Job | Page |
|---|---|---|
| Pump and motor | make flow, convert flow back to torque | Hydraulic Pumps and Motors |
| Cylinder | convert flow into linear force and motion | Cylinders and Actuators |
| Valves | limit pressure, direct flow and control speed | Valves and Flow Control |
| Accumulator and circuits | store energy, combine the parts | Circuits and Accumulators |
| Fluid, filter, reservoir, cooler | carry the power, keep it clean and cool | Fluids, Filtration and Reservoirs |
| Compressor and air tools | the pneumatic side | Pneumatic Systems |
Related
- Next: Hydraulic Pumps and Motors.
- Cylinder Force and Speed Calculator and Compressed Air Consumption Calculator.
- Pneumatic and Hydraulic Actuators.
- Hydraulics and Pneumatics: the whole guide.
FAQ
Does a bigger pump give more pressure?
No. The pump gives flow; the load creates pressure. A bigger pump moves the load faster at the same pressure, and the relief valve limits the maximum.
Why is hydraulic oil nearly incompressible?
Liquids resist volume change much more than gases: oil’s bulk modulus is about 1.4 GPa against about 1 bar for air, so oil holds a position stiffly.
Why do pneumatic cylinders move in jerks at low speed?
Air is soft and friction is high, so the cylinder sticks and slips (stick-slip). Use speed controls, low-friction seals or hydraulic checks for smooth slow motion.
What is the difference between gauge and absolute pressure?
Gauge pressure is measured above the atmosphere. Gas laws need absolute pressure (gauge + 1.013 bar); fluid power forces use gauge differences.
References
- Esposito, A., Fluid Power with Applications, Pearson; Parr, A., Hydraulics and Pneumatics: A Technician’s and Engineer’s Guide, Butterworth-Heinemann.
- ISO 1219-1 (fluid power graphical symbols) and ISO 5598 (vocabulary); NFPA/T2.6.1 and ISO 4413 (hydraulic) and ISO 4414 (pneumatic) safety requirements.
- Bosch Rexroth, Parker Hannifin and Eaton fluid power training manuals and catalogs.
Last reviewed: October 2026. Pressures and bulk modulus are typical.