Hydraulics and Pneumatics
Pressure, flow and force, pumps and motors, cylinders, valves, circuits and accumulators, oil and filtration, compressed air, design and troubleshooting, with worked numbers.
Fluid power uses a pressurized liquid or gas to move energy to where the work is done. Hydraulics (oil at 100 to 350 bar) gives enormous force from a compact actuator and holds a load stiffly; pneumatics (air at 6 to 10 bar) is clean, fast and simple but soft and costly to run. This guide covers the basics and Pascal’s law, pumps and motors, cylinders, valves, circuits and accumulators, oil, filtration and cooling, pneumatic systems, system design and energy, and troubleshooting and safety. One 160 bar press with an 80 mm cylinder runs through the pages, and every number is computed. The electric alternatives are in Motion Control and Actuators, and the water-side calculations in Fluid Mechanics and Pipe Flow.
Start here: what do you need?
| I need to… | Go to |
|---|---|
| understand pressure, flow, force and power in fluid power | Fluid Power Basics |
| choose a pump or motor and work out its drive power | Hydraulic Pumps and Motors |
| size a cylinder, check buckling and speed | Cylinders and Actuators |
| pick relief, directional and flow control valves | Valves and Flow Control |
| design a circuit or size an accumulator | Circuits and Accumulators |
| choose oil and filters, size the tank and cooler | Fluids, Filtration and Reservoirs |
| size an air cylinder, compressor or vacuum cup | Pneumatic Systems |
| size lines, pick hoses and cut the energy cost | Designing a Fluid Power System |
| find a fault or work safely | Troubleshooting and Safety |
A suggested order
Start with Fluid Power Basics, then the components in the order the oil meets them: Hydraulic Pumps and Motors, Valves and Flow Control and Cylinders and Actuators. Circuits and Accumulators puts them together, Fluids, Filtration and Reservoirs keeps the oil healthy, and Designing a Fluid Power System and Troubleshooting and Safety finish the job. Pneumatic Systems is the separate air-side chapter.
- Fluid Power Basics: Pascal’s law, force and pressure, flow and speed, power, hydraulic against pneumatic and the standard symbols.
- Hydraulic Pumps and Motors: Gear, vane and piston pumps, displacement, flow, torque, efficiency, drive power and hydraulic motors.
- Cylinders and Actuators: Force and speed, the annulus side, regeneration, cushioning, buckling of long rods and sizing a cylinder.
- Valves and Flow Control: Relief, directional, flow control, pressure reducing and check valves, orifice flow and proportional and servo valves.
- Circuits and Accumulators: Basic circuits, meter-in and meter-out, regeneration, sequencing and sizing an accumulator.
- Fluids, Filtration and Reservoirs: Oil viscosity and cleanliness, filters, reservoir size, heat generation and cooling.
- Pneumatic Systems: Compressed-air supply, air preparation, cylinder air use, valve flow, tubing, vacuum and sizing.
- Designing a Fluid Power System: Line sizing, pressure drops, efficiency, energy cost and a complete worked design.
- Troubleshooting and Safety: Common faults and their causes, how to measure, contamination, injection injuries and stored energy.
One press, from force to oil cooler
Worked example: the same press on every page
- The cylinder (Cylinders and Actuators): 80/45 mm at 160 bar pushes 80.4 kN; at 0.1 m/s it needs 30.2 L/min.
- The pump (Hydraulic Pumps and Motors): 21.2 cm³/rev at 1 500 rpm; 9.2 kW of shaft power at an overall efficiency of 87 percent.
- The accumulator (Circuits and Accumulators): 2 L between 161 and 201 bar needs about 14.7 L of gas volume.
- The oil (Fluids, Filtration and Reservoirs): ISO VG 46, a 100 L tank, a cooler if the oil passes 55 °C.
- The energy (Designing a Fluid Power System): unloading the pump when idle halves the electricity.
Quick reference: the equations
| What you need | Equation |
|---|---|
| Force | F = p A |
| Flow and speed | v = Q ÷ A Q = Vg N ηv |
| Fluid power | P (kW) = p (bar) × Q (L/min) ÷ 600 |
| Pump torque | T = Vg Δp ÷ (2π ηm) |
| Orifice | Q = Cd A √(2 Δp ÷ ρ) |
| Accumulator | ΔV = V₀ [ (p₀ ÷ p₁)1/n − (p₀ ÷ p₂)1/n ] |
| Rod buckling | Pcr = π² E I ÷ (K L)² |
| Laminar line loss | Δp = 32 μ L v ÷ d² |
| Free air | Vfree = V × (p + 1.013) ÷ 1.013 |
| Vacuum cup | F = Δp A |
Related guides and tools
- Cylinder Force and Speed Calculator and Compressed Air Consumption Calculator.
- Pneumatic and Hydraulic Actuators and Compressed Air and Gas Flow.
- Heat Transfer for the oil cooler and Columns and Buckling for the rod.
- The Pressure, Flow, Force and Power converters, and the Downloads library.
Safety first
- High-pressure oil injects through skin from a pinhole leak. It is a medical emergency; never check for leaks with a hand.
- Stored energy in accumulators, raised loads and trapped pressure: discharge, vent and lock out before any work.
- Accumulators are charged with nitrogen only, never air or oxygen.
- Hoses burst and whip; guard them, replace them on schedule and never exceed their rating.
- Fire and burns: oil sprays burn and hot oil scalds; use fire-resistant fluid where the risk calls for it.
- The numbers here are for study and preliminary design. Real systems need the machine safety standards (ISO 4413, ISO 4414), proper guarding and a competent designer.
FAQ
Where do I start?
Read Fluid Power Basics, then go to the component you need.
Hydraulic or electric?
Hydraulics wins at very high force and in harsh, mobile or compact applications; electric actuators win on precision, energy and cleanliness. See Pneumatic and Hydraulic Actuators.
Why is compressed air so expensive?
Compression turns most of the electrical energy into heat; typically only a tenth of it does work. See Pneumatic Systems.
Where are electrical power and control systems?
They are the next guides in this series.
References
- Esposito, A., Fluid Power with Applications, Pearson; Parr, A., Hydraulics and Pneumatics, Butterworth-Heinemann; Rabie, M. G., Fluid Power Engineering, McGraw-Hill.
- ISO 1219 (symbols), ISO 4413 and ISO 4414 (safety), ISO 4406 (cleanliness), ISO 6020/6022 (cylinders), ISO 4401 (valve interfaces).
- Bosch Rexroth, Parker Hannifin, Eaton, Hydac, SMC and Festo technical manuals and catalogs; US DOE compressed air sourcebook.
Last reviewed: November 2026. Values are typical; check the standard, the manufacturer’s data and the code that applies to your machine.