Pneumatic and Hydraulic Actuators
Cylinder force and air use, valves, speed control, hydraulic basics, electric linear actuators, and choosing between them.
Choosing an actuator type
An actuator turns a signal into force and motion. Electric motors (the rest of this guide) are the default, but pneumatic cylinders (compressed air) and hydraulic cylinders (pressurized oil) are often better for simple, strong, linear motion. Air is clean, fast, cheap and tolerates overload; oil carries enormous force in a small package and is stiff. Electric linear actuators sit in between, and win on control and energy efficiency. This page gives the formulas and the numbers to choose and size them.
Pneumatic cylinders
A double-acting cylinder is pushed in both directions by air; a single-acting cylinder is pushed one way and returned by a spring. The force is the pressure times the area: F = P × A. On the retract stroke the rod takes up part of the piston area, so the force is lower and the speed higher.
Worked example: force and air use of a 50 mm cylinder
- Areas: bore 50 mm: A = π × 0.050² ÷ 4 = 19.63 cm²; with a 20 mm rod the annulus is 16.49 cm².
- Force at 6 bar (6 × 105 Pa gauge): extend F = 6 × 105 × 19.63 × 10-4 = 1 178 N; retract 990 N. Seal friction takes about 10 percent, leaving 1 060 N and 891 N (roughly 120 kgf and 100 kgf).
- Air per cycle. Volume for a 200 mm stroke is 0.393 L to extend and 0.330 L to retract (compressed). At 6 bar gauge the air is compressed by (6 + 1.013) ÷ 1.013 = 6.92, so each cycle uses 0.723 × 6.92 = 5.00 L of free air.
- Air consumption. At 30 cycles per minute the cylinder uses 150 L/min of free air, about 0.2 m³/min. A compressor and a supply line must be sized for the total of all the cylinders (and leaks, which commonly waste 20 to 30 percent).
- Speed. Extending 200 mm in 0.5 s is 0.40 m/s, which needs a peak flow of A × v × 6.92 = 326 L/min of free air through the valve: choose a valve and tubing rated above this.
- Rod buckling and side load: a long, thin rod under compression can buckle (use the manufacturer’s stroke-and-load curve), and a side load wears the rod bearing and seals: guide the load externally.
- Cushions (adjustable or fixed) decelerate the piston near the end of the stroke; without them the piston bangs the end cap and shortens the life of the cylinder and the machine.
- Magnetic pistons allow reed or Hall switches on the barrel to sense the end positions (see Sensors).
- Rodless cylinders, grippers, rotary actuators and air muscles use the same air and valves in different shapes.
- Intermediate positions are hard because air is compressible. For a controlled position use a servo-pneumatic or an electric actuator.
Valves, circuits and speed control
- Directional valves are named by ports and positions: a 5/2 valve has five ports and two positions and drives a double-acting cylinder; a 3/2 valve drives a single-acting one. They are operated by a solenoid (direct or pilot), a pilot air signal, a lever or a mechanical cam. A 5/3 valve has a center position (closed, exhaust or pressure).
- Speed control: flow controls are needle valves with a check valve. Meter-out (restricting the air leaving the cylinder) is the norm because the exhaust pressure resists the load and keeps the motion steady; meter-in is used for single-acting cylinders or to avoid a back pressure.
- Quick-exhaust valves let the cylinder exhaust directly, for fast return strokes.
- Valve sizing uses the flow coefficient (Cv or the nominal flow in L/min) against the peak flow computed above, with the tubing (the largest bore possible) and the fittings.
- Control: a PLC output energizes the solenoid through a suppressed output (see I/O Wiring and Signals), and sensors confirm the position.
- Valve islands and fieldbus manifolds reduce wiring: valves and I/O on one block with a single network cable.
Air supply and consumption
- Pressure: typically 6 to 7 bar at the compressor and 5 to 6 bar at the machine, regulated to what each circuit needs. Higher pressure costs energy disproportionately (about 7 percent more power per bar).
- Conditioning: a filter-regulator (and a dryer upstream) keeps water, oil and dirt out. Lubrication is now often unnecessary with pre-lubricated cylinders; do not mix lubricated and unlubricated air.
- Leaks are the main waste: 20 to 30 percent of the output of a typical plant leaks. Find them with an ultrasonic detector or soapy water; shut off the supply when the machine stops.
- Receiver tanks near a large or fast consumer smooth the flow and prevent a pressure dip for others.
- Air is compressible, so force and speed vary with the load; a rigid hard stop or a position sensor is needed for repeatable positions.
Hydraulics in brief
A hydraulic system pushes oil at 70 to 350 bar. Since the oil is nearly incompressible the motion is stiff and the force enormous: a cylinder the size of a pneumatic one carries about a ten- to fifty-fold load. The price is a pump, a reservoir, filters, a relief valve, hoses and a heat problem.
Worked example: the same 50 mm cylinder on oil
- At 100 bar (107 Pa) F = P × A = 107 × 19.63 × 10-4 = 19 635 N (about 19.6 kN, nearly two tonnes), against 1 178 N with 6 bar of air.
- To get the same force from air at 6 bar a cylinder would need a bore of 204 mm.
- Flow and speed: Q = A × v. At 0.1 m/s the pump must deliver 0.20 L/s = 11.8 L/min.
- Power: hydraulic power = P × Q = 107 × 0.000196 = 1 963 W; with a pump efficiency of 85 percent the electric motor needs 2.31 kW. Any pressure drop across a valve, and the relief valve when it opens, turns the power into heat in the oil, so a cooler or a large reservoir is common.
- Components: pump (gear, vane or piston), reservoir, filter, relief valve (sets the maximum pressure: always fitted), directional and proportional valves, accumulators, cylinders or motors, hoses and fittings.
- Servo-hydraulics with proportional or servo valves and feedback give fast, strong, accurate motion: presses, flight simulators, test rigs, heavy robotics.
- Fluid care: clean oil and the right filtration (cleanliness class) are the keys to life. Most failures are from contamination, overheating or leaks.
- Electro-hydraulic and electric cylinders (a pump in a package) combine the power of oil with a simple electric connection.
Electric linear actuators, solenoids and voice coils
| Type | Force and stroke | Strengths | Limits |
|---|---|---|---|
| Electric cylinder / linear actuator (motor with a screw) | Tens of N to tens of kN; strokes of mm to a meter or more | Controllable speed and position, clean, efficient at holding, networked | Slower than air, cost, a screw’s wear and speed limit (see Mechanical Transmission: Screws, Belts and Gearboxes) |
| Solenoid | A few N to tens of N; strokes of a few mm | Cheap, fast, simple on-off | Force falls sharply with stroke; heat when held on |
| Voice coil | Grams to tens of N; mm strokes | Smooth, precise, fast, force proportional to current | Small force; needs a guide and feedback |
| Piezo and shape-memory | Tiny strokes (µm), or small forces | Nanometer resolution (piezo), silent | Very specialized |
Worked example: a solenoid coil
A 24 V, 6 W solenoid draws I = P ÷ V = 6 ÷ 24 = 0.25 A and its coil resistance is V ÷ I = 96 Ω. The pull force depends on the square of the current and falls rapidly with the gap, so a spec of “10 N” applies only at the start of its stroke. Most are rated for a duty cycle (for example 25 percent on): held on continuously they heat up. Use a flyback diode or other suppression across the coil (see Inductors, Transformers and Relays), and drive larger ones through a relay or transistor, not a PLC output directly.
Comparing the technologies
| Pneumatic | Hydraulic | Electric | |
|---|---|---|---|
| Force | Moderate (bar pressure) | Very high (hundreds of bar) | Moderate to high, set by the motor and screw |
| Speed | Fast, short strokes | Medium to fast, set by the pump | Medium to fast, set by the motor and lead |
| Position control | End positions; difficult in between | Excellent with servo valves | Excellent, programmable |
| Stiffness | Soft (air springs) | Stiff | Stiff, set by the mechanics |
| Efficiency | Low (about 10 to 15 percent overall) | Medium (50 to 70 percent) | High (70 to 90 percent), and no energy to hold position |
| Cost and complexity | Low, simple | High: pumps, filters, plumbing | Medium, falling |
| Environment | Clean, tolerant of heat and washdown | Leaks can contaminate | Clean; keep dust and water out of the motor |
| Best for | Clamping, pick and place, sorting, cheap two-position motion | Presses, lifts, heavy machinery, high force in small space | Precision, programmable motion, energy-aware machines |
Safety and stored energy
- Compressed air stores energy. Before working on a circuit, isolate and vent it; a cylinder can still move under gravity or trapped pressure. Use a lock-out valve with a vent.
- Hydraulic oil at 100 bar can inject through skin from a pinhole leak: never search for leaks with a hand. Hoses can whip if they fail; use guards and hose restraints.
- Vertical loads fall if the pressure is lost: use pilot-operated check valves, counterbalance valves or a mechanical lock.
- Pinch points: a cylinder’s force at 6 bar is more than enough to injure. Guard the motion and use two-hand or light-curtain controls where required (see Braking, Safe Stopping and Troubleshooting).
- Exhaust noise and oil mist are health matters: use silencers and clean exhaust.
Troubleshooting
| Symptom | Likely cause | Check |
|---|---|---|
| Cylinder is slow or weak | Low pressure at the cylinder, a restricted valve or hose, a leaking seal | Pressure at the port while moving, tubing size, seals |
| Jerky motion (stick-slip) | Poor lubrication, a side load, too tight a flow control | Lubrication and alignment; use meter-out with a moderate setting |
| Cylinder drifts | Valve center position leak, a leaking piston seal, a hydraulic check valve | Valve type, seals, pilot-operated checks |
| Valve does not shift | No coil voltage, a stuck spool, low pilot pressure | Voltage at the coil, a manual override, filtration |
| Slams at the end | No cushion, too much speed | Adjust the cushion and flow control |
| Hydraulic oil hot | Relief valve open, a blocked cooler, an oil level or viscosity problem | Pressure setting, cooling, filters |
| Air consumption too high | Leaks, a larger bore than needed, a high pressure | Ultrasonic leak survey, regulators set to what is needed |
Related
- Motion Control and Actuators: the overview of this guide.
- Choosing a Motor Technology and Sizing a Motion Axis: electric alternatives and sizing.
- Braking, Safe Stopping and Troubleshooting: stopping and guarding.
- Sensors and Field Devices: end-of-stroke and pressure sensors.
- I/O Wiring and Signals: driving valve solenoids from a PLC.
FAQ
Pneumatic or electric?
Pneumatic for a simple, fast, two-position, cheap motion in a plant that has compressed air. Electric when you need intermediate positions, control of speed and force, energy savings or networked diagnostics.
How do I size a cylinder?
Find the force needed (including friction and acceleration), divide by the working pressure (leave a 25 to 50 percent margin) to get the area, and choose the next standard bore. Check the speed against the air flow of the valve and tubing.
Why does compressed air cost so much?
Compressing air to 7 bar converts most of the electrical energy to heat; the delivered energy is only about 10 to 15 percent. Fix leaks and use the lowest pressure that works.
Can I stop a pneumatic cylinder mid-stroke?
Roughly, with a 5/3 closed-center valve, but the air spring makes it drift and move under load. Use a positioning cylinder with a servo valve or an electric actuator if the position matters.
References
- Festo and SMC, Pneumatics technical handbooks (cylinder forces, flow sizing, circuits).
- Esposito, A., Fluid Power with Applications, Pearson; Parker Hannifin, Industrial Hydraulic Technology.
- ISO 4414 (pneumatic fluid power safety), ISO 4413 (hydraulic fluid power safety).
Last reviewed: October 2026. Forces, flows and efficiencies are typical examples; follow the manufacturer’s data and the fluid-power safety standards.