Direct answer: Use a single-acting cylinder when air should drive one direction and a spring/gravity should define the return state. Use a double-acting cylinder when air must control force in both directions. Choose rodless, guided, compact, tandem or other designs when stroke, side load, space, rotation or force makes a standard rod cylinder unsuitable.
All pneumatic cylinders convert pressure into linear force at a piston. The meaningful differences are how air reaches each chamber, how load is supported, what happens when air/power fails, and how the PLC proves position.
The basic force relationship
Approximate extension force is:
Force = pressure × effective piston area
Actual usable force is lower because of friction, pressure losses, seal condition, backpressure and design margin. Retraction force on a rod cylinder is lower because the rod occupies part of the piston area.
Never size a cylinder at exactly the theoretical load. Dynamic acceleration, orientation, binding, pressure variation and safe failure behavior matter.
Single-acting cylinder
A single-acting cylinder has one pressurised working port. Air moves the piston in one direction; a spring or external force returns it.
Advantages
- defined spring-return position when air is removed;
- one working air line and a simple 3/2 valve;
- lower air use where work occurs only in one direction.
Limitations
- spring occupies space and limits practical stroke;
- available output force changes with spring compression;
- powered force exists in only one direction;
- exhaust/vent must remain clear.
Typical uses include ejecting, clamping, marking and light positioning where the return force is modest and a defined air-loss state is useful.
Double-acting cylinder
A double-acting cylinder has ports at both ends. A 5/2 valve sends pressure to one chamber while exhausting the other; shifting the valve reverses those paths.
Advantages
- controlled powered motion in both directions;
- long strokes and broad size range;
- more consistent force than spring return;
- independent extend/retract speed adjustment.
Limitations
- consumes air for both movements;
- air loss does not inherently define a safe position;
- rod-side retraction force is lower;
- load can move under gravity or external force if not restrained.
Festo identifies single-acting and double-acting cylinders as the two primary categories and notes the common 5/2 valve relationship for double-acting motion.
Rodless cylinder
A rodless cylinder transfers piston motion to an external carriage without a rod extending beyond the barrel. Designs use a mechanical slot/seal or magnetic coupling.
Choose it when:
- long stroke must fit in limited machine length;
- the moving carriage should be supported along the actuator;
- bending risk from a long extended rod is unacceptable.
Consider carriage loading, moment capacity, sealing, contamination and whether the load needs external guidance. A rodless actuator is not automatically a structural linear guide for every side load.
Guided cylinder / guided drive
A guided cylinder combines the piston drive with guide rods/bearings so the tooling cannot rotate and side/moment loads are carried more effectively.
It is well suited to pressing, pick-and-place, stops and fixtures where a standard piston rod would bind or rotate. Check the manufacturer's load/moment charts for orientation and stroke; “guided” does not mean unlimited side load.
Compact, tandem, multi-position and rotary variants
| Type | Main reason to choose it |
|---|---|
| Compact/short-stroke | limited installation length |
| Tandem | higher force at the same bore/pressure by combining piston areas |
| Multi-position | several defined stroke positions without a servo axis |
| Through-rod | more balanced area/load support or motion at both ends |
| Locking/clamping | hold position under defined loss-of-pressure conditions |
| Rotary actuator | convert pneumatic energy to limited-angle rotation |
Each adds constraints. A rod lock may hold a static load but is not automatically an emergency brake. Confirm its certified behavior for the exact risk.
Cushioning and end-of-stroke energy
End cushioning restricts exhaust near the end cap so the piston decelerates before impact. Adjustable pneumatic cushioning can be tuned for a load/speed range; elastomer bumpers handle smaller residual energy.
If a cylinder bangs at the end:
- reduce speed using correctly arranged flow control;
- verify load and pressure;
- adjust cushioning within the manufacturer's method;
- ensure exhaust is not causing unstable motion; and
- use an external shock absorber where kinetic energy requires it.
Cushioning cannot rescue a grossly undersized cylinder or uncontrolled falling load.
Meter-out flow control
Air's compressibility makes pneumatic motion prone to running away if only the inlet is restricted. In many horizontal/controlled applications, meter-out control restricts exhaust from the leaving chamber. Backpressure then stabilizes piston motion.
Place the flow controls at the cylinder ports as intended and orient one-way flow correctly. A reversed flow control is a common reason one direction is unexpectedly fast.
PLC sequence and feedback
A double-acting cylinder commonly exposes:
| Tag | Meaning |
|---|---|
CYL_EXTEND_CMD | solenoid command for extend path |
CYL_RETRACT_CMD | command for retract path, if double-solenoid |
CYL_EXTENDED | end sensor proves extended position |
CYL_RETRACTED | home sensor proves retracted position |
A safe sequence does not assume command equals position:
Command extend
→ start travel proof timer
→ wait for CYL_EXTENDED
→ if proof arrives, advance state
→ if timeout, remove/hold commands as designed and report EXTEND_TIMEOUT
Interlock opposing coils unless the valve specifically requires another strategy. On restart, determine actual position from sensors before continuing a sequence.
Choose from the required failure state
Ask what should happen if electrical power or air pressure disappears:
- spring return to a defined state;
- stop and remain trapped by a valve center condition;
- exhaust to remove force;
- hold with a mechanical lock/brake;
- lower under controlled gravity; or
- move to a safe state using stored energy.
There is no universally safest pneumatic state. Trapping air can hold a load but leave stored energy. Exhausting can remove force but allow gravity movement. The risk assessment determines the architecture.
Common faults by symptom
| Symptom | Likely checks |
|---|---|
| no movement | supply, valve command, manual override, pressure, jam |
| moves one direction only | valve/spool, blocked port/exhaust, seal, flow control |
| slow both ways | low pressure/flow, undersized valve/tube, high load, leakage |
| creeps | valve leakage, piston seal leakage, external load |
| chatters/sticks | contamination, side load, low pressure, damaged guide |
| misses sensor | sensor position, stopping energy, loose magnet/sensor, overspeed |
Practise valve, cylinder and sensors as one system
The Pneumatic Cylinder lesson lets Pro learners switch between single- and double-acting behavior, route air, and observe position feedback. Pair it with the Solenoid Valve lesson before entering a pick-and-place scenario.
Frequently asked questions
What are the two main pneumatic cylinder types?
Single acting and double acting. Single acting uses air for one direction and spring/external force for return; double acting uses air for both directions.
What valve is used for a double-acting cylinder?
A 5/2 directional valve is the common choice. Five ports provide pressure, two actuator ports and two exhausts; two positions swap extend/retract flow paths.
Why is cylinder retraction force lower?
On the rod side, the piston rod subtracts from the effective pressure area. At the same pressure, the smaller annular area produces less theoretical force.
When should I use a rodless cylinder?
Use it when a long stroke must fit into a shorter installation envelope or a carriage arrangement is preferable. Confirm load/moment support and contamination requirements.
Can a pneumatic cylinder hold a suspended load after air loss?
Do not assume it can. Leakage, hose failure and valve center state can allow motion. Vertical loads may require a risk-assessed mechanical rod lock, brake, counterbalance or another technology.
