Types of rotary pneumatic actuator
Rotary pneumatic actuators produce limited-angle movement, typically 90°, 180° or a set angle, rather than continuous rotation, which is the job of an air motor. The main designs are:
| Type | How it works | Torque characteristic | Typical use |
|---|---|---|---|
| Rack and pinion | Two pistons drive racks that turn a central pinion | Constant torque through the stroke (double-acting) | Quarter-turn ball, butterfly and plug valves; dampers; general automation |
| Scotch yoke | A piston drives a slotted yoke on the shaft | High torque at the ends of travel, lower in mid-stroke | Large valves with high breakaway and seating torque |
| Vane | Air acts on a vane inside a cylindrical chamber | Constant torque; compact; limited by vane sealing | Swivel units, light indexing, small dampers |
| Helical (spline) piston | Linear piston motion converted through a helical spline | Constant, high torque in compact form | Heavy swivel and positioning duties |
| Rack and pinion swivel units | Compact twin-piston units with end-position cushioning | Constant torque; adjustable end stops | Pick-and-place, part turning, gripper rotation |
Each may be double-acting (air drives both directions) or spring-return (air drives one way, springs drive back). The choice changes the torque calculation completely.
Understanding the torque requirement
The driven load rarely needs the same torque throughout its movement. For a quarter-turn process valve the valve maker typically publishes three values:
- Breakaway torque to unseat the valve from closed, usually the highest.
- Running torque through mid-travel, usually lower.
- End or seating torque to close tightly against the seat.
These values depend on line pressure, the medium (clean liquid, slurry, dry gas), temperature, seat material and how long the valve has stood without moving. Valves that sit closed for months, or handle sticky or crystallising media, can need considerably more breakaway torque than their clean-water rating. Many specifiers apply a margin over the valve maker’s torque, often in the range of 20–50% depending on service severity, following the valve maker’s guidance. Any margin used should be stated and justified.
For automation tasks such as swivelling a part, the requirement comes from the load: friction and gravity torque plus the torque needed to accelerate the inertia within the required time. A heavy offset load being lifted through 90° has its highest gravity torque when the arm is horizontal.
Actuator output: pressure, direction and springs
Actuator torque is roughly proportional to the supply pressure acting on the pistons. Makers publish torque tables at several pressures, commonly 3–8 bar (43.5–116 psig). Always read the table at the lowest pressure the actuator will reliably receive, not the nominal header pressure. In many plants that means checking the pressure at the far end of the network during peak demand, after the filter-regulator and solenoid valve.
For spring-return actuators, four numbers matter:
- Air stroke start torque (air pressure minus the spring preload, the highest air value).
- Air stroke end torque (air pressure minus compressed spring force, the lowest air value).
- Spring stroke start torque (spring at full compression).
- Spring stroke end torque (spring at its weakest, the lowest spring value).
The spring end torque must exceed the valve’s seating or breakaway torque in the fail direction. Spring sets are chosen to match the available air pressure: too strong a spring for low air pressure leaves the air stroke weak; too weak a spring fails to close the valve on loss of air.
Selection method step by step
- Define the movement. Angle, direction, start and end positions, and what must be true at each end (sealed valve, part at datum, damper closed).
- Build the load envelope. Torque against angle for both directions, at the worst process condition, with the chosen margin.
- Fix the minimum air supply. The lowest reliable pressure at the actuator, measured or derived from the system.
- Decide the fail position. Fail-close, fail-open or fail-last-position, from the process risk assessment. This sets double-acting or spring-return.
- Compare candidate actuators. Check every point of the load envelope against the actuator’s output at minimum pressure, including spring end values.
- Check motion. Required stroke time, inertia and the energy the actuator’s end stops or cushions can absorb.
- Check mechanical interfaces. Shaft and mounting standard, coupling backlash, bracket stiffness and bearing loads.
- Size the valve and air supply. Solenoid valve flow, tubing, exhaust path and air consumption per cycle.
- Verify after installation. Stroke times both ways, end positions and fail action on removal of air and power.
Speed, inertia and end-of-travel energy
A rotary actuator moving a load must stop it at the end of travel. The kinetic energy at the end is half the moment of inertia multiplied by the square of angular velocity. Doubling the speed quadruples that energy. Compact swivel units in automation often have a maker’s chart of permissible inertia against swivel time; exceeding it damages end stops, vanes or pinion teeth.
Ways to manage energy include flow control on the exhaust (meter-out), external shock absorbers, end-position cushioning and choosing a larger actuator that runs at lower relative stress. For process valves, very fast strokes on large valves can also cause water hammer in the pipeline; stroke time should be agreed with the process engineer.
Stroke time also depends on how fast air can enter and leave. A small solenoid valve, long narrow tubing or an undersized exhaust silencer can slow a large actuator considerably. For large actuators and fast emergency strokes, quick-exhaust valves or volume boosters are common.
Mounting, accessories and standards
For process valves, two interface conventions simplify selection and replacement:
- ISO 5211 defines the flange and drive (square or double-D) between actuator and valve.
- NAMUR (VDI/VDE 3845) defines the interface for mounting solenoid valves directly on the actuator and the top shaft and bracket for limit-switch boxes and positioners.
Check coupling or adaptor backlash, which reduces positioning accuracy, and confirm that brackets are stiff enough not to deflect under torque. Misalignment overloads actuator bearings and causes uneven wear.
Accessories should match the duty: limit switches or a sensor box to confirm positions, a positioner for modulating control, a manual override for maintenance and corrosion-resistant finishes for chemical or coastal sites such as those around Visakhapatnam and Kakinada.
Air consumption and supply
Air consumption per stroke is the swept volume of the actuator chamber (published by the maker in litres) multiplied by the absolute pressure ratio. As a working estimate:
Free air per stroke ≈ chamber volume × (gauge pressure + 1.013) ÷ 1.013
Example: a double-acting actuator with 0.5 litres per side at 6 bar (87 psig) uses about 0.5 × 7.013 ÷ 1.013, roughly 3.5 normal litres per stroke, or about 7 normal litres per open-close cycle. Add the volume of tubing between the valve and actuator. At 10 cycles a minute this is about 70 normal litres per minute, roughly 2.5 cfm. For a single-acting spring-return actuator only the air stroke consumes compressed air.
Total demand for a plant with many actuated valves is usually small on average, but emergency shutdowns or start-up sequences can move many valves at once. Size receivers and supply lines for that coincident demand so that pressure does not fall below the minimum used in the torque selection.
Air quality protects seals and solenoid pilots. Dried, filtered air, typically from a refrigerated dryer at about +3 °C pressure dew point with coalescing filtration, suits most plant duties; instrument-air systems may specify a lower dew point.
Common problems and causes
| Symptom | Likely cause | Check |
|---|---|---|
| Valve will not open after a long idle period | Breakaway torque higher than the selection allowed; low supply pressure | Pressure at actuator during operation; valve condition; margin used |
| Spring return does not fully close | Spring set too weak, broken spring, or exhaust blocked | Spring end torque versus seating torque; exhaust path |
| Slow stroke | Undersized solenoid valve or tubing, blocked silencer, low supply | Valve flow coefficient, tubing bore, exhaust condition |
| Hard impact at end of travel | Excess speed for the inertia; no cushioning | Flow controls, inertia chart, shock absorbers |
| Air leaking from exhaust when idle | Worn piston seals or solenoid valve seals | Seal kit; air quality |
| Position indication correct but process not isolated | Coupling slip or backlash; valve damage | Coupling and adaptor; independent process check |
How Reddy Compressors helps
Rotary actuators are only as reliable as the compressed air behind them. Reddy Compressors, at Tarnaka, Secunderabad since 2002, is an independent multi-brand compressed-air specialist. We sell (new and used/refurbished), rent and service compressors from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others, and supply dryers, filters, drains and receivers. We are not an authorised dealer of any brand, and actuator selection itself is normally finalised with the valve or machine maker. We have tie-ups with all major and reputed brands.
- Measuring the real minimum pressure at actuator locations during an air audit, so torque selections use the correct figure.
- Sizing receivers and supply for coincident valve movement during shutdown and start-up sequences.
- Supplying dryers and filtration to meet the air quality actuators and solenoid valves require.
- Service, AMC and breakdown support across Telangana and Andhra Pradesh with more than 20 service engineers and technicians, plus rental compressors from our own fleet, supplied across India.
What to send us
- The application and the number of actuated valves or swivel units.
- Actuator types and sizes, with stroke volumes if known.
- Required fail positions and any emergency stroke requirements.
- Plant air pressure at the compressor and at the actuators, and any low-pressure events.
- Compressor and dryer nameplate photos and the current air treatment arrangement.
Frequently asked questions
How do I size a pneumatic actuator for a ball valve?
Get the valve maker’s breakaway, running and seating torques for your line pressure and medium, apply the margin the valve maker recommends for your service, then choose an actuator whose output at the lowest reliable air pressure exceeds those values at every point of travel. For spring-return types, check that the spring end torque also exceeds the torque needed in the fail direction.
What is the difference between rack and pinion and scotch yoke actuators?
A rack and pinion actuator delivers nearly constant torque through its stroke and is compact, which suits most small and medium quarter-turn valves. A scotch yoke delivers its highest torque at the ends of travel and less in mid-stroke, which matches large valves that need high breakaway and seating torque. Scotch yoke designs are therefore common on large-bore valves.
Should I choose a double-acting or spring-return actuator?
Choose from the required behaviour on loss of air or power. If the valve must move to a safe position automatically, a spring-return actuator gives a defined fail-open or fail-close action. If the valve should stay where it is, or a spring would make the actuator too large, a double-acting type is used, sometimes with a reserve air tank for emergency movement.
How much compressed air does a rotary actuator use?
Multiply the chamber volume per stroke by the absolute pressure ratio, then by the number of strokes. At 6 bar (87 psig) the ratio is about 7, so an actuator with 0.5 litres per side uses roughly 3.5 normal litres per stroke plus tubing volume. Average plant demand is usually small, but coincident movement during shutdowns needs to be covered.
Why is my pneumatic actuator slow to open?
Slow strokes usually come from restricted air flow rather than the actuator itself: an undersized solenoid valve, long or narrow tubing, a blocked exhaust silencer, or low supply pressure during plant peaks. High valve torque from a stiff valve also slows movement. Measure pressure at the actuator during the stroke and check the air path before replacing the actuator.
What do ISO 5211 and NAMUR mean on an actuator?
ISO 5211 is the standard for the flange and drive interface between a quarter-turn actuator and the valve, so that actuators and valves from different makers can fit together. NAMUR, defined in VDI/VDE 3845, is the interface for mounting solenoid valves, limit-switch boxes and positioners directly on the actuator. Both make replacement and spares simpler.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Rotary actuator air consumption
Free air per stroke and per open-close cycle of a rotary actuator from the maker’s chamber volume, and the consumption at your cycle rate.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Swept volume per stroke, one direction (L): 0.5
- Action: Double-acting: air drives both directions
- Tubing inside diameter (mm; 0 if none): 0
- Tubing length, valve to actuator, each port (m): 0
- Supply pressure at the actuator (bar g): 6
- Atmospheric pressure (bar a): 1.013
- Required stroke time (s; 0 to skip): 0
- Operating rate in full cycles (per period below): 10
- Rate period: per minute
- Number of these actuators in total: 1
- Number of them moving together in the worst case: 1
Result
- Swept volume per stroke: 0.5 L
- Tubing and other volume per stroke: 0 L (tubing 0 L)
- Absolute pressure ratio: 6.923 (7.013 ÷ 1.013)
- Free air per stroke: 3.46 L litres of free air at the site atmospheric pressure entered (of which tubing etc. 0 L)
- Free air per full cycle: 6.92 L (two air strokes)
- Average demand: 69.2 L/min = 4,154 L/h = 0.069 m³/min = 2.44 cfm
Method. Free air per stroke = (swept volume + tubing volume + other volume filled each stroke) × (p + p₀) ÷ p₀, with p the supply pressure in bar g and p₀ ≈ 1.013 bar a. Cylinder swept volume: extend = π/4 × D² × stroke; retract = π/4 × (D² − d²) × stroke. Tubing volume = π/4 × (inside diameter)² × length, filled once per stroke on the side being pressurised. Double-acting: a full cycle is two strokes (both directions). Spring-return or single-acting: one air stroke per cycle. Average flow needed during a stroke = free air per stroke ÷ stroke time; this is the flow the solenoid valve, tubing and regulator must pass, before allowing for pressure build-up time and margin (valve Kv ≈ 0.86 × Cv; select from the maker’s flow charts at an acceptable pressure drop). Average demand = free air per cycle × cycle rate × number of actuators + steady positioner bleed. Reserve receiver for the worst case = free air for the actuators that move together × p₀ ÷ (normal pressure − lowest pressure the actuators accept); isothermal, add margin.
What to take from it. Average actuator demand is small; shutdowns and start-ups that move many valves at once set the receiver and supply-line size.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Rotary actuator torque margin check
Check a candidate actuator’s published torques at the lowest reliable supply pressure against the valve’s breakaway, running and seating torques with your margin, including the spring torques of a spring-return unit, and the end-of-travel energy of a swivelled load.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Actuator type: Spring-return (air opens, spring closes)
- Valve breakaway torque (N·m): 100
- Valve running torque (N·m): 60
- Valve seating (end) torque (N·m): 90
- Margin on valve torque (%): 30
- Actuator air stroke start torque at minimum pressure (N·m): 210
- Actuator air stroke end torque at minimum pressure (N·m): 150
- Spring stroke start torque (N·m; spring-return only): 160
- Spring stroke end torque (N·m; spring-return only): 125
- Moment of inertia of the swivelled load (kg·m²; 0 to skip): 0
- Swing angle (degrees): 90
- Swing time (s): 1
Result
- Required torques with 30% margin: 130 / 78 / 117 N·m breakaway / running / seating
- Air stroke start vs breakaway: OK 210 N·m available against 130 N·m required (62% spare)
- Air stroke end vs running: OK 150 N·m available against 78 N·m required (92% spare)
- Spring start vs running: OK 160 N·m available against 78 N·m required (105% spare)
- Spring end vs seating (fail direction): OK 125 N·m available against 117 N·m required (7% spare)
- Result: Suitable on the torques entered confirm with the valve and actuator makers
Method. Required torque = valve torque × (1 + margin ÷ 100), with the margin the valve maker recommends for the service (often 20–50%). Double-acting: air start torque must exceed breakaway, the lower of air start and air end must exceed running, and air end must exceed seating (closing by air). Spring-return (air opens, spring closes): air start ≥ breakaway, air end ≥ running, spring start ≥ running, spring end ≥ seating, so the spring can close the valve on loss of air. Read the actuator torques from the maker’s table at the lowest pressure the actuator reliably receives, not at header pressure. Energy at end of travel = ½ × J × ω²; with ω = angle ÷ swing time this is the mean-speed value; a load that accelerates uniformly arrives at twice the mean speed and four times this energy.
What to take from it. Select at the lowest reliable pressure at the actuator, check every point of the torque envelope, and on spring-return units make sure the spring end torque closes the valve.