A linear actuator pushes and pulls along an axis. A rotary actuator turns a shaft through a defined angle, usually 90° or 180°, and holds torque at both ends. Converting one into the other with a linkage is possible and often the wrong answer, because the linkage changes the force curve in ways the specification did not account for.
This guide covers the two rotary designs, how torque behaves through the stroke, where each type belongs, and the mistake that turns a valve actuator into a maintenance item.
Two ways to make rotation
Rack and pinion. Two opposed pistons drive a rack that turns a central pinion. Torque is essentially constant through the whole rotation, because the lever arm at the pinion never changes. Rotation is typically 90° or 180°, set by internal stops.
Vane. A vane sweeps a sealed chamber. Simpler, more compact and lighter for the same envelope, but sealing a moving vane is harder than sealing a piston, so leakage rises with age and the achievable torque per size is lower.
For valve automation, rack and pinion dominates, because the constant-torque characteristic matches what a quarter-turn valve needs: highest resistance at the moment of unseating and again at the moment of seating.
Torque is not one number
A rotary actuator's catalogue torque is quoted at a stated air pressure, and it varies through the stroke on some designs and with pressure on all of them.
Three figures matter when sizing for a valve:
- Break torque, the torque needed to unseat a valve that has been closed and possibly sitting for months.
- Running torque, much lower, through the middle of the travel.
- Seating torque, to close the valve against its seat.
Size on the highest of the three, then add margin. A commonly used starting point in valve automation is a safety factor of around 1.25 to 1.5 over the valve manufacturer's stated torque, more where the medium can leave deposits, more still for infrequently operated valves that will sit closed for long periods.
A valve that has not moved for six months needs more torque than the same valve cycling hourly. Deposits, dried product and corrosion all raise break torque. An actuator sized for running torque will stall on a valve that was left shut over a shutdown.
Double-acting or spring-return
Double-acting rotary actuators use air in both directions. Higher torque for a given size, and the position on air failure is wherever the actuator happened to be.
Spring-return actuators use air one way and a spring bank the other, giving a defined fail position: fail-closed or fail-open, chosen at order time.
Choosing between them is a process-safety decision, not a cost decision. Ask what the plant needs the valve to do when air fails or power is lost, then order accordingly. The spring absorbs a substantial share of the available torque, so a spring-return actuator is larger than a double-acting one for the same duty.
Where linear still wins
Rotary actuators are for rotation. Where the job is genuinely linear, a linear cylinder does it with fewer parts and a force that is trivially calculable.
Use a linear actuator for:
- Pushing, clamping, ejecting, pressing, lifting.
- Anything where the force must be constant along a straight path.
- Anything where stroke length is the defining dimension.
Use a rotary actuator for:
- Quarter-turn valves: ball, butterfly, plug.
- Indexing and part turning through a fixed angle.
- Flap, damper and gate operation through an arc.
Converting linear to rotary with a linkage is legitimate for simple flaps and dampers, and it is where the force curve stops being constant. At the extremes of travel, the lever arm shortens and the delivered torque collapses, exactly where a valve needs the most. If a linkage is unavoidable, calculate the torque at the worst angle, not at mid-travel.
Mounting to a valve
Two standards do most of the work and they are worth knowing by name.
ISO 5211 defines the mounting flange and drive shape between actuator and quarter-turn valve. An actuator and a valve that both conform will mate without a bespoke bracket.
VDI/VDE 3845 (NAMUR) defines the interface between the actuator and its accessories: solenoid valve, limit switch box, positioner. A NAMUR-compliant actuator takes a NAMUR solenoid directly, without tubing between them.
Specifying both is what makes a valve assembly serviceable years later. An actuator bolted to a valve through a fabricated bracket, with a solenoid piped in with fittings, is an assembly nobody will be able to source a replacement for.
Accessories that decide whether it is usable
- Solenoid valve, sized for the actuator's air volume. An undersized solenoid makes a fast actuator slow.
- Limit switch box, giving open and closed feedback to the control system. Mechanical, inductive or a combination.
- Positioner, only where the valve must modulate rather than switch. A positioner on an on/off valve is money spent on nothing.
- Manual override, where the process needs the valve moved with the air off.
- Speed control, throttling the actuator's exhaust to slow the stroke. On large valves this prevents water hammer, and it is the same meter-out principle used on linear cylinders.
A specification checklist
- Motion required: genuine rotation, or a linear job that should stay linear.
- Angle: 90°, 180°, or something a standard actuator does not do.
- Torque, sized on the highest of break, running and seating, with margin for a valve that sits idle.
- Fail position on air loss, which decides double-acting or spring-return.
- Air pressure available at the actuator, since torque is quoted at a stated pressure.
- ISO 5211 flange and drive size matched to the valve.
- NAMUR interface if accessories will be fitted.
- Ambient temperature and environment, which set seal material and body protection.
- Cycle rate, which decides whether seals and bearings are sized for duty or for occasional use.