When should I choose a spring return actuator over a double acting one?

Choose spring return when the valve has to reach a defined position after air or power is lost — closing on a shutdown, opening a vent or a cooling line. If no fail position is required and the air supply is reliable, double acting gives more torque from a smaller body and uses fewer components.

Does the spring reduce the actuator's output torque?

Yes. Part of the piston force works against the spring pack, so a spring return unit delivers less working torque than a double acting unit of the same size, and is usually one size larger for the same valve. Check the drive torque and the return torque separately; the return torque is set by the spring alone.

What air pressure do OMAL pneumatic actuators need?

The standard supply is 5.6 bar, which is also the pressure the catalogue torque figures are quoted at, with a maximum of 8.4 bar. On double acting units the output torque rises and falls in proportion to supply pressure. On spring return units the return torque does not change with pressure, because it comes from the springs.

What do the numbers after DA and SR mean?

The number is a torque in Nm at 5.6 bar supply. DA90 is a double acting actuator rated 90 Nm at that pressure, and DA8 to DA1920 covers the rack and pinion range. On spring return units the number refers to the starting torque at the same pressure, so an SR unit and a DA unit with the same number are not equivalent in working torque.

Can a double acting actuator be converted to spring return later?

No. The spring pack sits inside the end caps and the body is built for it. Converting means replacing the actuator rather than adding parts. If a fail position may be required in a future revision, size for spring return at the design stage instead of changing it later.

OMAL Actuator Selection: Spring Return vs Double Acting

Pick double acting unless the valve must reach a defined position when air or power fails. Spring return guarantees that position, spends part of the actuator’s torque on its springs and needs a single air line. Fail-safe first, then torque, then air supply.

This is the order we work through whenever a plant asks us to quote an OMAL actuator, and it is the order that avoids the two expensive mistakes: buying a double acting unit for a valve that had to close on shutdown, or buying a spring return unit too small to seat the valve.

Start with the fail-safe question

Every actuator selection ends in the same place: what happens to the valve when the air or the power goes away. Answer that and the type is usually decided.

Double acting (DA) has no fail position. Air drives the piston one way and air drives it back. Lose the air and the valve stays where it was. That is acceptable when the process can be stopped and the valve operated by hand, and unacceptable when it cannot.
Spring return, normally closed (SR/NC) uses springs to close the valve when air is lost. This is the usual choice for shut-off valves on a safety loop.
Spring return, normally open (SR/NO) does the opposite and opens the valve on air loss. It is used where the safe state is an open path — cooling water, vents, some heating circuits.

If the P&ID, the safety file or the customer specification names a fail position, that settles the argument. Only when no fail position is required does the decision fall back to torque, envelope and component count.

What spring return costs you in torque

The springs are not free. In a spring return actuator part of the piston force works against the spring pack, so the torque available to drive the valve is lower than in a double acting unit of the same size. An SR actuator is therefore often one size larger than the DA unit that would do the same job on paper — worth knowing before the bracket and the space around the valve are fixed.

Two torque figures matter on a spring return actuator, and the OMAL catalogue lists both:

Drive torque — what the air produces when it has to overcome the springs.
Return torque — what the springs produce with no air at all. On OMAL spring return actuators this value depends only on the spring and is independent of the air supply pressure. Four spring settings are available, so the same body can be ordered with different return torque.

Both have to be checked against the valve. A frequent mistake is to size on drive torque alone and then discover the valve will not close. Seating a valve often needs as much torque as breaking it open, sometimes more, because the seat and seal resist differently in each direction.

Reading the DA and SR codes

OMAL sizes its rack and pinion actuators by torque rather than by bore, and the code states it:

DA plus a number — double acting, where the number is the output torque in Nm at the standard 5.6 bar supply. The range runs from DA8 to DA1920, which covers 8 to 1920 Nm.
SR plus a number — spring return, where the number is the starting torque in Nm at the same 5.6 bar.

Because the springs consume part of the output, an SR unit does not deliver the same working torque as a DA unit carrying the same number. When you size, compare the return torque row as well as the drive torque row — that is the comparison the number alone hides.

Air supply and control

The air side is where double acting and spring return diverge in cost and in behaviour.

Valve count. Double acting needs a five-port, two-position solenoid valve to drive both directions. Spring return is normally driven by a single air line, so a three-port valve is enough, with the spring providing the other direction. One less function, one less failure point on a safety loop.
Pressure behaviour. OMAL actuators run at a standard 5.6 bar with a maximum of 8.4 bar. On a double acting unit the torque rises and falls in proportion to the supply pressure: run it at 4 bar and you get roughly four fifths of the rated figure. On a spring return unit the return torque is unaffected by pressure, because it comes from the springs — which is exactly why spring return is the safer answer on a plant with an unreliable air supply.
Air quality. Filtered, dried compressed air is specified. Lubrication is not required, and if the air is lubricated at all the oil has to be compatible with the NBR seals. On a new installation it is simpler to stay with dry, unlubricated air.
Temperature. Standard units work from 0 to +80 °C, or from -20 to +80 °C with dry air. High-temperature versions reach +150 °C with a different seal compound. Check the seal material against the ambient and the media temperature rather than carrying it over from an old spare.

Mounting, interfaces and options

Actuator selection is not finished until the mechanical interface is confirmed, and here the standard does most of the work.

Valve mounting. OMAL actuators use DIN/ISO 5211 and DIN 3337 flanges, sizes F03 through F16, with a square drive onto the valve stem. A standard quarter-turn ball or butterfly valve with an ISO 5211 pad accepts the actuator directly, without an adapter.
Accessory interface. The top and side interfaces follow NAMUR, so solenoid valves, positioners, limit switch boxes and manual override gearboxes bolt on without a bracket.
Rotation. Standard travel is 90°. Adjustable versions allow about ±10° at the ends, which is useful when a valve needs to be set slightly short of full travel or slightly past it.
Body material. Aluminium as standard, with carbon steel and stainless steel versions for more aggressive environments and for washdown or coastal sites.
Hazardous areas. Accessories for explosive atmospheres are available, so an actuator for an ATEX-rated area is a configured package rather than a special order.

Double acting vs spring return at a glance

Criterion	Double acting (DA)	Spring return (SR)
Fail position on air loss	None — valve stays put	Defined — opens or closes
Typical control valve	5/2 solenoid	3/2 solenoid, one air line
Working torque at equal size	Full rated output	Lower, springs take part of it
Effect of supply pressure on torque	Proportional	Drive torque proportional, return torque independent
Return torque source	Not applicable	Springs only, four settings
Pressure range	5.6 bar standard, 8.4 bar max	Same
Rotation	90°, adjustable about ±10°	90°, adjustable about ±10°
Mounting	DIN/ISO 5211, DIN 3337, F03–F16	Same
Typically one size	Smaller for the same duty	Larger for the same working torque

Which one to choose

We supply both, so the recommendation follows the duty rather than the stock position.

Choose double acting when:

No fail position is required, or the fail action is handled elsewhere in the loop.
The air supply is stable and monitored.
You want the maximum working torque from the smallest body and the shortest face-to-face space.
The valve cycles often and you would rather not have springs in the assembly.

Choose spring return when:

A safety study, P&ID or customer specification requires a defined position on air or power loss.
The valve is on a shutdown, vent, cooling or isolation duty where the fail action is the whole point.
The plant air supply is unreliable, or the consequence of losing it is unacceptable.
The valve is unattended, on a remote skid or a site without an operator on shift.

Decide the fail direction as carefully as the fail action. SR/NC and SR/NO are different orders, and getting that backwards is not a sizing error you can correct on site. On OMAL spring return units the springs close the valve clockwise as standard, so confirm the valve’s open and closed directions against the actuator before the order goes in.

What to send for a quote

The specification that settles an OMAL actuator in one pass is short: valve type and size, the valve’s break and reseat torque, media and temperature, required fail action and fail direction, available air pressure, and the ambient conditions at the valve. Add a photo of the valve’s mounting pad and the existing actuator nameplate if there is one, and we confirm the size, the mounting kit and the accessory set together.

Browse the OMAL brand page and the wider brand directory for the valves and actuators we carry.
See the quarter-turn and pneumatic valve product range and the full product catalogue.
Read more selection and cross-reference guides.
Or send the valve details straight to the inquiry form.

Every actuator and valve we supply is original and new, sourced through the manufacturer or established channels, with procurement documentation for verification on arrival.