Pneumatic actuator

01

Start from the valve's torque signature

Cross Company lists the valve torques to collect: break (opening) torque to start a closed valve moving, running torque mid-stroke, seating (closing) torque, and the maximum allowable stem torque (MAST) beyond which the stem may deform. It notes that different valve types have different signatures: a metal-seated butterfly needs high torque to break and seat but little in between, while a metal-seated ball valve also needs elevated torque while travelling. [1]

A valve maker may publish only a minimum opening torque in water service, or only the torque at maximum conditions, without the running torque. Some media increase torque. Ask the valve supplier for the torques in your service: pressure, temperature, media and seat material. [1]

Modulating (throttling) valves need more torque than on-off valves, and stop repeatedly mid-stroke. State which service the actuator is for. [1]

Cross Company gives a 25% safety factor as an example: a valve needing 100 in-lb to open would get an actuator sized for 125 in-lb break torque. Use the factor your project specifies, and keep actuator output below the valve's maximum allowable stem torque. [1]

02

Check the operating envelope and seal options on the data sheet

A rack-and-pinion data sheet states the limits the actuator is rated for, and these should match your plant air and site. FNW's AD-Series brochure, for an aluminium-bodied rack-and-pinion range offered double-acting or spring-return, sets a minimum supply pressure of 30 psi and a maximum of 115 psi, and accepts dry or lubricated air or non-corrosive gas. [3] It ties the temperature rating to the O-ring material: standard NBR seals cover -20 °C to +80 °C, low-temperature NBR -40 °C to +80 °C, and Viton seals -15 °C to 150 °C. [3] State your lowest and highest ambient and media temperatures so the supplier quotes the right seal set rather than the standard one.

The same brochure gives a standard 90-degree rotation with external travel-stop bolts adjustable by ±5° at both the 0° and 90° positions. [3] It tabulates the air volume for opening and closing each body size in litres, and notes that air consumption depends on air supply, so ask for air volume per stroke and stroke times at your own supply pressure and with your solenoid and tubing. [3]

For maintenance, FNW ships the actuator lubricated with EP-1 grease, recommends periodic checks that fasteners remain tight, and warns that spring fatigue may set in on spring-return units, with springs always to be replaced in new full sets. [3] Ask the supplier for seal and spring kits by part number.

03

Read the spring-return torque chart at your lowest air pressure

Ramén's Wirematic torque chart for its WM/TT rack-and-pinion actuators lists, for each size, the double-acting torque at 2–7 bar and, for each spring set, the spring start and end torque and the air-stroke start and end torque at each pressure. For the WM/TT8 size, double-acting torque is 60.0 Nm at 4 bar, 82.5 Nm at 5.5 bar and 90.0 Nm at 6 bar. [4]

The chart shows the trade-off: each added spring raises the fail-safe torque and lowers the air torque left to turn the valve. With eight springs the air-stroke end torque is 2.4 Nm at 4 bar, and at 2 bar the chart prints negative values (−1.7 / −27.6 Nm). [4]

Editorial calculation: a valve needing 40 Nm to break open, with Cross Company's 25% example factor, needs 50 Nm of air start torque. The three-spring WM/TT8SR gives 66.7 Nm at 5.5 bar but 44.2 Nm at 4 bar, so it falls short if the supply can drop to 4 bar; its spring end torque of 15.8 Nm limits the seating torque it can cover to 15.8 ÷ 1.25 ≈ 12.6 Nm. [4] [1]

04

Choose rack-and-pinion or scotch yoke, double-acting or spring-return

Cross Company notes that rack-and-pinion actuators give constant torque through the stroke, while scotch-yoke actuators give reduced torque mid-stroke. Compare the actuator's published torque curve with your valve's torque signature, not one figure. [1]

DONJOY's actuator page lists horizontal rack-and-pinion and vertical stainless-steel actuators in SS304, SS316L or aluminium alloy for sanitary butterfly and ball valves. Specify body material and coating for the environment, especially washdown or corrosive atmospheres. [2]

05

Size against every point of the stroke at minimum air pressure

For spring-return actuators Cross Company defines four torque points: start of air (air torque from the normal position), end of air (air torque at full travel), start of spring (spring torque at full travel when air is removed) and end of spring (spring torque back at the normal position). Each must exceed the valve torque needed at that position; MAST must exceed every actuator output. [1]

Read the actuator's torque tables at the lowest air pressure that will reach the valve, not the compressor set-point; Cross Company warns these can differ. As a calculation example only, a butterfly valve with 80 N·m break torque and a 25% factor needs at least 100 N·m start-of-air torque at minimum supply pressure; if the valve's MAST is 300 N·m, no actuator output may exceed 300 N·m. [1]

06

Define mounting, accessories and air quality

The actuator must fit the valve's mounting pad and stem, and carry the accessories the control system needs.

Ask for the mounting flange pattern and stem drive (square or flats) matched to the valve, and whether a bracket and coupling are needed. If your plant standardises on a mounting or accessory interface (for example ISO 5211 flanges or NAMUR solenoid mounting), name it and confirm the pattern on the drawing.

List accessories: solenoid valve, limit-switch box, positioner for modulating duty, manual override, speed controls and filter-regulator. Give the air supply pressure range, quality and the ambient temperature.

07

Body, pistons and rack or yoke, pinion, springs, seals, travel stops and accessory interfaces

The body and mechanism rows carry published facts (DONJOY's aluminium or stainless bodies; Cross Company's constant versus mid-stroke-reduced torque); seal and travel-stop details are model-specific, as FNW's seal-temperature table above shows. [2] [1] [3]

08

Air pressure, torque at each stroke point, seals, materials and mounting for a quarter-turn actuator

Pressure and torque rows are sized from Cross Company's four stroke points at your lowest air pressure; temperature, air volume and seal data belong to the model quoted, as FNW's brochure shows. [1] [3]

09

Send the valve torques and air supply together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Valve: [type; size; seat material; manufacturer and model; MAST]
Torques: [break; running; seating; at your pressure, temperature and media]
Service: [on-off or modulating; cycles per day]
Function: [double-acting, spring-return fail-closed or fail-open]
Air supply: [minimum and maximum pressure at the valve; air quality]
Safety factor: [percentage required by your project]
Mounting: [flange pattern; stem drive; bracket and coupling]
Accessories: [solenoid valve; limit switches; positioner; manual override; speed control]
Environment: [ambient temperature; washdown; corrosive or hazardous area]
Documents: [torque tables; sizing sheet; dimension drawing; certification]

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