Rotary Pneumatic Actuator Selection

1. Engineering answer: match torque, travel and the complete operating duty

A rotary pneumatic actuator should be selected by comparing the torque required by the driven equipment with the actuator's supported output at the relevant angle, direction, pressure and operating condition. The comparison must also address travel, movement time, inertia, stopping energy, bearing loads, mounting, air consumption and the required response when air or electrical power is lost. A single torque value cannot establish all of those conditions.

It also requires different starting, running and seating conditions to remain visible where they matter.

The practical selection sequence is to define the driven task, construct the load envelope, establish the available pneumatic conditions, compare candidate output through both directions of travel, then check motion, mechanical interfaces and abnormal states. The selected assembly is subsequently verified against its actual duty. An attractive nominal torque is a starting clue, not a completed selection.

A useful result may be a technically suitable candidate with specific open checks. It may instead identify a missing valve-torque curve, an excessive inertia, an inadequate exhaust path or an unsupported fail-position claim. Making those findings explicit is more valuable than assigning an actuator size from incomplete inputs. The examples in this document are fictional teaching cases, not manufacturer ratings, field measurements or recommendations for a real machine.

2. Identify the rotary task before selecting the mechanism

Limited-angle actuation includes swivelling a workpiece, turning a diverter, indexing a fixture between defined positions or operating a quarter-turn process valve. These boundaries prevent one product family from being used as a substitute for every rotational task.

Describe the movement at the driven member. State its initial and final orientation, required angle, direction of travel, working load and what must be true when motion ends. An actuator shaft reaching a stop is not necessarily proof that a flexible coupling has positioned the workpiece correctly. A valve position indication is not necessarily proof of the required process isolation.

Separate on/off movement from modulating or multi-position control. An actuator suitable for occasional endpoint movement may not have the friction, feedback, cycling capability or control arrangement needed for repeated small corrections. A timed air pulse is not, by itself, a positioning system. Likewise, selecting an intermediate-position accessory does not define the accuracy of the complete mounted mechanism.

The specification should describe both a productive movement and an acceptable stopped condition. During a dwell, the load may remain supported by another structure, require continuous torque, or need a separately engineered retention function. Those are different duties. Keep the functional description independent of the preferred catalogue solution until the necessary evidence supports that solution.

3. Create a duty brief with separate mechanical and pneumatic boundaries

Give the application, driven equipment and actuator candidates stable identifiers. Retain the source of each requirement: equipment drawing, process specification, supplier declaration, measurement or clearly labelled planning assumption. A value copied from a previous installation should not become a verified requirement merely because its units look appropriate.

Define the minimum, normal and maximum operating cases. The lowest available dynamic pressure may occur when several users operate together. The greatest mechanical resistance may occur after a long idle period, at a particular product condition or near an endpoint. The quickest required movement may not coincide with the largest load. Preserve those combinations as named cases rather than blending unrelated maxima into an unexplained design number.

Specify measurement locations early. “Five bar available” should identify whether the value is at the plant header, the regulator outlet or the actuator inlet while moving. Similarly, state whether torque is required at the actuator shaft or after a linkage. These boundaries determine which losses and transformations belong in the comparison.

4. Compare rack-and-pinion, vane and scotch-yoke arrangements

The mechanism changes how pneumatic force becomes shaft torque and how the actuator fits the application. It does not remove the need for a supported output curve. Festo identifies rack-and-pinion and vane arrangements within its rotary-drive families, with different cushioning and accessory choices. These are examples of mechanism characteristics, not universal performance guarantees for every product. [R1, R3]

Do not assume a nominally constant torque from every rack-and-pinion actuator or a suitable mid-travel torque from every scotch-yoke actuator. Friction, spring forces, geometry, pressure and the supplier's definition all matter. The appropriate comparison is between documented output and the actual required load case.

Packaging can be decisive even after torque is adequate. Through-shaft routing, moving hose clearance, access to stops, mounting orientation and the space needed for accessories should be reviewed in the real layout. A physically compact actuator can still create an inconvenient or highly loaded bracket. Record these consequences in the candidate matrix rather than ranking mechanisms by size alone.

5. Read torque terminology by position and direction

Maintain two distinct records: the driven equipment's required torque and the actuator's available output. Supplier tables can use similar abbreviations for both. A value labelled “break torque” must identify whether it is the resistance that must be overcome or the output offered to overcome it.

For valve-type duties, the following labels form a useful reading structure. Bray's sizing interface distinguishes opening and closing torque requirements and separate pneumatic and spring outputs, including intermediate minima.

The labels are not permission to assume that the largest resistance occurs at the seat or that the weakest output always occurs at the end. Use the actual driven-equipment and actuator profiles. For a fixture, replace valve terminology with the relevant contact, rotation and release stages.

Keep supplier angle conventions intact and map them into the project convention explicitly. “Start of spring” usually describes the start of the return stroke, not the same physical position as “start of air.” A comparison that reverses those positions can appear favourable while matching the wrong ends of the curves.

6. Construct the load torque from identifiable contributions

A force acting at a perpendicular distance from the rotation axis creates torque. The general vector relationship is torque = position vector × force; its magnitude is F r sin(phi). For a force perpendicular to a lever, this reduces to F r. Use metres and newtons to obtain newton-metres. A force that remains constant while its direction changes relative to the lever can produce a changing torque.

For a rigid assembly about a fixed axis, a useful signed equation is:

tau_actuator + tau_external = J_total alpha.

Here tau_external contains the signed gravity, process and friction contributions assigned to the external load. An equivalent required-output expression is J_total alpha − tau_external. Define the positive direction and do not count the same resistance twice.

Breakaway friction, moving friction and contact torque need separate consideration. A measured slow manual turn may help characterise resistance, but it does not reproduce acceleration, operating process pressure or every temperature condition. A load that assists the first half of a movement can oppose the return. An assisting load may also increase stopping duty instead of reducing the overall selection requirement.

For a valve or other supplier-defined mechanism, use the manufacturer's required-torque information for the actual configuration and operating service. Bray's ball-valve selection bulletin explicitly relates torque to factors such as seating material, medium, line pressure and operating frequency. Its own factor-combination procedure must be retained when it applies; it should not be replaced by an unrelated multiplier. [R5]

Document which torque contributions are already included in a supplied requirement. If a quoted torque already includes the applicable process and service allowance, adding those same terms again creates an artificial requirement. Conversely, omitting an unrepresented linkage or bearing resistance understates the load at the actuator shaft.

7. Establish available pressure at the actuator and preserve the output basis

Record the minimum dynamic inlet pressure for each direction, not only the compressor setpoint. Pressure can be lost through preparation equipment, the directional valve, tubing, fittings and moving connections. Record relevant exhaust backpressure as well.

Match the candidate data to those conditions. Identify whether published torque is theoretical, effective, minimum guaranteed, nominal or measured under a stated test. A theoretical piston-force calculation and a manufacturer's effective output are not interchangeable. If the published output already includes mechanism losses or spring opposition, do not subtract an assumed efficiency or spring torque again.

For an idealised rack-and-pinion illustration, shaft torque is the sum of effective rack forces multiplied by their pitch radius. That relationship can explain why changing pressure changes output. It does not authorise linear scaling of a complete spring-return actuator's net torque. Spring preload, changing geometry and friction can make that shortcut particularly misleading. Use the supplier's supported pressure rows or approved interpolation. [N1]

Minimum-pressure selection and maximum-pressure checking are separate. The first asks whether enough torque is available. The second asks whether the actuator pressure rating and the driven assembly's permissible torque can be exceeded. A regulator setting does not by itself establish what pressure is possible under every configuration or fault; the responsible designer must define the credible cases.

A static no-flow reading can be retained as context but must not be presented as proof of dynamic pressure. If movement pressure is unknown, preserve an open input and request the relevant measurement or supplier-confirmed supply assumption. An unmeasured low-pressure condition cannot be repaired by extra decimal places in the torque calculation.

8. Build the actuator torque and duty matrix

Each row should retain the source requirements, any explicit allowance, the candidate output and a separate maximum-permissible check. Do not replace the row results with one average margin.

For a row with positive design-required torque:

• Torque margin = available output − design-required torque
• Coverage ratio = available output / design-required torque

A negative margin means that the candidate fails that row's arithmetic comparison. A positive margin means only that the stated output exceeds the stated requirement. It does not establish an acceptable movement time, mounting arrangement, full-travel coverage or protective function. When the requirement is zero, the ratio is undefined for this worksheet; retain the difference and explain why no ratio is shown. [N2]

A finite list of angles is a sampling of the duty envelope. It is sufficient for full-travel acceptance only when the supporting data or an approved analytical envelope covers the intervening positions. A generous endpoint margin cannot compensate for an unknown or inadequate intermediate output.

Keep candidate identities complete. A different spring cartridge, yoke orientation, drive insert or accessory package can change the relevant selection. Do not combine a favourable torque row from one configuration with mounting or consumption data from another.

9. Use allowances deliberately and check the upper torque boundary

An allowance can address a specified service condition, documented variability or a supplier's prescribed selection method. It should have an owner, scope and revision. The fictional factors used in worked examples are teaching assumptions and are not recommendations.

Show the unadjusted requirement and the adjusted requirement separately. Where the governing method uses a sum of factors, preserve that method; where it uses a multiplier, preserve that instead. Applying several overlapping allowances without checking their definitions can lead to unnecessary oversizing. Removing an applicable allowance merely because the candidate fails is equally indefensible.

Excess torque can be a problem. The driven shaft, valve stem, key, coupling, bracket, stop or workpiece may limit the acceptable output. For process valves, obtain the applicable maximum allowable stem torque and its conditions. Do not invent that value from valve size. For a machine fixture, identify the corresponding mechanical limits from the design evidence.

Check upper output at the credible maximum pneumatic pressure and the relevant spring or stalled condition, not only at the minimum sizing pressure. Where an operating regulator or another protective arrangement is part of the proposed solution, its design, reliability and verification belong to the actual project. The worksheet should record that requirement without claiming that software arithmetic makes the arrangement safe.

A practical selection is therefore constrained on both sides: enough available torque to perform the duty, but no unacceptable mechanical overload in the relevant cases. “Choose the next larger actuator” is not a universal answer when the upper boundary is already limiting.

10. Evaluate double-acting operation in both directions

A double-acting actuator uses supplied air to drive both commanded directions. Check both pressure paths and their relevant output conditions. Symmetrical geometry does not automatically make the installed duty symmetrical: the driven load, exhaust routing, tubing, gravity and end requirements can differ. For a valve, opening and closing resistance may be different. For a fixture, a workpiece may be present in only one direction.

Record how the actuator is expected to behave during a dwell. A blocked control-valve state may retain air but does not establish indefinite position holding. Leakage, external forces, elastic movement and changes of pressure must be considered in the physical design.

Likewise, loss of air does not automatically mean that a double-acting actuator remains safely at its last position. The whole circuit and driven load determine the result. If a maintained position is required, define the permitted movement, retention duration, monitoring and any separate mechanical or pneumatic arrangement that must provide it.

Check restoration of supply as its own event. A valve can remain in a state that sends air to a chamber before the sequence is ready. Retained commands or an unreconciled mechanical position can produce movement.

The selection record should not merely say “double acting: fail last.” It should identify the actual specified loss-of-energy response and the evidence or unresolved design work behind that claim.

11. Evaluate spring-return operation throughout air and spring strokes

For a spring-return actuator, the powered stroke must move the driven equipment while loading the spring system. The return stroke must perform the required movement using the available spring output under its applicable conditions. Those are separate output envelopes. A successful air-stroke calculation does not prove that the return stroke can complete seating, release or another required endpoint action.

Request the exact spring configuration and the supplier's definitions of air-start, air-end, spring-start and spring-end output. Where the mechanism has an intermediate minimum, include it. Bray's sizing interface exposes separate pneumatic and spring minima; Rotork's yoke description reinforces why the shape of the curve matters. Do not assume that all spring-return products have the same torque profile. [R3, R4]

Spring torque is not an extra quantity to subtract from a table that already reports net air-stroke output. Confirm the definition first. Similarly, a spring-output table normally represents specified conditions for the exhausted or returning side; an obstructed exhaust or an undocumented pressure can alter installed behaviour. The physical return path must therefore be part of the selection and later trial.

Consider loss of supply partway through travel as well as at the fully operated position. The starting spring compression, load position and subsequent return path differ. A return requirement from every allowed position needs corresponding evidence, not just one successful endpoint-to-endpoint demonstration.

Selecting more springs is not a one-direction improvement. It can increase return capability while reducing net powered-stroke capability at the same air pressure. Any change to the spring set requires the complete matrix to be rerun using supported configuration data. Spring assemblies store mechanical energy; this document gives no spring-disassembly or preload-adjustment procedure.

12. Define travel, direction and end positions without ambiguity

Record the rotation datum and the viewing direction used to describe clockwise and counterclockwise movement. A technician looking from the opposite end of a shaft sees the opposite apparent direction. Connect the actuator convention to the actual open, closed, located or released equipment condition in a drawing or controlled description.

Distinguish nominal actuator travel, adjustable travel range and usable travel in the assembled mechanism. An actuator advertised for a particular angle may need different accessories, stops or adjustment for the required application. Check that the adjustment does not conflict with the driven equipment's endpoint requirements or the intended cushioning region.

For linkages, inspect the motion throughout travel. Torque amplification can change as the effective lever arm changes; a near-toggle position may introduce very high joint forces or poor motion authority. A simple perpendicular-force calculation at one position does not cover that geometry. Use the actual kinematic relationship and mechanical review.

For direct-mounted valves, reconcile stem position, actuator orientation, drive insert and travel-stop setting. A position indicator can be correctly aligned with the actuator while the valve connection is misassembled. Acceptance should verify the driven member rather than rely solely on the accessory indication.

Specify acceptable end behaviour: contact with a supported stop, controlled deceleration, seating within an allowed torque range, or confirmed location before the next machine action. Precision, repeatability and settling time should each have an explicit meaning. They are not interchangeable promises, and none should be inferred merely from a catalogue angle label.

13. Calculate inertia and acceleration on a declared motion model

Mass moment of inertia describes how the mass is distributed around the rotation axis. For point-like masses, J = sum(m r²). For a solid disc about its centre, J = m R²/2. For an offset rigid part, the parallel-axis relationship is J_axis = J_centre + m d². Add the fixture, product, gripper and other rotating contributions about the same axis. Use kg·m²; do not confuse this with an area moment used in structural calculations. [N1]

If the assembly has a transmission, define the speed ratio before referring inertia between shafts. For an ideal ratio n = actuator speed / load speed, the load inertia referred to the actuator is J_load/n². Gear and coupling inertia, losses, backlash and supported operation remain additional considerations. For a direct-mounted actuator, do not insert a fictitious gearbox efficiency or ratio.

A rest-to-rest triangular angular-velocity teaching profile over angle theta in total time t gives:

alpha = 4 theta / t²; omega_peak = 2 theta / t; tau_inertia = J alpha.

Use radians and seconds. This is an explicitly chosen kinematic model with equal acceleration and deceleration times, not a prediction of an unrestricted pneumatic stroke. SMC's rotary selection guide uses its own load-dependent selection multipliers and angle/time approximations. Those product-selection rules remain separate from this original mechanics illustration. [R2; N1]

The purpose is to reveal sensitivity. Halving a specified motion time multiplies the triangular-profile acceleration by four. Moving the same product farther from the shaft can greatly increase inertia. Those changes can invalidate a previously adequate actuator even without a change in product mass. Final response must be checked with the selected actuator, valve, flow controls, pressure and load.

14. Check stopping energy, permitted speed and repeated duty

Torque capability is not stopping capability. A rotating assembly carries kinetic energy E_k = J omega²/2. The allowable energy and movement-time range of the selected actuator or cushioning arrangement must come from its applicable data. SMC's selection process separately checks inertia, torque, rotation time, kinetic energy and imposed loads; passing one check does not replace the others. [R2]

For a defined stopping interval, an energy balance can include kinetic energy entering the interval plus work added by an assisting external load or continuing pneumatic drive, less separately demonstrated energy removed elsewhere. With a constant net assisting torque tau_a over a stopping angle delta_theta, the added work is tau_a delta_theta. Both quantities must refer to the same motion direction and interval. [N1]

A mean resisting torque calculated as stopping energy / stopping angle is not a peak stop-load rating. Real force and torque histories can be uneven, and the actuator's internal geometry can transfer loads differently from an external stop. Do not size a shaft, bracket or shock absorber from that mean alone.

For repeated motion, consider both energy per event and events per unit time. A device able to absorb one event may have a lower permitted repeated-duty capability. Record the actual duty definition and cooling or recovery requirements from its manufacturer. Do not convert an energy value in joules directly into a permitted number of cycles without that evidence.

Use the speed at cushion entry or the selected stopping event, not automatically the average speed over the whole stroke. For the ideal rest-to-rest profile, the final target speed is zero; its peak kinetic energy is an intermediate energy state. Keeping these definitions distinct prevents a motion calculation from assigning the wrong energy to an endpoint.

15. Treat shaft, bearings, coupling and bracket as selection constraints

The rotary output shaft transmits torque, but it may also experience axial force, radial force and an overturning moment. The permissible combination depends on the specific product and where the load acts. A long overhung fixture can exceed a bearing or mounting limit while its torsional demand remains modest.

Record the distance from the bearing reference to each external load. An apparently small side force can create a significant bending moment at a large offset. Do not assume that a torque rating is also a bending-moment rating, even though both can be expressed in newton-metres. They represent different load modes and require different supporting data.

Check the drive interface: shaft shape, key or spline, engagement length, coupling, fasteners, alignment, bracket stiffness and access for assembly. Matching a mounting flange does not by itself establish compatible torque transmission or permissible loading. An adapter can introduce extra clearance, flexibility or leverage that must be included in the assembly review.

For valve duties, obtain the applicable valve-stem and mounting-kit limitations. For handling equipment, include product-support and bearing arrangements. Where external support is needed, design it so that misalignment does not force the actuator shaft to become the primary structural guide.

Specify acceptable backlash, torsional deflection and output-position behaviour at the actual driven member. A sensor mounted on the actuator can report an endpoint while clearance or elasticity prevents the load from reaching the required condition. These mechanical checks should stay open until evidence is available, rather than being absorbed into a general torque-margin number.

16. Match the valve, tubing and exhaust to the required response

The complete air path determines how rapidly pressure can build and decay in the actuator chambers. Record the directional valve, pilot arrangement, flow controls, manifold passages, tubing length and internal diameter, connectors and exhaust devices. A nominal connection thread is not a flow-performance specification.

Separate electrical switching time, chamber pressurisation, mechanical travel and feedback confirmation. A data sheet reporting a bare-actuator stroke time under a particular valve and supply arrangement cannot automatically predict the installed assembly's end-to-end response. The timing requirement should say which start and finish events it measures.

Speed control should be selected for the actual load behaviour. Restricting exhaust can affect pressure difference, torque and motion stability as well as time. An assisting load may behave differently from a resisting load. A quick-exhaust accessory changes the physical circuit and its noise, release path and stopping behaviour; it is not a generic cure for every slow actuator.

For spring-return equipment, the return movement needs a suitable discharge or breathing path under the intended conditions. Protecting that path from blockage and inappropriate contamination belongs in the design. A spring does not eliminate the effect of the air trapped ahead of its movement.

Use supplier sizing tools or supported flow data for final component selection and verify the installed response under relevant pressure and load cases. The reference-volume-per-stroke divided by stroke time is a useful average flow over that movement; it is not the instantaneous peak flow, a valve conductance calculation or proof that the target time will be achieved.

17. Estimate air consumption without confusing it with torque or speed

Start with the manufacturer's consumption declaration when it matches the exact actuator, angle, pressure and cycle definition. Determine whether the value represents one powered stroke, one complete out-and-back cycle, chamber displacement or an already converted reference-air quantity. Do not multiply a stated reference-air consumption by a pressure ratio again.

For a transparent screening model, a chamber's swept volume V_s at approximately constant absolute pressure p_s and temperature T_s contributes reference volume:

V_s,ref = V_s (p_s / p_ref) (T_ref / T_s) (Z_ref / Z_s).

For a fixed dead volume V_d raised from residual state r to final state f, the additional reference-equivalent mass is:

V_d,ref = V_d (Z_ref T_ref / p_ref) [p_f/(Z_f T_f) − p_r/(Z_r T_r)].

These expressions are newly presented gas-mass accounting relationships, consistent with the series' explicit pressure and reference-basis discipline. They are not an actuator manufacturer's consumption test method. Swept-volume and fixed-dead-volume definitions must not overlap. [S1; N1]

Sum only the powered chambers and events actually supplied. A complete double-acting cycle normally includes two powered movements. A spring return does not automatically add a second air-powered stroke, although pilot demand, purging or other supplied functions may add consumption. Include leakage and other losses separately when the agreed boundary requires them.

Average reference flow is reference volume per cycle multiplied by cycles per unit time. For plant demand, also retain the timing and simultaneity of events. A small hourly average can coexist with a substantial short-duration requirement at the valve. Lowering pressure may reduce a screening consumption figure but is acceptable only when torque, response and the complete operating requirement remain satisfied.

18. Specify air quality, environment and maintainability for the actual location

The installation environment can narrow the candidate set before torque is compared. Record ambient and equipment temperatures, water exposure, corrosive agents, dust, washdown, outdoor weather, product-contact restrictions and any application-specific hazardous environment. Keep these as requirement fields and obtain the selected assembly's applicable evidence. A broad product-family description does not establish every accessory's suitability.

The air-quality requirement belongs at the actuator or relevant consumer boundary. Filtration, drying and lubrication should follow the actual equipment and process needs rather than a habitual “fit an FRL” instruction. A lubricator appropriate to one device may conflict with an adjacent quality-sensitive use.

Include silencers, breathers and exhaust routing in the environmental review. An accessible, replaceable accessory can still become a performance restriction as it becomes contaminated. Maintenance criteria should relate to the exact product and observed condition, not an invented universal replacement interval.

Check access for mounting bolts, adjustment, position sensors, tubing and the identification label. A selection that fits only before the machine enclosure is installed can create difficult future maintenance. Preserve the space needed for the approved removal or service method, especially around stored-energy spring modules.

Maintainability also means configuration control. Record the exact actuator and spring set, seals or temperature option, accessory identities and approved replacements. A physically interchangeable replacement can change torque, direction, response or environmental capability. The selection record should make those controlled attributes apparent to both engineering and procurement.

19. Select feedback and positioning around the required evidence

Specify what must be known: a commanded state, an actuator endpoint, the driven member's angle, contact with a stop or the success of a process action. These are different signals. A limit switch confirms its own detection condition; it should not silently acquire the meaning of measured torque or verified valve sealing.

For on/off operation, define the accepted endpoint signals, their relationship to physical positions and the response when neither or both are indicated unexpectedly. Include detection tolerance, adjustment, wiring, signal validity and the effect of a failed or disconnected sensor.

For modulating or intermediate-position duty, identify the feedback variable, controller or positioner, required resolution, accuracy, repeatability, settling time and disturbance conditions. Pneumatic compressibility, friction and the driven mechanism influence the response. A catalogue accessory labelled “positioner” is not a complete performance specification.

A remote supervisory value can be useful without being suitable for rapid motion control or a protective function. Keep the data path, update rate and permitted use explicit. Do not assign a safety role to an ordinary website selection field or monitoring signal.

Feedback should be verified on the completed assembly after adjustments. If the output linkage is changed, the earlier relationship between actuator position and driven-equipment state may no longer hold. Retain the as-left configuration and the acceptance evidence rather than recording only the sensor part number.

20. Define loss-of-energy behaviour, isolation and restart separately

Write separate cases for loss of main air, low air during movement, loss of electrical power, loss of command, loss of feedback and restoration of each service. The required response may differ. The state table must identify what the load can physically do, not merely which software output becomes false.

Spring-return construction is a candidate means of achieving a defined return, not proof that every attached machine is safe. The load, spring output through travel, exhaust path, permitted movement and final condition must all be appropriate. A double-acting arrangement similarly requires a designed retention or alternate response where the application demands it. [S1, S2; N2]

A commanded stop is not the same as energy isolation. Air can remain trapped downstream of valves, while a spring or external load can store mechanical energy. Safe maintenance and access require the site's approved arrangements and competent assessment of those sources. This article does not provide instructions for dismantling a spring pack or defeating a protective device.

Restart should require reconciliation of physical state and valid prerequisites. Restoration of pressure should not be treated as permission to resume a movement from an assumed position. The approved circuit specification should determine the preparation and deliberate start sequence.

Keep ordinary production acceptance, abnormal-state verification and safety-function validation distinct. A torque spreadsheet can document capability comparisons but cannot assign a safety integrity level, approve an emergency action or certify an assembled machine. Those decisions remain with the responsible project disciplines and their required evidence.

21. Worked example 1: endpoint torque hides a mid-travel shortfall

Fictional teaching case. A driven rotary mechanism has the following accepted opening torque requirements. For this example only, the project has selected a 1.25 multiplier on the raw values. Two candidate output sets are assumed to be supplier-supported at the same minimum dynamic pressure and temperature. No actual product is represented.

Candidate A exceeds the adjusted requirement at both endpoints but fails the intermediate comparison. Its minimum coverage ratio over these rows is 80/87.5 = 0.914286. Selecting from its largest torque value would conceal the shortfall. Candidate B clears the three stated rows; its smallest coverage ratio is 100/87.5 = 1.142857.

These are pointwise comparisons, not proof of full-travel suitability. Before accepting Candidate B, the reviewer must establish the intervening curve envelope, closing-direction requirements, maximum-pressure output, shaft and mounting limits, movement time and abnormal response. Three favourable rows do not create missing evidence elsewhere.

Suppose the driven mechanism has a separately documented maximum permissible transmitted torque of 240 N·m, and the synthetic maximum-pressure checks give Candidate A an output of 238 N·m and Candidate B an output of 260 N·m at a relevant stop condition. A has 2 N·m nominal upper margin but already fails the minimum-capability test. B has sufficient output at the listed minimum-pressure points but exceeds the upper boundary by 20 N·m.

The correct outcome is not “B selected.” It is that neither candidate has demonstrated suitability across the stated cases. A different configuration or a properly engineered pressure-control arrangement may need assessment. No regulator setting or protective architecture is prescribed by this fictional comparison.

22. Worked example 2: spring return needs six matched checks

Fictional teaching case. Consider a spring-to-close assembly with approved raw opening requirements of 80, 40 and 35 N·m at start, intermediate and end positions. Closing requirements are 45, 30 and 70 N·m. The example uses an explicitly assumed factor of 1.20, applied once to those requirements. The candidate data below are invented net output values for one pressure and configuration—not quantities to be reduced again by subtracting springs.

Set X fails the final spring-closing comparison even though it has considerable margin earlier in the return. Set Y clears that closing requirement but fails the final powered-opening comparison. Its stronger spring set is not an overall cure. Both candidates therefore remain unsuitable for the complete stated six-row duty.

A third fictional combination has outputs of 120, 82, 50, 135, 104 and 90 N·m in the same row order. Its margins are 24, 34, 8, 81, 68 and 6 N·m. It passes the six numerical comparisons, with the smallest ratio 90/84 = 1.071429. That identifies a candidate for the remaining checks, not an approved assembly.

The pressure basis matters. Raising supply pressure would not directly increase the mechanical spring-return torque of an unchanged fully stroked spring configuration. It may also create an upper-load or pressure-rating problem. Use the manufacturer's actual configuration data rather than scaling net spring-return rows by a gauge-pressure ratio.

Finally, verify the required return from allowed intermediate positions, the discharge path, completion time, mechanical limits and final driven-equipment state. The example assumes only three checkpoints in each direction; it does not claim continuous-curve coverage or safety validation.

23. Worked example 3: a faster swivel changes torque and stopping duty

Fictional mechanics illustration. An assembly has a fixture inertia of 0.0500 kg·m² and a compact 2.0 kg load represented as a point mass 0.18 m from the shaft. The total inertia is 0.0500 + 2.0 × 0.18² = 0.1148 kg·m². It turns through 90°, or pi/2 radians, about a vertical axis. A stated 0.8 N·m resisting torque is assumed; gravitational torque about that axis is excluded by this simplified geometry.

For an ideal rest-to-rest triangular velocity profile completed in 0.60 s, alpha = 4 × (pi/2)/0.60² = 17.453293 rad/s². Peak speed is 2 × (pi/2)/0.60 = 5.235988 rad/s. Inertial torque is 0.1148 × 17.453293 = 2.003638 N·m. During acceleration against the specified resistance, the corresponding output requirement is 2.803638 N·m before any applicable product-selection allowance.

At a proposed 0.30 s movement, acceleration becomes 69.813170 rad/s² and peak speed becomes 10.471976 rad/s. Inertial torque becomes 8.014552 N·m; the acceleration-plus-resistance requirement is 8.814552 N·m. Peak kinetic energy increases from approximately 1.573654 J to 6.294614 J. The timing change doubles peak speed and quadruples both inertial torque and peak kinetic energy.

That peak energy is not automatically the energy entering the normal endpoint cushion. The ideal profile decelerates to zero at the final position. To illustrate a separate stopping event, suppose actual speed at entry to a 10° stopping zone is 4.0 rad/s, and the net continuing drive after already accounting for resistance assists the movement by a constant 0.8 N·m. Entry kinetic energy is 0.9184 J. Added work is 0.8 × pi/18 = 0.139626 J, giving 1.058026 J to be absorbed in the stated simplified boundary.

Mean resisting torque over that zone is approximately 6.062044 N·m. It is not a peak-load rating. The selected stop or cushioning arrangement still requires its own permissible-energy, load, speed and repeated-duty checks. These calculations describe assumed mechanics; they do not establish the actual pneumatic velocity profile or select a manufacturer model.

24. Worked example 4: cycle consumption is not movement-flow capacity

Fictional gas-accounting illustration. A double-acting rotary actuator has assumed swept volumes of 0.180 L in the outward direction and 0.160 L on return. Fixed dead volumes, not included in those swept volumes, are 0.020 L and 0.015 L. The supplied chamber pressure is assumed constant at 6 bar gauge with a 1 bar absolute atmosphere and a 1 bar absolute reference. Gas and reference temperatures are equal, compressibility factors are one, and each dead volume starts at 1 bar absolute.

The outward stroke's reference consumption is 0.180 × 7 + 0.020 × (7 − 1) = 1.380 L. The return stroke contributes 0.160 × 7 + 0.015 × 6 = 1.210 L. A complete cycle therefore uses 2.590 L reference air within this defined model. Pilot consumption, leakage, extra volumes and real pressure trajectories are excluded.

At 20 complete cycles per minute, average demand is 51.8 L/min, or 0.0518 m³/min. If the outward movement alone must occur in 0.25 s, dividing its reference volume by that time gives 331.2 L/min averaged over that movement. This is already much larger than the cycle average, yet it still does not determine the instantaneous peak or the required valve conductance.

Six identical units with the same cycle rate would average 0.3108 m³/min. If their outward movements overlap, the sum of the movement-average figures is 1.9872 m³/min over the common 0.25 s window. Actual dynamic sizing must use the relevant pressure histories and component data; it cannot rely only on those averages.

Reducing the assumed chamber pressure to 5 bar gauge gives cycle consumption 0.340 × 6 + 0.035 × 5 = 2.215 L, a calculated reduction of 14.478764%. This is an air-quantity comparison under unchanged assumptions, not a verified electricity saving or permission to lower operating pressure. The complete torque and response matrix must still pass at the lower pressure.

26. Commission the assembled duty and diagnose changes systematically

The trial should address the actual driven equipment with its final mounting, valve, tubing, exhaust and feedback. Define the permitted test conditions with the responsible machine or process owner. Do not improvise an unsafe movement or expose people to an unsupported rotating load merely to measure a response time.

Observe the relevant minimum dynamic pressure case, maximum credible output case and required movement pattern. Record which conditions were demonstrated and which remain untested. A light-load workshop trial can establish basic operation without proving a loaded production cycle or a process-valve torque condition.

Treat these as hypotheses, not automatic diagnoses. A changed load or obstructed connection can mimic inadequate actuator size. Record what changed before replacing the actuator or raising common pressure.

Closeout should contain the accepted duty, the completed matrix, remaining limitations, exact assembly identities and baseline observations. Reopen the assessment after changes to product mass, lever arm, rotation time, spring set, pressure, valve, tubing, stops or protective response. Preserve earlier versions rather than silently revising the record of an already issued decision.

27. Frequently asked engineering questions

Is the largest catalogue torque enough to select a rotary actuator?

No. It may describe one angle, pressure or configuration. Compare the required and available torque at corresponding positions in both directions, then check motion, stopping energy, mechanical limits and abnormal conditions. A strong endpoint output can coexist with inadequate intermediate capability.

How is a rotary actuator different from an air motor?

This page concerns limited-angle movement between defined positions. Similar torque units do not make their duty definitions interchangeable.

Does a larger actuator always provide a better result?

No. It may increase possible transmitted torque, air consumption, moving loads or impact. The selected assembly must meet the minimum functional requirement without exceeding the relevant mechanical and pressure limits. A bigger unit can also require a different valve or stopping arrangement.

Can one air-start torque prove a spring-return selection?

No. Powered and spring-return outputs must be checked against their corresponding loads. Start, intermediate and end conditions can matter in each direction. Use the exact spring configuration and identify whether listed powered output is already net of spring opposition.

Can higher supply pressure fix inadequate spring-end torque?

Not directly for the mechanical return torque of an unchanged fully stroked spring configuration. Higher pressure can affect the powered stroke and may introduce overload. Reassess the configuration with supplier data rather than treating a pressure increase as a universal solution.

Should every actuator use the same selection factor?

No. Apply the governing supplier or project method with its actual scope. Keep existing allowances visible and avoid counting the same service condition twice. The factors in this document's examples are fictional assumptions, not a default for field selection.

Why is load inertia needed when the static torque is small?

Accelerating and decelerating the rotating assembly requires torque, and a moving load carries energy that must be controlled when stopping. Inertia depends on mass distribution about the shaft, not just total mass. A faster movement or a longer offset can materially change the duty.

Does a spring-return actuator automatically create a safe machine?

No. The complete load, return path, output envelope, final condition and control response need competent assessment. Spring construction is one component characteristic. It does not, by itself, validate the required protective function or permit access to stored energy.

Can cycle air consumption determine the valve size?

Not by itself. It supports a consumption estimate. Dynamic flow, pressure change, tubing, exhaust, load and required movement time determine the response problem. Even volume divided by movement time is only an average over that movement, not a complete valve-sizing calculation.

What information should be sent for a selection review?

Send the driven-equipment identity, torque-versus-position requirements, angle, direction, pressure range, movement time, cycle pattern, inertia or load geometry, mounting and permitted load information, required failure response, and available supplier data. Mark assumptions and missing inputs explicitly.