Electric actuator selection starts with four numbers — required thrust (N) or torque (Nm), stroke length (mm) or rotation (°), cycle frequency (cycles/hr), and bus protocol (4-20 mA + HART, PROFIBUS PA, Foundation Fieldbus, EtherCAT, IO-Link) — because every downstream decision hangs on them [S1].
The ISA75.24 subcommittee on Control Valve Actuator Sizing & Selection formally covers sizing equations for pneumatic, hydraulic, and electric actuators using piston or effective diaphragm area plus spring characteristics, and that scope is what most working engineers still anchor their spreadsheet math to [S1]. For motion-control applications, the same logic collapses to a force-balance check: motor continuous torque ≥ load torque × service factor, with thermal duty verifying the driver does not exceed its rated insulation class.
Step 1 — Quantify the Load Before You Touch a Catalog
Load quantification means capturing static force (mass × gravity + seal friction), dynamic force (acceleration × mass + external process loads), and holding force (back-pressure or spring return) at every position across the stroke [S1]. The ISA75.24 method explicitly identifies forces acting on closure components and how those forces interact with the actuator during positioning, and that force ledger is what every credible sizing sheet is built on [S1].
A common field error is sizing on nameplate torque alone, then discovering that a 25° differential pressure across a ball valve demands nearly double the breakaway torque the catalog lists. A working rule: take the worst-case differential pressure at shutoff, multiply catalog breakaway by 1.5–2.0, and verify the electric actuator still hits the required cycle time at that derated output. For linear slides carrying a 5 kg payload at 500 mm/s with 0.1 s accel/decel, peak thrust from a 20 mm-lead ball screw at 70% efficiency lands near 35 N before the guide-rail friction term is added.
Step 2 — Match Drive Type to Duty Profile
Three drive families dominate: stepper (open-loop, holding torque at standstill, low cost), servo (closed-loop, high dynamics, encoder feedback), and brushless DC with integrated driver (mid-cost, mid-dynamics) [S2]. For an indexed indexing duty at 60 cycles/min with 50 ms move time, a stepper with closed-loop encoder is the cost-effective default; for continuous sinusoidal motion above 5 rps, a servo is the only honest answer [S2].
Oriental Motor's product family partitions cleanly along that line: αSTEP AZ (battery-free absolute encoder, closed-loop stepper), RKII (AC servo), NX Series (servo), and PKP/CVD (2-phase/5-phase stepper) [S2]. Each family carries its own sizing spreadsheet on the vendor site, and the inputs are always the same — load mass, lead, max speed, duty, accuracy, and electrical interface. Picking a drive family before the load profile is locked down is the fastest path to a redesign.
For rotary valve duties, sizing equations from ISA75.24 produce a required effective area and stem thrust; the same logic applies whether the actuator is pneumatic, hydraulic, or electric, only the output stage changes [S1]. Where the duty is modulating rather than on/off, add 25–50% torque margin to absorb stem-packing friction hysteresis that builds after 50,000 cycles.
Step 3 — Stroke, Speed, and Resolution: Three Coupled Numbers
Stroke (mm or °), max speed (mm/s or rpm), and resolution (µm or arc-min) are coupled through the lead screw or gear ratio, and trading one against the other is where most catalog screens are entered incorrectly. A 20 mm-lead ball screw at 3000 rpm delivers 1000 mm/s linear speed but only 50 µm per step at 1.8° step angle; halve the lead and the speed drops to 500 mm/s while resolution doubles — that is a design choice, not a free lunch. [S2]
For electric linear slides, linear guide selection has to keep pace with the actuator: a low-friction linear guide with <0.01 friction coefficient preserves motor torque for the actual work, while a misaligned or overloaded guide can burn 15–20% of the rated thrust as parasitic drag. For multi-axis gantry or pick-and-place, crossed roller guide stages give 4-way equal load capacity and <0.005 mm repeatability, which is the reason they are the default for semiconductor and metrology builds.
A shortlist sanity check: for a 100 mm stroke, 0.5 m/s, 5 kg payload, 0.05 mm repeatability duty, a closed-loop stepper with a 10 mm-lead screw and a 25 mm-profile linear guide will close; the same duty on a 5 mm-lead screw with the same motor will stall. The drive and the mechanics are not independent — overspec the motor and you underload the screw, underspec the screw and the motor cooks.
Step 4 — Bus Protocol and Integration Stack
Process-plant electric actuators default to 4-20 mA analog with HART (FSK overlaid on the 4-20 mA loop) for diagnostics, while discrete-factory actuators default to EtherCAT, PROFINET, or IO-Link [S2]. These are different physical and protocol layers — HART is not a digital fieldbus like Foundation Fieldbus or PROFIBUS PA, and trying to force a HART-only device onto a PROFIBUS PA segment will fail at commissioning. Get the bus decision locked in writing before the PO, because retrofitting a different protocol board often costs more than the actuator.
Vendor matrix for reference: Oriental Motor's CVD Series driver supports EtherCAT, RS-485, and pulse-input variants; the AZ Series multi-axis driver consolidates two axes into one footprint for space-constrained panels [S2]. For an in-panel automation build with a single controller, IO-Link single-drop wiring cuts cabinet hours by 30–50% versus parallel I/O on a multi-axis machine.
Step 5 — Environment, Enclosure, and Hazardous Areas
Enclosure rating (IP65, IP67, IP65K for washdown), ambient temperature range (typical industrial: -10 °C to +60 °C), and hazardous-area certification (ATEX 2014/34/EU, IECEx for Zone 1/2; UL/CSA Class I Div 2 for North America) collectively determine the SKU suffix. In a Zone 1 area, an electric actuator without an Ex d or Ex e certification cannot be installed regardless of how well it is mechanically sized. [S1]
For sub-freezer or outdoor exposure, derate the duty cycle by 20–30% below 0 °C unless the actuator is explicitly cold-rated; grease viscosity alone can push breakaway torque above the motor's peak. Corrosive chemical exposure (e.g. CIP loops, marine) demands 316 stainless hardware or a third-party epoxy coating, and 304 stainless is not a substitute — chloride pitting will eat the stem in 18 months.
Step 6 — Comparison Table: Drive Family vs Decision Criteria
Below is the criteria-based comparison a process engineer should walk into a vendor meeting with; it lines up the three drive families against four decision axes so the shortlist is defensible. [S3]
Stepper (closed-loop) on cost lands at 1× baseline, peak torque at 1.5×, accuracy at ±0.05 mm with 5 mm-lead, and bus support at pulse / RS-485 / EtherCAT (CVD Series). Servo (AC) on cost lands at 2.5× baseline, peak torque at 3×, accuracy at ±0.01 mm, and bus support at EtherCAT / PROFINET (NX Series). Brushless DC integrated on cost lands at 1.7× baseline, peak torque at 2×, accuracy at ±0.02 mm, and bus support at RS-485 / pulse (BLS/BLE2 Series) [S2].
Selection logic from that table: if cost is the binding constraint and accuracy of ±0.05 mm is acceptable, closed-loop stepper wins; if the duty is continuous high-dynamic, the 3× peak torque on servo justifies the 2.5× spend; if the cabinet is space-limited and bus is RS-485, the integrated brushless platform is the cleanest fit. Whoever picks a drive family that fails on the axis that matters most will be back at the vendor inside a year.
Step 7 — When Not to Use an Electric Actuator
Electric actuators fail as the right answer when continuous power-on duty exceeds 50% (motor thermal limits), when ambient exceeds 80 °C without active cooling, when the area is explosion-risk and certified electric options are unobtainable, or when the duty demands >50,000 Nm torque (where hydraulic still owns the upper envelope) [S1]. For an electric ball valve on a 24" Class 150 line with a tight-shutoff requirement, the actuator torque can be specified but the reality is that hydraulic and pneumatic dominate above 24" line size for capital-cost reasons.
For pallet-handling and AGV-style transfer where a small electric actuator is paired with a powered roller, an electric pallet truck duty profile (intermittent, high inrush, battery-powered) is closer to a vehicle traction sizing problem than a stationary valve problem, and a sizing method developed for EV battery packs — energy demand, range, battery technology fit — illustrates how the same load profile logic transfers across domains [S3]. For high-force industrial pressing where electric stroke times run into the minutes, a hydraulic actuator remains the cost-correct answer despite the energy efficiency penalty.
A practical last filter: total cost of ownership over 10 years, not purchase price. Electric actuators draw energy only on movement (pneumatic bleed losses consume compressed air continuously), so the operating-cost gap widens every year the unit is in service.
Trackable signals worth monitoring: the next revision of IEC 60079-0 (the explosive-atmospheres general requirements standard governing electric actuator certification) and any update to ISA75.24's working equations for electric-only sizing, since the current scope still groups electric with pneumatic and hydraulic [S1]. If the supplier base consolidates further in 2026, expect a 5–10% list-price move on the second-tier servo lines by Q4.
See also our earlier report, Swing Check Valve Sizing and Selection: DN, Cracking Pressure, and Water-Hammer Tradeoffs.