Duty cycle for an electric linear actuator equals on-time divided by total cycle time, expressed as a percentage, with most catalog ratings referenced to a 10-minute window [S1][S4][S5].
Apply a minimum 1.5x safety factor between calculated application duty and actuator rating, then derate further when ambient temperature exceeds 25°C or load approaches the actuator's published maximum [S2][S3].
Core Formula and Reference Window
Duty cycle (%) = (on-time) / (on-time + off-time) × 100 [S4][S5]. The 10-minute reference window is industry standard: a 25% rating means 2.5 minutes of operation and 7.5 minutes of rest within any 10-minute slice [S2][S5]. A 10% rating, common on compact rod-style units used in furniture and solar trackers, allows only 1 minute on and 9 minutes off [S2][S3]. A 50% rating gives 5 minutes on and 5 minutes off; a 100% rating permits continuous energising, but only on units explicitly designed with thermal-management provisions (oversized windings, forced-air or fan-cooled housings, or gearboxes rated for sustained input power) [S3][S5].
For an example calculation, if a linear actuator extends in 8 seconds and rests 52 seconds, the duty is 8 / (8 + 52) × 100 = 13.3%; an actuator rated at 20% would cover it with margin [S3].
Step-by-Step Calculation for an Intermittent Application
Step 1: measure the active stroke time (extend plus retract) for one full motion. Step 2: measure the rest period until the next motion request. Step 3: divide active time by total cycle time and multiply by 100 [S1][S5]. A worked example: 2 minutes active plus 18 minutes rest yields 2 / 20 = 0.10 = 10% [S7]. The Firgelli calculator also exposes derived figures, with cycles per hour N = 3600 / Tcycle and total operating time per hour Toperating = (DC% / 100) × 3600 seconds [S8].
For engineering reference, travel time itself is computed as stroke length divided by no-load speed, which feeds back into the active window [S9]. When the stroke is short (under 50 mm) and the speed is moderate (5-30 mm/s typical for a 12 V or 24 V DC rod actuator), the per-cycle on-time often falls well under 10 seconds, making the duty cycle very low even at high cycle frequency.
What Changes the Effective Duty Cycle in the Field
Published duty cycle assumes rated load, rated voltage, and 20-25°C ambient air with free convection [S2][S5]. Each of three variables can independently push the actuator into thermal overload. Load: heavier loads draw more current and dissipate more I²R heat in the windings, so running at 80% of rated load roughly doubles the motor's heat generation per second compared to no-load. Voltage: over-voltage (common when a 24 V unit is fed from a 28 V bus) raises speed and current draw, increasing heat. Ambient: at 40°C ambient the safe duty can drop 20-30% versus the 25°C baseline [S3].
Operating below rated load, in contrast, can effectively raise the safe duty cycle because less heat is generated per unit of on-time [S2][S5]. The actuator's linear guide and gearbox also participate in the thermal budget: grease viscosity drops with temperature, accelerating wear on the lead screw or ball screw if the cycle pushes the gearbox above its continuous-rating lubricant limit.
Rating Tiers Compared Against Application Demands
Catalog tiers cluster into two practical bands, and the decision is driven by cycles per hour and per-cycle on-time, not by marketing copy [S3]. Low-duty (10-20%) units suit bed lifts, recliners, solar trackers, agricultural gates, and machine-guard doors, all of which fire intermittently with long rest gaps. High-duty (50-100%) units target conveyor diverters cycling every 5-10 seconds, robotic pick-and-place arms, and continuously modulating ventilation dampers; these require larger motors, better thermal paths, and heavier gear trains [S3].
A 10% rating gives 6 seconds of on-time per minute; 20% gives 12 seconds; 25% gives 15 seconds; 50% gives 30 seconds; 100% permits continuous operation, with the caveat that very few standard electric actuators ship in this class [S3][S5]. Selecting on the boundary is risky: the same reference recommends a 50% margin, meaning a calculated 13.3% application should be matched to a 20% rated actuator, not a 10% unit [S3].
Failure Modes When the Rating Is Exceeded
Exceeding rated duty causes four observable failure paths, in order of how quickly they appear [S2][S5]. First, winding insulation degrades; class A or class B insulation rated for 105-130°C will eventually short if the motor's internal temperature repeatedly spikes past its limit. Second, lubricant in the gearbox and on the linear bearing surfaces thins and oxidises, accelerating wear on the lead screw or acme screw. Third, electronic components (limit switches, Hall sensors, encoder discs) drift or fail when their solder joints thermal-cycle beyond design. Fourth, brushes (in brushed DC units) and bearings see accelerated mechanical wear from sustained elevated temperature [S2][S5].
Two monitorable signals warn before catastrophic failure: the housing temperature, which a calibrated thermistor can track, and the no-load current, which climbs as internal friction rises. A field rule of thumb is that housing temperature should not exceed 70-80°C in continuous service, well below the 130°C winding limit, leaving margin for transient overloads.
Selection Criteria and the Right Sizing Margin
Use this four-criterion comparison when picking between catalog ratings: rated duty percentage (must exceed calculated duty by 1.5x), thermal time constant (larger housings recover faster from short bursts), continuous-force derate curve (some vendors publish force versus duty charts), and IP rating (sealed units trap heat more than open-frame units of the same power class) [S3][S9]. A unit that barely passes the duty math on paper will fail in service if any of the other three criteria is borderline.
For applications where the duty math yields 10-15%, a 20%-rated unit is the practical floor; for 20-30% applications, step to 50%-rated industrial units; above 50%, plan on continuous-duty hardware with forced-air or oil-bath gearboxes, not catalog rod actuators [S3][S5]. When the actuator is paired with a VFD duty motor for higher-power mechanical work, remember the VFD and the actuator have independent thermal budgets, and both must be sized to the application's true cycle, not to its peak load alone.
Standards, Documentation, and Common Pitfalls
No single IEC or ISO standard prescribes a uniform duty-cycle test method for general-purpose linear actuators, so cross-vendor comparison requires reading the fine print: reference window (10 minutes is typical, but not universal), ambient assumption (25°C unless noted), and load assumption (rated load, not no-load) [S2][S3][S5]. Vendors using a 5-minute or 15-minute reference window will publish higher duty percentages for the same physical hardware, which is why two "20% rated" actuators from different vendors can have very different thermal headroom [S2].
Two pitfalls recur in the field. First, designers compute duty from stroke time alone and forget to add retract time; for a symmetric stroke the on-time is double the one-way travel [S5][S7]. Second, designers ignore end-of-stroke dwell under load (the moment the actuator reaches limit and the motor stalls against the internal clutch or stops drawing current). Holding a stalled actuator under full load for even a few seconds can dump more heat into the windings than the entire moving stroke, and that dwell must be included in the on-time numerator [S5].
Trackable signals to watch: vendor datasheets publishing ambient-corrected duty curves (more common in 2025-2026 industrial catalogs than in earlier furniture-grade literature) [S2][S9], and the gradual migration of low-cost rod actuators from 10% to 20% ratings as brushless DC and improved gear lubricants allow higher continuous dissipation in the same form factor [S5][S9].
See also our earlier report, Hall vs Optical Encoder Feedback in Electric Linear Actuators.