A 25% duty cycle linear actuator is permitted to run for 2.5 minutes and must rest 7.5 minutes inside any 10-minute reference window, while a 100% rated unit is built for unrestricted continuous operation inside its thermal envelope [S2][S4].
The two ratings sit at opposite ends of the same thermal curve, and the selection decision is driven by heat dissipation in the gearbox and DC or servomotor windings, not by stroke length or force alone [S1][S3]. Standard electric actuators in the 10-25% range deliver peak performance only for short indexing moves, whereas continuous-duty servomotor actuators are designed for conveyor diverters, valve modulation, and process-line positioning where the actuator never sees a meaningful cool-down period [S3][S4].
What Duty Cycle Actually Measures
Duty cycle is defined as the ratio of on-time to total cycle time, expressed as a percentage: D = T on ÷ (T on + T off) [S1][S5]. A 25% rating therefore means the unit can be energized for 25% of the reference window and must be off for 75% of that same window.
The reference window is not universal, and this trips up most first-time specifiers. Firgelli publishes cycle tables against a 5-minute window, where a 25% rating equals 75 seconds of run and 225 seconds of rest [S1]. Actuonix and Firgelli's February 2026 guidance publish against a 10-minute window, where 25% equals 2.5 minutes on and 7.5 minutes off [S2][S4]. Thomson's worked example uses a 60-second window with 15 s on and 45 s off to land on the same 0.25 ratio [S5]. The math is identical, but the absolute seconds change, so a 25% rating on a 5-min reference is not the same allocation of active time as a 25% rating on a 10-min reference.
25% Duty Cycle: The Workhorse of Indexing Motion
Most standard electric linear actuators on the market ship rated for duty cycles between 10% and 25%, meaning they can operate continuously for 1-2.5 minutes before mandatory rest [S4]. The physical reason is I²R losses in the copper windings: as current squared through winding resistance generates heat, the motor's internal temperature rises faster than the housing can shed it to ambient air.
At a 10% rating on a 10-minute reference, the actuator is allowed only 1 minute of energized motion and 9 minutes of cooling [S2][S4]. Compact micro linear actuators often land in this 10% band because their small motor housings have very little thermal mass [S4]. Pushing a 25%-rated unit to continuous operation produces three classic failure modes: winding insulation breakdown once the varnish coating exceeds Class F (155°C) or Class H (180°C) temperature limits, carbon brush structural failure, and permanent demagnetization of the rotor magnets, which leaves the actuator weaker even after it cools [S1]. All three are irreversible.
100% Continuous Duty: Servomotor Architecture
A 100% duty-cycle actuator is engineered so the motor can run indefinitely at rated load without tripping a thermal limit, and the typical design choice is a servomotor instead of a stepper motor, with closed-loop feedback to keep the winding current inside the thermal envelope [S3].
The architecture trade is not free. Servomotors add an encoder, a position-loop controller, and usually a higher-resolution gearbox to deliver the continuous torque. The win is that the unit's I²R losses are matched to its dissipation path, so the winding temperature stabilises below the insulation class limit instead of climbing [S1][S3]. For a process valve that modulates every few seconds, a solar tracker that adjusts every minute through the day, or a conveyor diverter that fires on every package, a 25% unit will cook itself inside weeks; a 100% servomotor actuator runs cold for years.
Selection Criteria: When 25% Is Enough and When It Is Not
Pick a 25% duty cycle unit when the application is intermittent, the stroke is short, the cycle time is well above 10 minutes, and the load is below roughly 70% of the actuator's rated force so the motor draws less current and runs cooler than the lab-rating test condition [S1][S2][S4]. Examples are TV lifts, cabinet openers, adjustable desks, RV bed lifts, and agricultural hatches that move a few times per hour. The published 25% is then a generous worst case, and the unit will usually outlive its mechanical bearings long before it sees a thermal failure.
Pick a 100% continuous-duty servomotor actuator when the application is high-cycle, the load sits above 70% of rated force, the ambient temperature is above 30°C, or the cycle time is below 10 minutes with the actuator never seeing a meaningful rest period [S1][S3][S4]. The cost premium is real, but it is cheaper than a service call to swap a burnt-out actuator inside a machine that runs three shifts. The supporting linear bearing and linear guide hardware must be specced to the same continuous duty, because a continuous-duty actuator pushing a marginal bearing will simply move the failure point downstream.
Real-World Use Cases by Duty Rating
Adjustable hospital bed actuators typically run at 10-25% because the stroke fires a few times per shift and the unit rests for hours between patients [S4]. Solar tracker actuators fall in the 25% band when the tracker steps every 5-10 minutes through daylight, but a tracker that slews continuously during cloud-following mode needs a 100% rating. Industrial valve modulation on chemical loops, where a 4-20 mA signal can command continuous position changes, is 100% territory by definition. Packaging line diverters, depending on line speed, sit at the boundary: a slow line at 30 packages per minute can survive on 25% with margin, while a high-speed line at 200+ per minute needs 100% because the rest interval collapses below the cool-down requirement [S3]. For motion-control subsystems, linear encoder feedback on a 100% actuator allows closed-loop position correction that a 25% open-loop unit cannot match.
Limitations, Failure Modes, and Misreading the Spec
Catalog duty cycle is measured in clean lab air at 20-25°C at rated load, and real installations run hotter, dustier, and often above rated load, so the practical duty cycle is always lower than the published number [S4]. The "5-minute rule" is the most commonly misread clause: a 25% rating does not entitle the user to run the actuator for 2 hours and rest for 6 hours; it entitles the user to 25% of any 5-minute reference window [S1].
Three predictable failure paths follow misapplication. First, insulation breakdown in the windings once internal temperature crosses the Class F 155°C or Class H 180°C threshold; this is the most common and is permanent [S1]. Second, carbon brush disintegration on brushed DC motors when the brush holder exceeds its thermal rating. Third, permanent magnet demagnetization, which leaves the actuator producing less torque even after cool-down, and which is irreversible without a rotor replacement. None of these are warranty issues if the published duty cycle was exceeded, and most manufacturers explicitly disclaim them.
Standards, Sourcing, and What to Confirm on the Datasheet
There is no single ISO or IEC standard that defines "duty cycle" for linear actuators; the rating is a manufacturer-published thermal specification derived from internal life testing [S2]. The relevant adjacent standards are the IEC 60034 series for rotating electrical machine thermal classes, which is what the Class F (155°C) and Class H (180°C) winding limits trace back to, and ISO 13849 for safety-related control integration when the actuator is part of a machine safety function.
On the datasheet, confirm three things before placing the order: the explicit reference window (5 min vs 10 min), the rated load at which the duty cycle was measured, and the ambient temperature range. If the datasheet only prints "25% duty cycle" without the window, ask the vendor, because the answer changes the absolute run-time budget by a factor of two. For a side-by-side spec of linear actuator options, anchor the comparison on cycle window, rated load, ambient rating, and motor type, in that order; stroke, speed, and force are secondary once duty class is fixed. The full decision matrix for linear module selection follows the same logic, with the additional constraint of travel per minute averaged over the reference window.
Track two signals over the next spec refresh cycle: vendor migration of mid-range 25% units to 50% ratings as brushless DC motors replace brushed designs, and the publication of an industry-wide duty-cycle test standard that removes the 5-min versus 10-min reference window ambiguity that currently lets two "25%" ratings mean very different things.
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