How do electric actuators "stop accurately and hold position"? The industry uses three routes: (1) worm-gear mechanical self-locking — the mainstream; Rotork, AUMA, Limitorque, JPS and Huakong multi-turn and quarter-turn models all rely on it and fit no brake at all, holding position by mechanics even on power loss; (2) motor brake (electromagnetic) — added only when the gear ratio is too low to self-lock (DKJ/SKJ series, high-speed ratio steps); (3) electronic braking — inverter-based smart actuators such as SIPOS 5 Flash and AUMA TIGRON ramp down and DC-inject, doing away with the mechanical brake for modulating duty. The core selection question is self-locking: at high output speeds (56/120/192 rpm steps) the worm lead angle grows, self-locking is lost, and a brake or electronic braking becomes mandatory.
Two scenarios: positioning accuracy — the rotor coasts on inertia after power-off, so a modulating actuator that overruns misses the setpoint; and back-driving — differential pressure pushes the output shaft backwards (gate and butterfly valves especially), and if the gear train can be back-driven the valve creeps open. The actuator must therefore either make the gear train back-drive-proof (self-locking) or make the motor stop on command (braking) — or both.
| Route | Principle | Pros | Cons / duty |
|---|---|---|---|
| Worm self-locking | Worm lead angle below the friction angle; output cannot back-drive | Simplest; holds position on power loss by mechanics; maintenance-free; low cost | Lower efficiency (5–30% friction loss); only valid at small lead angles (low speeds); fails at high speed steps |
| Motor brake (electromagnetic) | Solenoid + spring lever + friction disc: released when energised, engaged when de-energised | Holds on power loss (safe); independent of ratio; precise positioning | Bulk, complexity; friction wear; heating; used on DKJ/SKJ/ZKJ and high-speed steps |
| Electronic braking | Inverter ramp-down + DC injection (stator DC energy dissipation) or braking resistor | No wear parts; controllable torque; soft accurate stops; frequent-start friendly | Needs inverter hardware; fails with power — still needs a self-locking gear train as back-up |
| Brand / series | Drive & braking |
|---|---|
| Rotork IQ/IQ3 (UK) | Oil-bath worm gear, self-locking against back-driving, no brake |
| AUMA SQ/SA (DE) | Proven worm principle, self-locking; TIGRON inverter types use electronic control |
| Limitorque LQI (US) | AC/DC motor + self-locking epicyclic gears; MXFS fail-safe holds the spring with self-locking mechanics |
| SIPOS 5 Flash (DE) | Built-in inverter controls speed/direction/torque; modulating duty without mechanical brake or frequent-start motors |
| JPS / SJMC (CN) | Double worm-gear self-locking, no brake (per brochure) |
| Huakong HKC (CN) | Two-stage worm self-locking; linear thrust units self-lock throughout travel |
| Chuanyi M8000 (CN) | Oil-bath worm gear, mechanically self-locking, no brake required |
| Zhonghuan Tianyi (CN) | Final worm / screw-nut self-locking; dynamic braking standard on the control board |
| Xingli LT (CN) | Worm + NGWN planetary self-locking; 56 rpm+ steps non-self-locking, not for modulating duty |
| DREHMO (DE) | Self-locking at standard ratios; 120/160 rpm steps non-self-locking |
| DKJ/SKJ/ZKJ (CN, legacy) | The classic electromagnetic brake machines |
| Bernard 2SA3-type (FR/CN) | Motor code A = standard 380 V (no brake); code Z = special motor (brake optional) |
Worm self-locking has a strict geometric condition: the worm lead angle must be smaller than the meshing friction angle (≈5–6° for steel on bronze). At low ratios and high output speeds designers enlarge the lead angle for efficiency, and self-locking is lost. Hence the recurring industry pattern — the same brand’s high-speed steps are marked "non-self-locking": Xingli above 56 rpm not recommended for modulating duty, DREHMO 120/160 rpm non-self-locking, Shangyi No.11 144/192 rpm "not advised where self-locking is required". Limitorque’s LQI keeps self-locking with an efficient epicyclic train only through its multi-stage large ratio. Always verify the chosen ratio step self-locks; for non-self-locking modulating or pressurised duty, fit a brake or use electronic braking.
(1) Conventional multi-turn/quarter-turn: worm self-locking plus torque switches is a complete solution — power-loss holding is mechanical, no extra brake needed. (2) High-speed steps: check the brochure for "non-self-locking"; fit a brake motor (e.g. 2SA3 code Z) or dynamic braking for modulating duty. (3) Inverter-based smart types (SIPOS, TIGRON, Zhonghuan with dynamic braking): positioning comes from electronic deceleration curves — accurate and maintenance-free, but verify the gear train self-locks as the power-loss back-up. (4) Linear actuators: trapezoidal screws self-lock naturally, but as the thread wears the friction angle drops — re-check holding on old units. (5) Fail-safe duty: prefer mechanical self-locking and power-off brakes; electronic braking alone must never be the means of holding on power loss.
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References: "Motor braking forms & principles" (Electric Actuator Expert, Chongqing Lanpu); AUMA catalogue and Limitorque LQI/MXFS official documents; SIPOS 5 Flash technology manual; JPS / Huakong HKC / Chuanyi M8000 / Xingli / DREHMO / Zhonghuan brochures (downloadable on this site); compiled by actuators.cn.
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