An electric winding device is a small motorised assembly that periodically rewinds the mainspring or lifts the driving weight of a mechanical clock so the timepiece runs indefinitely without hand-winding. It works by coupling a low-RPM gearmotor to the winding arbor through a slip clutch and a one-way ratchet, then triggering on a limit switch or timed cycle. Designers fit them to museum clocks, tower movements, and long-running display pieces where daily winding is impractical. A correctly tuned unit holds mainspring torque within a 5-8% band, which keeps amplitude — and rate — stable across the run.
Electric Winding Device Interactive Calculator
Vary the mainspring safe torque and slip-clutch limit to see the clutch release torque and protected torque margin.
Equation Used
The clutch release torque is set as a percentage of the mainspring safe maximum torque. In the article example, the slip clutch is set to 60%, so a 400 mNm safe maximum gives a 240 mNm clutch release setting and 160 mNm of protected overrun margin.
- Slip clutch is adjusted as a percentage of the mainspring safe maximum torque.
- Ratchet blocks back-drive and does not change the torque limit calculation.
- Limit switch cuts motor power at the target wind state.
How the Electric Winding Device Works
The job is simple in principle: replace the human hand with a motor, but without slamming the going train. A typical unit uses a micro gearmotor running at 2-15 RPM at the output, coupled through a slip clutch (set to release at, say, 60% of the mainspring's safe maximum torque), then through a sprag or pawl-and-ratchet so the mainspring can never drive the motor backwards. A limit switch — either a microswitch riding on the winding arbor or a Hall sensor watching a magnet on the barrel — kills power once the spring reaches a target wind state. On weight-driven clocks the same architecture lifts the weight cable through a drum, and the limit switch trips when the weight reaches the top of its travel.
The reason for all this complexity comes down to torque control. A mainspring at full wind in an 8-day movement might develop 250-400 mNm at the barrel arbor. If your motor punches past full wind because the limit switch chattered or the slip clutch seized, you will snap the mainspring or shear a centre wheel pinion. We see this fail mode in field returns more than any other — a frozen slip clutch (usually from old grease turning waxy) means the motor delivers full stall torque straight into the going train. The other common failure is reversed-polarity wiring on the limit switch which lets the motor wind continuously past the cutoff. Tolerances matter here: the ratchet click must engage within 1-2 teeth of motion, and the slip clutch friction surfaces should be set with a torque wrench, not by feel.
Most modern designs run on a remontoire-style cycle — wind a little, often — rather than letting the spring fully unwind before rewinding. A 30-second wind every 60 minutes keeps the torque curve almost flat, which is exactly what an escapement wants. That flat torque is the whole point of fitting an electric winder rather than just leaving the clock to run down between weekly visits.
Key Components
- Micro Gearmotor: Provides the low-speed, high-torque rotation that drives the winding arbor. Output speeds typically 2-15 RPM with 200-1000 mNm stall torque. We size for roughly 1.5× the peak winding torque the spring demands at full wind, never less.
- Slip Clutch (Torque Limiter): Disengages the motor from the arbor once a preset torque is exceeded, preventing overwind damage. Typical setpoint is 60-70% of the mainspring's rated maximum. The friction surfaces must be kept clean of oil — contaminated clutch faces can drift the slip torque by 30%.
- Pawl and Ratchet (Sprag Clutch): Allows the motor to wind in one direction only, while letting the mainspring drive the going train freely between winding cycles. The click must engage within 1-2 ratchet teeth or you risk back-driving the gearmotor on power-off.
- Limit Switch: Cuts motor power when the mainspring reaches its target wind state or when the weight hits its top travel position. Microswitches are common on traditional builds; Hall-effect sensors with a barrel-mounted magnet are cleaner for modern installs and avoid mechanical wear on the contact arm.
- Cycle Controller: Triggers a winding pulse on either a fixed timer (every 30-60 minutes) or on a torque-drop sensor reading. A simple 555-based timer works for hobby builds; commercial units like the Kieninger AMS auto-winder use a small microcontroller that logs cycle counts for service intervals.
- Winding Arbor Coupling: Transfers torque from the clutch output to the original square winding arbor of the movement. Must be a slip-fit with 0.05-0.10 mm radial clearance — too tight and you bind the arbor against the plate, too loose and the coupling cams the arbor sideways and chews the bearing hole.
Industries That Rely on the Electric Winding Device
Electric winding devices show up wherever a mechanical clock has to run unattended, where the movement is physically inaccessible, or where the rate stability that comes from a near-constant mainspring torque matters more than the romance of hand-winding. The applications below all share one common driver: somebody decided that the cost of fitting a motorised winder was lower than the cost of either a horologist's weekly visit or the rate drift from an unwound spring.
- Heritage Horology: A clock conservator in Bristol fits Kieninger AMS-pattern auto-winders to longcase clocks in National Trust properties so they keep time between curator visits without staff training to hand-wind every 8 days.
- Public Tower Clocks: Smith of Derby retrofits motorised winding gear to Victorian tower clock movements where the original gravity weights would otherwise need a winch operator twice a week — common on civic buildings across the UK.
- Watch Industry Display: Patek Philippe boutiques run window-display chronometers on continuous-cycle electric winders so the rotor-wound automatic movements never run down during showroom-closed hours.
- Museum Exhibits: The Science Museum in London uses purpose-built remontoire-style electric winders on demonstration chronometers, cycling once an hour, so visitors always see the escapement running at near-constant amplitude.
- Architectural Installations: A hotel lobby in Singapore commissioned a 4-metre skeleton clock with a 12-RPM micro gearmotor winding the mainspring every 45 minutes through a slip clutch — eliminates the visible weight drop a public guest would otherwise see.
- Astronomical Regulators: Observatory regulators driving sidereal-time displays use electric remontoire winders to hold mainspring torque within a 5% band, since rate drift from torque variation directly corrupts the displayed star time.
The Formula Behind the Electric Winding Device
The core sizing question is: how much motor torque do you need at the winding arbor, and what gear ratio gets you there at a sensible speed? At the low end of typical winding speeds — say 2 RPM — you can use a smaller motor but the wind takes longer, which means the limit switch and clutch see more cumulative wear cycles. At the high end, 15 RPM, the wind is fast and the motor sees fewer running hours, but inertia and clutch slip become harder to control and you risk overshooting the cutoff. The sweet spot for most 8-day mainsprings sits around 5-8 RPM at the arbor with a clutch torque set to roughly 1.5× the spring's full-wind demand.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tmotor | Required motor output torque before the gear reduction | mNm | oz·in |
| Tspring | Peak winding torque demanded by the mainspring at full wind | mNm | oz·in |
| ksafety | Safety factor for clutch setting and friction variation, typically 1.4-1.6 | dimensionless | dimensionless |
| η | Combined efficiency of gearbox and coupling, typically 0.55-0.75 | dimensionless | dimensionless |
| i | Gear ratio between motor shaft and winding arbor | dimensionless | dimensionless |
Worked Example: Electric Winding Device in a marine chronometer display rig
A maritime museum in Halifax is commissioning a continuously running display chronometer based on a 1955 Hamilton Model 22 movement with a 56-hour mainspring. The curator wants the spring rewound automatically every 30 minutes so visitors always see full amplitude on the balance. Measured peak winding torque at the arbor is 320 mNm at full wind. We need to size the gearmotor and pick an arbor RPM.
Given
- Tspring = 320 mNm
- ksafety = 1.5 dimensionless
- η = 0.65 dimensionless
- i = 100 dimensionless (gearbox)
Solution
Step 1 — compute the required motor output torque at the nominal operating point, with arbor speed 6 RPM (motor shaft 600 RPM through a 100:1 gearbox):
That is well within the rating of any standard 12 V Maxon or Faulhaber-class micro gearmotor. At 6 RPM the arbor needs roughly 8 turns to bring a Hamilton 22 mainspring from half-wind to full, which means a wind cycle of about 80 seconds — short enough not to compete with the next 30-minute trigger, long enough that the slip clutch never sees a slamming engagement.
Step 2 — at the low end of the typical operating range, drop arbor speed to 2 RPM (motor 200 RPM, same gearbox). Required torque is unchanged because gear ratio is unchanged:
A 4-minute wind cycle every 30 minutes means the motor runs 13% of the time. Over a year that is 1,140 motor-hours and roughly 17,500 clutch engagements — you will be replacing the clutch friction discs inside 3 years.
Step 3 — at the high end, push arbor speed to 15 RPM (motor 1500 RPM):
Fast wind, low duty cycle, but at this speed the rotational inertia of the motor armature alone can carry the arbor 1-2 teeth past the limit switch trip point. On a Hamilton 22 that is enough to spike spring torque 15-20% above the clutch setpoint and you will hear the clutch chattering on every cycle.
Result
The nominal motor torque requirement is 7. 4 mNm at the motor shaft, easily met by a 10-15 mNm rated gearmotor. At 6 RPM arbor speed the wind takes about 80 seconds and the clutch engages cleanly — this is the sweet spot. Drop to 2 RPM and the cycle stretches to 4 minutes with heavy clutch wear; push to 15 RPM and motor inertia overshoots the limit switch and the clutch chatters audibly. If your installed unit fails to wind to full or stalls early, the three most common causes are: (1) a contaminated slip clutch with oil migration from the movement plates, dropping clutch torque below the spring demand; (2) a sticking limit-switch contact arm tripping early on a partial wind; and (3) gearbox efficiency dropping below 0.5 from cold, dried grease in winter installs — measured current draw will run 40% above nominal as the symptom.
Electric Winding Device vs Alternatives
An electric winding device is one of three reasonable options for keeping a mechanical clock running unattended. Each carries a different cost-vs-precision tradeoff, and the right choice depends on whether you care about pure rate stability, install simplicity, or maintaining the original character of the movement.
| Property | Electric Winding Device | Mechanical Remontoire | Hand-Winding Schedule |
|---|---|---|---|
| Mainspring torque variation across cycle | 5-8% (with 30-60 min cycle) | 1-3% (cycle every 30-60 s) | 30-50% over 8-day run |
| Installation cost (typical retrofit) | $200-800 | $3,000-15,000 (custom) | $0 |
| Service interval | 3-5 years (clutch + switch) | 8-15 years (mechanical wear) | Weekly human labour |
| Power requirement | 12-24 V DC, ~5 W peak | None — driven by going train | None |
| Reliability over 10 years | Good if clutch maintained; motor fails first | Excellent — proven on Riefler regulators | Depends entirely on operator |
| Visible to public viewing the clock | Hidden if installed behind dial | Visible — often a feature | Visible weekly intervention |
| Application fit | Museum displays, tower clocks, hotel installs | Precision regulators, observatories | Domestic clocks with engaged owner |
Frequently Asked Questions About Electric Winding Device
Use the mainspring itself as your reference. Wind the spring fully by hand, measure the torque at the arbor with a small inch-ounce torque wrench (a Wiha 28501 or similar reads to 1 oz·in), then set the clutch to slip at roughly 1.5× that value. If you don't have a torque wrench, use a known weight on a lever arm of measured length attached to a dummy arbor — a 100 g weight at 50 mm gives you 49 mNm reference torque.
Skipping this step and setting the clutch by feel is the single biggest cause of broken mainsprings in field-installed auto-winders. The friction faces on most slip clutches have a 20-30% torque tolerance from new, and that drift gets worse with age.
That symptom points to limit switch contact welding or a stuck microswitch arm. When the switch contacts pit from arc-erosion, they eventually fuse closed during a high-current motor inrush event. The motor then runs until something physically stops it — usually the slip clutch saving the mainspring, but not always.
Fix is to fit a snubber diode (1N4007 across the motor terminals) to kill the back-EMF spike, and replace mechanical microswitches with Hall-effect sensors on installs that cycle more than 50 times per day. We see this failure on hotel-lobby installs running 30-minute cycles within 2-3 years.
Periodic fast wind almost always wins. A continuous slow wind sounds elegant but means the slip clutch is engaged 100% of the time, which doubles or triples wear on the friction surfaces. It also means the motor runs at low duty in a region where most DC gearmotors have poor efficiency and overheat.
The exception is precision regulators where you want maximum torque flatness — there a continuous wind through a high-ratio worm drive can hold variation under 2%. For everything else, a 30-60 second wind every 30-60 minutes is the right answer.
High current at predicted torque almost always means efficiency has dropped. Three usual suspects: gearbox grease has thickened (common in installs that drop below 5°C overnight, switch to a synthetic like Mobil SHC 100), the winding arbor coupling is binding because thermal expansion closed the 0.05-0.10 mm radial clearance, or the motor brushes are worn and arc-resistance is climbing.
Quick diagnostic: disconnect the arbor coupling and run the motor unloaded. If unloaded current is more than 30% of loaded current, the motor or gearbox is the problem. If unloaded current is normal, the binding is downstream in the coupling or arbor bearing.
You can, but the design is fundamentally different from a standard barrel install. On a fusee, you wind the chain from the barrel back onto the fusee cone, which means the winding direction reverses the natural going-train torque path. You need a stronger one-way clutch — a sprag bearing rather than a pawl-and-ratchet — because pawl chatter under reverse load will eat the ratchet teeth fast.
Also size the clutch torque conservatively. Fusee chains are weaker than mainsprings and a runaway motor will snap the chain before it stresses the spring. Set clutch slip at no more than 80% of the chain's working load rating.
Two effects you might be missing. First, magnetic interference — the gearmotor magnets and switching transients can perturb a steel hairspring or the balance staff's pivots. Keep the motor at least 150 mm from the balance and use a mu-metal shield if you can't.
Second, the wind cycle itself imparts a small mechanical shock through the plates every time the clutch engages. On a sensitive escapement that shock can momentarily change amplitude by 5-10° and shift the rate for the next few minutes. The fix is a softer clutch engagement — fit a small flywheel between motor and clutch to smooth the engagement profile, or extend ramp-up time on the controller to 0.5 seconds.
References & Further Reading
- Wikipedia contributors. Remontoire. Wikipedia
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