A pin-geared watch stop is a horological mechanism that arrests the balance wheel the instant the crown is pulled to the time-setting position, freezing the seconds hand for precise synchronisation. A small pin or lever — driven by the keyless works through a geared or cammed link — drops onto the balance rim or hairspring collet within 5 to 20 ms of crown travel. It exists so the wearer can set the time to the second against a reference signal, and it appears in calibres from the ETA 2824-2 to the modern Sellita SW200-1.
Pin-geared Watch Stop Interactive Calculator
Vary the movement beat rate and required stop window to see the allowable hacking stop time for the balance wheel.
Equation Used
The article states that a 28,800 vph movement must be stopped within about one beat, giving 3,600,000 / 28,800 = 125 ms. Use the beat-rate slider to compare slower or faster watch movements, and the stop-window slider to allow a fraction or multiple of one beat.
- VPH is vibrations per hour, so one beat interval is 3,600,000/VPH milliseconds.
- The selected stop window is expressed as a number of beat intervals.
- This timing calculator does not model contact friction, spring force, rebound, or balance amplitude.
How the Pin-geared Watch Stop Actually Works
The mechanism lives on the dial side of the movement, tucked between the keyless works and the balance cock. When you pull the crown to position 2 or 3, the sliding pinion shifts the setting lever, the setting lever rotates the yoke, and a pin on that yoke — geared or cammed against a stop spring — pushes a thin steel finger against the rim of the balance wheel. That finger is the brake. Contact friction stops the balance in well under one full oscillation, which on a 28,800 vph movement means inside 125 ms. The stop spring holds light constant pressure so the balance stays parked while you set the minute hand against a time signal.
Geometry matters more than people expect. The stop lever's contact face must meet the balance rim at a tangent — not a stab. If the angle of attack drifts past about 5° off tangent, the lever either skips the rim entirely or digs in and bends the balance staff pivot, which is a 0.07-0.09 mm part you do not want to replace in the field. The stop spring force sits in a narrow window: typically 8 to 15 millinewtons. Below 8 mN the balance creeps under residual mainspring torque. Above 15 mN you risk scoring the polished rim and producing a visible witness mark under 10× loupe inspection.
The common failure modes are predictable. A bent stop lever from a clumsy hand-setting pulls the contact off-tangent and the watch will not hack reliably — you pull the crown, the seconds hand twitches, then keeps drifting. A fatigued stop spring loses pressure and the balance restarts on its own within a few seconds. A burr on the lever tip, often left after rough servicing, will catch the hairspring instead of the rim and that is a full strip-down to fix.
Key Components
- Stop Lever (Pin Finger): The thin steel arm that physically contacts the balance rim. Tip thickness is usually 0.15-0.25 mm and the contact face is polished to Ra 0.2 µm or better to avoid scoring. It pivots on a jewelled or steel pivot with 0.02 mm radial play maximum.
- Stop Spring: A flat leaf spring that biases the lever against the balance when the crown is pulled. Designed force range is 8-15 mN at full deflection. Made from blued steel or Nivaflex; loses 10-15% of its preload over 10 years of service and is the part most often replaced in hacking-fault repairs.
- Setting Lever / Yoke: Translates crown axial motion (typically 0.6-0.9 mm of pull travel) into rotation of the stop lever. The yoke pin engages a slot or cam on the stop lever. Slot clearance must be 0.03-0.05 mm — too tight and the action binds, too loose and the lever drops late.
- Sliding Pinion: The component on the winding stem that shifts position when the crown is pulled, transferring drive from winding to setting and triggering the stop lever via the setting lever. Engagement depth tolerance is ±0.05 mm.
- Balance Wheel Rim: The receiving surface for the stop lever. Glucydur or beryllium-copper alloy, polished. The contact arc is restricted to the outer rim — never the spokes or the hairspring collet, where contact would damage the regulating organ.
Real-World Applications of the Pin-geared Watch Stop
Hacking seconds is non-negotiable on any watch sold for time-keeping accuracy against a reference. That covers military issue watches, observatory chronometers, dive watches synchronised to bezel timing, and any modern automatic where the buyer expects to set the time against an atomic clock app. Pin-geared stops appear across the entire price spectrum because the part count is low and the mechanism scales from 28,800 vph wristwatch calibres up to slow-beat pocket movements. Where you do NOT see them is in vintage pre-1960s designs, traditional Swiss bumper automatics, and a handful of intentionally lever-free Seiko movements where the brand made a stylistic choice.
- Mass-market mechanical watches: ETA 2824-2 and its Sellita SW200-1 clone — both use a pin-geared stop lever activated at crown position 2.
- Military timepieces: The MIL-W-46374 spec field watches required hacking seconds for radio time synchronisation across squad members.
- Diving instruments: Rolex Submariner reference 16610 introduced hacking seconds with the calibre 3135 — divers set runtime against the bezel.
- High-end horology: A. Lange & Söhne calibre L901.0 in the Lange 1 uses a pin stop lever on the balance for second-precise setting.
- Microbrand assembly: Christopher Ward, Farer and Halios all build on Sellita SW200-1 movements where the hacking lever is a buyer-expected feature.
- Observatory regulators: Slow-beat precision regulators built by workshops in Glashütte use scaled-up pin stop levers for chronometer-trial start synchronisation.
The Formula Behind the Pin-geared Watch Stop
The practitioner's question is not 'what does the formula say' — it is 'how hard does my stop spring need to push to brake the balance reliably without damaging it'. The formula computes the minimum tangential braking force at the balance rim. At the low end of the typical range, with a small ladies' calibre balance and weak mainspring residual torque, you can get away with 6-8 mN. At the high end, on a high-torque chronograph balance running 4 Hz with strong residual torque, you need 12-15 mN. The sweet spot for a standard 11½ ligne automatic sits at 10-11 mN — enough to stop the balance inside one oscillation, gentle enough to leave no witness mark on a polished Glucydur rim.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Fstop | Required tangential braking force at the balance rim | N (typically mN) | lbf (typically µlbf) |
| Ibal | Moment of inertia of the balance wheel | kg·m² | lb·in² |
| ωbal | Peak angular velocity of the balance at the moment of contact | rad/s | rad/s |
| rcontact | Radius from balance staff to the lever contact point on the rim | m (typically mm) | in |
| Δt | Permitted braking time (target ≤ one half-oscillation) | s (typically ms) | s |
| μ | Coefficient of friction between steel lever tip and balance rim alloy | dimensionless | dimensionless |
Worked Example: Pin-geared Watch Stop in an ETA 2824-2 service rebuild
A small independent service shop in Solothurn is rebuilding a batch of ETA 2824-2 movements and wants to verify the replacement stop spring delivers the right braking force. The balance wheel has Ibal ≈ 1.1 × 10⁻⁹ kg·m², runs at 28,800 vph (4 Hz) with peak ωbal ≈ 25 rad/s at maximum amplitude swing, contact radius rcontact = 4.0 mm, target braking time Δt = 60 ms (well inside one half-oscillation of 125 ms), and μ ≈ 0.18 for blued steel against Glucydur.
Given
- Ibal = 1.1 × 10⁻⁹ kg·m²
- ωbal = 25 rad/s
- rcontact = 0.004 m
- Δt = 0.060 s
- μ = 0.18 dimensionless
Solution
Step 1 — compute the angular momentum that has to be dumped at nominal operating point:
Step 2 — at nominal 60 ms braking time and 4.0 mm contact radius, divide angular momentum by (r × Δt × μ):
That sits squarely in the 8-15 mN sweet spot — strong enough to bite, gentle enough to leave no witness mark on Glucydur. Step 3 — check the low end of the typical operating range. On a smaller 9 ligne ladies' calibre with Ibal ≈ 6 × 10⁻¹⁰ kg·m² and lower ωbal ≈ 18 rad/s:
That is too low for a steel-on-Glucydur contact — the balance will creep under residual mainspring torque before you finish setting the minute hand. The fix is a smaller rcontact or a longer Δt budget, not more spring force. Step 4 — check the high end, a high-torque chronograph balance with Ibal ≈ 2 × 10⁻⁹ kg·m² and ωbal ≈ 30 rad/s:
Still inside the safe window, but you are now close to the scoring threshold and the lever tip needs Ra 0.2 µm finish or better, otherwise you will see a polished band develop on the rim within 5 years.
Result
Required nominal braking force is approximately 10. 6 mN — right in the middle of the 8-15 mN design window for a standard 11½ ligne automatic. In practice that translates to a stop spring you can flex with light tweezer pressure and barely feel resistance, which is exactly the tactile signature watchmakers learn to expect. Across the operating range, the small ladies' calibre at 2.9 mN risks creep, the standard 2824-2 at 10.6 mN is the sweet spot, and the chronograph balance at 12.3 mN is approaching the upper safe limit. If you measure stopping action that fails to bite — seconds hand drifts after crown pull — check first for stop-lever pivot friction (a sticky pivot eats spring force before it reaches the rim), second for incorrect lever-tip geometry (a chamfer larger than 0.1 mm changes the effective contact radius and starves the friction calculation), and third for hairspring contamination dragging the balance back into motion after the lever lifts.
When to Use a Pin-geared Watch Stop and When Not To
Hacking seconds is not the only way to stop a balance, and the pin-geared lever is not the only way to drive a hack. The two main alternatives are a friction-style stop (a soft brush or felt pad acting on the rim) and a hairspring-side stop (lever contacts the hairspring collet). Each has a clear application window.
| Property | Pin-geared Watch Stop | Friction Pad Stop | Hairspring-collet Stop |
|---|---|---|---|
| Stopping time after crown pull | 20-125 ms | 100-300 ms | 10-50 ms |
| Risk of damage to regulating organ | Low — contacts rim only | Very low — soft contact | High — hairspring is delicate |
| Required stop spring force | 8-15 mN | 20-40 mN | 3-6 mN |
| Service life before component replacement | 20-30 years | 5-10 years (pad wear) | 20-30 years |
| Manufacturing complexity / part count | Medium — 3-4 dedicated parts | Low — 2 parts | High — tight hairspring tolerances |
| Typical applications | ETA 2824-2, Sellita SW200-1, Lange L901.0 | Vintage pin-lever budget movements | A few high-end Swiss calibres only |
| Cost to manufacture (relative) | 1.0× | 0.6× | 1.8× |
Frequently Asked Questions About Pin-geared Watch Stop
That is almost always a fatigued stop spring. The lever drops, makes initial contact, but the spring's preload has decayed below about 6 mN and residual mainspring torque overcomes the static friction at the rim. The balance creeps forward one tooth of the escape wheel at a time.
Quick diagnostic — pull the crown, watch the seconds hand under a loupe for 30 seconds. If it advances in discrete jumps rather than smooth drift, you have residual torque pushing through a weak brake. Replace the stop spring; do not try to re-tension it. Blued steel that has set will not return.
Outer rim, always. The braking torque scales linearly with rcontact, so doubling the radius halves the spring force you need for the same stopping time. That means lower contact pressure, less rim wear, and a wider tolerance window on spring manufacture.
Practical rule — aim for rcontact at 85-95% of the balance rim radius. Below 70% you are fighting the formula and need a stiffer spring, which raises the witness-mark risk. Above 95% the lever can slip off the rim edge during the contact transient.
The formula assumes all the spring force reaches the rim. In a real movement, a measurable fraction is lost to pivot friction in the stop lever itself and to flex in the setting lever yoke. On a worn movement that loss can hit 30-40%.
Check the stop lever pivot first — it should turn under its own weight when the spring is detached. If it does not, the pivot hole has gummed varnish or dried oil from the last service. A clean and one drop of Moebius 9010 usually restores correct timing.
Generally no. The keyless works on pre-1960s movements were not designed with the cam profile or the lever-clearance pocket required for a stop function. Retrofitting means re-cutting the setting lever, which destroys originality and risks breaking through into the dial-side jewel seats.
If the customer absolutely needs hacking, the cleaner path is to source a service-grade modern calibre (an ETA 2824-2 drop-in is often dimensionally close) and keep the original movement preserved. Watchmakers who routinely modify keyless works on vintage pieces tend to ruin resale value within one service cycle.
Notchiness comes from cam-on-pin geometry where the setting lever pin rides over a peak before dropping into the stop position. Smooth action comes from a continuous-cam profile where engagement is gradual.
The ETA 2824-2 uses a near-flat cam transition and feels smooth. Some Sellita variants and many older calibres use a peak-and-valley cam that snaps the lever into place — that is not a fault, it is the design. If a normally-smooth movement starts feeling notchy, suspect debris in the cam track or a burr on the setting-lever pin.
Anything above about 15 mN of contact force at the rim starts to load the balance staff pivots beyond their fatigue limit. Pivot diameter on a standard wristwatch is 0.07-0.09 mm and they are designed for axial load, not radial impact. A staff with hairline cracks at the pivot shoulder will fail catastrophically within months.
Practical lower bound on Δt is roughly 20 ms for a 4 Hz movement. Below that you are slamming the balance hard enough to be heard as a faint click under a loupe-mounted microphone. Stay between 40 and 100 ms and the staff lasts the life of the watch.
References & Further Reading
- Wikipedia contributors. Movement (clockwork). Wikipedia
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