A spiral stop-motion gear is an intermittent-motion gear pair where a spirally cut driver tooth engages a slotted or partially toothed follower for one portion of each input revolution, then disengages so the follower dwells. Hamilton Watch Company refined the form in early 20th-century pocket watch keyless works to safely advance the date wheel without backlash. The spiral profile pulls the follower smoothly into engagement, indexes it through a fixed angle, then locks it via the smooth driver hub. The result is precise, repeatable stepping with zero creep during dwell — exactly what you need on calendar wheels, fuze timers, and small turret indexers.
Spiral Stop-motion Gear Interactive Calculator
Vary follower tooth count, engagement share, and cycle time to see the index angle, dwell share, and locked dwell time.
Equation Used
The follower advances one tooth space per driver revolution, so the index angle is 360 divided by follower tooth count. The engagement share is the moving part of the cycle; the remainder is locked dwell, converted to dwell time using the cycle time.
- One spiral driver tooth produces one follower index per input revolution.
- Follower teeth are evenly spaced around 360 degrees.
- Engagement and dwell shares sum to one full input cycle.
How the Spiral Stop-motion Gear Works
The spiral stop-motion gear works on a simple idea: most of the time, the driver isn't actually driving anything. The driver is a disc with a single spiral-cut tooth (or a short helical pinion segment) sitting on an otherwise smooth locking hub. The follower is a wheel with cut teeth around part of its circumference and concave locking surfaces between those teeth. As the driver rotates, the spiral tooth catches one follower tooth, sweeps it through a defined index angle — usually 30°, 36°, or 45° depending on tooth count — then disengages. The smooth hub then rides against the concave locking surface of the follower, holding it dead still until the spiral comes around again.
The geometry has to be right or the mechanism either binds or skips. The spiral lead angle controls how aggressively the follower accelerates. Too steep and you slam the follower tooth, chip the corners, and watch the indexing accuracy drift over a few thousand cycles. Too shallow and the spiral disengages before the index is complete, leaving the follower short of its locking pocket — which means the next cycle starts misaligned and you get cumulative position error. We typically hold the spiral lead within ±0.5° of nominal on a precision indexer, and the locking-hub-to-pocket clearance must be 0.02 to 0.05 mm. Tighter than 0.02 mm and thermal expansion jams it; looser than 0.05 mm and the follower wobbles during dwell.
The most common failure modes are tooth corner chipping from over-steep lead angle, locking-hub wear that lets the dwell position drift, and pinion-tooth fatigue at the engagement entry point. If you notice the indexed position walking by a few arc-minutes per hundred cycles, the locking hub is the first thing to check — not the spiral.
Key Components
- Spiral driver pinion: A single spirally cut tooth or short helical segment mounted on a continuously rotating shaft. The spiral lead angle is typically 15° to 25° on a watch-grade unit, held to ±0.5° tolerance. This is the part that does all the work in roughly 60° to 90° of each input revolution.
- Locking hub: The smooth cylindrical body of the driver that surrounds the spiral tooth. It rides against the follower's concave locking face during dwell, preventing any rotation. Diameter must match the follower locking radius within 0.02 to 0.05 mm — tighter binds, looser drifts.
- Slotted follower wheel: The driven wheel with partial tooth cuts (typically 6, 8, 10, or 12 indexing teeth) and concave locking pockets between them. The pocket radius must equal the locking hub radius to within 5 µm on a watch movement, more like 20 µm on an industrial indexer.
- Index pawl or detent (optional): On larger industrial implementations, a spring-loaded pawl backs up the locking hub during high-inertia dwells. Sized so the pawl spring force exceeds expected back-driving torque by 2× minimum.
- Mounting plate and pivots: Centre distance between driver and follower shafts must be held to roughly 0.01 mm on a watch and 0.05 mm on a machine-tool indexer. Any drift here changes the engagement geometry and cascades into position error.
Industries That Rely on the Spiral Stop-motion Gear
Spiral stop-motion gears show up wherever a small, accurate, repeatable angular index has to happen at modest speed without backlash. They compete with Geneva drives, ratchets, and indexing cams. The reason engineers reach for the spiral form specifically is when the dwell must be absolutely rigid — no creep, no float — and the index motion must be smoother than what a Geneva can deliver, especially in compact spaces where a Geneva's slot path won't fit.
- Horology: Hamilton 992B railroad pocket watch date-wheel advance — the spiral stop-motion advances the date once per 24-hour cycle and locks it solidly the rest of the day.
- Munitions & fuzing: Mechanical time fuzes such as the M501 series used spiral stop-motion gears to step the timing wheel through discrete settings without permitting the wheel to drift between settings.
- Camera shutters: Compur and Prontor leaf-shutter timing trains used miniature spiral stop-motion gears to gate the speed-selection cam between exposure values.
- Small-part assembly: Hauni cigarette-tip attachment heads use spiral stop-motion indexers on the tip-rolling drum to advance the drum 36° per cycle while locking it during the rolling stroke.
- Vending and counting machines: Older Rowe-AMI and Wurlitzer jukebox record-selection mechanisms used spiral stop-motion gears to index the selector wheel one position per coin pulse.
- Precision instrumentation: Decade counter wheels in Veeder-Root mechanical totalisers — the carry-over from one decade to the next is driven by a spiral stop-motion pinion that steps the next wheel exactly one digit and holds.
The Formula Behind the Spiral Stop-motion Gear
The output you actually care about is the index angle per input revolution and the engagement time fraction. Index angle tells you how many follower teeth you need for a full follower revolution; engagement fraction tells you how much of the input revolution is spent moving versus dwelling. At the low end of the typical range — 6-tooth followers giving 60° per index — engagement fraction is high, dwells are short, and the mechanism feels almost like a continuous gear. At the high end — 24-tooth followers giving 15° per index — dwell dominates and the mechanism behaves like a true stop-motion. The sweet spot for most industrial indexers sits at 8 to 12 follower teeth, where you get a clean stop with enough dwell time for downstream operations to complete.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θindex | Angular advance of the follower per single index event | degrees | degrees |
| Nf | Number of indexing teeth on the follower wheel | count | count |
| fengage | Fraction of input revolution during which the spiral tooth is engaged with the follower | dimensionless | dimensionless |
| θspiral | Angular sweep of the spiral driver tooth around the input shaft | degrees | degrees |
| ωin | Input shaft angular velocity | rad/s or RPM | RPM |
Worked Example: Spiral Stop-motion Gear in a postal-meter date-stamp indexer
You are sizing the spiral stop-motion gear that drives the day-of-month wheel on a Pitney Bowes-style postal meter date-stamp head, where the operator pulls a hand lever once per cycle and the day wheel must advance exactly 1/31 of a turn, lock dead solid against the stamping force, and return zero creep when the stamp head slams down at roughly 200 N peak.
Given
- Nf = 31 teeth
- θspiral = 75 degrees (nominal)
- ωin = 60 RPM (nominal hand-cycle equivalent)
- Stamping load = 200 N peak
Solution
Step 1 — compute the per-index angle. With 31 teeth on the follower for a full month wheel:
That is a tight index — 11.61° per cycle is on the fine end of what a spiral stop-motion can hold reliably. It works here because the postal meter only cycles once per stamp, not at speed.
Step 2 — compute the engagement fraction at the nominal 75° spiral sweep:
So the spiral is moving the wheel for about 21% of each input revolution and dwelling for 79%. That dwell is when the stamping force hits, and that is exactly when you want the locking hub fully seated.
Step 3 — at the low end of the typical spiral sweep range, 45°:
Engagement drops to 12.5%. The follower has to accelerate harder to cover the same 11.61° in less input rotation — peak tooth contact stress climbs roughly 65%, and on a brass follower you'll see corner deformation within 50,000 cycles. Bad trade for a postal meter that sees a million cycles in its life.
Step 4 — at the high end, 110° spiral sweep:
Now you are engaged for nearly a third of every revolution. Dwell shrinks. If the stamping head is timed even slightly early, it lands while the spiral is still pushing the follower — and the wheel jumps under the hammer. You'll see ghost prints offset by 0.3 to 0.5 mm. The 75° nominal sits in the sweet spot: enough sweep that tooth stress stays low, enough dwell that the stamp lands on a locked wheel.
Result
Nominal index is 11. 61° per cycle with the spiral engaged for 20.8% of each input revolution. That feels right on a hand-cranked postal meter — the operator senses a smooth turn-and-click, and the day wheel snaps to its next position with no audible buzz or rebound. Compared to the 12.5% engagement at a 45° spiral (harsh, high-stress, short life) and the 30.6% engagement at a 110° spiral (mushy dwell, vulnerable to ghost prints), the 75° nominal hits the design sweet spot. If you measure the day wheel drifting backward by more than 0.1° between strikes, suspect three things in this order: locking-hub-to-pocket clearance opened past 0.05 mm from wear, the spiral-driver shaft pivot has developed radial slop above 0.03 mm, or the follower's concave pocket radius has worn oversize from repeated stamping shock loading.
Spiral Stop-motion Gear vs Alternatives
Spiral stop-motion gears compete head-on with Geneva drives and pawl-and-ratchet indexers. Each has a niche. The choice usually comes down to dwell rigidity, smoothness of engagement, and how much axial space you have.
| Property | Spiral stop-motion gear | Geneva drive | Pawl-and-ratchet |
|---|---|---|---|
| Typical operating speed | 1-300 RPM input | 10-1000 RPM input | 1-100 cycles/min |
| Index accuracy (after 10k cycles) | ±2 to ±5 arc-min | ±5 to ±15 arc-min | ±10 to ±30 arc-min |
| Dwell rigidity (back-driving resistance) | Excellent — full hub lock | Excellent — slot lock | Poor — pawl can lift |
| Engagement smoothness | Very smooth, spiral pulls in gradually | Moderate, abrupt at slot entry | Harsh, instantaneous |
| Axial / radial footprint | Compact, fits in watch movements | Moderate, slot path is bulky | Compact but needs return spring |
| Manufacturing cost | High — spiral cutting needs CNC or spec tooling | Moderate — slotted plate is standard | Low — stamped parts work fine |
| Lifespan at rated load | 1-10 million cycles | 5-50 million cycles | 0.5-5 million cycles |
| Best application fit | Precision low-speed indexing under load | High-speed packaging indexers | Cheap counters and ratcheting tools |
Frequently Asked Questions About Spiral Stop-motion Gear
The spiral tooth is disengaging before the follower fully seats into its locking pocket. This almost always traces back to the spiral lead angle being slightly too shallow for the follower's inertia — the tooth slips out of contact while the follower is still decelerating into the pocket.
Check the spiral's exit geometry first. If the trailing edge of the spiral has worn or was undercut at manufacture by more than 0.05 mm, the engagement window shortens with every cycle. The fix is to increase the spiral arc by 5-10° or to add a light detent spring that pulls the follower the last fraction of a degree into the pocket once the spiral releases.
Geneva drive, in most cases. At 40 cycles/min and 6 stations you are well inside Geneva territory, and the slotted Geneva plate is cheaper to make and easier to time. The spiral stop-motion only wins if you need the dwell to resist a heavy axial or tangential load while a downstream operation hits the turret — the full hub lock of a spiral resists back-drive better than a Geneva slot under shock.
Rule of thumb: under 50 cycles/min with significant dwell loading, spiral wins. Above 50 cycles/min or with light dwell loading, Geneva wins on cost and durability.
Peak torque demand happens at the steepest point of the spiral, usually around 30-40% through the engagement arc. As a working approximation, peak torque equals the follower's reflected inertia times the angular acceleration imposed by the spiral profile, plus friction.
For a typical 12-tooth follower indexing 30° in 75° of input rotation at 60 RPM, the angular acceleration on the follower peaks at roughly 8-12 rad/s². Multiply by your follower's mass moment of inertia, add 20% for friction and locking-hub drag, and size the motor for at least 2× that peak. Undersizing here causes stalls right at the start of engagement, which then chips the leading face of the spiral tooth.
The follower is unseating from its locking pocket during dwell because the locking-hub-to-pocket clearance is too loose for the vibration environment. On the bench there is no excitation; on the machine, the resonant frequency of the follower wheel coincides with vibration from the downstream punch or stamp, and the wheel rocks just enough to climb out of the pocket.
Two fixes. First, tighten the locking clearance — go from 0.05 mm down to 0.02-0.03 mm, but watch for thermal binding. Second, add a sprung detent acting on a separate index ring on the follower shaft. The detent doesn't take the indexing load but kills the rocking motion during dwell.
You cannot. The locking hub geometry is non-reversible by design — the follower's concave pocket sits flush against a smooth cylindrical surface, and there is no tooth on the follower side that can engage the driver to push it the other way. Try to back-drive and you'll either stall the system or, if you apply enough torque, deform the locking-pocket edge as it tries to climb the smooth hub.
This non-reversibility is actually one of the reasons engineers pick the spiral form for safety-critical indexing — calendar wheels, fuze timers, ammunition counters — where you specifically want the output to refuse any back-drive attempt.
Tighter than you'd think. On a watch-grade movement we hold centre distance to ±0.01 mm; on an industrial indexer ±0.05 mm is the practical limit. Drift beyond that and two things happen: the spiral engages early or late (changing the effective index angle), and the locking hub either binds against the pocket or floats above it.
If you are seeing inconsistent index quality across nominally identical units, measure centre distance on each before blaming the gears. Plate machining variation is the usual culprit, not the gear cuts themselves.
References & Further Reading
- Wikipedia contributors. Intermittent mechanism. Wikipedia
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