10-Slot External Geneva Mechanism: The Classic Quick-Tick, Long-Pause Index
- Breno Cruz
- Aug 10
- 6 min read
This is the Geneva mechanism most people picture when they hear the name: a small crank spinning continuously, its pin darting into a slot just long enough to tick a star-shaped wheel forward by one step, then pulling back out while the wheel locks solid for a pause before the next engagement. It's the external Geneva drive — the same mechanism family behind the Maltese cross — and this is the 10-slot version of it.
We already covered a different, less-common internal Geneva configuration in a separate case study, using a 6-slot design. This article covers a full-size external version specifically, building on the fundamentals from our main Geneva Drive guide — with ten slots instead of six, which changes the numbers even though the underlying principle is identical.
The 10-Slot External Geneva, Specifically
For an external Geneva mechanism, the governing relationship for the driver's rotation during engagement is:
θ_motion (external) = 180° − 360°/n
For n = 10 slots:
Driven wheel step angle: 36° per index (360°/10) — and since each full driver revolution advances the wheel by exactly one slot, it takes 10 full turns of the driver crank for the Geneva wheel to complete one full revolution.
Motion phase (driver rotation while engaged): 180° − 36° = 144°.
Dwell phase (driver rotation while locked): 360° − 144° = 216°.
Motion-to-dwell ratio: 144:216, which simplifies to 2:3 — the wheel dwells for one and a half times as long as it spends moving, a gentler balance than the shorter-dwell rhythm you'd get from a lower slot count on the same configuration.
More slots means a smaller step angle and a shorter motion phase relative to the dwell than a coarser wheel would have — trading a larger, more complex part for smoother, lower-shock indexing at each step.
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How It Works, Step by Step
Continuous input. The driver crank rotates continuously at constant speed, carrying a single pin near its outer edge.
Tangential entry. As the pin approaches one of the star wheel's ten slots, it enters tangentially — moving in the same direction as the slot's centerline at the instant of contact, avoiding impact loading at entry.
Drive phase. For 144° of the driver's rotation, the pin stays engaged in the slot, pushing the star wheel through its 36° step. The star wheel's angular velocity isn't constant through this arc — it starts at zero, accelerates, then decelerates back to zero by the time the pin exits.
Disengagement and locking. As the pin exits the slot, a convex locking arc on the driver engages a matching concave cutout on the star wheel, holding it rigidly in place without needing a separate brake.
Dwell. The star wheel holds position for the remaining 216° of the driver's rotation, giving whatever downstream process needs to happen at that station a stable window before the next index — and the cycle repeats nine more times before the wheel has completed one full revolution.
Why the Locking Arc Matters as Much as the Slot
It's easy to focus entirely on the slot-and-pin engagement, but the locking arc is doing just as much work for well over half the cycle. The convex arc on the driver and the concave seat on the star wheel need to share a common center for the lock to hold without play — if those two surfaces don't seat cleanly, the star wheel can rock slightly during what's supposed to be a fully stationary dwell, which shows up downstream as position drift at whatever process depends on that dwell.
How Slot Count Changes the Rhythm
The same external-Geneva formula applies at any slot count — only the numbers shift:
Slot count (n) | Step angle | Motion phase | Dwell phase | Motion:dwell |
4 | 90° | 90° | 270° | 1:3 |
6 | 60° | 120° | 240° | 1:2 |
8 | 45° | 135° | 225° | 3:5 |
10 | 36° | 144° | 216° | 2:3 |
More slots produces a smaller step angle and a motion phase that takes up a larger share of the cycle relative to the dwell — the 10-slot design here sits toward the smoother, higher-slot-count end of that range, favoring gentler, lower-shock indexing over a long, generous dwell window.
Real-World Applications
Automated bottling and packaging lines — indexing containers through fill, cap, or label stations.
CNC automatic tool changers — precise, repeatable positioning between tool stations.
Rotary indexing tables with many stations — a higher slot count like this 10-slot design suits applications needing more, smaller indexed steps around a full rotation rather than a few large ones.
Traditional cinema projectors — advancing film frames with a controlled, repeatable dwell for each frame.
The CAD Design Challenge
Calculating the exact entry angle for the pin so it slides into the slot without binding or crashing is the fundamental kinematic exercise for this mechanism, and it gets more demanding as slot count increases:
Set the center distance to satisfy tangential entry exactly. The pin's entry point, its radius from the driver's center, and the center distance between driver and star wheel are all linked — getting one wrong throws off the tangential-entry condition.
Watch tolerance stack-up more closely with ten slots than you would with fewer. Ten smaller slots packed around the same wheel diameter leave less material and less margin per slot than a coarser design would — small errors in slot spacing are more likely to compound into a visibly uneven index from one station to the next.
Model the locking arc and its concave seat from a shared reference, not as independently sketched features, so they share a common center and seat cleanly through the full 216° dwell.
Simulate a full ten-step revolution, not just one engagement — with more slots involved, this is the more realistic way to catch any single slot that was generated slightly differently from the rest.
Common Problems and Troubleshooting
Wheel drifts slightly during dwell: Almost always a locking arc that isn't seating cleanly against its concave counterpart — check that both surfaces share the same center and haven't worn unevenly.
Harsh impact or "clunk" at slot entry: Usually means the center distance or pin radius doesn't satisfy true tangential entry — recheck the geometry rather than just adding clearance, which masks the problem without fixing it.
One or two steps feel different from the rest: With ten slots on the same wheel, this points to inconsistent slot spacing rather than a fundamental geometry error — verify all ten slots and their matching locking cutouts were generated from a single patterned reference rather than sketched individually.
Frequently Asked Questions
How many turns of the crank does it take for the Geneva wheel to complete one full revolution? Ten — each full rotation of the driver crank advances the star wheel by exactly one slot (36° for a 10-slot wheel), so it takes ten complete driver revolutions for the star wheel to come back around to its starting position.
Why does this 10-slot design dwell for less of the cycle, proportionally, than a 6-slot Geneva? Because the motion phase (180° − 360°/n) grows relative to the dwell phase as slot count increases — more, smaller steps around the wheel mean the driver spends a larger share of each cycle actively engaged rather than locked in dwell.
Is this the same mechanism as a Maltese cross? Yes — "Maltese cross mechanism" and "external Geneva drive" refer to the same slot-and-pin indexing principle; the classic four-pointed star shape is just the most commonly pictured slot count, not the only one.
What's the most common failure mode in a physical build with this many slots? Slot-to-slot inconsistency — with ten slots sharing the same wheel diameter, small variations in how each slot and its locking cutout were generated are more likely to produce a visibly uneven step from one station to the next than they would on a coarser, lower-slot-count design.
Related Mechanisms
Geneva Drive Mechanism: How It Works + Animation and Design Explained — the full pillar guide covering external, internal, and spherical Geneva variants, plus an interactive calculator.
6-Slot Internal Geneva Mechanism: Why the Motion Phase Beats the Dwell — a companion case study showing how the internal configuration inverts the motion-dwell balance.
Explore more in 3D Mechanisms and Mechanical Movements.
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