6-Slot Internal Geneva Mechanism: Why the Motion Phase Beats the Dwell

Internal Geneva Mechanism

Most people who've seen a Geneva mechanism have seen the classic version: a small pin-driven crank spinning fast, ticking a star wheel forward in short, sharp steps, then holding it locked for a long stretch before the next tick. That's the external Geneva drive — and it's the version that shapes most people's intuition about how the mechanism behaves.

This is a 6-slot internal Geneva mechanism, and it breaks that intuition in a specific, verifiable way: the wheel actually spends more of the cycle moving than it does dwelling. This article covers the full breakdown of our Geneva Drive pillar guide applied to this specific internal-type case, including where the 240°/120° split comes from and why it flips the usual rule.

Internal vs. External: The Geometry That Changes Everything

In an external Geneva mechanism, the driver crank and the driven star wheel sit side by side, with the crank's pin reaching outward into slots cut into the star wheel's outer edge. In an internal Geneva mechanism, the driven wheel's slots face inward instead, and the driver sits inside the wheel's own diameter, engaging the slots from within.

That single geometric flip changes the balance between motion and dwell. For a wheel with n slots:

• External Geneva: motion angle (driver) = 180° − 360°/n

• Internal Geneva: motion angle (driver) = 180° + 360°/n

For n = 6, that's the difference between a driver that's engaged for 120° (external) and one engaged for 240° (internal) — out of the same 360° cycle. The dwell phase is simply whatever's left: 240° dwell for the external case, or 120° dwell for the internal case. Same slot count, same 60° step angle for the driven wheel — completely different rhythm.

Internal Geneva Mechanism - Motion

Download the CAD File

Grab the CAD files (it’s free)

Internal Geneva Mechanism 1.zip

Download ZIP • 121KB

The 6-Slot Internal Geneva, Specifically

Applying that to this mechanism:

• Driven wheel step angle: 60° per index — set purely by the slot count (360°/6), the same for internal or external designs.

• Motion phase (driver rotation while engaged): 240°.

• Dwell phase (driver rotation while locked): 120°.

• Motion-to-dwell ratio: 2:1 — the wheel is actively moving for twice as long as it sits locked, the reverse of what most people assume a Geneva mechanism does.

This is worth sitting with for a second, because it directly contradicts the "quick tick, long pause" mental model most engineers default to. That model is accurate for external Genevas — it's simply not how the internal variant behaves.

How It Works, Step by Step

1. Continuous input. The driver crank, mounted inside the driven wheel's diameter, rotates continuously and at a constant speed.

2. Engagement. As the crank's pin reaches the entry point of one of the six internal slots, it engages tangentially — the same tangential-entry requirement that applies to external Genevas, just approached from inside the wheel rather than outside it.

3. Extended drive phase. Because of the internal geometry, the pin stays engaged for a full 240° of the driver's rotation — more than two-thirds of the total cycle — smoothly advancing the driven wheel through its 60° step.

4. Disengagement and locking. As the pin exits the slot, a locking arc on the driver engages a matching concave cutout on the driven wheel, holding it rigidly in place.

5. Short dwell. The driven wheel dwells for only 120° of driver rotation before the next slot's engagement begins — a comparatively brief pause relative to the long motion phase that just occurred.

Why This Matters for Design Choices

The practical implication is direct: if a machine design needs a long, gentle indexing motion with a comparatively short pause — rather than the sharp tick-then-long-pause behavior of an external Geneva — an internal Geneva with the same slot count delivers exactly that, from the same basic mechanism family. The tradeoff is packaging: an internal Geneva needs the driver to fit inside the driven wheel's diameter, which generally means a larger overall footprint than an equivalent external design.

	External Geneva (6-slot)	Internal Geneva (6-slot)
Driver engagement (motion phase)	120°	240°
Dwell phase	240°	120°
Motion:dwell character	Short motion, long dwell	Long motion, short dwell
Best suited for	Processes needing a long, stable pause per station	Applications needing a smoother, more gradual index with less idle time

Real-World Applications

• Automated assembly and packaging lines — where a smoother, more gradual index reduces shock loading on the conveyed product compared to a sharp external-Geneva tick.

• CNC automatic tool changers — precise, repeatable indexed positioning between tool stations.

• Mechanical watches and film projectors — the classic Geneva application family, more commonly using the external form, but built on the same underlying principle.

• Educational and demonstration mechanisms — a clear, physical way to show that "Geneva mechanism" isn't a single fixed rhythm, but a family of behaviors depending on internal vs. external configuration.

The CAD Design Challenge

Calculating the exact entry angle so the pin slides into the slot without binding or crashing is the fundamental kinematic exercise for any Geneva design — internal or external:

1. Verify tangential entry at the modeled center distance. The pin must enter and exit each slot moving tangentially to the slot's centerline; getting the center distance between driver and driven wheel even slightly wrong throws off this condition and causes impact loading at entry instead of a smooth tangential slide.

2. Model the locking arc and its matching concave cutout as a mated pair, not independently. These two surfaces have to share a common center for the dwell lock to hold without play — model them from the same reference geometry rather than sketching each separately.

3. Run a full motion simulation through several complete cycles, not just one, to confirm the pin consistently enters and exits cleanly at the calculated angles as tolerances and any small assembly variation come into play.

4. Double-check which formula applies — internal or external — before finalizing dimensions. Because the two configurations use different motion-angle formulas for the same slot count, using the wrong one at the design stage produces a mechanism that doesn't hit the intended step timing at all.

Frequently Asked Questions

Why does an internal Geneva mechanism move for longer than it dwells, when an external one does the opposite? The internal configuration's driver engages the slot for 180° + 360°/n of its rotation, compared to 180° − 360°/n for an external Geneva — for a 6-slot wheel, that's 240° of motion versus 120° for the internal case, the reverse of the external design's 120° motion and 240° dwell.

Does the driven wheel still step by 60° per cycle in the internal version? Yes — the step angle (360° divided by the number of slots) depends only on the slot count, not on whether the mechanism is internal or external. What changes is how much of the driver's rotation it takes to produce that step, and how much is left over for dwell.

When would a design use an internal Geneva instead of the more common external version? When the application benefits from a longer, more gradual indexing motion with a shorter dwell — for example, reducing shock loading on a conveyed product — and the larger footprint required to fit the driver inside the driven wheel's diameter is acceptable.

Is the 240°/120° split specific to this 6-slot design, or does it generalize? The formula (180° + 360°/n for motion, 180° − 360°/n for dwell) applies to any internal Geneva with n slots — for a 6-slot wheel specifically, that works out to 240° motion and 120° dwell, but the same relationship holds at other slot counts with the corresponding numbers.

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.

• Explore more in 3D Mechanisms and Mechanical Movements.