An internal Geneva drive is an intermittent motion mechanism where the driving crank pin engages slots cut into the inside rim of a driven wheel, converting continuous input rotation into precise stop-and-go output indexing. Internal Genevas commonly run 60–300 indexes per minute with positional repeatability under ±0.05° on quality builds. The geometry shortens dwell time and lengthens motion time compared to an external Geneva, which is why it shows up in high-cycle indexers like Stokes tablet press turrets and older 35 mm film projector pulldowns where dwell-to-motion ratio matters more than compactness.
Internal Geneva Drive Interactive Calculator
Vary slot count and completed driver turns to see output indexing travel plus the internal Geneva motion and dwell angles.
Equation Used
For an ideal internal Geneva drive with N slots, each completed driver revolution advances the output by alpha = 360 / N. The internal version dwells for the same crank angle alpha and moves for the remaining crank angle, 360 - alpha.
FIRGELLI Automations - Interactive Mechanism Calculators.
- Internal Geneva geometry with one output index per full driver revolution.
- Slots are equally spaced around the driven wheel.
- Driver Turns counts completed full revolutions.
- Ideal kinematic angles; clearance, inertia, and acceleration are not included.
Operating Principle of the Geneva Drive (internal)
The internal Geneva runs on the same principle as the external (Maltese cross) version — a driving crank with a single pin engages radial slots in a driven wheel — but the slots open inward from the rim instead of outward. The pin enters a slot tangentially, drags the driven wheel through one index, then exits. While the pin is disengaged, a circular locking arc on the crank rides against a matching concave arc on the driven wheel, holding the output dead-still. No slop, no drift, no need for a brake.
The defining trick of the internal version is the dwell-to-motion ratio. On a 4-slot external Geneva the driven wheel moves for 90° of crank rotation and dwells for 270°. Flip to a 4-slot internal Geneva and the driven wheel moves for 270° of crank rotation and dwells for only 90°. That is a much gentler index — lower peak acceleration, lower jerk, less hammering on the slot walls. You buy that smoothness with a bigger driven wheel diameter and a more cramped layout, which is why internal Genevas show up where space allows it and cycle rate punishes shock loading.
Tolerances matter more than people expect. The pin-to-slot clearance has to sit in the 0.02–0.05 mm range on a 6 mm pin — tighter and the pin binds at slot entry, looser and you get a clack at engagement that walks the indexing position over thousands of cycles. The locking arc radius must match the driven wheel cup radius to within 0.01 mm or the wheel rocks during dwell. If you see indexing position drift over a shift, suspect three things in this order: pin wear (measure with a pin gauge), slot mouth peening (inspect with a 10× loupe), and locking-arc-to-cup gap opening up from a loose crank fastener.
Key Components
- Driving Crank (Driver): The continuously rotating input member, carrying the engagement pin and the locking arc. Crank is typically hardened steel, 45–55 HRC, with the pin pressed into a reamed bore held to H7 fit. Crank rotation is steady — usually direct from a gearmotor at 60–300 RPM.
- Engagement Pin: A hardened, ground steel pin (typically 4–8 mm diameter, 60 HRC minimum) that slides into the slot to drag the driven wheel through one index. Surface finish matters — Ra below 0.2 µm is the rule for slot longevity. Pin wear of more than 0.03 mm on diameter is the scrap point.
- Driven Wheel (Slot Wheel): The output member with internally-facing radial slots, one slot per index station. A 4-slot wheel indexes 90° per cycle, a 6-slot indexes 60°, an 8-slot indexes 45°. Slot walls are ground, parallel within 0.01 mm, and the slot bottom radius matches the pin radius plus running clearance.
- Locking Arc and Locking Cup: The crank carries a convex locking arc; the driven wheel rim carries a matching concave cup between every pair of slots. During dwell these two arcs ride in contact and hold the output rigidly stationary. Concentricity between locking arc and crank centerline must be within 0.01 mm or the output wobbles during dwell.
- Output Shaft and Bearings: The driven wheel rides on a precision output shaft, usually two angular-contact or deep-groove ball bearings preloaded to remove axial play. Radial runout at the indexing station must stay under 0.02 mm — anything more and downstream tooling misregisters.
Where the Geneva Drive (internal) Is Used
Internal Genevas show up wherever you need a high cycle rate with a smoother index than an external Geneva can deliver, and where you can afford the larger driven wheel. They were everywhere in 20th-century film and photographic equipment, and they are still found in pharmaceutical, packaging, and certain horological applications. The mechanism is purely mechanical — no servos, no encoders, no firmware — which is why it survives in machines that need to run for decades with minimal electronics.
- Cinema Equipment: The Bell & Howell Model 2709 35 mm camera used an internal Geneva-style intermittent to pull each frame down behind the gate, holding it dead still during exposure.
- Pharmaceutical Manufacturing: Older Stokes B2 tablet-press feed turrets used internal Geneva indexers to advance die positions between fill, compression, and ejection stations at 80–120 indexes per minute.
- Watchmaking: The date ring jumper on certain ETA 2824-2 movement variants uses a miniature internal-style Geneva action so the date snaps over instantly at midnight rather than dragging through the change.
- Packaging Machinery: Bartelt horizontal pouch fillers used internal Geneva drives on the pouch-transport carousel to index pre-formed pouches through fill and seal stations with smoother acceleration than an external Geneva would allow.
- Textile Machinery: Cop-changing mechanisms on Schweiter automatic winders used internal Geneva-style indexers to rotate fresh bobbin holders into position without shock-loading the yarn package.
- Coin and Token Counters: Klopp coin-counting and bagging machines historically used internal Genevas in the bag-positioning carousel, where the long dwell-to-motion ratio gave the diverter time to dump a count cleanly.
The Formula Behind the Geneva Drive (internal)
The angular displacement of the driven wheel as a function of crank angle is the formula that tells you whether your index will run smoothly or hammer itself to death. At the low end of the typical operating range — say 60 RPM input — peak angular acceleration on the driven wheel is modest and the slot walls see manageable contact stress. At the nominal 150 RPM most production indexers run at, accelerations climb into the range where slot wall hardness and pin finish actually matter. Push toward 300 RPM and the inertia torque rises with the square of speed; that is where you start chipping pin tips and rounding slot mouths. The sweet spot for a typical 4-slot internal Geneva sits around 120–180 RPM input — fast enough to be productive, slow enough that mechanical life beats 10⁸ cycles.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θdriven | Angular position of the driven (slot) wheel during indexing | radians | degrees |
| θcrank | Angular position of the driving crank | radians | degrees |
| a | Crank pin radius (distance from crank center to pin center) | mm | in |
| R | Center distance between crank axis and driven wheel axis | mm | in |
| n | Number of slots on the driven wheel (governs index angle = 360°/n) | count | count |
Worked Example: Geneva Drive (internal) in a 6-slot internal Geneva for a pinion-stamping turret
Sizing a 6-slot internal Geneva indexer for a Bruderer BSTA 30 high-speed stamping turret feeding small brass pinion blanks into a coining die. The turret must advance 60° per index, hold each blank stationary while the ram cycles, and run at the press's nominal 150 strokes per minute. Crank pin radius a = 50 mm, center distance R = 100 mm.
Given
- n = 6 slots
- a = 50 mm
- R = 100 mm
- Nnom = 150 RPM
Solution
Step 1 — calculate index angle per cycle from the slot count:
Step 2 — at nominal 150 RPM input, calculate cycle time and motion time. For an internal 6-slot Geneva, the driven wheel moves through (180° + 360°/n) = 240° of crank rotation per index, so motion time tm is 240/360 of one input revolution:
Step 3 — compute peak driven-wheel angular velocity at nominal input speed. Peak ωdriven occurs at the slot's deepest engagement; for an internal Geneva it works out to ωcrank × a / (R − a):
At the low end of the typical operating range — say 60 RPM, which is what you would see during a slow setup run — peak driven angular velocity scales linearly to ωpeak,low = 6.28 rad/s. That is a gentle, audibly quiet index, and slot wall stress is well within infinite-life territory. At the high end, push to 300 RPM (some Bruderer setups do run there), and ωpeak,high = 31.42 rad/s. Peak angular acceleration scales with N2, so going from 150 to 300 RPM does not double the contact stress at the pin-slot interface — it quadruples it.
Step 4 — interpret the dwell time at nominal:
The press ram needs to enter, coin, and clear in 0.133 s. That is tight but workable for a Bruderer at 150 SPM. At 60 RPM dwell stretches to 0.333 s, comfortable. At 300 RPM dwell shrinks to 0.067 s, and now the ram timing has to be perfect or you crash a partially-indexed station.
Result
Nominal peak driven angular velocity is 15. 71 rad/s with 0.133 s of usable dwell per index at 150 RPM input. In practice that means the turret advances cleanly with about 90 ms of safety margin around a typical 40 ms coining stroke — comfortable for production. At 60 RPM the index feels languid (0.333 s dwell), at 300 RPM the dwell collapses to 67 ms and ram-to-turret timing tolerance shrinks below 5 ms — past that point a single timing belt stretch will crash a die. If your measured peak velocity comes in 10–15% below this prediction, three failure modes account for almost all of it: (1) crank pin-to-slot clearance opened past 0.05 mm from wear, allowing the pin to lag at engagement; (2) the locking arc fastener has loosened and the driven wheel rocks during dwell, eating part of the next motion phase; or (3) the input gearmotor is slipping on its keyway under peak load — check the keyway for fretting marks and the key for rollover.
Choosing the Geneva Drive (internal): Pros and Cons
Internal Geneva is one of three close cousins for delivering intermittent rotary motion. The decision usually comes down to dwell-to-motion ratio, packaging, and budget. Here is how it stacks up against the external (Maltese cross) Geneva and a cam-driven indexer like the Camco RDM series.
| Property | Internal Geneva | External (Maltese Cross) Geneva | Cam-Driven Indexer (Camco RDM-style) |
|---|---|---|---|
| Typical input RPM range | 60–300 RPM | 30–200 RPM | 30–1,200 RPM |
| Indexing repeatability | ±0.03–0.05° | ±0.05–0.10° | ±0.005–0.01° |
| Dwell-to-motion ratio (4-slot) | 1:3 (short dwell, long motion) | 3:1 (long dwell, short motion) | Tunable from 1:5 to 5:1 by cam profile |
| Peak angular acceleration | Lower than external — smoother index | Higher — sharp acceleration spike at engagement | Lowest — modified-sine cam profile |
| Packaging | Larger driven wheel diameter | Compact, smallest envelope | Largest — needs cam box and oil bath |
| Relative cost | Medium | Low | High (3–10× the Geneva) |
| Service life at nominal load | 10⁸ cycles typical | 5×10⁷ cycles typical | 10⁹ cycles typical |
| Best application fit | High-cycle indexers needing smooth motion | Simple, compact, low-cost indexing | Precision indexing turrets, machine tools |
Frequently Asked Questions About Geneva Drive (internal)
The motion angle on the crank is longer for an internal Geneva — 240° versus 120° on a 6-slot — so the same 60° driven-wheel index is spread across twice as much crank rotation. Peak angular acceleration on the driven wheel is roughly proportional to ωcrank2 × (a / (R − a)) for the internal case versus a / (R + a) for the external. The denominator is smaller for the internal Geneva, so peak velocity is higher, but the acceleration profile is gentler at engagement and disengagement because the pin enters and leaves the slot at a shallower path angle.
The practical effect is less audible clack at slot entry and lower peening stress on the slot mouth. That is exactly why high-cycle film cameras adopted the internal arrangement.
The textbook ratio a / R = sin(180°/n) is the geometric ideal where the pin enters tangent to the slot. It assumes zero clearance and a perfectly rigid frame. In a real build with bearing play, frame flex, and a 0.03 mm pin-to-slot clearance, the pin does not arrive exactly tangent — it arrives at a slight angle and tries to wedge.
The fix is to reduce a / R by about 0.5–1.0% (so for a 4-slot, target 0.700 instead of 0.7071) and add a lead-in chamfer of 0.2 mm × 30° at the slot mouth. That gives the pin a gathering surface and forgives the small geometric errors of a real assembly.
Slot count must equal station count — that is non-negotiable, because each slot corresponds to one index. Within that constraint, more slots means smaller index angle (45° for 8, 60° for 6, 90° for 4) and shorter motion time per index, which means higher peak velocity and acceleration on the driven wheel for a given input RPM.
Rule of thumb: if you have a choice between adding more stations or running faster, more slots wins on smoothness. A 6-slot at 150 RPM is mechanically gentler than a 4-slot at 100 RPM doing the same throughput, because the smaller index angle keeps peak acceleration lower.
Hum during dwell almost always means the locking arc is not fully seating against the locking cup. The two most common causes are: (1) the locking arc radius and locking cup radius differ by more than 0.02 mm, leaving a line contact instead of arc contact that lets the wheel rock under any disturbance torque from downstream tooling; or (2) the crank locking arc and the driven wheel cup are not on parallel axes — a 0.05° axis misalignment turns full arc contact into a single-point contact that buzzes at crank frequency.
Quick check: with the input stopped mid-dwell, try to rotate the driven wheel by hand. Any detectable rotation means the arcs are not seating. Re-grind or re-shim until the wheel is dead solid.
No, and trying it will destroy the mechanism within a few cycles. The Geneva relies on the locking arc to hold position during dwell; if you drive the slot wheel, there is no equivalent locking surface on the driven crank, so the crank free-spins and the pin slams into the slot wall on every engagement.
If you need bidirectional intermittent motion, use a cam-driven indexer with a roller-gear cam — those are reversible by design. The Geneva, internal or external, is a one-way mechanism.
Drift over thousands of cycles, with parts measuring in tolerance, points to fastener creep rather than wear. The crank-to-shaft connection (typically a key plus a setscrew or a tapered locking bushing) is the usual suspect. Any micro-rotation of the crank on its shaft accumulates as a position offset on every index.
Diagnostic: scribe a witness line across the crank-to-shaft joint after assembly. Run 5,000 cycles, then inspect. If the witness line has moved, the fix is a positive-engagement coupling — a Tsubaki Power-Lock bushing or a properly-fitted woodruff key with thread-locked setscrews against a flat. Don't rely on a setscrew biting into a round shaft.
Around 300 RPM input for a 4-slot, 350–400 RPM for a 6-slot, in a typical industrial-grade build. The limit is not bearing speed — it is impact energy at slot engagement, which scales with the square of input RPM. Above that range, slot mouth peening starts to show up within 10⁶ cycles even with hardened slots, and the pin tip starts mushrooming.
If your application needs faster indexing than that, switch to a cam-driven indexer with a modified-sine profile. A Camco or Sankyo unit will run 600–1,200 RPM at the same indexing accuracy with a 10× life improvement, at roughly 5× the cost.
References & Further Reading
- Wikipedia contributors. Geneva drive. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
