Geneva Drives: Turning Spinning into Perfect Stepping
Some jobs cannot tolerate continuous motion. A film frame must sit perfectly still while the shutter opens, then jump — instantly — to the next. A bottling line must hold each bottle under the filler, then snap it forward. The mechanism of choice for two centuries has been the Geneva drive: one wheel that turns continuously, and another that only ever steps.
The parts and the catch
A Geneva mechanism has just two members:
- The driver — a disc turning at constant speed, carrying a pin near its rim and a concave locking crescent.
- The Geneva wheel — a plate with radial slots and convex bumps between them (the classic Maltese-cross shape).
Once per revolution, the driver’s pin slides into a slot and carries the wheel around through a precise arc — exactly one slot-pitch, 60° for a six-slot wheel. Then the pin lifts out, and the driver’s crescent seats against the wheel’s convex bump, locking it dead still until the next catch. Rotate continuously; move in steps; dwell in between.
The dwell ratio is designed, not fixed
The share of time the wheel spends moving versus still is a direct function of geometry. More slots mean a shorter index and longer dwell; fewer slots (the classic 4-slot cross) index faster and dwell shorter. The rule of thumb:
- 6 slots → the wheel moves about 1/3 of the cycle (as in the animation above).
- 4 slots → the wheel moves about 1/4 of the cycle, with violent acceleration peaks.
- 12 slots → gentle motion, but the dwell shrinks and the wheel is large for its output angle.
Below four slots the pin cannot exit cleanly — the geometry breaks — which is why four is the classic minimum. Designers pick the slot count to match the rhythm the process needs.
Why not just use a motor that stops and starts?
Because accelerating a real load from zero to speed and back, sixty times a second, destroys motors and shakes mechanisms apart. The Geneva drive’s genius is that the driver never stops: the brutal acceleration happens mechanically, in a shaped arc, with the pin’s path designed so the wheel’s velocity starts and ends at exactly zero — no impacts, no backlash at the stops, no electronics. It is motion profiling, baked into steel.
Where Genevas have worked
- Film projectors — the archetype: 16–24 steps per second, frame held while the shutter flashes light through it.
- Indexing tables — machining centres that rotate a part to the next station and clamp it dead still.
- Bottling and packaging lines — carousels that pause under each filler, capper, or labeler.
- Mechanical counters and watches — some date wheels and counting mechanisms are pure Geneva.
- Banknote printing and numbering — exact one-step-at-a-time feeding.
Key takeaways
- Two members: a constantly rotating driver and a wheel that only steps and dwells.
- The pin indexes the wheel; the crescent-and-bump pair locks it motionless.
- Dwell-to-motion ratio is chosen by slot count — four is the geometric minimum.
- It converts steady rotation into perfectly profiled steps with zero electronics.