Geneva Drive

How a Geneva drive works

The mechanism has two parts: a driver disk that rotates continuously, and a star-shaped output wheel with radial slots cut into its rim. A single pin protrudes from the driver. As the driver turns, the pin enters one of the slots, drags the output wheel along for one slot-pitch (90° for a four-slot star, 60° for six-slot), then exits — leaving the output stopped.

Two complementary geometric features make the mechanism work:

• Tangential entry and exit. The slot is oriented so the pin enters along a line that's tangent to the slot's path. This means the output's angular velocity at entry is zero, and rises smoothly. No clunk, no infinite acceleration.
• Locking arc. The driver disk has a circular boss on its outer face that mates with a concave arc machined into each pair of slot lands on the star. While the pin is between slots, the boss-arc contact prevents the star from rotating — the output is positively held, not just at rest.

For a Geneva drive with n slots, each input revolution produces one slot's worth of output motion. Total output motion to traverse 360° therefore takes n input revolutions. The angular extent of the active (engaged) phase is (n − 2)π/n on the input side (the pin sweeps through this arc while engaged) and 2π/n on the output side.

Worked example: dwell ratios for 4, 6, 8 slots

The motion phase fraction of the input cycle is (n − 2) / (2n) and dwell is (n + 2) / (2n):

Slots (n)	Output step	Motion phase	Dwell phase	Cycles per output revolution
3	120°	17%	83%	3
4	90°	25%	75%	4
5	72°	30%	70%	5
6	60°	33%	67%	6
8	45°	38%	62%	8
12	30°	42%	58%	12

Note the trade. More slots means smaller steps (finer resolution) but a smaller share of dwell time per cycle. A four-slot Geneva gives 75% dwell — three quarters of every input rotation, the output is stationary. That's why projectors use four-slot designs: at 24 fps, the input shaft rotates at 24 RPS, and the output (advancing the film) is stationary for 75% of every cycle, giving the shutter time to project the frame.

To compute the exact velocity profile during the motion phase, with input angle θ measured from pin-entry, the output angle φ for a 4-slot Geneva of center-distance d and crank radius r = d·sin(45°) = d/√2 follows:

tan(φ) = (r·sin θ) / (d − r·cos θ)
φ̇ = (r·d·cos θ − r²) / (d² − 2·r·d·cos θ + r²) × θ̇

The output angular velocity is zero at θ = ±45° (entry and exit) and peaks at θ = 0 (radial alignment). For typical Geneva geometry, peak output angular velocity is roughly 1.4 to 2.4 times the input angular velocity, depending on slot count.

Geneva drive vs alternative indexing mechanisms

	Geneva drive	Cam-driven indexer	Ratchet and pawl	Servo motor	Stepper motor	Roller-gear cam
Position accuracy	±0.05° (precision unit)	±0.01° (custom cam)	±tooth pitch	±arc-second class	±half-step	±0.005°
Speed limit	~1,000 cycles/min	~3,000+ cycles/min	~300 cycles/min	Software-bound	Software-bound	~3,000 cycles/min
Direction	One only	One only	One only	Both	Both	One
Locking	Mechanical (arc)	Mechanical (cam)	Pawl friction	Servo torque	Detent + holding torque	Mechanical
Programmable step count	Fixed by slots	Fixed by cam	Variable	Yes	Yes	Fixed
Cost	Low	High	Very low	High	Medium	High
Best for	Fixed-ratio indexing	Custom motion law	Rough advance	Flexible cells	Open-loop steps	High-speed indexing

The Geneva drive's killer feature is mechanical positivity: no software, no power, no PID tuning. The output stops exactly where the locking arc puts it, and stays there until the next pin entry. For applications where you need precisely n positions per revolution and you have continuous rotary input available, nothing is simpler.

Real-world specifications

• 35 mm film projectors. Four-slot Geneva, output advances 90° per input revolution. Input shaft at 24 RPS gives 24 frames per second; the dwell phase coincides with the rotating shutter open-phase so the audience sees a still image. Replaced largely by digital cinema projectors after 2010.
• Mechanical wristwatches. The Maltese cross was an 18th-century stop-work device — a kind of Geneva mechanism — that limited mainspring winding to its central, most linear region. The "Geneva stripes" finish on movements is named after this same mechanism's home town.
• Industrial assembly indexing tables. Cam-driven indexers have largely displaced Geneva drives at the high end, but Geneva drives remain in pharmacy bottle-fillers, glass tube insertion machines, and assembly fixtures where a 60° or 90° step is acceptable.
• Vending-machine carousels. A motor-driven Geneva ensures the snack carousel advances exactly one slot per coin, with the locking arc keeping it stationary against vending pressure.
• Roll-feed presses. A Geneva-driven feed roller advances stock by a fixed pitch each press cycle.

Variants

• External Geneva. The classic version: pin and slots on opposite sides of the center distance. Compact, easy to manufacture, used in most projectors.
• Internal Geneva. The driving pin is on the inside of a ring, with the star wheel inscribed inside. Allows a wider range of r/d ratios and gives smoother kinematics for some slot counts.
• Spherical Geneva. Driver and follower axes intersect at right angles. Found in some watch complications and old computing devices.
• Multi-pin Geneva. Multiple pins on the driver multiply the index frequency. A four-slot star with a two-pin driver indexes twice per input revolution.
• Modified Geneva (curved slots). Curved slots smooth the velocity profile and reduce peak acceleration. Used in high-speed packaging.
• Stop-work / Maltese cross. Limits maximum revolutions using similar tangent-engagement geometry. Found on watch mainsprings to bound winding.

Common failure modes

• Pin shear under high inertial loads. The pin is the highest-stressed element. Sudden output deceleration (jamming, dropped tools, or contact with a workpiece) puts large lateral force on the pin and can shear it. Hardened steel pins and shock-load fuses (a sacrificial shear pin) mitigate this.
• Slot wear at entry edges. The slot mouth experiences highest contact stress because pin entry happens with finite relative velocity; over millions of cycles the entry corner wears, increasing positioning error and impact noise.
• Locking-arc wear. The boss-on-arc sliding contact during dwell wears the convex boss, eventually allowing residual rotation of the output during dwell — a fatal flaw for vending or printing applications that depend on exact alignment.
• Backlash in the output bearing. The pin can position the output precisely only if the output shaft is free of bearing slop. Worn bearings or loose shaft couplings introduce error that the Geneva mechanism can't correct.
• Lubrication failure. The high pressures at pin-slot contact and boss-arc sliding require continuous lubrication. Dusty environments need sealed enclosures or frequent regreasing.
• Star-wheel cracking. Brittle materials under repeated shock loading can develop fatigue cracks at slot roots. Modern designs use through-hardened tool steel or shot-peened surfaces to extend life.
• Pin-mounting failure on the driver. Press-fit pins loosen over time, especially when the input shaft is reversible. Set-screws or threaded shoulders are the fix.

Common misconceptions

• The output velocity is constant during the motion phase. It isn't — output velocity is zero at entry and exit, with a peak in between.
• Geneva drives are silent. The pin-slot contact at entry and the boss-arc contact at exit produce audible clicks that grow louder with wear.
• You can run a Geneva drive in reverse. Mechanically possible but impractical: the pin re-engages the slot's exit edge, accelerating wear.
• The locking arc only holds the output during dwell. The same arc also pre-positions the output before pin entry, which is why Geneva drives don't drift between cycles.