Presets
Playback
Mechanism
More slots mean a shorter dwell but a gentler start and stop.
Layers
Model checks
The science
How a Geneva drive indexes
A driver turns continuously. Once per revolution its pin enters a radial slot of the driven wheel, turns the wheel by exactly 2π/N and leaves; a circular lock disc then holds the wheel still until the next entry. With N slots the geometry is fixed by λ = sin(π/N), the ratio of the crank radius to the centre distance (60 mm here).
Kinematics
With the driver angle θ measured from the line of centres, the wheel angle is ψ = atan2(λ sinθ, 1 − λ cosθ). Differentiating gives the speed ratio ωwheel/ωdriver = λ (cosθ − λ) / (1 − 2 λ cosθ + λ2), which starts from zero at entry (the pin arrives at a right angle to the slot, so there is no impact), peaks at λ/(1 − λ) on the line of centres and returns to zero at exit. The acceleration ratio λ (λ2 − 1) sinθ / (1 − 2 λ cosθ + λ2)2 is not zero at entry, so the jerk is infinite there: the reason Geneva drives are limited in speed.
Dwell and the number of slots
The pin is engaged while the driver is within ±(90° − 180°/N) of the line of centres, so the wheel moves for a fraction (N − 2)/(2N) of each revolution and dwells for (N + 2)/(2N). More slots give gentler peak acceleration but a shorter dwell; a 3-slot drive has the longest rest and the harshest motion.
Honest simplifications: rigid parts without clearance, friction or wear, a constant driver speed, and a lock disc drawn as a simple relieved circle (the 3D page uses an exactly conjugate profile). The kinematics are exact for the ideal geometry.
What to try
- Switch between 3, 4, 6 and 8 slots and watch the dwell fraction and the peak acceleration graph change.
- Raise the driver speed: acceleration grows with the square of the speed, so fast drives need more slots.
- Use Reverse crank: the wheel always turns opposite to the driver.
- Show the slot centre lines and check the pin enters exactly when its arm is perpendicular to the slot.