The Geneva Drive consists of a continuously driven rotor with a drive pin and a crescent locking disk, alongside a driven wheel (Maltese cross) with radial slots.
- Dwell Phase: The convex locking disk on the driver slides smoothly against the concave arc of the cross, locking it motionless without backlash.
- Slot Entry: The drive pin enters the radial slot tangentially at 90° to the slot centerline, ensuring smooth velocity transition ($\omega_2 = 0$).
- Index Phase: The pin accelerates the driven wheel through an angle of $360^\circ / n$, then exits smoothly at the opposite boundary.
Kinematic relationship during engagement ($\theta \in [-\alpha_0, +\alpha_0]$):
m = r / a = sin(π / n)
tan(φ) = (m sin θ) / (1 - m cos θ)
ω₂ / ω₁ = (m cos θ - m²) / (1 + m² - 2m cos θ)
α₀ = (n - 2)π / 2n
Notice that at entry ($\theta = -\alpha_0$), angular velocity is zero, but angular acceleration spikes instantly. This acceleration jump produces dynamic jerk, limiting high-speed use.
Key Mechanical Characteristics:
- Film Projectors: 4-slot Geneva wheels advance 35mm film frame-by-frame 24 times per second (1/96 sec pull-down, 3/96 sec dwell).
- Tool Changers: Used in CNC turrets and automated rotary index tables for precise positioning.
- Design Constraint: Slots $n < 3$ are geometrically impossible for external Geneva mechanisms. Higher $n$ increases dwell percentage.