A Geneva mechanism is a compact mechanical device that turns smooth, continuous rotation into step-by-step motion. It is useful in robotics and automation when a part must move to an exact position, stop briefly, and then move again. This kind of intermittent indexing is common in rotary tables, tool changers, film projectors, and turret systems.
The mechanism is popular because it gives precise angular steps using only simple rotating parts.
In a typical external Geneva drive, a drive wheel rotates continuously and carries a pin near its edge. As the pin enters one slot of the Geneva wheel, it pushes the wheel through a fixed angle, then exits the slot so the Geneva wheel remains still during the dwell period. A locking surface on the drive wheel often fits against the Geneva wheel between steps to hold it in position.
The number of slots determines the indexing angle, so a 6-slot Geneva wheel advances 60 degrees each time it is engaged.
Key Facts
- A Geneva drive converts continuous rotation into intermittent rotary motion.
- Index angle per step = 360 degrees / number of slots.
- For N slots, each full engagement advances the Geneva wheel by 1/N of a full turn.
- Dwell is the time interval when the Geneva wheel is stationary while the drive wheel keeps rotating.
- The drive pin provides motion only while it is inside a slot of the Geneva wheel.
- Average indexing speed during one full drive revolution depends on step angle and cycle time: omega_avg = theta_step / T.
Vocabulary
- Geneva mechanism
- A mechanical linkage that changes continuous rotation into intermittent stepwise rotation.
- Drive wheel
- The continuously rotating wheel that carries the pin and supplies motion to the Geneva wheel.
- Drive pin
- The protruding pin on the drive wheel that enters a slot and pushes the Geneva wheel through one indexing step.
- Geneva wheel
- The slotted output wheel that rotates by a fixed angle each time the drive pin engages one of its slots.
- Dwell
- The part of the cycle when the output wheel remains stationary between indexing steps.
Common Mistakes to Avoid
- Confusing continuous input with continuous output is wrong because the drive wheel rotates continuously while the Geneva wheel moves only during short engagement intervals.
- Using the drive wheel angle as the index angle is wrong because the output step angle is set by the number of slots in the Geneva wheel.
- Ignoring dwell time is wrong because the stationary interval is a major reason Geneva drives are used in indexing tables and turret mechanisms.
- Assuming the drive pin is always pushing the Geneva wheel is wrong because the pin only transfers motion while it is inside a slot.
Practice Questions
- 1 A Geneva wheel has 8 slots. What is the angular index step in degrees for each engagement?
- 2 A 5-slot Geneva wheel is driven by a motor rotating at 60 rpm, with one indexing step per drive wheel revolution. How many output indexing steps occur per minute, and what angle does the Geneva wheel move per step?
- 3 Explain why a Geneva mechanism is useful for a robotic indexing table that must hold parts still while a sensor or tool performs an operation.