A Geneva drive converts continuous input rotation into a fixed angular step followed by a stationary dwell. In the standard single-pin external arrangement, a driver pin enters a radial slot in the output wheel, advances it one position, then withdraws. Mating locking surfaces hold the output during dwell. A four-slot wheel advances 90° per input revolution and, at constant input speed, dwells for 75% of each cycle.
External Geneva Drive Calculator
Choose the slot count, driver speed and pin-orbit radius to calculate each step, its motion and dwell times, and the distance between shafts. Watch the pin enter a radial slot while the locking disc clears the moving wheel, then hold the wheel between steps.
Equations Used
C = R / sin(Ï€ / n)
T = 60 / RPM
Dwell = T(n + 2) / (2n)
Motion = T(n − 2) / (2n)
These equations describe a standard single-pin external Geneva with tangential pin entry. At zero RPM, cycle times are undefined. Animation runs at one fifth of the entered speed; the calculated times are real times. The drawing scales to fit, so changing R changes shaft spacing in millimetres without changing the displayed proportions. Pin size, slot clearance and locking-disc relief are illustrative; they do not establish torque, wear or a safe operating speed. The rear crank disc lies below the slotted wheel.
How the pin and slots produce a step
The driver rotates continuously around one fixed centre. Its pin travels in a circle and enters a slot in the wheel on the second centre. While engaged, the pin moves along the slot and turns the wheel. Once it exits, the wheel remains stationary while the driver completes its revolution.
For the standard external geometry with n equally spaced radial slots, pin-orbit radius R and centre distance C satisfy R/C = sin(180°/n). At four slots, C = R√2. This relation gives zero output velocity at ideal entry and exit; it does not remove the acceleration change at those transitions.
Main parts
- Driver: carries the pin and rotates about the input shaft.
- Drive pin or roller: enters successive radial slots.
- Slotted wheel: advances by 360°/n per engagement.
- Locking surfaces: restrain output motion during dwell.
- Frame and bearings: maintain the required shaft positions.
Real clearance, deflection and wear affect positioning. The lock is not an unlimited torque restraint, and the ideal geometry does not specify machining tolerances or a maximum operating speed.
Where intermittent indexing is useful
Geneva drives are associated with intermittent film transport and rotary indexing. The dwell provides time for a process to act while the output is stationary. Slot count sets the number of equally spaced output positions.
Watch winding stopwork uses a related arrangement with a blocked position to limit rotation. It must be distinguished from an ordinary Geneva drive that can index continuously; slot count alone does not establish a watch’s permitted winding turns.
For an inspection or assembly table, compare required processing and settling time with the available dwell. Load inertia, input-speed variation, backlash, strength and vibration need separate analysis before selecting a drive.
Step angle and dwell time
These expressions apply to a standard single-pin external Geneva with n slots and constant driver speed N in revolutions per minute.
Output step =360°/n
Input rotation during motion =180°−360°/n
Input rotation during dwell =180°+360°/n
Cycle time =60/N seconds
Motion fraction =(n−2)/(2n)
Dwell fraction =(n+2)/(2n)
Dwell time =(60/N) × (n+2)/(2n)
| Slots | Output step | Motion fraction | Dwell fraction |
|---|---|---|---|
| 3 | 120° | 1/6 | 5/6 |
| 4 | 90° | 1/4 | 3/4 |
| 6 | 60° | 1/3 | 2/3 |
| 8 | 45° | 3/8 | 5/8 |
More slots give smaller steps and reduce the dwell fraction toward one-half. Internal Geneva mechanisms have different engagement timing; these external-drive expressions should not be applied to them.
Example: a four-position inspection table
Consider a hypothetical four-slot external Geneva with a station requiring 0.6 seconds of stationary time. At 40 rpm input, each cycle takes 1.5 seconds. Dwell fraction is (4+2)/(2×4) =6/8 =0.75, giving 1.125 seconds of dwell and 0.375 seconds of motion.
| Input speed | Dwell | Motion | Dwell minus 0.6 s |
|---|---|---|---|
| 20 rpm | 2.25 s | 0.75 s | 1.65 s |
| 40 rpm | 1.125 s | 0.375 s | 0.525 s |
| 60 rpm | 0.75 s | 0.25 s | 0.15 s |
Each case has more than 0.6 seconds of ideal dwell, but usable processing time also depends on settling and process timing. For unchanged geometry, acceleration scales with input speed squared: increasing speed from 40 to 60 rpm multiplies ideal acceleration by (60/40)² =2.25. These timings do not establish that the pin, slots or load can withstand the faster cycle.
Comparing indexing methods
| Method | Motion and dwell | What to evaluate |
|---|---|---|
| Standard external Geneva | Step and dwell fractions follow slot count and geometry | Entry/exit acceleration changes, pin contact, locking clearance and inertia |
| Cam indexer | Motion law and dwell can be designed into the cam | Cam profile, followers, preload and rated loads |
| Ratchet and pawl | Intermittent advances follow the driving stroke | Tooth engagement, holding arrangement and backlash |
| Servo-driven indexer | Timing can be programmed | Motor torque, feedback, control and transmission stiffness |
No universal accuracy, speed, life or cost follows from the mechanism name. Compare specified products or analyze the actual geometry and load.
Frequently asked questions
What sets the output step angle?
For n equally spaced slots, each engagement advances the output by 360°/n. A single-pin driver produces one engagement per revolution.
Does an eight-slot external Geneva have more dwell than a four-slot one?
No. At the same constant input speed, the standard four-slot dwell fraction is 75%, while the eight-slot fraction is 62.5%.
Can the driver run in reverse?
An ideal standard Geneva can be driven in either direction by reversing its input. The output cannot simply be forced through its locked dwell. Check the actual drive’s load and operating restrictions.
Is a Geneva winding stop identical to a continuously indexing drive?
They share a pin-and-slot principle, but winding stopwork includes a blocked position or equivalent stop feature. A normal open-slot indexing wheel does not impose a fixed number of winding turns.
Why can a Geneva produce shock even when the geometry is correct?
Ideal output velocity reaches zero at entry and exit, but acceleration changes abruptly. Clearance, compliance and inertia can add contact shock in a real mechanism.
Can these equations specify pin size or allowable speed?
No. They give ideal geometry and timing. Pin contact stress, slot strength, locking loads, inertia, bearings, lubrication and tolerances need separate calculations.
Technical references
- NPTEL: Geneva mechanism, pin-and-slot operation and external/internal distinctions.
- Cornell engineering collection: Geneva mechanisms, geometry and motion.
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