A Geneva Drive (external) is a mechanical indexer that converts continuous input rotation into stepped output rotation using a driving wheel with a single pin and a driven wheel with radial slots. The pin enters a slot, sweeps the driven wheel through one fixed angular step, then exits while a circular locking arc holds the output stationary. Engineers use it when a station needs to advance, dwell, advance again — without an electronic servo or clutch. Its repeated step-and-dwell motion is useful when an output must pause between advances.
Geneva Drive External Interactive Calculator
Vary slot count to compare output step and driver index/dwell timing. Compare the selected pin clearance with an illustrative band, not a fit or risk rating.
Equation Used
The calculator follows the worked 4-slot Geneva relationship. Slot count sets the output index angle, the driver angle spent indexing, and the remaining dwell period. The dwell KPI is shown as index:dwell = 1:x, so the default 4-slot case gives 1:3.
- External Geneva drive with one drive pin.
- Ideal tangential pin entry and exit.
- Dwell ratio is reported as index:dwell = 1:x.
- The 0.05 to 0.10 mm clearance band is an illustrative comparison, not a recommended fit.
How the external Geneva drive works
The continuously rotating driver carries a pin that enters one radial slot in the output wheel. While engaged, the pin turns the wheel through one station. After the pin leaves, the driver’s circular locking surface fits against a concave portion of the output wheel and holds it during the dwell.
The classic geometry gives zero ideal output velocity at entry and exit. This does not eliminate contact loads, acceleration changes or clearance effects in a real mechanism. The pin, slots, bearings and locking surfaces all contribute to positioning behaviour.
Parts to follow in the animation
- Driver and pin: the blue driver rotates continuously, carrying the gold pin around its circular path.
- Slotted wheel: each pin engagement advances the wheel by one slot pitch. Four slots give a quarter turn per driver revolution.
- Locking disc: its relieved portion permits indexing; its circular portion restrains the wheel during dwell.
- Pin-to-slot gap: the enlarged detail shows the selected clearance. The timing equations do not predict contact force or positional error from that gap.
- Shaft supports: their spacing and alignment must match the chosen geometry. They are essential to an actual assembly even though the illustration focuses on the moving parts.
Where intermittent indexing is useful
A Geneva drive suits a repeated sequence of movement followed by a stationary interval. An indexing table, film transport or demonstration model can use that interval for an operation while the output rests. The required station count determines the output step, while the available dwell must be long enough for the operation.
The worked example below is a hypothetical four-station carousel. It demonstrates timing only; it is not a specification for a particular bottling line, tool changer or projector. Load inertia, speed, balance, guarding and the strength of the holding arrangement require separate assessment.
Slot count sets the index and dwell angles
For an ideal single-pin external Geneva with n equally spaced slots, the output step is 360°/n. The driver rotates through 180° − 360°/n while indexing and through 180° + 360°/n during dwell. These are driver angles: the output itself is stationary throughout the dwell.
At constant driver speed, the time ratio is the same as the angle ratio. The displayed index:dwell ratio is 1 : [(180° + 360°/n)/(180° − 360°/n)]. Four slots give 1:3, six give 1:2, and eight give 3:5.
The clearance comparison reports the distance of the entered gap outside the example band of 0.05–0.10 mm. A zero result means only that the input is inside that band. It does not establish a suitable running fit, positioning accuracy, thermal allowance or resistance to jamming. Those depend on dimensions, materials, temperature, lubrication and loading.
Worked example: a four-station carousel
Suppose an ideal four-slot mechanism has a driver running steadily at 60 rpm. Each driver revolution takes one second. The following calculation uses that chosen speed to convert the calculator’s angular results into times.
- The output advances by 360°/4 = 90° per index.
- The driver’s indexing sweep is 180° − 90° = 90°.
- The driver’s dwell sweep is 180° + 90° = 270°.
- Index time is 90°/360° × 1 s = 0.25 s.
- Dwell time is 270°/360° × 1 s = 0.75 s.
The carousel therefore advances one station per second and completes a full output revolution in four seconds. At 30 rpm driver speed, both times double; at 120 rpm, both halve. This timing comparison does not size the pin, establish a maximum speed or predict fatigue life.
Comparing indexing approaches
| Feature | External Geneva | Cam indexer | Servo indexer |
|---|---|---|---|
| Motion sequence | Pin and slot geometry set the step and dwell | Cam profile sets the motion law | Controller sets the motion command |
| Changing the step | Usually requires different mechanism geometry | May require a different cam or arrangement | Can be commanded within machine limits |
| Holding during dwell | Locking-disc contact | Depends on the cam and follower arrangement | Depends on the drive, brake and load |
| Design checks | Pin/slot contact, locking fit and inertia | Follower contact, cam geometry and inertia | Motor torque, feedback, braking and control |
Accuracy, cost and service life are properties of a specified assembly and operating duty. This timing calculator does not provide those ratings for any of the alternatives.
Geneva drive questions
Why is the wheel stationary for part of each cycle?
The drive pin is clear of the slots during dwell. The circular locking surface restrains the slotted wheel until the next engagement.
What changes when I select more slots?
Each output step becomes smaller. The driver spends a larger fraction of each revolution indexing and a smaller fraction dwelling. The required station count and available work time must both suit the application.
Does zero clearance error mean the fit is acceptable?
No. It means the entered gap lies inside the example comparison band of 0.05–0.10 mm. The calculator does not assess real contact, temperature, lubrication or manufacturing tolerances.
Does the timing calculation determine pin diameter?
No. Pin design requires contact and bending loads, support geometry, material properties and the operating duty. Slot count and clearance alone do not establish those loads.
Can an external Geneva run in reverse?
The ideal geometry can traverse the same path in reverse. Whether a particular assembly may reverse, especially under load or during engagement, depends on its drive, backlash, locking arrangement and operating instructions.
What should I investigate if a real mechanism knocks or binds?
Pin and slot geometry, shaft alignment, locking-disc clearance, bearing condition, lubrication and load can all contribute. Timing results alone cannot identify the cause or prescribe a machining change.
References & Further Reading
- Wikipedia contributors. Geneva drive. Wikipedia
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