BIZOMAT MAKEOS · Mechanics
Geneva Wheel Indexing Mechanism
What crank radius and wheel size does a Geneva drive need, and how long are its index and dwell?
Geometry and timing of an external Geneva wheel with slots tangent to the roller path at entry and exit: crank radius, wheel radius, minimum slot length, driver angle for indexing, index and dwell times, and the peak speed ratio of the wheel.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Number of slots | Number of radial slots in the driven wheel (indexes per revolution of the wheel). | |
| Centre distance | mm | Distance between the driver and wheel centres. |
| Driver speed | rpm | Speed of the driving crank. |
| Roller diameter | mm | Diameter of the drive roller (sets the slot width). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Number of slots
- What it is
- The number of radial slots in the driven Geneva wheel. The wheel turns one slot pitch, 360° ÷ slots, for each turn of the driver.
- Units and format
- Enter a plain number with no unit. The form accepts 3 to 24. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More slots shorten the crank radius and lower the peak wheel speed, but the driver spends a larger angle indexing, so the dwell is shorter.
What this means: Centre distance
- What it is
- The distance between the axis of the driving crank and the axis of the Geneva wheel; the crank and wheel radii are derived from it.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The crank and wheel radii scale in proportion; the index and dwell times do not change.
What this means: Driver speed
- What it is
- The steady rotational speed of the driving crank that carries the roller. One driver revolution gives one index and one dwell of the wheel.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Index and dwell times are inversely proportional to it; the peak wheel speed rises in proportion.
What this means: Roller diameter
- What it is
- The diameter of the drive roller (or pin) on the crank that enters the slots. It sets the slot width and adds its radius to the minimum slot length.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Make the slot width a sliding fit on this diameter; the tool adds no clearance to the slot length.
Method and formulas
- For slots tangent to the roller path: crank radius a = C sin(180° ÷ n); wheel radius to the slot mouth b = C cos(180° ÷ n).
- Driver angle while indexing = 180° − 360° ÷ n; index time = that fraction of one driver revolution.
- Peak wheel-to-driver speed ratio (roller on the centre line) = λ ÷ (1 − λ), with λ = sin(180° ÷ n).
- Minimum slot length from the mouth = a + b − C, plus the roller radius and clearance.
- Example: a 4-slot wheel on 100 mm centres needs a 70.711 mm crank and indexes in 0.25 s at 60 rpm.
Limitations
- External Geneva with radial slots; internal and spherical Geneva drives differ.
- Acceleration at roller entry is high for 3 and 4 slots; check inertia loads.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Lay out a Geneva drive for an indexing table
- Check the index and dwell times against a process cycle
- Estimate peak wheel speed for inertia loads
Who it is for
- Machine designers
- Maintenance engineers
- Automation engineers
What you get
- Crank radius and wheel radius
- Index and dwell time
- Peak wheel speed
Interpreting the result
- More slots give a smoother index but a smaller index angle per driver revolution.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 171. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Index with a Geneva drive
In Printed Mechanisms That Run
What it covers: Size a six-slot Geneva mechanism, calculate its motion and dwell, set the centre distance and locking clearance that decide station accuracy, and print it to last.
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