- Difficulty
- Advanced
- Build time
- 90-150 min
- Estimated cost
- $0-$20
- Age range
- 13-18
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
A four-slot wheel advances about 90 degrees per input revolution, rests without drift between indexes, and completes eight consecutive engagements without a collision.
Learning goals
- Identify how continuous rotation of a drive disk produces quarter-turn indexed rotation.
- Construct and explain a continuous rotary-to-intermittent rotary system.
- Measure how the drive-pin radius changes performance.
- Diagnose losses caused by pin-slot impact and slot rubbing.
Before you build
Materials, tools, and safety
Reuse-material cost: Usually under $5 with an existing kit. Supervision: Adult help recommended for sharp or heated tools.
Tools
- Ruler
- Removable tape for motion marks
Low-cost swaps
- Use equivalent brick-compatible parts from any kit.
- Use cardboard beams and straw bearings for a larger demonstration model.
- Build a large cardboard Geneva pair from a printed geometric template you draw yourself with compass and ruler.
Project-specific safety
- Keep fingers, hair, and loose sleeves clear of moving parts.
- Turn the mechanism by hand; do not attach a high-speed motor.
- Turn only by hand and stop at the first collision; the drive pin can snap or eject if forced.
Orient the build
Place the build so continuous rotation of a drive disk is on your left and quarter-turn indexed rotation is on your right. Call the side facing you the front, the far side the back, the tabletop the bottom, and the opposite face the top.
Build it
Step-by-step instructions
Step 1
Build a rigid two-shaft base
Brace parallel bearing walls around the planned shaft centers.
Keep both axles perpendicular to one reference face.
Step 2
Mount the Geneva wheel
Center the four-slot wheel on the output shaft and mark each index.
Limit side movement with collars.
Step 3
Assemble the drive disk
Place one offset pin on the input disk and add its locking surface.
Verify the pin is straight and fully secured.
Builder checkpoint: After assemble the drive disk, the first subassembly should stay aligned when handled gently.
Step 4
Set the center distance
Position shafts so the pin reaches the slot center without bottoming out.
Temporarily turn through one engagement before bracing.
Watch for: If this stage binds or drifts, inspect disk misalignment before adding more parts.
Step 5
Align the locking dwell
Rotate the pin clear and confirm the blocking arc fits between slots.
Leave slight clearance so it locks without rubbing hard.
Step 6
Brace both shafts
Add cross supports close to each bearing and recheck the full cycle.
The center distance must not change under light pressure.
Builder checkpoint: After brace both shafts, operate the build slowly and confirm that quarter-turn indexed rotation begins without binding.
Step 7
Mark engagement phases
Label entry, midpoint, exit, and dwell positions on the input disk.
Move between marks slowly and observe output behavior.
Step 8
Run eight indexes
Turn at one input revolution every four seconds for eight cycles.
Stop and correct any pin strike, missed slot, or output drift.
Builder checkpoint: At the final checkpoint, A four-slot wheel advances about 90 degrees per input revolution, rests without drift between indexes, and completes eight consecutive engagements without a collision.
See the engineering
Why it works
- Input
- continuous rotation of a drive disk
- Output
- quarter-turn indexed rotation
- Motion
- continuous rotary-to-intermittent rotary
- Energy losses
- pin-slot impact, slot rubbing, axle backlash, disk misalignment
Why this works
Geneva indexing
The drive pin engages one radial slot for part of each input turn. A matching locking surface holds the Geneva wheel during the remaining dwell, creating a fixed angular index determined by slot count.
Look for: Watch the output acceleration rise and fall during engagement, then stop completely while the input continues.
Where the energy goes
Efficiency and losses
The ideal model leaves out pin-slot impact, slot rubbing, axle backlash, disk misalignment. These effects turn some input energy into heat, sound, vibration, or unwanted motion, so measured performance will be lower than an ideal calculation.
Look for: Run the build slowly and locate the first place where pin-slot impact becomes visible or audible.
Math bite
Find the index angle
Formula: index angle = 360° / number of slots
- Full circle = 360°
- Slot count = 4
Substitute: index angle = 360°/4 = 90°
Result: Each successful engagement advances the output by one quarter turn.
The output dwells for the portion of the input cycle when the pin is outside a slot.
Clearance and backlash create small index error.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Turn one full input cycle over four seconds with no output load.
Success looks like: The output advances one 90-degree index and remains fixed through the dwell.
Measure: Index angle, dwell duration, and error after eight indexes.
Change: the drive-pin radius
Keep constant: slot count, center distance, frame, input rate, and output marker
- slightly short pin radius
- designed pin radius
- slightly long pin radius
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The pin strikes a slot edge | Timing or center distance is wrong | Approach entry one degree at a time | Shift the input shaft and realign locking phase |
| The output drifts during dwell | Locking clearance is excessive or arc missing | Rock the output while the pin is clear | Adjust the locking disk for light clearance |
| The pin bottoms in the slot | Pin radius is too long | Stop at engagement midpoint and inspect depth | Shorten radius or increase center distance |
| The wheel skips an index | Frame flex moves shafts apart | Apply light output resistance during entry | Brace bearings directly beside both disks |
Choose your tradeoff
Reliable entry matters more than speed. Increase input rate only after eight slow indexes succeed; small changes to pin radius and center distance strongly affect impact and locking clearance.
Keep experimenting
Try another version
Manual indexer
Move the pin through one engagement without a locking disk.
Six-slot wheel
Build a 60-degree indexer and compare dwell fraction.
Motion timing graph
Record output angle at equal input-angle intervals.
Build together
Classroom and access options
Classroom version
Teams can compare the drive-pin radius while keeping slot count, center distance, frame, input rate, and output marker. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Use high-contrast tape to distinguish input and output parts.
- Replace a small crank with a wider handle for an easier grip.
- Add raised quarter-turn marks to the output dial and a large slow crank.
Reflect on the design
- How did the drive-pin radius change the measured result?
- Where did pin-slot impact affect the build most strongly?
- What evidence shows that geneva indexing explains the motion?
- Which change would improve quarter-turn indexed rotation without creating a new problem?
Glossary
- Geneva indexing
- The drive pin engages one radial slot for part of each input turn.
- Input
- The action or energy supplied to a system; here it is continuous rotation of a drive disk.
- Output
- The useful response produced by a system; here it is quarter-turn indexed rotation.
- Efficiency
- The fraction of input energy that becomes useful output instead of friction, sound, heat, or unwanted motion.
Build your dreams
One build can start the next.
Share what you learned, change one variable, and help another builder understand what worked.
Explore more guidesSources and build notes
An original BrickLabClips interpretation of a standard mechanical mechanism.
- Mechanism verification: Standard kinematics were checked for motion direction, constraint, clearance, and likely friction points.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.