- Difficulty
- Intermediate
- Build time
- 60-90 min
- Estimated cost
- $0-$18
- Age range
- 11-17
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The chain completes ten input turns without derailing, excessive sag, or tooth jumping, and the measured sprocket ratio stays consistent under light load.
Learning goals
- Identify how rotation of a driving sprocket produces rotation of a distant driven sprocket.
- Construct and explain a rotary-to-rotary through linked chain system.
- Measure how the center distance changes performance.
- Diagnose losses caused by link articulation and sprocket misalignment.
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.
- Make a paper chain and cardboard pin sprockets for a low-load visual model.
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.
- Keep fingers away from sprocket entry points and turn only with a side-mounted crank.
Orient the build
Place the build so rotation of a driving sprocket is on your left and rotation of a distant driven sprocket 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 parallel bearing rails
Create rigid input and sliding output supports on one base.
Keep both axle centerlines parallel.
Step 2
Install the sprockets
Secure one sprocket to each shaft in the same vertical plane.
Add collars outside the chain path.
Step 3
Size the chain loop
Wrap the chain around both sprockets and connect a full-link loop.
Avoid forcing a half-link mismatch.
Builder checkpoint: After size the chain loop, the first subassembly should stay aligned when handled gently.
Step 4
Set working tension
Slide the output until the slack span moves slightly by hand.
Lock the bearing block without twisting it.
Watch for: If this stage binds or drifts, inspect chain rubbing before adding more parts.
Step 5
Check tooth seating
Turn one loop slowly and inspect every entry and exit point.
Correct any repeating lifted link.
Step 6
Add a slack guide
Place a smooth guide under the returning span only if sag contacts the frame.
Leave the driven span clear.
Builder checkpoint: After add a slack guide, operate the build slowly and confirm that rotation of a distant driven sprocket begins without binding.
Step 7
Count the ratio
Reset both flags and turn the input through the driver tooth count.
Record output rotations.
Step 8
Apply light load
Add gentle pointer resistance and run ten more turns.
Stop if the chain jumps or links pull tight.
Builder checkpoint: At the final checkpoint, The chain completes ten input turns without derailing, excessive sag, or tooth jumping, and the measured sprocket ratio stays consistent under light load.
See the engineering
Why it works
- Input
- rotation of a driving sprocket
- Output
- rotation of a distant driven sprocket
- Motion
- rotary-to-rotary through linked chain
- Energy losses
- link articulation, sprocket misalignment, excess tension, chain rubbing
Why this works
Positive chain engagement
Sprocket teeth engage individual chain spaces, so ideal speed ratio follows tooth counts and does not depend on friction. Correct pitch and alignment are required for each link to seat.
Look for: Watch one marked link enter and leave both sprockets during a full loop.
Where the energy goes
Efficiency and losses
The ideal model leaves out link articulation, sprocket misalignment, excess tension, chain rubbing. 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 link articulation becomes visible or audible.
Math bite
Predict sprocket speed
Formula: output/input = driver teeth / driven teeth
- Driver = 12 teeth
- Driven = 24 teeth
Substitute: output/input = 12/24 = 0.5
Result: Ten input turns produce five ideal output turns.
The chain enforces this ratio unless it jumps teeth.
Link and bearing friction reduce available torque but not ideal kinematic ratio.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Rotate the input for ten slow turns with no load.
Success looks like: The chain stays seated and output turns match the tooth ratio without a jump.
Measure: Output turns, maximum slack-span deflection, and derailments.
Change: the center distance
Keep constant: chain length, sprockets, frame, crank rate, and output load
- slightly loose
- moderate tension
- overtight reference stopped before long running
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The chain derails sideways | Sprockets are not coplanar | Sight along both tooth rows | Shift collars until sprocket faces align |
| Links ride over teeth | Pitch is incompatible or tension is too high | Turn one marked link through entry slowly | Use matching parts and reduce tension |
| The chain slaps the frame | Slack span is too loose | Measure midpoint deflection while stopped | Increase center distance slightly or add a smooth guide |
| Bearings feel rough | Chain is overtightened | Remove chain and compare shaft motion | Move sprockets closer until slight slack returns |
Choose your tradeoff
A chain needs controlled slack, not belt-like stretch. Align sprockets first, then set enough slack for links to articulate without allowing the return span to strike the frame.
Keep experimenting
Try another version
Equal sprockets
Build a one-to-one positive drive.
Idler tensioner
Add a free idler on the slack span and compare stability.
Three-sprocket path
Route around an obstacle and predict every shaft direction.
Build together
Classroom and access options
Classroom version
Teams can compare the center distance while keeping chain length, sprockets, frame, crank rate, and output load. 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.
- Mark one chain link with high-contrast tape and add a large crank handle.
Reflect on the design
- How did the center distance change the measured result?
- Where did link articulation affect the build most strongly?
- What evidence shows that positive chain engagement explains the motion?
- Which change would improve rotation of a distant driven sprocket without creating a new problem?
Glossary
- Positive chain engagement
- Sprocket teeth engage individual chain spaces, so ideal speed ratio follows tooth counts and does not depend on friction.
- Input
- The action or energy supplied to a system; here it is rotation of a driving sprocket.
- Output
- The useful response produced by a system; here it is rotation of a distant driven sprocket.
- 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.
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