- Difficulty
- Beginner
- Build time
- 45-70 min
- Estimated cost
- $0-$12
- Age range
- 10-16
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The output runs for ten input turns without the belt leaving either pulley, and changing pulley size creates a measurable speed difference.
Learning goals
- Identify how rotation of a driving pulley produces rotation of a distant driven pulley.
- Construct and explain a rotary-to-rotary through a flexible loop system.
- Measure how the belt tension changes performance.
- Diagnose losses caused by belt slip and belt bending.
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.
- Use rubber bands on smooth bottle-cap pulleys for a household-scale demonstration.
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.
- Wear eye protection and use a relaxed belt; an overstretched band can snap or leave the pulley.
Orient the build
Place the build so rotation of a driving pulley is on your left and rotation of a distant driven pulley 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 the fixed input support
Mount the driver axle in a rigid pair of bearings.
Attach the crank beyond the frame.
Step 2
Build a sliding output mount
Guide the driven bearing block along the base without allowing rotation.
Leave at least 4 cm of tension adjustment.
Step 3
Install both pulleys
Align pulley grooves in one vertical plane and secure side collars.
Sight along the belt path from above.
Builder checkpoint: After install both pulleys, the first subassembly should stay aligned when handled gently.
Step 4
Fit the belt loosely
Loop the belt around both pulleys with the output mount close.
Rotate by hand to seat it in both grooves.
Watch for: If this stage binds or drifts, inspect pulley misalignment before adding more parts.
Step 5
Set moderate tension
Slide the output away until slack disappears but the belt still deflects by hand.
Lock the mount in that position.
Step 6
Mark shaft positions
Point both flags upward and add one contrasting mark to the belt.
Keep flags clear of the belt.
Builder checkpoint: After mark shaft positions, operate the build slowly and confirm that rotation of a distant driven pulley begins without binding.
Step 7
Count unloaded turns
Rotate the input ten times at one turn per second.
Record output turns and any belt wandering.
Step 8
Test controlled slip
Apply very light output resistance and repeat at two tension settings.
Stop before the belt heats or stretches permanently.
Builder checkpoint: At the final checkpoint, The output runs for ten input turns without the belt leaving either pulley, and changing pulley size creates a measurable speed difference.
See the engineering
Why it works
- Input
- rotation of a driving pulley
- Output
- rotation of a distant driven pulley
- Motion
- rotary-to-rotary through a flexible loop
- Energy losses
- belt slip, belt bending, bearing friction, pulley misalignment
Why this works
Belt speed relationship
Without slip, both pulley rims move at the same linear speed. A larger driven pulley therefore turns more slowly in inverse proportion to its diameter.
Look for: Add matching rim marks and watch whether the belt and pulley move together under light output resistance.
Where the energy goes
Efficiency and losses
The ideal model leaves out belt slip, belt bending, bearing friction, pulley 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 belt slip becomes visible or audible.
Math bite
Predict the pulley ratio
Formula: output speed / input speed = driver diameter / driven diameter
- Driver diameter = 20 mm
- Driven diameter = 40 mm
Substitute: output/input = 20/40 = 0.5
Result: Ten input turns should produce about five output turns.
A larger driven pulley reduces speed and raises ideal torque.
Belt slip and elastic stretch reduce real output turns.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Run ten unloaded input turns at one turn per second.
Success looks like: The belt stays centered and measured output turns are within 10 percent of the ideal ratio.
Measure: Output turns, belt deflection, and visible slip marks.
Change: the belt tension
Keep constant: pulleys, center alignment, crank rate, turn count, and output load
- low tension
- moderate tension
- moderate tension with light load
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The belt walks off | Pulleys are not coplanar or axles are tilted | Sight across pulley faces while stopped | Realign bearings and grooves |
| Output slows under light load | Tension or belt friction is too low | Watch rim marks for relative motion | Increase tension slightly or use a grippier belt |
| Input is hard to turn | Belt is overtightened | Remove belt and compare shaft effort | Reduce center distance until bearings run freely |
| The ratio varies each trial | Belt stretch or counting marks are inconsistent | Reset flags and inspect permanent stretch | Use a fresh belt and ten-turn trials |
Choose your tradeoff
Use the lowest tension that prevents unacceptable slip. Extra tension can reduce slip but raises bearing friction and belt stress, so alignment comes before tightening.
Keep experimenting
Try another version
Equal pulleys
Verify a one-to-one open-belt drive.
Crossed belt
Cross the belt to reverse output direction at low speed.
Slip curve
Graph output turns against several measured tension settings.
Build together
Classroom and access options
Classroom version
Teams can compare the belt tension while keeping pulleys, center alignment, crank rate, turn count, 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.
- Use a light-colored belt on dark pulleys and large tactile shaft flags.
Reflect on the design
- How did the belt tension change the measured result?
- Where did belt slip affect the build most strongly?
- What evidence shows that belt speed relationship explains the motion?
- Which change would improve rotation of a distant driven pulley without creating a new problem?
Glossary
- Belt speed relationship
- Without slip, both pulley rims move at the same linear speed.
- Input
- The action or energy supplied to a system; here it is rotation of a driving pulley.
- Output
- The useful response produced by a system; here it is rotation of a distant driven pulley.
- Efficiency
- The fraction of input energy that becomes useful output instead of friction, sound, heat, or unwanted motion.
Build your dreams
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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.