- Difficulty
- Advanced
- Build time
- 80-120 min
- Estimated cost
- $0-$20
- Age range
- 13-18
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The output follows the input under a light pointer load, then slips repeatedly at a chosen resistance without gear skipping or frame damage.
Learning goals
- Identify how hand rotation of a driving disk produces limited-torque rotation of a second disk.
- Construct and explain a rotary-to-slip-limited rotary system.
- Measure how the compression setting changes performance.
- Diagnose losses caused by disk rubbing and uneven pressure.
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 cardboard disks, felt circles, and a binder clip on a shared skewer for a low-load 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.
- Use only hand speed and stop if surfaces become warm; continuous slipping converts motion into heat.
Orient the build
Place the build so hand rotation of a driving disk is on your left and limited-torque rotation of a second disk 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 coaxial support
Align input and output hubs on the same centerline using a rigid frame.
Check face parallelism before adding friction material.
Step 2
Prepare friction faces
Attach equal flat pads to the contacting disk surfaces.
Keep glue or fasteners away from the contact area.
Step 3
Assemble the disk pair
Slide disks together with alignment hubs but no compression.
Rotate and check for wobble.
Builder checkpoint: After assemble the disk pair, the first subassembly should stay aligned when handled gently.
Step 4
Add adjustable pressure
Install a spring, elastic element, or movable collar that presses faces together.
Begin with the lightest setting.
Watch for: If this stage binds or drifts, inspect surface wear before adding more parts.
Step 5
Mark relative position
Draw one line across both disk edges at the start angle.
Attach a lightweight pointer to the output.
Step 6
Test normal drive
Turn the input slowly with no resistance and confirm both marks stay aligned.
Correct wobble before increasing pressure.
Builder checkpoint: After test normal drive, operate the build slowly and confirm that limited-torque rotation of a second disk begins without binding.
Step 7
Find the slip point
Hold the output pointer with a spring scale or light finger force while turning.
Record the resistance when marks separate.
Step 8
Compare three settings
Repeat with low, medium, and higher safe compression.
Allow surfaces to cool between repeated slip tests.
Builder checkpoint: At the final checkpoint, The output follows the input under a light pointer load, then slips repeatedly at a chosen resistance without gear skipping or frame damage.
See the engineering
Why it works
- Input
- hand rotation of a driving disk
- Output
- limited-torque rotation of a second disk
- Motion
- rotary-to-slip-limited rotary
- Energy losses
- disk rubbing, uneven pressure, axle misalignment, surface wear
Why this works
Friction-limited torque
Friction between pressed surfaces transmits torque until tangential demand exceeds the maximum static friction. After slipping begins, the output torque stays limited while input motion continues.
Look for: Draw matching lines across both disks and watch them separate only after output resistance crosses the limit.
Where the energy goes
Efficiency and losses
The ideal model leaves out disk rubbing, uneven pressure, axle misalignment, surface wear. 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 disk rubbing becomes visible or audible.
Math bite
Estimate friction torque
Formula: torque ≈ μ × N × r
- Friction coefficient μ ≈ 0.4
- Normal force N = 5 N
- Effective radius r = 0.02 m
Substitute: torque ≈ 0.4 × 5 × 0.02 = 0.04 N·m
Result: The ideal clutch begins slipping near 0.04 newton-metres.
More pressure or radius raises the limit.
Actual pads have distributed pressure, changing friction, and surface wear.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Turn the input with a free paper pointer at the lowest pressure setting.
Success looks like: The pointer follows normally and slips before the frame or drive train deforms under resistance.
Measure: Slip force at a known output radius.
Change: the compression setting
Keep constant: disk radius, pad material, input rate, frame, and output lever
- low pressure
- medium pressure
- higher safe pressure
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The clutch slips with no load | Compression is too low or pads are contaminated | Clean surfaces and test at the next setting | Increase pressure slightly and keep pads dry |
| It never slips safely | Compression is excessive or disks mechanically lock | Remove pads and inspect for interlocking parts | Reduce pressure and restore free face contact |
| Slip is jerky | Faces wobble or friction is uneven | Watch edge spacing during rotation | Align hubs and replace uneven pads |
| The threshold changes quickly | Pads are heating or wearing | Pause and compare cool versus warm trials | Use shorter tests and replace damaged pads |
Choose your tradeoff
Set the lowest pressure that carries normal load. A high slip threshold defeats overload protection, while too little pressure wastes motion during ordinary use.
Keep experimenting
Try another version
Marker-only clutch
Drive only a paper pointer and feel three pressure settings.
Pad comparison
Test felt, rubber, and paper at equal compression.
Torque calibration
Use a spring scale and known lever radius to plot slip torque.
Build together
Classroom and access options
Classroom version
Teams can compare the compression setting while keeping disk radius, pad material, input rate, frame, and output lever. 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 large compression knob with three tactile setting marks.
Reflect on the design
- How did the compression setting change the measured result?
- Where did disk rubbing affect the build most strongly?
- What evidence shows that friction-limited torque explains the motion?
- Which change would improve limited-torque rotation of a second disk without creating a new problem?
Glossary
- Friction-limited torque
- Friction between pressed surfaces transmits torque until tangential demand exceeds the maximum static friction.
- Input
- The action or energy supplied to a system; here it is hand rotation of a driving disk.
- Output
- The useful response produced by a system; here it is limited-torque rotation of a second disk.
- 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.