- Difficulty
- Intermediate
- Build time
- 55-80 min
- Estimated cost
- $0-$15
- Age range
- 11-17
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The output follows ten hand turns through a small controlled misalignment without joint separation, while a marked twist shows elastic compliance under light load.
Learning goals
- Identify how rotation and slight displacement of one shaft produces compliant rotation of a second shaft.
- Construct and explain a rotary-to-rotary through elastic bending system.
- Measure how the shaft misalignment angle changes performance.
- Diagnose losses caused by material hysteresis and beam 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.
- Use a short section of silicone tubing over smooth shaft ends for a low-torque flexible coupling.
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 misalignment and twist small; replace any beam that shows white stress marks, cracks, or permanent bending.
Orient the build
Place the build so rotation and slight displacement of one shaft is on your left and compliant rotation of a second shaft 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 adjustable towers
Mount two shafts nearly collinear with one movable support.
Brace both towers against rotation.
Step 2
Assemble the input hub
Secure a centered hub and attach one end of the flexible element.
Keep attachment symmetric around the shaft.
Step 3
Assemble the output hub
Attach the other flexible end to a matching hub.
Set zero misalignment and check for rubbing.
Builder checkpoint: After assemble the output hub, the first subassembly should stay aligned when handled gently.
Step 4
Add phase flags
Align two paper flags at the same angle with no load.
Mark a safe relative-twist limit.
Watch for: If this stage binds or drifts, inspect excess misalignment before adding more parts.
Step 5
Test aligned rotation
Turn ten slow cycles while the output is free.
Confirm the flexible element returns to its original shape.
Step 6
Introduce small angular error
Tilt the output shaft about 5 degrees while keeping centers close.
Rotate one cycle and inspect every beam section.
Builder checkpoint: After introduce small angular error, operate the build slowly and confirm that compliant rotation of a second shaft begins without binding.
Step 7
Apply light resistance
Hold the output pointer gently and measure relative flag angle.
Release and check elastic recovery.
Step 8
Compare configurations
Test one flexible link and a crossed pair at the same safe load.
Record twist, smoothness, and visible stress.
Builder checkpoint: At the final checkpoint, The output follows ten hand turns through a small controlled misalignment without joint separation, while a marked twist shows elastic compliance under light load.
See the engineering
Why it works
- Input
- rotation and slight displacement of one shaft
- Output
- compliant rotation of a second shaft
- Motion
- rotary-to-rotary through elastic bending
- Energy losses
- material hysteresis, beam rubbing, hub play, excess misalignment
Why this works
Compliant coupling
Thin beam sections bend elastically to accommodate small alignment errors while their overall geometry transmits torque. Stored elastic energy returns when load is removed.
Look for: Draw phase marks on both hubs and watch the relative angle grow under load, then recover after release.
Where the energy goes
Efficiency and losses
The ideal model leaves out material hysteresis, beam rubbing, hub play, excess 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 material hysteresis becomes visible or audible.
Math bite
Estimate twist stiffness
Formula: torsional stiffness k = torque / twist angle
- Applied torque = 0.02 N·m
- Twist angle = 10° = 0.175 rad
Substitute: k = 0.02/0.175 = 0.114 N·m/rad
Result: The coupling twists about 0.175 radians under this load.
Higher stiffness reduces phase lag but accommodates less misalignment.
The estimate assumes elastic behavior and a known applied torque.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Run ten turns with aligned shafts and no output load.
Success looks like: The output follows, the element clears the frame, and all beams return to their start shape.
Measure: Relative twist angle and visible permanent set.
Change: the shaft misalignment angle
Keep constant: coupling element, input rate, output load, towers, and flags
- zero degrees
- about five degrees
- five degrees with light load
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The coupling stays twisted | The elastic limit was exceeded | Release load and compare phase marks | Reduce load and replace the damaged element |
| Rotation pulses | The flexible element rubs or is asymmetric | Turn one cycle and inspect clearance | Center attachments and widen space |
| A hub slips on its shaft | Connection cannot carry torque | Hold the beam and rock the shaft | Strengthen the keyed or pinned hub |
| Bearings still bind | Misalignment exceeds coupling range | Disconnect coupling and test shafts | Reduce angle and realign tower centers |
Choose your tradeoff
Thinner or longer compliant sections increase flexibility but also twist and fatigue. Use only enough compliance to cover the measured misalignment, then keep load safely below permanent-deformation limits.
Keep experimenting
Try another version
Tubing coupler
Join two aligned shafts with soft tubing.
Crossed beams
Compare single and symmetric flexible layouts.
Stiffness graph
Plot torque against twist angle within the elastic range.
Build together
Classroom and access options
Classroom version
Teams can compare the shaft misalignment angle while keeping coupling element, input rate, output load, towers, and flags. 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 large hub flags and high-contrast safe-limit marks for twist angle.
Reflect on the design
- How did the shaft misalignment angle change the measured result?
- Where did material hysteresis affect the build most strongly?
- What evidence shows that compliant coupling explains the motion?
- Which change would improve compliant rotation of a second shaft without creating a new problem?
Glossary
- Compliant coupling
- Thin beam sections bend elastically to accommodate small alignment errors while their overall geometry transmits torque.
- Input
- The action or energy supplied to a system; here it is rotation and slight displacement of one shaft.
- Output
- The useful response produced by a system; here it is compliant rotation of a second shaft.
- 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.