- Difficulty
- Intermediate
- Build time
- 60-90 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 input crank completes full rotations while the output rocker swings repeatedly between two clear limits without crossing a toggle lock.
Learning goals
- Identify how full rotation of the shortest crank produces limited angular swing of a rocker.
- Construct and explain a rotary-to-oscillating angular system.
- Measure how the coupler length changes performance.
- Diagnose losses caused by pivot friction and link 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 craft sticks with measured paper-fastener holes to explore the same center-to-center lengths.
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 by the crank handle, not by pushing links near folding pivots where pinch points form.
Orient the build
Place the build so full rotation of the shortest crank is on your left and limited angular swing of a rocker 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
Mark two ground pivots
Build a rigid base and place fixed pivots 8 modules apart.
Brace both pivot towers on two sides.
Step 2
Attach the short crank
Mount the 4-module link at the left ground pivot with a free joint.
Add a handle beyond the pivot.
Step 3
Attach the rocker
Mount the 7-module link at the right ground pivot.
Point both moving links upward for assembly.
Builder checkpoint: After attach the rocker, the first subassembly should stay aligned when handled gently.
Step 4
Join with the coupler
Connect the 9-module link between free crank and rocker ends.
Use collars that prevent separation without squeezing.
Watch for: If this stage binds or drifts, inspect joint clearance before adding more parts.
Step 5
Find the assembly branch
Move the linkage slowly and keep the coupler on the same side of the ground line.
Stop before forcing a crossed configuration.
Step 6
Check a full input turn
Rotate the crank through 360 degrees while observing the rocker limits.
Recheck center distances if any joint reaches a hard lock.
Builder checkpoint: After check a full input turn, operate the build slowly and confirm that limited angular swing of a rocker begins without binding.
Step 7
Mark swing limits
Place paper markers at the rocker's two extreme angles.
Measure the total swing with a printed protractor.
Step 8
Change one link
Replace the coupler with a longer or shorter link and repeat the motion test.
Record whether full crank rotation remains possible.
Builder checkpoint: At the final checkpoint, The input crank completes full rotations while the output rocker swings repeatedly between two clear limits without crossing a toggle lock.
See the engineering
Why it works
- Input
- full rotation of the shortest crank
- Output
- limited angular swing of a rocker
- Motion
- rotary-to-oscillating angular
- Energy losses
- pivot friction, link bending, poor transmission angle, joint clearance
Why this works
Four-bar geometry
A four-bar linkage has a fixed ground link and three moving links. When the shortest plus longest link is no longer than the other two combined, at least one link can usually rotate fully in an appropriate assembly.
Look for: Watch the angle between coupler and rocker become shallow near the swing limits, where force transmission weakens.
Where the energy goes
Efficiency and losses
The ideal model leaves out pivot friction, link bending, poor transmission angle, joint clearance. 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 pivot friction becomes visible or audible.
Math bite
Check the Grashof sum
Formula: s + l ≤ p + q
- Shortest s = 4
- Longest l = 9
- Other links p = 7 and q = 8
Substitute: 4 + 9 = 13 and 7 + 8 = 15; 13 ≤ 15
Result: The length set satisfies the Grashof condition.
With the shortest link adjacent to ground, a crank-rocker arrangement is possible.
Joint thickness and collisions can still prevent ideal motion.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Turn the crank once slowly with no load on the rocker.
Success looks like: The crank makes a complete turn and the rocker reaches both marked limits without a forced snap.
Measure: Rocker swing angle and input force near each limit.
Change: the coupler length
Keep constant: ground spacing, crank, rocker, joint clearance, and input rate
- 7-module coupler
- 9-module coupler
- 11-module coupler
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The crank will not complete a turn | Link lengths or assembly branch create a non-Grashof lock | Move by hand to both limiting positions | Return to the verified lengths and uncross the coupler |
| The linkage snaps between shapes | Joints are loose near a toggle position | Rock it slowly at the snap angle | Reduce side play and avoid crossing the ground line |
| The rocker barely moves | Transmission angle is poor through much of the cycle | Watch coupler and rocker alignment | Change coupler or ground spacing to improve angle |
| Links scrape each other | All bars lie in the same layer | Inspect overlap during a full cycle | Add spacers so moving links occupy separate planes |
Choose your tradeoff
Length changes affect swing range, transmission angle, and whether rotation remains possible. Adjust one center-to-center distance at a time and never force a linkage through a geometry that locks.
Keep experimenting
Try another version
Rocker-only model
Move the crank through a half-turn and focus on output swing.
Best transmission angle
Compare link sets for the most even input force.
Coupler curve
Attach a pen to the coupler and trace its path over one cycle.
Build together
Classroom and access options
Classroom version
Teams can compare the coupler length while keeping ground spacing, crank, rocker, joint clearance, and input rate. 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 contrasting colors and raised labels for crank, coupler, rocker, and ground.
Reflect on the design
- How did the coupler length change the measured result?
- Where did pivot friction affect the build most strongly?
- What evidence shows that four-bar geometry explains the motion?
- Which change would improve limited angular swing of a rocker without creating a new problem?
Glossary
- Four-bar geometry
- A four-bar linkage has a fixed ground link and three moving links.
- Input
- The action or energy supplied to a system; here it is full rotation of the shortest crank.
- Output
- The useful response produced by a system; here it is limited angular swing of a rocker.
- 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.