- Difficulty
- Intermediate
- Build time
- 70-100 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 beam rocks through a repeatable angle for ten crank cycles, and two end flags alternate high and low without the drive rod buckling.
Learning goals
- Identify how rotation of a crank and connecting rod produces rocking beam motion.
- Construct and explain a rotary-to-oscillating angular system.
- Measure how the output pivot distance from beam center changes performance.
- Diagnose losses caused by center-pivot friction and rod side force.
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 craft-stick beam, cardboard tower, and paper-fastener pivots for a hand-cranked 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 the center pivot and moving rod, and use only lightweight output flags.
Orient the build
Place the build so rotation of a crank and connecting rod is on your left and rocking beam motion 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 center tower
Brace a tall pivot support on a wide base.
Check side-to-side stiffness at the pivot height.
Step 2
Balance the beam
Mount a rigid beam at its center and add collars with free clearance.
Confirm both ends swing through equal space.
Step 3
Build the crank support
Place a low crank axle beside one beam end.
Leave enough distance for the connecting rod.
Builder checkpoint: After build the crank support, the first subassembly should stay aligned when handled gently.
Step 4
Set the crank radius
Attach an offset pin that creates a modest vertical stroke.
Rotate once to inspect frame clearance.
Watch for: If this stage binds or drifts, inspect joint play before adding more parts.
Step 5
Connect rod to beam
Join the crank pin to the near beam end with free pivots.
Avoid a rod length that reaches a straight-line lock.
Step 6
Mark beam limits
Turn to both crank extremes and mark each end height.
Adjust tower or rod before adding output pieces.
Builder checkpoint: After mark beam limits, operate the build slowly and confirm that rocking beam motion begins without binding.
Step 7
Add opposite indicators
Attach light flags to both ends at equal lever-arm distance.
Point them toward separate height scales.
Step 8
Run ten cycles
Crank steadily and record high-low timing and swing angle.
Stop if the rod bows or tower twists.
Builder checkpoint: At the final checkpoint, The beam rocks through a repeatable angle for ten crank cycles, and two end flags alternate high and low without the drive rod buckling.
See the engineering
Why it works
- Input
- rotation of a crank and connecting rod
- Output
- rocking beam motion
- Motion
- rotary-to-oscillating angular
- Energy losses
- center-pivot friction, rod side force, beam flex, joint play
Why this works
Crank-driven rocking lever
The crank converts rotation into rod reciprocation, and the beam's central pivot converts that vertical motion into opposite angular movement at its two ends.
Look for: When the driven end rises, the far end falls by an amount set by their lever-arm lengths.
Where the energy goes
Efficiency and losses
The ideal model leaves out center-pivot friction, rod side force, beam flex, joint play. 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 center-pivot friction becomes visible or audible.
Math bite
Compare end travel
Formula: travel ratio = output arm / input arm
- Input arm = 80 mm
- Output arm = 120 mm
Substitute: ratio = 120/80 = 1.5
Result: The far end ideally moves 1.5 times the driven-end distance.
Its ideal force is lower by the reciprocal ratio.
Angular geometry and flex make the exact path an arc.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Turn one unloaded crank cycle over four seconds.
Success looks like: Both beam ends alternate smoothly and return to the same height marks.
Measure: Swing angle and end travel distances.
Change: the output pivot distance from beam center
Keep constant: crank radius, rod, tower, input arm, rate, and flags
- equal arm lengths
- long output arm
- short output arm
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The rod locks in line | Rod length and crank position create a toggle | Move slowly to the lock angle | Change rod length or crank location |
| The tower sways | Pivot support lacks diagonal bracing | Hold the crank and push beam sideways | Add braces to the base |
| One end has less travel than expected | Pivot distances are unequal or flexing | Measure center-to-flag lengths | Reposition flags and stiffen beam |
| The beam rubs its tower | Side collars or spacers are wrong | Swing by hand without the rod | Center beam with thin running gaps |
Choose your tradeoff
Longer beam arms increase end travel but also flex and space demands. Keep the drive rod away from near-straight toggle positions and brace the center tower before adding load.
Keep experimenting
Try another version
Hand-rocked beam
Move the beam directly and compare lever arms.
Twin guide rods
Add a second vertical indicator at the far end.
Pump linkage
Connect one end to a guided slider and graph its stroke.
Build together
Classroom and access options
Classroom version
Teams can compare the output pivot distance from beam center while keeping crank radius, rod, tower, input arm, rate, 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.
- Add contrasting flags that tap different textured cards at each beam end.
Reflect on the design
- How did the output pivot distance from beam center change the measured result?
- Where did center-pivot friction affect the build most strongly?
- What evidence shows that crank-driven rocking lever explains the motion?
- Which change would improve rocking beam motion without creating a new problem?
Glossary
- Crank-driven rocking lever
- The crank converts rotation into rod reciprocation, and the beam's central pivot converts that vertical motion into opposite angular movement at its two ends.
- Input
- The action or energy supplied to a system; here it is rotation of a crank and connecting rod.
- Output
- The useful response produced by a system; here it is rocking beam motion.
- 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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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.