- 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
Both jaws travel within 3 mm of equal distance, remain nearly parallel, and lift a 30-gram cardboard block without crushing it.
Learning goals
- Identify how rotation of a central handle or gear produces equal inward translation of two jaws.
- Construct and explain a rotary-to-symmetric linear system.
- Measure how the jaw-pad material changes performance.
- Diagnose losses caused by guide friction and link asymmetry.
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 folded-cardboard sliders and string tied to a centered winding drum.
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.
- Limit jaw force and keep fingers out of the closing gap during operation.
Orient the build
Place the build so rotation of a central handle or gear is on your left and equal inward translation of two jaws 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 symmetric base
Mark a centerline and brace equal guide lengths on both sides.
Measure from the center for every support.
Step 2
Make matched carriages
Build two jaw sliders with equal width and guide clearance.
Label left and right without changing geometry.
Step 3
Attach parallel jaw faces
Mount flat pads perpendicular to the guides.
Set both faces at equal height.
Builder checkpoint: After attach parallel jaw faces, the first subassembly should stay aligned when handled gently.
Step 4
Install the synchronizer
Add a centered pinion with two racks or mirrored links to both sliders.
Check equal engagement on each side.
Watch for: If this stage binds or drifts, inspect backlash before adding more parts.
Step 5
Add travel stops
Limit closure before carriages collide and opening before they leave guides.
Set stops symmetrically.
Step 6
Mark displacement scales
Place zero at the open position and matching scales beside both jaws.
Move in small input steps.
Builder checkpoint: After mark displacement scales, operate the build slowly and confirm that equal inward translation of two jaws begins without binding.
Step 7
Close on a soft block
Center the block and close until foam just compresses.
Measure left and right travel.
Step 8
Lift and release
Raise the 30-gram block 5 cm over a tray, then open smoothly.
Inspect jaw parallelism after five cycles.
Builder checkpoint: At the final checkpoint, Both jaws travel within 3 mm of equal distance, remain nearly parallel, and lift a 30-gram cardboard block without crushing it.
See the engineering
Why it works
- Input
- rotation of a central handle or gear
- Output
- equal inward translation of two jaws
- Motion
- rotary-to-symmetric linear
- Energy losses
- guide friction, link asymmetry, jaw flex, backlash
Why this works
Symmetric constraint
Mirrored geometry drives both jaws with equal and opposite displacement. Parallel guides or matched parallelograms prevent jaw faces from rotating as they close.
Look for: Mark both jaw positions and compare their distance from the centerline after each input step.
Where the energy goes
Efficiency and losses
The ideal model leaves out guide friction, link asymmetry, jaw flex, backlash. 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 guide friction becomes visible or audible.
Math bite
Check symmetric travel
Formula: center error = |left travel - right travel| / 2
- Left travel = 22 mm
- Right travel = 18 mm
Substitute: error = |22 - 18|/2 = 2 mm
Result: The grip center shifts 2 mm from the original centerline.
Matched travel keeps objects centered.
Backlash and pad compression affect measured positions.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Close empty jaws from the fully open marks in five equal handle steps.
Success looks like: Jaw faces stay parallel and left-right travel differs by no more than 3 mm.
Measure: Each jaw travel, face angle, and held mass up to 30 grams.
Change: the jaw-pad material
Keep constant: guides, synchronizer, input steps, block, and start marks
- bare jaws
- foam pads
- rubberized paper pads
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| One jaw moves farther | Synchronizer engagement or link length differs | Move one input step and measure both sides | Match rack mesh, links, and start positions |
| Jaw faces tilt | Carriages are short or guides loose | Press one corner while stopped | Lengthen guide contact and reduce side play |
| The object slips | Pads are smooth or force is uneven | Lift a light block over a tray | Add compliant high-friction pads |
| Closing binds near center | Stops or carriages collide early | Operate empty and inspect the center gap | Move stops outward and align guides |
Choose your tradeoff
Parallelism and symmetry come before force. Softer pads improve contact on uneven objects but consume travel; tighter guides improve orientation but raise friction.
Keep experimenting
Try another version
Pointer jaws
Move two paper flags without gripping an object.
Self-centering rack
Use one pinion between opposing racks.
Force comparison
Measure grip force at several handle radii.
Build together
Classroom and access options
Classroom version
Teams can compare the jaw-pad material while keeping guides, synchronizer, input steps, block, and start marks. 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 central handle and wide foam-faced jaws for easy placement.
Reflect on the design
- How did the jaw-pad material change the measured result?
- Where did guide friction affect the build most strongly?
- What evidence shows that symmetric constraint explains the motion?
- Which change would improve equal inward translation of two jaws without creating a new problem?
Glossary
- Symmetric constraint
- Mirrored geometry drives both jaws with equal and opposite displacement.
- Input
- The action or energy supplied to a system; here it is rotation of a central handle or gear.
- Output
- The useful response produced by a system; here it is equal inward translation of two jaws.
- 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.