- Difficulty
- Beginner
- Build time
- 30-50 min
- Estimated cost
- $0-$6
- Age range
- 10-15
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The grabber picks up three different lightweight objects, holds each for five seconds, and releases without the extension arms twisting off.
Learning goals
- Identify how hand squeeze at clothespin handles produces opening or closing at extended jaws.
- Construct and explain a angular squeeze-to-angular jaw motion system.
- Measure how the jaw-pad texture changes performance.
- Diagnose losses caused by extension flex and pivot friction.
Before you build
Materials, tools, and safety
Reuse-material cost: $0-$3 with reused materials. Supervision: Adult help recommended for sharp or heated tools.
Tools
- Ruler
- Pencil
- Scissors
- Masking tape
Low-cost swaps
- Use clean recycled packaging whenever it has similar stiffness.
- Substitute paper clips, binder clips, or twist ties for specialty fasteners.
- Use a binder clip with covered handles when a clothespin is unavailable.
Project-specific safety
- Wear eye protection when stretched elastic, magnets, or spinning parts are present.
- Test at floor or tabletop height and keep the path clear of people.
- Keep the spring clip away from fingers and faces, and use only lightweight objects.
Orient the build
Place the build so hand squeeze at clothespin handles is on your left and opening or closing at extended 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
Inspect the clothespin
Open and close it ten times and check the spring is seated.
Discard cracked wood or rusty springs.
Step 2
Prepare equal arms
Cut or fold two matching 15 cm extensions and mark the clothespin overlap.
Keep both straight.
Step 3
Attach the extensions
Secure one arm to each jaw with tape plus a rubber-band wrap.
Align them in the same plane.
Builder checkpoint: After attach the extensions, the first subassembly should stay aligned when handled gently.
Step 4
Add jaw pads
Place foam at the extension tips facing each other.
Keep pad centers at equal distance from the pivot.
Watch for: If this stage binds or drifts, inspect misaligned arms before adding more parts.
Step 5
Check open clearance
Squeeze handles and confirm extensions separate without crossing.
Trim any rubbing edge.
Step 6
Test near and far grip
Hold one object near the original jaw, then at the extended tips.
Compare hand effort and slip.
Builder checkpoint: After test near and far grip, operate the build slowly and confirm that opening or closing at extended jaws begins without binding.
Step 7
Try three shapes
Pick up a paper ball, empty cup, and marker over a tray.
Hold each for five seconds.
Step 8
Tune the tips
Change pad texture or angle while keeping arms the same.
Repeat the object that slipped most.
Builder checkpoint: At the final checkpoint, The grabber picks up three different lightweight objects, holds each for five seconds, and releases without the extension arms twisting off.
See the engineering
Why it works
- Input
- hand squeeze at clothespin handles
- Output
- opening or closing at extended jaws
- Motion
- angular squeeze-to-angular jaw motion
- Energy losses
- extension flex, pivot friction, pad slip, misaligned arms
Why this works
Lever-arm tradeoff
Moving a contact farther from the pivot increases jaw travel for a given angle but reduces available force at that point. The clothespin spring supplies the restoring force.
Look for: Compare grip near the original jaw tip with grip at the end of a long extension.
Where the energy goes
Efficiency and losses
The ideal model leaves out extension flex, pivot friction, pad slip, misaligned arms. 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 extension flex becomes visible or audible.
Math bite
Estimate tip force
Formula: tip force = spring moment / tip distance
- Spring moment = 0.06 N·m
- Tip distance = 0.15 m
Substitute: tip force = 0.06/0.15 = 0.4 N
Result: The ideal extension-tip force is about 0.4 newtons.
A shorter extension would give more force but less reach.
Arm flex and jaw angle reduce real force.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Operate the empty grabber ten times after securing the arms.
Success looks like: It holds three lightweight objects for five seconds and returns fully.
Measure: Successful pickups, hold time, and tip opening.
Change: the jaw-pad texture
Keep constant: clothespin, arm length, objects, pickup position, and hold time
- bare card
- foam
- rubberized paper
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| Arms twist sideways | Attachments are loose or off-center | Open slowly and view from above | Realign and add a second wrap |
| Objects slip | Pads are smooth or not parallel | Close gently around the object | Add foam and angle pads |
| The grabber will not open far | Extensions collide near the pivot | Operate empty and mark contact | Trim or offset the arms |
| The clothespin spring shifts | Extensions overload or mounting covers spring | Inspect after each trial | Reduce mass and clear the spring |
Choose your tradeoff
Long extensions increase reach and tip travel but reduce force and stiffness. Improve pad contact before shortening the arms, then choose the minimum length that meets the reach goal.
Keep experimenting
Try another version
Short soft jaws
Add only foam pads to the original clothespin.
Curved jaws
Shape tips for cups versus round objects.
Sorting task
Move ten objects into bins by shape.
Build together
Classroom and access options
Classroom version
Teams can compare the jaw-pad texture while keeping clothespin, arm length, objects, pickup position, and hold time. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Offer pre-cut parts and tactile or high-contrast measurement marks.
- Split roles so one builder can hold, another assemble, and another measure.
- Add wide foam handle sleeves and larger jaw pads.
Reflect on the design
- How did the jaw-pad texture change the measured result?
- Where did extension flex affect the build most strongly?
- What evidence shows that lever-arm tradeoff explains the motion?
- Which change would improve opening or closing at extended jaws without creating a new problem?
Glossary
- Lever-arm tradeoff
- Moving a contact farther from the pivot increases jaw travel for a given angle but reduces available force at that point.
- Input
- The action or energy supplied to a system; here it is hand squeeze at clothespin handles.
- Output
- The useful response produced by a system; here it is opening or closing at extended 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
A familiar household engineering activity implemented with original instructions and controlled tests.
- Classroom engineering basis: A common educational challenge implemented with original dimensions, tests, diagrams, and instructions.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.