- Difficulty
- Intermediate
- Build time
- 75-110 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 grabber picks up a paper cup from 40 cm away, holds it through a short transfer, and opens fully when the handles are released.
Learning goals
- Identify how squeezing motion at two handles produces closing motion at remote jaws.
- Construct and explain a angular hand input-to-angular jaw output system.
- Measure how the jaw pivot-hole position changes performance.
- Diagnose losses caused by long-link flex and pivot friction.
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 doubled corrugated-cardboard rails and paper-fastener pivots for a larger household version.
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.
- Use only lightweight objects and keep jaw tips rounded; never use the grabber near faces or animals.
Orient the build
Place the build so squeezing motion at two handles is on your left and closing motion at remote 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 two stiff rails
Create matching long rails and brace each against bending.
Keep rail width equal from handle to jaw end.
Step 2
Join the arm frame
Connect rails with spacers that preserve a straight open channel.
Check that the frame does not twist.
Step 3
Mount handle levers
Pivot two handles near the hand end with comfortable spacing.
Add stops for open and closed positions.
Builder checkpoint: After mount handle levers, the first subassembly should stay aligned when handled gently.
Step 4
Mount jaw levers
Pivot matching jaws at the far end and add soft inward-facing pads.
Set tips wider than a paper cup when open.
Watch for: If this stage binds or drifts, inspect jaw-pad slip before adding more parts.
Step 5
Connect handle to jaws
Run one rigid link along each side between matching lever arms.
Use separate planes so links never cross.
Step 6
Check direction and range
Squeeze halfway and confirm both jaws close evenly.
Move pivot holes if one jaw over-travels.
Builder checkpoint: After check direction and range, operate the build slowly and confirm that closing motion at remote jaws begins without binding.
Step 7
Tune unloaded return
Use a light band or gravity to reopen the handles and jaws.
Avoid return force that tires the hand.
Step 8
Pick and transfer
Grip an empty paper cup 40 cm away and move it 20 cm over a tray.
Repeat five times and inspect loose pivots.
Builder checkpoint: At the final checkpoint, The grabber picks up a paper cup from 40 cm away, holds it through a short transfer, and opens fully when the handles are released.
See the engineering
Why it works
- Input
- squeezing motion at two handles
- Output
- closing motion at remote jaws
- Motion
- angular hand input-to-angular jaw output
- Energy losses
- long-link flex, pivot friction, joint play, jaw-pad slip
Why this works
Linked lever transmission
Connected levers transfer angular movement along the arm. The ratio of handle and jaw lever arms sets the ideal relationship between hand travel, jaw travel, and grip force.
Look for: Compare how far the handles move with how far the jaw tips move during one closing cycle.
Where the energy goes
Efficiency and losses
The ideal model leaves out long-link flex, pivot friction, joint play, jaw-pad slip. 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 long-link flex becomes visible or audible.
Math bite
Estimate force ratio
Formula: ideal jaw force / hand force = handle arm / jaw arm
- Handle arm = 80 mm
- Jaw arm = 40 mm
Substitute: force ratio = 80/40 = 2
Result: Ideal jaw force is twice the hand force.
Jaw movement is correspondingly smaller than handle movement.
Long-link flex and pivot friction reduce real grip.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Close the unloaded jaws five times while watching both link sides.
Success looks like: Jaws close evenly, reopen fully, and then move an empty paper cup without slipping.
Measure: Handle travel, jaw travel, and successful transfers.
Change: the jaw pivot-hole position
Keep constant: arm length, handles, links, cup, pads, and squeeze speed
- wide jaw travel
- balanced setting
- higher-force setting
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The jaws open when squeezed | One link is attached to the wrong lever side | Move handles slowly and trace motion | Move the link across the pivot line |
| One jaw lags | Link lengths or pivot positions differ | Overlay corresponding links | Match center distances and hole choices |
| The arm bends | Rails are too flexible for the length | Hold horizontally and watch sag | Add depth, triangulation, or reduce reach |
| The cup slips | Jaw faces are hard or misaligned | Close on the cup without lifting | Add foam and square both pads |
Choose your tradeoff
Moving a link closer to a jaw pivot can increase jaw travel but lower force and worsen alignment. Keep both sides mirrored and stiffen the long arm before raising grip force.
Keep experimenting
Try another version
Short grabber
Build a 20 cm arm for easier alignment.
Interchangeable jaws
Compare flat, curved, and soft jaw faces.
Remote parallel jaws
Add parallelogram links so pads stay parallel.
Build together
Classroom and access options
Classroom version
Teams can compare the jaw pivot-hole position while keeping arm length, handles, links, cup, pads, and squeeze speed. 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 a wide springy handle loop and large foam jaw pads.
Reflect on the design
- How did the jaw pivot-hole position change the measured result?
- Where did long-link flex affect the build most strongly?
- What evidence shows that linked lever transmission explains the motion?
- Which change would improve closing motion at remote jaws without creating a new problem?
Glossary
- Linked lever transmission
- Connected levers transfer angular movement along the arm.
- Input
- The action or energy supplied to a system; here it is squeezing motion at two handles.
- Output
- The useful response produced by a system; here it is closing motion at remote jaws.
- 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.