Build a String-Operated Gripper
The mechanical linkage behind many real robotic end-effectors, operated by your own hand instead of a motor.
Related field: Robotics & Mechatronics Engineering
Mission Briefing
Objective
Build a hand-operated gripper that can pick up and hold a small object using only a string-pull mechanism — no batteries or motors.
Constraints
- No batteries, motors, or electronics of any kind — all motion must come from you pulling a string
- Must be able to pick up and hold an object without dropping it for at least 5 seconds
- Only cardboard, straws, string, and tape or brads (fasteners) are allowed
Concept You're Testing
Mechanical linkages — how pulling a string at one point can transfer force through a structure to close a gripper at another point. This same basic idea underlies many real robotic end-effectors, before motors and code ever get added.
What You'll Need
- Cardboard or stiff paper
- 2–4 straws
- String or thin cord
- Tape and/or paper fasteners (brads) to act as pivot points
Instructions
- 1Cut two cardboard "finger" shapes that will act as the gripper's jaws.
- 2Attach them to a base with a paper fastener at each pivot point, so each finger can rotate.
- 3Thread straws along each finger as guides, and run string through them, anchoring the string to the tip of each finger.
- 4Route both strings back to a single pull point.
- 5Pull the string and check whether both fingers close together evenly. Adjust the pivot points or string routing if one side closes faster than the other.
- 6Test picking up objects of different sizes — a small ball, a block, a pencil.
How to Measure Results
Test picking up at least 3 different small objects of different sizes or shapes, and record how many the gripper could successfully hold for at least 5 seconds without dropping.
Skills You'll Practice
- How a simple pull mechanism transfers force through a linkage
- Why symmetry matters — uneven pivot points make a gripper close unevenly
- Iterating on a mechanical design based on what actually happens when you test it
- The mechanical groundwork that comes before adding motors and sensors to a real robot
If It Doesn't Work the First Time
That's normal — this challenge follows the same design process real engineers use. A failed test just tells you where to go back and improve.
Figure out what problem you're actually solving, and what would even count as success. Skip this step and jump straight to building, and a lot of bad designs happen right here.