A Pulley-Driven Claw Machine Built to Show Prospective Humboldt Engineering Students

Two people pull handles to move a claw across a wooden frame, while buttons on a control board wired to a Raspberry Pi lower, raise, open and close it. That is the Cal Poly Humboldt Claw Machine, built in Spring 2024 by a student team called the Thermodynamic Outlaws for the ENGR 205: Intro to Design course. It is meant to be shown to prospective students at Cal Poly Humboldt's preview events and to spark their interest in the school of engineering.

The brief

The client was the head of the school of engineering, whose prompt was an interactive model of mechanical engineering applications. The client's representative weighted the criteria from 1 to 10:

Two prototypes

The first prototype was about a third of the final scale. It had a non-functional claw and wires wrapped around two popsicle sticks that slid across the top of the frame, meeting where the claw hung. That proved the frame and sliders worked, but it could not move the claw in the y direction or open and close it.

Top view of prototype #1. Photo: Mcooper on Appropedia, CC BY-SA 4.0.

The second prototype was to scale, with a working pulley system tying the sliders to handles and weights: pulling a handle moves its slider toward the user, and weights on the far side return the slider and raise the handle when it is released. Tracks on the frame cut the friction of wood on wood, though the team found friction was still significant. A pulley-operated claw was tried and dropped as too complex and unattractive, and the client asked for more portability. The final machine therefore became a wagon on four wheels, two of which lock, with a rope for pulling.

Prototype #2. Photo: Mcooper on Appropedia, CC BY-SA 4.0.

The final machine

The base is about 2 ft by 2 ft and a little over 2 ft tall, with an all-wood frame and wooden sliders. Two users each pull a handle to move the claw in the flat x-z plane. The 3D-printed plastic claw, on the sliders' intersection, is raised, lowered, opened and closed by servo motors that a Raspberry Pi runs from buttons on a wooden control board. Staff can reach the Pi's functions through a touchscreen.

The frame. Photo: Mcooper on Appropedia, CC BY-SA 4.0.

Building and running it

The page gives a step-by-step build: cut and assemble the wooden frame, cut sliders, 3D print tracks and glue them to the sliders and frame, run cords through pulleys to weights on one side and to handles on the other, add the wheels and rope, then print the claw, wire its servo motors to the Pi and code them. Operation takes three steps: lock the wheels, start the Pi and its program, then use the control-board buttons and the handles. Maintenance is only wiping off dust after long storage. For troubleshooting: if the sliders stick, check that the string is not caught in a pulley; if the claw does not respond, check that the computer is on and the code running.

Materials cost, excluding donated items:

Sources

• "Humboldt SoE claw machine demo," Appropedia contributors, by the Thermodynamic Outlaws (Mason Cooper, Finn Lyall, Miguel Pulido and Lucas Sira), Cal Poly Humboldt ENGR 205, Spring 2024, https://www.appropedia.org/Humboldt_SoE_claw_machine_demo, licensed under CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/). Changed here: rewritten in hubnx's own words; this version stays CC BY-SA 4.0. Photos by Mcooper on Appropedia, CC BY-SA 4.0.