There are dozens of mechanical principles that help turn consistent rotary motion into controllable and variable linear and rotary outputs.

The redshoebox YouTube channel specializes in creating dynamic LEGO models, such as transmissions and robotic arms powered by electric and pneumatic sources, and has created assemblies of some common and many uncommon principles, which are both educational and beautiful.

In some cases, these principles are exceptionally rare, such as a Peaucellier–Lipkin linkage, which means it can be difficult to offer real word, visual examples of complex motion control techniques. Meanwhile, something like a spring-pusher mechanism can be found in a variety of equipment, from pianos to vending machines. No matter the complexity or ubiquity, redshoebox aims to turn it into something understandable and even reproducible for LEGO fans. The account owner of reshoebox typically offers plans for featured models if requested.

This is actually the third in a series of videos focused on demonstrating mechanical principles in LEGO. The first and second episodes are also available.

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