Gears without teeth: that's the surprising invention from a team at New York University. This system uses cylinders set in rotation by a fluid, replacing traditional toothed wheels. The concept could transform machine design by offering unprecedented adaptability.
Classic gears have existed for millennia, from early examples in ancient China to the Antikythera mechanism. Their rigid teeth, whether made of wood, metal, or plastic, require perfect alignment and easily wear out or jam. These limitations drove researchers to imagine an alternative with no direct contact.
Two cylinders in a circular container, surrounded by a liquid with bubbles to visualize the flows. The left one is driven by a motor, while the right one rotates passively thanks to the currents. Credit: NYU's Applied Mathematics Laboratory Jun Zhang, professor of mathematics and physics, explains that his team invented gears that mesh by spinning a fluid, rather than using teeth. The movements of air and water are already used in turbines, but here the researchers aimed to precisely control the flows to mimic gear behavior.
The experiment used cylinders immersed in a mixture of water and glycerol. By adjusting viscosity and density, they could change how the fluid transmits motion. Microscopic bubbles allowed visualization of the currents. The active cylinder was set spinning, and scientists observed the reaction of the passive cylinder at different distances and speeds.
The results show two distinct behaviors. When the cylinders are close, the fluid acts like gear teeth: the passive cylinder rotates in the opposite direction. If the distance increases and speed is higher, the fluid envelops the passive cylinder like a belt, making them rotate in the same direction. This dual possibility does not exist with solid gears.
The advantages are numerous. Leif Ristroph, co-author, points out that classic gears require perfect meshing, while fluid gears avoid any jamming issues due to dust or misalignment. Moreover, the speed and even the direction of rotation can be changed in real time, opening up novel design possibilities.