Two spinners inside a circular container and surrounded by liquid with bubbles. Credit: NYU’s Applied Mathematics Laboratory

Fluid Gears: A New Mechanical System for Rotational Motion

Introduction

Most of the machines that can transmit rotational motion and torque make use of an essential component called gears. Recently, researchers at New York University have developed a gear mechanism that uses fluid flow instead of traditional gear teeth.

“We invented new types of gears that engage by spinning up fluid rather than interlocking teeth — and we discovered new capabilities for controlling the rotation speed and even direction,” says Jun Zhang, a professor of mathematics and physics at NYU and NYU Shanghai.

Advantages of fluid gears

Fluid gears have many benefits as compared to traditional toothed gears. There is no noticeable direct contact between the parts. Therefore, there will be less wear and tear, which increases durability and reduces the need for maintenance. The absence of friction also suggests that more useful energy is conserved or saved, making the system more efficient, consistent, and quieter during operation. In addition, normal gears must be precisely designed in order for the teeth to fit together perfectly. Even small defects or particles can cause them to jam.

Exploring fluid-based gears

To overcome problems such as the potential to wear out, Zhang, Leif Ristroph, a mathematics professor at NYU, Jesse Etan Smith, a doctoral student, and his team investigated whether it is possible to rotate gears in a system without the use of teeth or physical contact.

Moving fluids, such as air and water, are already used to rotate turbines for generating electricity and energy. With this in mind, they decided to design a detailed experiment where they placed cylindrical rotors into a liquid made from a glycerol-water mixture. This will then allow them to adjust fundamental properties like the viscosity and density of the solution, which acts like the “teeth” of gears if done properly.

In the setup, two cylinders were used. One was powered to rotate, while the other was left unpowered. The team hypothesized that the fluid motion generated by the active rotor should be enough to cause the passive rotor to revolve.

To clearly examine the motion of the fluid around the whole mechanism, tiny bubbles were added into the solution. These bubbles made it possible to visualize the flow patterns and understand how the fluid acted like gear teeth. The experiments were repeated with different distances between the cylinders and varying rotation speeds of the active rotor.

Observations and potential applications

In the end, the whole research team discovered that the active cylinders, together with the flow of the fluid, could cause the passive cylinder to rotate in two different ways. In some cases, the behavior of the motion resembled gears, while it could act more like pulleys connected by a belt.

When the cylinders were placed closely together, the fluid flow between them acted like gear teeth. The swirling liquid effectively gripped the passive cylinder, making it rotate in the opposite direction to the active one.

On the other hand, when the cylinders were positioned farther apart and the active cylinder rotated at higher speeds, the fluid moved around the outside of the passive cylinder. This motion was similar to a belt wrapped around a pulley, which in the end caused the passive cylinder to spin in the same direction as the active cylinder.

Written by Josh Bennett Kusuma