For thousands of years, gears have functioned with solid teeth to turn the wheels of the world, from chariots to clocks to engines. Now, scientists have shown that it is possible to transmit motion with those teeth never meeting at all, using only carefully channeled streams of liquid.
Engineers at New York University created a system in which rotation is transmitted from one part to another by fluid dynamics, without any direct contact. In their tests, two cylinders were placed in a mixture of glycerol and water. As one cylinder turned with a motor, the fluid around it transmitted rotation to the second one. Depending on how far apart the cylinders were and how fast they turned, either pushed them apart in a manner similar to gear teeth or pulled them in a manner similar to a belt.
“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,” said Jun Zhang, professor of mathematics and physics at NYU and NYU Shanghai. Our gears do not contact one another, so there’s no possibility of chipped teeth or jammed assemblies from debris. As co-author Leif Ristroph explained, Even grit in the system gets bypassed by the fluid flow.
However, the effects of this go beyond durability. Conventional gear contacts work in elasto-hydrodynamic lubrication conditions, where the possibility of fatigue, pitting, or spalling can result because of the combination of rolling and sliding contact. Such wear phenomena are a common concern for gear contacts in precise applications. Fluid-based gear contacts do not face any of these contact-related stress considerations at all.
One of the applications that shows potential is in soft robotics, in which metal gears might restrict flexibility. Fluid gears might be used in soft robots, which would enable changing gear ratios in an instant by simply altering fluid properties. This would be particularly beneficial in robots meant to handle delicate items or in robots meant to operate in cramped spaces. Research in millimeter-scale fluid-driven actuators indicates that tiny, precise motions can be achieved in biomedical applications, ranging from surgical equipment to bronchoscopes.
The enabling technology is also being developed in support of such systems. Miniature soft peristaltic pumps have been developed that provide fluid output at pressures that are suitable for actuating a robotic system. These soft peristaltic pumps are powered by dielectric elastomer actuators, enabling bi-directional fluid output without requiring rigid components, thus being suitable for fluid gear mechanisms in mobile devices.
From an engineering point of view, the substitution of solid gear teeth with liquid flows questions the traditional paradigm of gear design. It also raises questions about torque, speed, and direction in gear systems designed to withstand contamination, misalignment, or changing loads. The designers now have the flexibility to work with materials and geometries that would otherwise be difficult in traditional gear systems. Although it is still a concept that is only just being explored, the union of fluid gear and some of the latest ideas for soft actuators might well represent the start of a whole new era for motion control. In manufacturing, robotics, and medical devices, for example, the opportunity to transmit power through a fluid that will never wear out, as opposed to the usual sound of teeth meshing together, is extremely attractive.