Soft pistons improve efficiency and reduce friction

Pistons incorporating soft materials improve efficiency by 40 per cent

The team showed in an object-crushing comparison between a conventional piston (air cylinder; left) and a tension piston (right) that the tension piston can produce greater forces at the same air-pressure. Image: Wyss Institute at Harvard University

Pistons have been a fundamental component for machines of most types since the age of steam. However, they are not without their problems. Researchers at Harvard have now redesigned this apparently simple mechanism with new materials which, they claim, can eliminate or reduce most of these long-standing drawbacks.

Several phenomena reduce the efficiency of pistons. The friction between the moving piston head and the chamber wall can lead to breakdown of the seal between moving static components, leakages and gradual or sudden malfunctions. Moreover, when operating at low pressure, energy efficiency and response speed are often limited.

Until now, such limitations have been accepted and worked around: perfecting the seal between piston components made up a great deal of the original research of steam engine pioneers such as James Watt and Richard Trevithick, three centuries ago; basic piston design has changed little since then.

The Harvard team, working at the Wyss Institute for Biologically Inspired Engineering, the John A Paulsen School of Engineering and Applied Sciences (SEAS) and collaborating with colleagues from the computer science and artificial intelligence laboratory (CSAIL) at Massachusetts Institute of Technology, has designed what they call a “tension piston” that replaces conventional rigid elements with a mechanism using compressible structures inside a membrane made of soft materials, and described this invention in a paper in Advanced Functional Materials.

The tension piston idea builds upon previous work by the same team known as fluid-driven, origami-inspired artificial muscles (FOAMs), which are flexible materials that give soft robots more power and motion control while maintaining their flexible architecture.

Made from a geometrically-folded skeletal structure embedded in a liquid within a flexible and hermetically sealed skin, FOAMs work when the fluid pressure is changed, which triggers the skeletal structure to unfold or collapse along a pre-configured geometrical path. This changes the shape of the entire FOAM.

“In principle, we explored the use of FOAMs as pistons within a rigid chamber,” said Shuguang Li, a postgrad working on the project. “By using a flexible membrane bonded to a compressible skeletal structure inside, and connecting it to one of the two fluid ports, we can create a separate fluid compartment that exhibits the functionality of a piston.”

Increasing the driving pressure in this separate fluid compartment increased the tension forces in the membrane material that are directly transmitted to the folded skeletal structure. Physically linking the structure to an actuating element that reaches out of the chamber couples the compression or relaxation of the structure to mechanical movement outside the chamber.

The team tested the tension piston by comparing it against a conventional rigid piston in an object-crushing task. These showed that piston could crush an object like a wooden pencil with a lower input pressure (in this case, the pressure in the skin-surrounding fluid compartment) than the conventional mechanism. When the same input pressures were used, particularly in the lower pressure range, the tension piston generated three times greater output forces and displayed more than 40 per cent energy efficiency than the conventional mechanisms.

“By configuring the compressible skeletons with very different geometries such as a series of discrete discs, as hinged skeletons, or as spring skeletons, the output forces and motions become highly tuneable,” said Li. “We can even incorporate more than one tension piston into a single chamber, or go a step further and also fabricate the surrounding chamber with a flexible material like an air-tight nylon fabric.”

Rus believes that tension pistons could have far-reaching implications for machinery. “Better pistons could fundamentally transform the way we design and utilise many types of systems, from shock absorbers and car engines to bulldozers and mining equipment,” she commented. “We think that an approach like this could help engineers devise different ways to make their creations stronger and more energy-efficient.”