NASA's glass gears have cracking potential
US researchers, led by scientists and engineers from NASA’s Jet Propulsion Laboratory in California, are developing gears made out of special materials that that can act both as a metal and as a glass. The materials can operate efficiently without lubricants in extremely cold conditions, such as those found on the surface of planets and moons. They could also slash the cost of strain-wave gears used in applications such as robotics.
The materials, known as metallic glasses or amorphous metals, were originally developed at Caltech in 1960. Since then they have been used in a wide range of applications from golf clubs to transformers.
Metals have an organised, crystalline arrangement at the atomic level. But if you heat them to a liquid form, they melt and the atoms become randomised. If you then cool them rapidly enough – at about 1,000°C per second – they can get trapped in a “liquid” form which has an amorphous, or non-crystalline microstructure.
By being cooled so rapidly, the material is technically a glass. It can flow easily and be blow-moulded when heated, like glass. When the glassy material is thicker than about 1mm, it is called bulk metallic glass, or BMG.
Among their attractive qualities, BMGs have low melting temperatures. That allows parts to be cast using injection-moulding techniques, similar to those used for plastics, but with much higher strength and wear-resistance. Also, BMGs do not get brittle in extreme cold temperatures, at which conventional metallic gear teeth can fracture.
“Although BMGs have been explored for a long time, understanding how to design and implement them into structural hardware has proven elusive,” explains Nasa technologist, Douglas Hofmann. “Our team of researchers and engineers at JPL, in collaboration with groups at Caltech and UC San Diego, have finally put BMGs through the necessary testing to demonstrate their potential benefits for Nasa spacecraft. These materials may be able to offer us solutions for mobility in harsh environments – like on Jupiter's moon Europa.”
According to Hofmann, gears made from BMGs can run “cold and dry”, without needing any lubricants. Initial testing has demonstrated that they can deliver powerful torque and rotate smoothly without using lubricants, even at –200°C. For robots sent to frozen landscapes, this could save precious energy. Nasa’s Mars Curiosity rover, for example, needs to expend energy heating up grease lubricants every time it moves.
“Being able to operate gears at the low temperature of icy moons, like Europa, is a potential game-changer for scientists,” says Peter Dillon, a technologist and programme manager in JPL’s Materials Development and Manufacturing Technology Group. “Power no longer needs to be siphoned away from the science instruments for heating gearbox lubricant, which preserves precious battery power.”
The second potential application for the new materials is to lower the cost of manufacturing strain-wave (or “harmonic”) gears, that are used widely in robotics. These gears, which incorporate a metal ring that flexes as the gear spins, are tricky to mass-produce.