Gears play an essential role in precision robotics, and they can become a limiting factor when the robots must perform in space missions. In particular, the extreme temperatures of deep space pose numerous problems for successful gear operation. At NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, CA, technologist Douglas Hofmann and his collaborators aim to bypass the limitations of existing steel gears by creating gears from bulk metallic glass (BMG).
BMG is a specially crafted metal alloy that has the properties of glass. Metals have an organized, crystalline arrangement. When they are heated up into a liquid, they melt and the atoms become randomized. Cooling them rapidly — about 1832 °F (1000 °C) per second — traps their non-crystalline “liquid” form in place. The resulting amorphous material is technically a glass. It can flow easily and be blow molded when heated, just like windowpane glass. When this glassy material is produced in parts greater than about 0.04" (1 mm), it’s called bulk metallic glass.
“Although BMGs have been explored for a long time, understanding how to design and implement them into structural hardware has proven elusive,” said Hofmann. That’s largely because researchers didn’t understand the subtleties that composition and impurities during manufacturing play in the mechanical properties of BMGs. Metallic glasses have been used commercially for years, but almost all of the applications were composed of a single alloy. Once it was determined that the composition of the material dramatically affects the performance, design teams could then create metallic glasses explicitly for particular applications.
The JPL researchers and engineers, in collaboration with groups at Caltech and the University of California, San Diego, have finally put BMGs through the testing needed to demonstrate their potential benefits in creating robotic gears for space. BMGs are very hard, have good wear resistance, and they can be used at extremely low temperatures without becoming severely embrittled (Figure 1). This combination of properties makes them attractive for robotic gears.
Hofmann explained that metallic glass is a compromise in properties between ceramics and conventional crystalline metals like steel or titanium. Ceramics are very hard and they make excellent gears and bearings, but they are very brittle. Traditional metal gears are strong, but have problems with wear and temperature, and they require lubricant. Metallic glasses fit in between. They’re much tougher than ceramics, but much less brittle, and also have better wear performance than titanium or steel.
BMGs have a glass transition temperature that is lower than the melting temperature of the metals used to make them. The BMGs typically will begin to flow around 300 to 400 °C. That allows parts to be cast using injection-molding technology, similar to what’s used in the plastics industry. BMGs also don’t get brittle in extreme cold, a factor that can lead to a gear’s teeth fracturing.
According to Hofmann, initial BMG gear testing has demonstrated strong torque and smooth turning without lubricant, even at –328 °F (–200 °C). For robots sent to frozen landscapes, that can be a power-saving advantage. NASA’s Mars Curiosity rover, for example, expends energy heating up grease lubricant every time it needs to move.
BMGs could also lower the cost of manufacturing strain wave gears (Figure 2). This type of gear, which includes a metal ring that flexes as the gear spins, is tricky to mass produce and is ubiquitous in expensive robots. Mass producing strain wave gears is difficult because parts of them have to be precision machined from steel using a very complicated manufacturing process. JPL intends to cast the BMG gears. Casting parts exploits the economics of injection molding, in which increased production lowers the price per part. This can significantly reduce the cost of robots that use strain wave gears, including those in the consumer robotics market.
The project’s proof of concept and alloy material development were done at JPL, but they partnered with industry to manufacture the metallic glass gears when they reached the limits of JPL’s manufacturing capabilities. Materion of Elmore, OH makes the BMG feedstock material. That raw material is then used by Visser Precision in Denver, CO, a company that specializes in injection molding of metallic glasses.
Besides gears, there are other potential applications for BMG materials in deep space missions. The JPL researchers have been prototyping many applications for metallic glass, including compliant mechanisms and spacecraft shielding.
There’s also the possibility of commercial implementation of the same materials into gears that would be used on Earth. A NASA spinoff out of Caltech is trying to commercialize the strain wave gears for low-cost robotics. Also, metallic glass can create low-density planetary gears that are useful in applications where weight is a factor. Gears made from titanium-based metallic glass are 40% lighter than steel gears, and deliver similar performance. Hofmann believes they have the technology to make the first viable titanium-based planetary gears, which would be key for applications like drones and low-cost air vehicles.
The Bulk Metallic Glass Gears project is funded by NASA’s Space Technology Mission Directorate’s Game Changing Development Program. The objective of that funding is to help get through the testing necessary to be able to use these gearboxes in future missions. Proposed missions that could potentially use the new gears include putting a lander on the surface of Jupiter’s moon Europa, the asteroid redirect mission, a Mars sampling mission, or a comet sample return mission.
For more information, visit: http://scienceandtechnology.jpl.nasa.gov/metallurgyfacility.