Castable Bulk Metallic Glass Strain Wave Gears: Towards Decreasing the Cost of High-Performance Robotics

The use of bulk metallic glasses (BMGs) as the flexspline in strain wave gears (SWGs), also known as harmonic drives, is presented. SWGs are unique, ultra-precision gearboxes that function through the elastic flexing of a thin-walled cup, called a flexspline. The current research demonstrates that BMGs can be cast at extremely low cost relative to machining and can be implemented into SWGs as an alternative to steel. This approach may significantly reduce the cost of SWGs, enabling lower-cost robotics. The attractive properties of BMGs, such as hardness, elastic limit and yield strength, may also be suitable for extreme environment applications in spacecraft.

Since the geometry of the flexspline is fixed, the strain is fixed. This means that materials with higher elasticity are stresses less when integrated into the strain wave gear. This is how a BMG alloy, which is known to be more brittle than steel, can avoid premature fracture due to its lower loading stress.
Supplementary Figure 4 -Images of the prototype casting of the BMG flexsplines at NASA JPL. A mold was created with the outer dimensions of the flexspline and the BMG alloys were cast over brass inserts. The final part was then conventionally machined. This technique allowed for the creation of many flexsplines from customized compositions of metallic glass.
Supplementary Figure 5 -Images showing the prototyping process that was used at NASA JPL to create the blanks needed for the final machining of the flexspline. The upper image shows a progression of learning to cast a cup using BMG. From the left to right, casting procedures were modified to improve flow, which included increasing the cup wall thickness from 1 mm (at the left) to 2 mm (at the right). The lower image shows the progression of casting the cup as well as the teeth of the flexspline from BMG.
Supplementary Figure 6 -A steel outer spline sitting on a blank of the same outer dimensions cast from a Ti-based metallic glass. Although the primary purpose of the study was on the creation of the flexspline, machining models were also created for the outer spline so that it could be made from metallic glass. Supplementary Figure 11 -Enlarged images of three BMG flexsplines cast with specialty alloys. The two on the left are cast from non-Be Zr-based BMGs, which have a higher melting temperature than Be-bearing alloys. The flexspline on the right is a tough, Zr-Ti-based BMG.
Supplementary Figure 12 -As described in the text, the commercial casting had to go through a few iterations to successfully cast the 50 mm diameter flexspline. These images, provided by Visser Precision, Denver CO, show early attempts to cast the flexspline teeth and wall. In this case, the walls were too thin to allow for full filling of the part.
Supplementary Figure 13 -An as cast 50 mm diameter BMG flexspline (left) compared to a machined steel version (right). The location for the drilling of the holes was cast into the part.
Supplementary Figure 14 -As described in the text, fatigue testing of the flexsplines showed premature cracking, which was most likely caused by machining the cast flexspline walls so they would fit with the commercial wave generators. This figure shows the holder that was used for the machining operation but the "quilting" or "printthrough" of the teeth is shown at the image on the right. New strategies are being developed to limit this problem.
Supplementary Figure 15 -A crack which as developed in a GHDT flexspline after fatiguing it in the gear rig. Optical micrographs show the evolution of the crack from the edge of the cup during the cycling.
Supplementary Figure 16 -Optical profilometry was used to characterize the casting variance in the teeth of every cast BMG flexspline. For each part, the shape of the teeth was profiled and compared back to the model. The BMG teeth are rounded when compared to machined steel teeth but show low variance between casts.

Supplementary Video Legends
Video 1 -Size 50 BMG Hybrid SWG Bottom View -Operation of a 50 mm diameter strain wave gear with a bulk metallic glass flexspline showing bottom view Video 2 -Size20 BMG Flexspline Commercially Cast -Flexing of a 20 mm diameter BMG flexspline that has been cast commercially Video 3 -Size20 BMG Hybrid SWG -Operation of a hybrid strain wave gear with a bulk metallic glass flexspline and conventional steel outer spline and wave generator Video 4 -Size20 BMG Lifting Robot Arm -JPL wall gripping robot being moved by a bulk metallic glass hybrid strain wave gear Video 9 -Size50 BMG Hybrid SWG Cryogenic Operation -Operation of a 50 mm diameter bulk metallic glass flexspline integrated into a standard strain wave gear after submersion in liquid nitrogen Video 10 -Size50 BMG Hybrid SWG -Operation of a strain wave gear with a bulk metallic glass flexspline that has been cast commercially