## 1 Introduction  
Harmonic drives (HD) are power transmission mechanisms able to develop high ratios, providing a high positional precision, with a relatively low weight/volume ratio, high torque capability, and near-zero backlash. Invented by [PERSON] in 1955 [1] for aerospace applications, HD are currently widely used in robotics, medical equipment, printing presses, vehicles, and defense. They are used in Space mainly because of a good performance in terms of reduction ratio and good efficiency with respect to conventional gear trains [2,3]. Typically, the temperature operation range of HD in space is limited from -40 \(\,\)C to +100 \(\,\)C [4,5].  
The space and aerospace industries are increasingly demanding mechanisms able to work in cryogenic environments (T \(<\) 100 K) [6]. As signal-to-noise ratios in sensors increase as temperature decreases, cryogenic environments are very desirable for accurate and precise measurements such as those required by far infrared interferometer spectroscopy [7,8]. Solar array drive mechanisms or large antenna deployment and pointing mechanisms in which HD are traditionally used require increasingly demanding operational conditions [9].  
At very low temperatures, conventional mechanisms present severe tribological problems in bearingss and teeth like cold spots, fatigue, and wear [10; 11]. Only solid lubricants such as Teflon (PFTE) or Molybdenum disulfide (MoS2) can be a solution at low temperatures [12; 13]. However, for long life-time operation, solid lubricants turn out not to be a very reliable solution and the decrease of the efficiency in the mechanism is very significant.  
Specifically, the application of HD mechanisms is limited by the severe drop of the efficiency at very low temperatures (T \(<\) 100 K). For instance, the efficiency of conventional HD falls below 20% at temperatures below \(-\)40 \(\ \mathrm{C}\), even when using dry lubricants like the MAPLUB SH050a Grease [14].  
Additionally, dry lubricants present grating, clutching, rapid wear out, instability of the friction coefficient, formation of cold weld centers, and losses or decomposition at cryogenic conditions. This prevents conventional HD with dry lubricants from rotating faster than 500 rpm in the input axle. Moreover, fatigue associated with the intrinsic flexural functioning of the HD also limits their effective work life at cryogenic temperatures.  
The objective of the Magnetic-Superconductor Cryogenic Non-contact Harmonic Drive (MAGDRIVE) project, funded by the EU Space FP7, is to design, build, and test a new concept of HD. Non-contact interactions among magnets, soft magnetic materials, and superconductors are efficiently used to provide a high reduction ratio gear that smoothly and naturally operates at cryogenic environments. The limiting elements of conventional HD (teeth, flexspline, and ball bearings) are substituted by contactless mechanical components (magnetic gear and superconducting magnetic bearings). The absence of contact between moving parts prevents wear, lubricants are no longer required, and the operational life time is greatly increased.  
In order to achieve a Technology Readiness Level (TRL) 5, a prototype was designed, built, and tested at a temperature below 60 K and 10\({}^{-3}\) Pa. Mechanical performance, including maximum speed, speed ratio, efficiency, torque, and load capability, was characterized in order to establish the criteria for the usage of these MAGDRIVE mechanisms for Space.