416 between -100°C and +200°C. Radiation testing showed no failures after exposure to γ -radiation (absorbed  doses of 300 J/kg for 64h).  

maxon’s flight heritage from the MER mission does not include gearboxes , however, maxon gained  considerable experience during the ExoMars development [ 13] and qualification program. The program  demonstrated that although COTS planetary gearboxes, in particular those developed for high- temperature  “downhole” oil and gas industry applications, can be easily modified for space applications, significant  lifetime problems arise when switching to vacuum- rated lubricants such as Braycote 601EF. ExoMars  development work had shown that using a slurry of grease and oil as previously described for Harmonic  Drive applications [14] partially mitigate d this lubricant issue. The root cause of the lifetime issues is that  Braycote 601EF is fundamentally unsuitable for lubricating sliding (as opposed to rolling) surfaces. In order  to achieve the lifetime requirements f or the ExoMars drill drive, the COTS design sliding motion between  each planetary gear and its shaft/axis was converted to rolling motion by the addition of needle bearings ,  which  was then used as the basis for the Mars 2020 gearbox  (Figure 4) . 

Figure 4. Exploded view of M20 planetary gearbox stage  

Given the mission- critical nature of the M20 application, the JPL and maxon review teams decided that  excessive residual risk remained in connections between sun gears and planetary carriers as well as  between the planet axes pins and the carriers. In the COTS design,  these connections are press fitted and  welded, however , due to the hardened nature of the materials used for the axes and sun gears , the welds  always exhibit ed surface cracks. This is an accepted compromise for industrial applications (and is not  known to have ever caused a field failure), but due to the extreme Martian diurnal temperature cycling, this  was considered to present an unacceptable risk of weld failure followed by axial movement of the   components. The sun gear and planetary carriers’ connection was easily solved by combined these into a  one-piece part (by using shaping manufacturing techniques). The problem of how to secure the axes was  solved by adding a cage to the design. Both the ca ge and the planetary carrier have blind (vented) holes  that the axes are pressed into, without requiring any further retaining mechanism. The cage is pressed onto  an axial hard stop on the planetary carrier and is the n cold formed (swaged) into position. T he resultant  design is then both weld-  and adhesive- free and enables a free choice of gear and axes materials and  hardening processes.  

No part involved more complexity to manufacture than the combined ring gear and motor flange for the  M20 gearmotor. For industrial products , gearbox ring gears and motor flanges are usually screwed together  on a large diameter thread and then either glued or welded. In the case  of the M20 , however , the gearmotor   needs  to fit inside a ring of springs, which requires  a star shape pattern of wires to be precisely oriented  relative to the mounting lugs on the front of the gearbox. The disadvantage of the standard assembly  method is that the large diameter thread does not allow a precise angular orientation of the gearbox to the  motor. The solution to this problem was to combine the flange and the ring gear into one piece. This had