© 2020 Sierra Nevada Corporation   398 The M45S has two primary functions: (1) p rovide the required torque and speed when powered and  commanded to move, and (2) hold position when unpowered. Motor holding torque capability, necessary  for maintaining the robotic arm’s position when the motor is unpowered,  was a crucial requirement in the  design and development of the M45S  motor and brake  assembly , but is not covered in this paper . Rather,  the operational motor requirements and the failure to demonstrate margined  life capability against those  requirements will be discussed.  

In operation, t he M45S motor was required to produce a minimum torque of 0.13 N •m at a minimum speed  of 3,00 0 rpm across an operational temperature range of -70°C to +70° C. Given the bias of the Martian  environment to colder  temperatures and the associated increase in drag torque of the downstream gear ing,  a second performance requirement was given for temperatures at or below - 30°C: minimum torque of   0.20 N•m at a minimum speed of 2,400 rpm. These two performance points were the basis of the motor  design and sizing.  

Additional design goals and test requirements regarding operation at colder temperatures increased the  difficulty of the program. Specifically, there was a program goal to minimize torque variation over the wide  operational temperature range. A limited tor que uncertainty across operating conditions would allow JPL to  calculate output torque produced by the motor solely as a function of the current input. In real terms, this  translated to design direction  to avoid adding excessive lubricant to the motor bear ings, understanding that  additional lubricant would increase the motor drag and therefore the torque uncertainty at colder  temperatures.   

Additionally , one of the most challenging test requirements, necessary to represent the bias of operation at  colder t emperatures,  was to satisfy the following distribution of temperatures in the execution of  the  margined  life test (105.2M motor shaft revolutions ):  • Minimum 25%  of the revolutions at the hot extreme (+70° C).  • Minimum 25%  of the revolutions at the cold extreme ( -70°C).  • Minimum 25%  of the revolutions at the nominal operating temperature ( -55°C). 

Design and Evaluation  

The M45S motor  and brake assembly wa s comprised of a brushless DC motor with Hall commutation and  a fricti on brake assembly . The rotor at the core of the M45S was supported by two radial ball bearings,  spring  preloaded  in a face- to-face orientation . Each double- shielded, SR3 size bearing contained  a crown  steel retainer and was lubricated with a grease plate of Braycote 600EF followed by the addition of a 1:1  by volume slurry  mixture  of Braycote 600EF grease and Brayco 815Z oil to fill 5% to 10% of the bearing’s  void volume.  The quantity of lubricant was minimized in order to keep torque losses low, and to reduce drag  torque variation over temperature extremes.  It should be noted that the combination of a grease plate prior  to the addition of 5% to 10% fill of 1:1 grease/oil slurry resulted in a total lubricant condition of 8% to 13%  volume fill and a grease/oil ratio closer  to 2:1.  

This lubrication approach was selected based on its heritage success  [1] [2] [3] . Additionally,  the crown  steel retainer had heritage, being  successfully used in Mars Science Laboratory gearboxes  and in  developmental testi ng for Mars 2020 gearbox designs . However,  the combination of  this retainer  configuration, the lubricant fill,  and the high number of revolutions required to be performed at the margined  operating temperature, - 70°C, pushed this bearing configuration beyond the acceptable limits of operation.  This will be explained in detail.  

A simplified cross -section of the M45S motor is shown in Figure 2.