189 A post -life teardown of the gearbox revealed the bearing races showed signs of use, however nothing  atypical was observed. On both inner and outer races of the roller bearings, the contact zone was evident  by a visibly darker band  visible in Figure 11. The grease within the PEEK cages around the rollers was  darker, but still wet with adequate lubricant for proper operation. The PEEK cages appeared in good  condition as well, maintaining their integrity.  

STIG Testing Results  

The assembled qualificatio n model corer has completed a 1x life test program of actuator/gear train  rotations  and gear shifts. At the time of writing, the unit has also performed approximately 20% of a life’s  worth of coring and drilling operations. This testing will continue.  

The flight model corer has completed dyno testing, completed thermal -vac testing, and has been integrated  to the flight rover. There it has performed basic operations, such as shifting gears. Figure  12 shows   performance data for the f light unit taken across the protoflight temperature range during testing.  >100%  margins were proven for dril ling and core breakoff operating points.  Drilling operations were performed  while connected to a dynamometer . Core break operating points wer e measured quasi -statica lly against a  load cell, since that operation can be performed arbitrarily slow in flight.  

Challenges and Lessons Learned  

Several challenges were encountered during the design of STIG.  

Shift Energy Compensation   Spring preload allows for shifting into gear without pre -alignment of selector and actuator output splines .  However, energy on the order of 0.3 J is stored and released once the splines align rotationally. This energy  manifests in the form of axial velocity of t he selector of about 2.5 m/s . Accommodations must be made for  this kinetic energy; allowing the selector to dissipate energy via impact with the actuator  output interface   (specifically , at the termination of the depth of the selector’s  internal splines)  was not acceptable. Damping  was considered, for example with piston seals, but the large range of interference fits that would be  encountered over the operating temperature range of the mechanism on Earth and on Mars and over the  life of these seals meant th at constant  friction coefficients could not be guarantee d. 

A compact absorber spring of high stiffness was designed and incorporated in order to release energy into  other STIG  parts with a load path to corer structure. The absorber spring, along with a few small parts  contained within it, surround the selector  and can be impacted without blocking the selector’s interfaces to  the spindle actuator. This particular load path also included the transmission’s output bearings, whose  truncation loads of 3 kN became the limiting factor for maximum impact forces. The design of the absorber  spring was made difficult by the need to absorb a fixed amount of energy in a small packaging volume. As  the spring geometry became more complex to reduce stresses, increases to tolerances were necessary to  allow the part to remain machinable. Given that the spring needs to flex in order to absorb energy, stresses  are quite sensitive to tolerances in part thickness. A great deal of iteration was used in the design of the  absorber spring. Figure 13  shows the absorber spring final design, which stores energy through torsional   and radial flexing.  Qualification model testing has shown the bearings to be capable of sustaining the 3 kN  max loads without impacting bearing performance.