© 2020 Sierra Nevada Corporation   405 The results of this and other unit teardowns led to the development of the following failure sequence theory :  1. Operation at the - 70°C temperature plateau, very  close to the lubricant pour point of - 72°C,  rendered the lubrication scheme ineffective, resulting in lubricant  starvation conditions.   2. Starvation conditions , never a desir ed condition for long life bearing operation,  had the most  significant effects on the crown steel retainer, wearing first  at the inner diameter (ID) of bearing  retainer spine, where guided.   3. Retainer w ear debris mixe d with grease , accelerating  wear  as the grease became an abrasive  mixture with the wear particles.   4. Retainer ID continued to grow  with wear, allowing it to move radially out of position.   a. Ball pockets began to wear against balls, generat ing debris and burrs .  b. Outer diameter faces of the retainer  began to wear on the outer ring and inside of shields .  5. Wear debris led to increased drag in the bearings , causing increased noise in the motor current  signature.   6. Eventually lubricant  was depleted  and dry debris  was ejected from the bearing .  7. Two types of failure were observed:   a. The bearing  retainer was jammed between the bal l and outer ring, stalling the motor.  This  mode  was the initial life test failure observed in the original ShEl DV M45S motor.   b. The b uild-up/plating of wear particles and depleted  grease on raceways removed radial  play from the bearing and caused a significant increase in drag/motor current, but not an  outright motor stall.  This failure occurred in the M45S SN013 motor that had replaced the  original ShEl DV motor to complete the margined gearbox life test.  

Response to M45S Bearing Failure  

This life test failure occurred on one of the last gearmotor DV  life tests and  after the delivery and integration  of the flight gearmotors into the Mars 2020 rover’s flight robotic arm. The state of the program and the  severity of the issue  resulted in  parallel recovery paths being pursued:   1. “Use as is” – Determine an operating tempe rature range that would allow the baseline, delivered  bearing configuration to achieve the margined life test requirement . The goal was ensure the  lubricant was able to flow and protect the crown steel retainer from lubricant starvation wear.   2. “Refurbish” – Determine a superior bearing configuration that w ould achieve the margined  life test  requirement without failure. Adjust operating temperature range as necessary to maintain  acceptable torque uncertainty.  

“Use as is” was t he option with least impact to t he assembly and integration work already accomplish ed by  the Mars 2020 rover team. However, the refurbishment option was simultaneously pursued as a backup  plan in the event  there was no reasonable operating temperature range that allowed the delivered hardware  configuration to achieve the margined life test requirement . 

SNC and JPL had a number of M45S motors available as test beds for testing various bearing  configurations  across various temperature ranges . In addition, SNC possessed a number of motor  controller s, test consoles , and thermal chambers  to support simultaneous testing. Among many options,  the collective program team considered the following variables valuable t o compare:   • Minimum mission life test  temperature   • Speed   • Lubricant mix ratio and percentage of fill   • Retainer material  

Within three weeks of determining the front mot or bearing to be the cause of the ShE l DV life test stall, the  JPL-SNC team had begun a modified life test on two available M45S motors of the baseline bearing  configuration. B oth the flight spare motor (SN019) and a non- flight thermal correlation motor (SN032) began  life testing , alternating between two less severe  temperat ure plateaus: - 35°C and +25° C. To increase the  fidelity of these life tests and match the test flow , the motors were expose d to the random vibration