Tribology  

There are four areas of tribology for the PMA mec hanisms:  bearings, gears, hard-stops, pin-pullers. The  mechanisms must all operate under the same c onditions and therefore have similar operational  requirements. The most significant requirement is t he lower operating temperature of -70ºC. In addition,  the mechanisms must function throughout ground testi ng under various temperatures, survive the hard  vacuum during the trip to Mars and then function in the dusty Martian atmosphere. The mechanisms are  mostly slow moving and the mission life is fairly short. 

Several lubricants were considered for use on the PMA mechanisms. Silicone lubricants eventually break  down and become abrasive. Silicone contamination on optical components is considered a major risk.  Perfluoropolyether (PFPE) lubricants such as Bray co 815Z oil and Braycote 601 grease also polymerize,  but generally outperform silicone lubricants. The lower operating temperature limit is -80ºC. Dry film  lubricants such as those based on Molybdenum Disulfide (MoS 2) or thin metal films can operate well at  temperatures near 0 Kelvin. However, this type of lubricant has a lower load capability than wet  lubricants. It is also a less robust lubricant because it s life is compromised by humidity. Hybrid bearings  such as those with Silicon Nitride balls in metal race s can sometimes operate with little or no lubricant if  the contact stress is very low. This was not feas ible for the PMA mechanisms because the contact stress  was too large due to the mass of the payloads and  the Martian gravity. 

PFPE lubrication was selected for use on all ball  bearings and gears in the PMA mechanisms.  Molybdenum Disulfide was added to the PFPE  lubricant for sliding components such as the gears  and the 4-Point contact bearing discussed in the  Mast Deployment Drive sect ion. Dry film lube in the  form of Molybdenum Disulfide was used for the  launch lock mechanism in which the pin-pullers have  sliding pins within spherical bearing bores. No  lubrication was used on the hard stops. Material  combinations with anti-galling properties were  carefully selected for both the pin-puller interface and  the hard stops. 

Testing 

Testing was critical on the program. Even with the  tight schedule, an extensive test program was  established. The philosophy behind the test program  was that since the design phase was short, testing  was even more important. Working jointly with JPL, a  comprehensive test program was put into place  starting with testing of the gear-motor actuator at the  subcontractor (Figure 16). The actuators for each  mechanism were exposed to qualification levels for  vibration and speed toque curves were generated  over the operating temperat ure range. After delivery,  the actuators were integrated into their respective  mechanisms. Safe to mate testing was performed to  ensure the mechanisms were wired correctly.  Thermal tests were then performed to characterize the mechanisms as an assembly.  Figure 16.  Program  test flow  

Actuator Testing   • Integrate Actuator  • Safe-to-mate verification  • Actuator Run-in  • Vibration testing  • Drive characterization   (speed/torque at temp) 

Mechanism Testing   • Integrate Actuator to mechanism   (Establish current limits)  • Safe-to-mate verification  • Drive run-in  • Drive characterization at temperature  • Pointing accuracy at temperature 

System Testing   • System integration  • Continuity & isolation check  • Optical alignment  • Ambient functional test  • Vibration tests  • Pin-puller test  • Functional test  • Load testing  • Thermal testing  • Ambient functional 

Three Pancam Mast Assemblies were built and test ed. The first unit was the Engineering Development  Unit (EDU) followed by the two flight units. All three were fabricated to the same specifications. One of  the units was designated the EDU and additional testi ng including life cycle testing was performed on it.  The EDU could be refurbished to flight standards, if needed.   274