Evaluation of Perflouropolyether Lubricant Lifetime in the High Stress and High  Stress-Cycle Regime for Mars Applications.

Jason Herman* and Kiel Davis* 

Abstract 

The successful operation of long-life, highly loaded mechanisms used for planetary exploration or autonomous structures assembly will depend upon the ability to effectively lubricate rolling-element  bearings. As new tools are developed (i.e. drill, ab raders, robotic manipulators, etc.) that interact with  their environment in a more direct manner, lubricants will be pushed past the bounds that current  scientific literature has published. This paper details results from bearing lubrication lifetime testing  performed in support of Honeybee Robotics’ development of the Mars Science Laboratory (MSL) Surface Removal Tool (SRT). This testing was done due to the la ck of available data in research literature that is  applicable to the lubrication regime the SRT bearings are being designed for. Based on the test results,  the chosen bearing arrangement can be used for the SRT Grind Shaft bearings with the use of a Braycote Micronic 601EF grease-plate with a 10 vol% grease slurry fill (50/50 wt% Braycote Micronic  601EF and Brayco 815Z). This arrangement showed no signs of detrimental degradation over the course  of the 3x life test. The purely grease-plated bearing ran at a consistently higher torque and showed signs of failure beginning at ~2.2 x 10 7 revs (~6.3 x 107 stress-cycles) with a torque over-limit failure at  ~4.5 x  107 revs (~1.3 x 108 stress-cycles). Barring cold-start torque margin limitations, it is recommended that  any long-life bearing application include some vol% grease-pack in addition to a standard grease-plate to reduce parasitic torque and increase bearing life. While these results are specific to a particular  environment and loading condition, they demonstrate the extended capabilities of a commonly used flight  lubricant outside of the range that is published in current research literature.     Introduction  

In 2009, the National Aeronautics and Space Administration (NASA) plans to launch the Mars Science  Laboratory (MSL) spacecraft that will include a roving vehicle and payload designed for a surface mission  of one Martian year (~669 Martian sols or ~687 Earth days) at a yet to be selected landing site. During  the nominal surface mission, the vehicle will be expected to traverse multiple kilometers and payload  instruments and tools will be required to perform dozens and in some case hundreds of operations. The  combination of MSL’s mission life requirement and potentially extreme surface environment at higher  latitudes is unprecedented for the Mars Exploration Program. As a quick comparison, NASA’s Viking  landers, Pathfinder Sojourner rover, Mars Explorat ion Rovers (MER) Spirit and Opportunity and the  Phoenix Lander (currently en route) were all designed for surface missions of 90 sols or less. Instruments  and tools of past missions were designed for dozens of hours of operation as opposed to hundreds of  hours as is the case with MSL. While many of the past landed Mars missions have far exceeded their design life, the same (if not more stringent) reliability assurance requirements apply to MSL mechanism  designs. At a fundamental component level, MSL’s mechanism design challenges exceed those of past  landed Mars missions by nearly an order of magnitude. In other words, what worked for MER will likely not work for MSL. 

MSL originally planned to carry a Surface Removal Tool (SRT) as part of its payload for brushing and grinding rock surfaces in a manner similar to the MER Rock Abrasion Tool (RAT) [1]. The desired  performance improvements over the MER RAT led to an increase in both grinding loads and lifetime  requirements (see Table 1). Outside of designing a longer lasting grinding bit, the most significant challenge faced by the design team was to develop a drive train design that would meet the life  requirements given all of the constraints. Specifically, the bearings, which supported the grinding bit shaft,                                                       

* Honeybee Robotics Spacecraft Mechanisms Corporation, New York, NY 

Proceedings of the 39th Aerospace Mechanisms Symposium, NASA Marshall Space Flight Center, May 7-9, 2008  69