59 An Evaluation of Liquid, Solid, and Grease Lubricants for Space Mechanisms  Using a Spiral Or bit Tribometer 

Michael Buttery * 

    Abstract   We present the findings of the test program pe rformed by The European Space Tribology Laboratory  (ESTL) to evaluate the performance (f riction and lifetime) of a number of space lubricants under vacuum  using a Spiral Orbit Tribometer (SOT). Focus was gi ven to a comparison of various popular space oils, a  comparison study between the old and new MAPLUB  grease formulations, and the performance of  commonly used solid lubricants under various conditions.  Tests demonstrated that the lifetimes  of hydrocarbon NYE oils 2001 & 2001A outperformed those of the  perfluroropolyalkylether (PFPE) oils Fomblin Z25 & Z60, though these pairs displayed similar behavior. This relationship was also generally seen for grease s; with the lifetimes of the multiple alkylated  cyclopentane (MAC)-based greases being extended in comparison to the PFPE-based greases. Testing on greases also demonstrated similar performance between the old (-a) and new (-b) formulations when considering PFPE-based MAPLUB greases, and indeed for all tested PFPE-based non-MAPLUB greases, but significantly shorter lifetimes for the new formulations when considering MAC-based  MAPLUB greases. MAPLUB MAC greases containing molybdenum disulphide (MoS 2) thickener were  also found to display reduced lifetimes.  For solid lubricants, lead displayed si gnificantly extended lifetimes over MoS 2, speculated to be caused by  redistribution of lead from the ball onto all contact surf aces during the test. Friction coefficients were seen  to be some 2.5x higher for lead than for MoS 2 under similar conditions, a result that corresponds well with  conventional bearing tests.  The work described was performed under contract for the European Space Agency as part of the Tribology Applications Program, with all funding for testing and apparatus pr ovided by European Space  Agency (ESA).    Introduction   Selection of an appropriate lubricant is a vitally important stage of mechanism design. Due consideration must be given to the lubricant properties such as lifetime, friction coefficient, and vapor pressure to avoid unforeseen mechanism failure, a potentially disastrous consequence. As such there is a great need to accurately understand the behavior of space lubricants, and to comparat ively assess their performance  under representative test cond itions. This paper details the findings of a test program performed by ESTL  to assess the performance of a number of space lubric ants (liquid, grease and solid), using a Spiral Orbit  Tribometer.  Two commonly used space oils are Fomblin Z25, a Z-type PFPE oil, and the MAC oil NYE 2001A. Additionally the oil NYE 2001 is also frequently used; an oil similar in composition to that of 2001A, but  containing phosphate ester boundary lubricants and antio xidant additives to improve this oil’s boundary  lubrication performance.                                                     * The European Space Tribology Laboratory (ESTL), ESR Technology, Warrington, UK 

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010   NASA/CP-2010-216272