Performance and Analysis of Perfluorinated Grease used on Space Shuttle Actuators

摘要

Actuators used on the United States space shuttle fleet are lubricated with grease consisting of a perfluoropolyalkyl ether (PFPE) base oil thickened with a polytetrafluoroethylene (PTFE) filler. The actuators were designed to operate for life without periodic relubrication and some have been in use for over 20 years. Visible inspection of a partially dissembled actuator, however, raised concerns over possible grease degradation due to discoloration of the grease on several places on the surfaces of the gears. Whereas new grease is beige in appearance, the discolored grease consisted of both grey and reddish colors. A number of grease samples were taken from various locations in the actuators and subjected to a variety of physical and chemical analytical tests. Gravimetric tests show that base oil separation from the actuator PFPE grease was not significant after two decades within the sealed actuators. The gray color of grease samples taken from the actuators was due to metallic iron. The red color was due to oxidation of the metallic wear particles from the gears and the bearings comprising the actuators. Both infrared spectroscopy and size exclusion chromatography analyses show no chemical differences between the used and unused grease samples. Little or no degradation of the PFPE grease had occurred in the actuators over the decades of operation.