Performance and Analysis of Perfluoropolyalkyl Ether Grease Used on Space Shuttle Actuators-A Case Study

摘要

Actuators used on the U.S. space shuttle fleet are lubricated with unspecified amounts of Braycote 601 grease consisting of a perfluorinatedpolyalkyl ether (PFPAE) base oil thickened with polytetrafluoroethylene (PTFE). Each shuttle has four body flap actuators (BFAs; two on each wing) on a common segmented shaft and four rudder speed brake (RSB) actuators. The actuators were designed to operate for 10 years and 100 flights without periodic relubrication. Visible inspection of two partially disassembled RSB actuators in continuous use for 19 years raised concerns over possible grease degradation due to discoloration of the grease on several places on the surfaces of the gears. Inspection revealed fretting, micropitting, wear, and corrosion of the bearings and gears. A small amount of oil dripped from the disassembled actuators. New grease is beige in appearance, whereas the discolored grease was both grey and reddish. Grease samples taken from the actuators together with representative off-the-shelf new and unused grease samples were analyzed for oil content using gravimetry; for metals content using inductively coupled plasma (ICP) analysis; for base oil decomposition using Fourier transform infrared (FTIR) spectroscopy; and for determination of the molecular weight distributions of the base oil using size exclusion chromatography (SEC). The Braycote 601 grease was physically and chemically stable after 19 years of continuous use in the sealed RSB actuators and was fit for its intended purpose. There were no significant chemical differences between the used grease samples and new and unused samples. Base oil separation was not significant within the sealed actuators and no iron fluoride was detected. The grey color of grease samples 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.