Effects of hardness on the lifetime of graphite brushes used for aircraft starter generators
This study concentrates on the relationship between the hardness of three different carbon/graphite brushes and the wear rate of the brushes and heat generated within a 400 Amp starter generator. The method being proposed was to alternate the starter generator from the starting mode and the generating mode, and run each carbon/graphite brush in the starter generator for a set time and document both wear rates and heat created. The study originated with a starter generator overheating during normal usage. The original hypothesis was that either excess carbon dust caused arcing, or the energy required to form the carbon dust was releasing too much heat, and thus the brush was too hard. The three carbon/graphite brushes chosen for this researched spanned the range of hardness for this type of material. This thesis analyzed the surfaces of the carbon/graphite brushes to correlate grooves and pitting with the frictional coefficient. An optical microscope, AFM, and SEM were used in the analysis of the surfaces. It was found that as the hardness of the brush went up, the wear rate decreased while the overall heat on the inside of the starter generator increased. From this research it is evident that the hardness of the brushes is the key factor in the excess heat in the starter generator. The energy that is expelled as heat as the carbon brush gradually wears is directly correlated with the increase of operating temperature. A harder brush will prolong the life of the brushes, but also increases the operating temperature to more than the unit is capable of handling.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering.