be spring loaded and made of silver/niobium diselenide carrying from amp/m2 ( ). Brush pressure would be from (4 to 10 psi). Brush speed is low, so no arcing problems are anticipated. Brush height would be designed for the life of the SSPS at very small penalty, or life/reliability goals could be met by an unloaded set of standby brushes to be actuated by command when wearout of the initial set was imminent. Life predictions for such designs have been estimated to be possible for over 100 years. The main problem is providing for oil vapor lubrication to supplement the solid lubricant. Oil vapor lube extends the life of brushes, but re- quires some form of reservoir and labyrinth seal to minimize vapor loss. Mass estimate has yet to be made, but the specific weight will probably be small relative to structure, bearings, and buses. Figure 3.2-8 is a schematic of the slip ring and brush design concept for the azimuth rotary joint. The total weight of brushes and slip rings is estimated at 1100 lb (504 Kg). Slip rings and brushes possess a well-developed technology and have unlimited rotational freedom in one axis. Their performance is not degraded by stopping, starting or reversing. Slip rings have high reliability over long operating periods. Slip rings, however, are relatively heavy and because of their large size would present problems in assembly. The major consideration to overall system design is the high thermal inputs to the structural interface due to ^R losses at the brush slip ring interface. Figure 3.2-9 and 3.2-10 summarize the operating temperatures and voltage drop for some candidate brushes and slip ring combinations. Considering a system power level of 5 GW and a voltage level of 20 KV, there will be approximate 20,000 amperes per bus bar. To achieve a medium current density of 7.75 amperes per square meter requires 3,23 square meters of brush/slip ring contact area. According to Fig. 3.2-10, the voltage drop across the brush/slip ring interface will be approximately 0.2 volt which will generate 40 KW of waste heat at each interface. To ensure reasonable operating temperatures for the brushes, methods for efficiently "dumping" this waste heat should be considered. 3.2.2.6 Liquid Metal Slip Rings No information on successful application of this concept has been uncovered to date. The state-of-the-art is such as to leave most questions unanswered, so development risk is considered high. Reference 8 gave no technical information relative to liquid slip rings but did not list them with SKF as origin. SKF had tried to use a mercury liquid metal slip ring for instrumentation noise suppression on a 24, 000 rpm bearing research program but had dropped development in favor of a silver/silver graphite solid brush system.
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