Liquid sodium has potential as a slip ring liquid because of good conductivity, wetting and viscosity characteristics. Theoretically, such a device would carry current with low power loss and low torque drag. The main problem is foreseen as development of a long life, low friction seal to keep the liquid contained. Such a seal would be sensitive to temperature change, contamination, and corrosion. It might prove difficult to lubricate and on-line maintenance or seal replacement could prove complicated. Because of high development risk, lack of design information, constraints on structural configuration, and lower life/reliability rating than conventional slip rings, liquid metal slip rings are not recommended for baseline development but should be considered as a potential technology study. 3.2.2.7 Power Clutch and Flexible Cable A power clutch holds two contacts fixed together while they move through part of a revolution, and then the contacts break apart and reset to an initial position, and the cycle repeats. The relative motion is allowed for by a flexible cable. For SSPS application, two sets of contacts would be required so one could carry the load while the other reset. The advantage is a lower contact resistance and lower wear rate than with brushes and slip rings, but the greater mechanical complexity and redundant mechanism required would probably trend toward higher weight and lower reliability. 3.2.2.8 Rotary Ti ansformer A rotary transformer could be designed to operate at high efficiency with nc wear (no contact), but the high efficiency would require heavy7 core material and a close tolerance gap. Since SSPS is a DC system, an additional penalty would have to be added for DC ' C-DC power conversion. Rotary transformers do provide low friction and should be considered as a potential technology7 study. 3.2. 2.9 Rolling Contact A rolling contact device transferring power through either gears or bearings is inefficient as an electrical conduction path because contact areas are essentially lines or points unless there is deformation - in which case metal fatigue becomes a problem. It is not recommended. 3.2.2.10 Lubrication Lubricant options include oil, grease and solids. Figures 3.2-11 and 3.2-12 (based on data from Ref 8) summarize test data on various condidate oils and grease. The 30-year life
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