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Operation - 14001_95
Retractable Sheave

Aviation Boatswains Mate E 3 & 2 - Aviation theories and other practices
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As the sheave damper crosshead reaches its BATTERY position, the battery-position indicator limit switch is engaged, and lights the battery-position indicator light at deck edge. PURCHASE CABLE The purchase cable is the wire rope reeved onto the arresting engine sheaves and fed through fairlead tubing and over the fairlead sheave to the deck gear on the flight deck. The purchase cable transmits the force of the landing aircraft from the deck gear to the arresting engine. The purchase cables are 1 7/16-inch diameter, 6 x 25 filler wire construction, right-hand lay, lang lay cables with a minimum breaking strength of 195,000 pounds. The 6 x 25 filler wire construction means that the cable is made up of 6 strands with 19 major wires and 6 filler wires per strand. (See fig. 3-21.) The filler wires provide shape and stability to the strand.    Lang lay denotes cables in which the wires of the strand and the strand are twisted in the same direction so that the outer wires in the lang lay cables run diagonally across the longitudinal axis of the cables. The purchase cables are made from high-strength, uncoated plow steel. A hemp center, made from resilient oil-impregnated hemp, serves as a foundation for the strands, keeps the strands evenly spaced, and prevents them from bearing against each other. The hemp center also aids in lubrication of the inner wires. Within the hemp center is buried a paper or plastic strip bearing the name of the manufacturer. Poured threaded terminals are fitted on each end of the purchase cables. A clevis socket is Figure 3-21 .—Cross section of preformed purchase cable. screwed onto the threaded terminals to connect the purchase cables to the crossdeck pendants and to the single-reeved engines. See figure 3-22. The Mk 7 arresting engines have an 18:1 reeve ratio, which means for every foot of ram travel there are 18 feet of purchase cable payout. The reeve ratio is determined by the number of sheaves on the crosshead. The types of reeves used on the Mk 7 arresting engines are 18:1 single reeve on pendant engines, and 18:1 endless reeve on barricade engines. The endless reeve barricade engines use only one purchase cable with the two bitter ends terminating on the flight deck and connected to the barricade. The single-reeved pendant engines require two purchase cables, one reeved around the 28-inch pitch diameter sheaves and one reeved around the 33-inch pitch diameter sheaves of the fixed sheave and crosshead assemblies. One end of each of the two purchase cables is connected to a torque-release coupling on the cable anchor damper operating piston rod. The opposite ends are connected to the crossdeck pendant. DECK GEAR The function of the deck gear is to provide a means of engaging a landing aircraft and directing the force of arrestment to the arresting engine. The deck gear consists essentially of deck pendants, wire supports, fairlead and deck sheaves, and the barricade system. The deck gear is a series of devices (used for engagement of landing aircraft) that direct and control the energies developed during arrestment. Figure 3-22.—Clevis and socket assembly. 3-26







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