Figure 5-9.—The quadrant davit.wheel turns a long threaded shaft. Half the threads—those nearer the wheel end of this shaft-are right-handthreads. The other half of the threads-those fartherfrom the wheel—are left-hand threads. Nut A has aright-hand thread, and nut B has a left-hand thread.Notice that two steering arms connect the crosshead tothe nuts; the crosshead turns the rudder. If you stand infront of the wheel and turn it in a clockwise direction—to your right—arm A moves forward and arm B movesbackward. That turns the rudder counterclockwise, sothe ship swings in the direction you turn the wheel. Thissteering mechanism has a great mechanical advantage.Figure 5-9 shows you another practical use of thescrew. The quadrant davit makes it possible for two mento put a large lifeboat over the side with little effort. Theoperating handle attaches to a threaded screw that passesthrough a traveling nut. Cranking the operating handlein a counterclockwise direction (as you face outboard),the nut travels outward along the screw. The travelingnut fastens to the davit arm by a swivel. The davit armand the boat swing outboard as a result of the outwardmovement of the screw. The thread on that screw is theself-locking type; if you let go of the handle, the nutremains locked in position.SUMMARYYou have learned the following basic informationabout the screw from this chapter; now notice thedifferent ways the Navy uses this simple machine:The screw is a modification of the inclined plane—modified to give you a high mechanicaladvantage.The theoretical mechanical advantage of the screwcan be found by the formulaAs in all machines, the actual mechanical advantageequals the resistance divided by the effort.In many applications of the screw, you make use ofthe large amount of friction that is commonlypresent in this simple machine.By using the screw, you reduce large amounts ofcircular motion to very small amounts ofstraight-line motion.5-4
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