Summary

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-hand threads. The other half of the threads-those farther from the wheel—are left-hand threads. Nut A has a right-hand thread, and nut B has a left-hand thread. Notice that two steering arms connect the crosshead to the nuts; the crosshead turns the rudder. If you stand in front of the wheel and turn it in a clockwise direction— to your right—arm A moves forward and arm B moves backward. That turns the rudder counterclockwise, so the ship swings in the direction you turn the wheel. This steering mechanism has a great mechanical advantage. Figure 5-9 shows you another practical use of the screw. The quadrant davit makes it possible for two men to put a large lifeboat over the side with little effort. The operating handle attaches to a threaded screw that passes through a traveling nut. Cranking the operating handle in a counterclockwise direction (as you face outboard), the nut travels outward along the screw. The traveling nut fastens to the davit arm by a swivel. The davit arm and the boat swing outboard as a result of the outward movement of the screw. The thread on that screw is the self-locking type; if you let go of the handle, the nut remains locked in position. SUMMARY You have learned the following basic information about the screw from this chapter; now notice the different ways the Navy uses this simple machine: The screw is a modification of the inclined plane— modified to give you a high mechanical advantage. The theoretical mechanical advantage of the screw can be found by the formula As in all machines, the actual mechanical advantage equals the resistance divided by the effort. In many applications of the screw, you make use of the large amount of friction that is commonly present in this simple machine. By using the screw, you reduce large amounts of circular motion to very small amounts of straight-line motion. 5-4