Ever wrestled with a length of wire rope? Obstinate and unwieldy, wasnt it? Riggers have dreamed up tools to help subdue wire rope. One of these tools-the riggers vise-is shown in figure 5-6. This riggers vise uses the mechanical advantage of the screw to hold the wire rope in place. The crew splices a thimble-a reinforced looponto the end of the cable. Rotating the handle causes the jaw on

Figure 5-8.The screw gives a tremendous mechanical advantage.

that screw to move in or out along its grooves. This machine is a modification of the vise on a workbench. Notice the right-hand and left-hand screws on the left-hand clamp.

Figure 5-7 shows you another use of the screw. Suppose you want to stop a winch with its load suspended in mid-air. To do this, you need a brake. The brakes on most anchor or cargo winches consist of a metal band that encircles the brake drum. The two ends of the band fasten to nuts connected by a screw attached to a handwheel. As you turn the handwheel, the screw pulls the lower end of the band (A) up toward its upper end (B). The huge mechanical advantage of the screw puts the squeeze on the drum, and all rotation of the drum stops.

One type of steering gear used on many small ships and as a spare steering system on some larger ships is the screw gear. Figure 5-8 shows you that the

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 wheelare 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 rightarm 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.

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.

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.