Figure 1-13.-An electric crane.Figure 1-14.-A. A pelican hook; B. A chain stopper.The wrecking bar is a first-class lever. Notice that it hascurved lever arms. Can you figure the mechanicaladvantage of this one? Your answer should be M.A. = 5.The crane in figure 1-13 is used for handlingrelatively light loads around a warehouse or a dock. Youcan see that the crane is rigged as a third-class lever; theeffort is applied between the fulcrum and the load. Thisgives a mechanical advantage of less than 1. If it’s goingto support that 1/2-ton load, you know that the pull onthe lifting cable will have to be considerably greater than1,000 pounds. How much greater? Use the formula tofigure it out:LR— = —lEGot the answer? Right. . . E = 1,333 poundsNow, because the cable is pulling at an angle ofabout 22° at E, you can use some trigonometry to findthat the pull on the cable will be about 3,560 pounds tolift the 1/2-ton weight! However, since the loads areFigure 1-15.-An improvised drill press.generally light, and speed is important, the crane is apractical and useful machine.Anchors are usually housed in the hawsepipe andsecured by a chain stopper. The chain stopper consistsof a short length of chain containing a turnbuckle and apelican hook. When you secure one end of the stopperto a pad eye in the deck and lock the pelican hook overthe anchor chain, the winch is relieved of the strain.Figure 1-14, part A, gives you the details of thepelican hook.Figure 1-14, part B, shows the chain stopper as awhole. Notice that the load is applied close to thefulcrum. The resistance arm is very short. The baleshackle, which holds the hook secure, exerts its force ata considerable distance from the fulcrum. If the chainrests against the hook 1 inch from the fulcrum and thebale shackle is holding the hook closed 12 + 1 = 13inches from the fulcrum, what’s the mechanicaladvantage? It’s 13. A strain of only 1,000 pounds on thebase shackle can hold the hook closed when a 6 1/2-tonanchor is dangling over the ship’s side. You’ll recognizethe pelican hook as a second-class lever with curvedarms.Figure 1-15 shows you a couple of guys who areusing their heads to spare their muscles. Rather thanexert themselves by bearing down on that drill, they pickup a board from a nearby crate and use it as asecond-class lever.If the drill is placed halfway along the board, theywill get a mechanical advantage of 2. How would youincrease the mechanical advantage if you were usingthis rig? Right. You would move the drill in closer to thefulcrum. In the Navy, a knowledge of levers and how toapply them pays off.1-7
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