Quantcast Figure 1-14.-A. A pelican hook; B. A chain stopper.

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 has curved  lever  arms.  Can  you  figure  the  mechanical advantage of this one? Your answer should be M.A. = 5. The  crane  in  figure  1-13  is  used  for  handling relatively light loads around a warehouse or a dock. You can see that the crane is rigged as a third-class lever; the effort is applied between the fulcrum and the load. This gives a mechanical advantage of less than 1. If it’s going to support that 1/2-ton load, you know that the pull on the lifting cable will have to be considerably greater than 1,000 pounds. How much greater? Use the formula to figure it out: L R — = — l E Got the answer? Right. . . E = 1,333 pounds Now, because the cable is pulling at an angle of about 22° at E, you can use some trigonometry to find that the pull on the cable will be about 3,560 pounds to lift the 1/2-ton weight! However, since the loads are Figure 1-15.-An improvised drill press. generally light, and speed is important, the crane is a practical  and  useful  machine. Anchors are usually housed in the hawsepipe and secured by a chain stopper. The chain stopper consists of a short length of chain containing a turnbuckle and a pelican hook. When you secure one end of the stopper to a pad eye in the deck and lock the pelican hook over the anchor chain, the winch is relieved of the strain. Figure 1-14, part A, gives you the details of the pelican  hook. Figure 1-14, part B, shows the chain stopper as a whole.  Notice  that  the  load  is  applied  close  to  the fulcrum. The resistance arm is very short. The bale shackle, which holds the hook secure, exerts its force at a considerable distance from the fulcrum. If the chain rests against the hook 1 inch from the fulcrum and the bale shackle is holding the hook closed 12 + 1 = 13 inches   from   the   fulcrum,   what’s   the   mechanical advantage? It’s 13. A strain of only 1,000 pounds on the base shackle can hold the hook closed when a 6 1/2-ton anchor is dangling over the ship’s side. You’ll recognize the pelican hook as a second-class lever with curved arms. Figure 1-15 shows you a couple of guys who are using their heads to spare their muscles. Rather than exert themselves by bearing down on that drill, they pick up  a  board  from  a  nearby  crate  and  use  it  as  a second-class  lever. If the drill is placed halfway along the board, they will get a mechanical advantage of 2. How would you increase the mechanical advantage if you were using this rig? Right. You would move the drill in closer to the fulcrum. In the Navy, a knowledge of levers and how to apply them pays off. 1-7


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