Figure 2-6.-A gun tackle.

Applications Afloat and Ashore - 14037_22

Basic Machines - Intro to machines and motion theories
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Figure 2-6.-A gun tackle. The single movable block in this setup is a second-class lever. See figure 2-5. Your effort (E) acts upward upon the arm (EF), which is the diameter of the sheave. The resistance (R) acts downward on the arm (FR), which is the radius of the sheave. Since the diameter is twice the radius, the mechanical advantage is 2. When the effort at E moves up 2 feet, the load at R is raised only 1 foot. That’s something to remember about blocks and falls—if you are actually getting a mechanical advantage from the system. The length of rope that passes through your hands is greater than the distance that the load is raised. However, if you can lift a big load with a small effort, you don’t care how much rope you have to pull. The sailor in figure 2-4 is in an awkward position to Figure 2-7.-A luff tackle. want to get. For example, a luff tack consists of a double block and a single block, rigged as in figure 2-7. Notice that the weight is suspended by the three parts of rope that extend from the movable single block. Each part of the rope carries its share of the load. If the crate weighs 600 pounds, then each of the three parts of the rope supports its share—200 pounds. If there’s a pull of 200 pounds downward on rope B, you will have to pull downward with a force of 200 pounds on A to counterbalance the pull on B. Neglecting the friction in the block, a pull of 200 pounds is all that is necessary to raise the crate. The mechanical advantage is: pull. If he had another single block handy, he could use it to change the direction of the pull, as in figure 2-6. This second arrangement is known as a gun tackle. Because the second block is fixed, it merely changes the direction of pull—and the mechanical advantage of the Here’s a good tip. If you count the number of parts of rope going to and from the movable block you can whole system remains 2. figure the mechanical advantage at a glance. This simple You can arrange blocks in several ways, depending rule will help you to approximate the mechanical on the job to be done and the mechanical advantage you advantage of most tackles you see in the Navy. 2-3

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