Figure 3-7.-Valves.andE1 = 900 = 112/5 pounds8Slim, the smart sailor, has to lift only 112.5pounds. There’s a sailor who really puts hisknowledge to work.THE COUPLETake a look at figure 3-6. It’s another capstan-turning situation. To increase an effective effort,place a second capstan bar opposite the first andanother sailor can apply a force on the second bar.The two sailors in figure 3-6 will apparently bepushing in opposite directions. Since they are onopposite sides of the axle, they are actually causingrotation in the same direction. If the two sailors arepushing with equal force, the moment of force is twiceas great as if only one sailor were pushing. Thisarrangement is known technically as a couple.You will see that the couple is a special exampleof the wheel and axle. The moment of force is equal tothe product of the total distance (Ln between the twopoints and the force (E1) applied by one sailor. Theequation for the couple may be writtenE1 x LT = E2 x L2APPLICATIONS AFLOAT AND ASHOREA trip to the engine room important the wheeland axle makes you realize how is on the modernship.Figure 3-8.—A simple torque wrench.Everywhere you look you see wheels of all sizes andshapes. We use most of them to open and close valvesquickly. One common type of valve is shown in figure3-7. Turning the wheel causes the threaded stem torise and open the valve. Since the valve must closewatertight, airtight, or steamtight, all the parts mustfit snugly. To move the stem on most valves withoutthe aid of the wheel would be impossible. The wheelgives you the necessary mechanical advantage.You’ve handled enough wrenches to know thatthe longer the handle, the tighter you can turn a nut.Actually, a wrench is a wheel-and-axle machine. Youcan consider the handle as one spoke of a wheel andthe place where you take hold of the handle as a pointon the rim. You can compare the nut that holds in thejaws of the wrench to the axle.You know that you can turn a nut too tight andstrip the threads or cause internal parts to seize. Thisis especially true when you are taking up onbearings. To make the proper adjustment, you use atorque wrench. There are several types. Figure 3-8shows you one that is very simple. When you pull onthe handle, its shaft bends. The rod fixed on thepointer does not bend. The pointer shows on the scalethe torque, or moment of force, that you are exerting.The scale indicates pounds, although it is reallymeasuring foot-pounds to torque. If the nut is to betightened by a moment of 90 ft-1b, you pull until thepointer is opposite the number 90 on the scale. Theservicing or repair manual on an engine or piece ofmachinery tells you what the torque—or moment offorce—should be on each set of nuts or bolt.The gun pointer uses a couple to elevate anddepress the gun barrel. He cranks away at ahandwheel that has two handles. The right-handhandle is on the opposite side of the axle from theleft-hand handle—180° apart.3-5
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