Figure 6-13.—Automobile valve gear.The total, or overall, theoretical mechanicaladvantage of a compound machine is equal to theproduct of the mechanical advantages of theseveral simple machines that make it up. In thiscase you considered the winch as two machines—one having a mechanical advantage of 6 and theother a mechanical advantage of 2. Therefore, theoverall theoretical mechanical advantage of thewinch is 6 x 2, or 12. Since friction is alwayspresent, the actual mechanical advantage may beonly 7 or 8. Even so, by applying a force of 100pounds on the handle, you could lift a load of 700to 800 pounds.CAMYou use gears to produce circular motion.However, you often want to change rotary motioninto up-and-down, or linear, motion. You can usecams to do this. For example, in figure 6-12 thegear turns the cam shaft. A cam is keyed to theshaft and turns with it. The design on the cam hasan irregular shape that moves the valve stem upand down. It gives the valve a straight-line motionas the cam shaft rotates.When the cam shaft rotates, the high point(lobe) of the cam raises the valve to its openposition. As the shaft continues to rotate, the highpoint of the cam passes, lowering the valve to aclosed position.A set of cams, two to a cylinder, driven bytiming gears from the crankshaft operate theexhaust and intake valves on the gasolineautomobile engine as shown in figure 6-13. We usecams in machine tools and other devices to makerotating gears and shafts do up-and-down work.ANCHOR WINCHOne of the gear systems you’ll get to seefrequently aboard ship is that on the anchorwinch. Figure 6-14 shows you one type in whichyou can readily see how the wheels go around. Thewinch engine or motor turns the driving gear (A).This gear has 22 teeth, which mesh with the 88teeth on the large wheel (B). Thus, you know thatthe large wheel makes one revolution for everyfour revolutions of the driving gear (A). You get a4-to-1 theoretical mechanical advantage out ofthat pair. Secured to the same shaft with B is thesmall spur gear (C), covered up here. The gear (C)has 30 teeth that mesh with the 90 teeth on thelarge gear (D), also covered up.Figure 6-14.—An anchor winch.6-7
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