THE WORM AND WORM WHEELFigure 6-7.—Worm gears.Worm and worm-wheel combinations, like those infigure 6-7, have many uses and advantages. However,it’s better to understand their operating theory beforelearning of their uses and advantages.Figure 6-7, view A, shows the action of asingle-thread worm. For each revolution of the worm,the worm wheel turns one tooth. Thus, if the wormwheel has 25 teeth, the gear ratio is 25:1.Figure 6-7, view B, shows a double-thread worm.For each revolution of the worm in this case, the wormwheel turns two teeth. That makes the gear ratio 25:2 ifthe worm wheel has 25 teeth.A triple-thread worm would turn the worm wheelthree teeth per revolution of the worm.A worm gear is a combination of a screw and a spurgear. You can obtain remarkable mechanical advantageswith this arrangement. You can design worm drives sothat only the worm is the driver-the spur cannot drivethe worm. On a hoist, for example, you can raise orlower the load by pulling on the chain that turns theworm. If you let go of the chain, the load cannot drivethe spur gear; therefore, it lets the load drop to the deck.This is a nonreversing worm drive.GEARS USED TO CHANGE DIRECTIONThe crankshaft in an automobile engine can turn inonly one direction. If you want the car to go backwards,you must reverse the effect of the engine’s rotation. Thisis done by a reversing gear in the transmission, not byreversing the direction in which the crankshaft turns.A study of figure 6-8 will show you how gears areused to change the direction of motion. This is aschematic diagram of the sight mounts on a Navy gun.If you crank the range-adjusting handle (A) in aclockwise direction, the gear (B) directly above it willrotate in a counterclockwise direction. This motioncauses the two pinions (C and D) on the shaft to turn inthe same direction as the gear (B) against the teeth cutin the bottom of the table. The table is tipped in thedirection indicated by the arrow.As you turn the deflection-adjusting handle (E) in aclockwise direction, the gear (F) directly above it turnsFigure 6-8.-Gears change direction of applied motion.6-4
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