Gears used to change speed

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The worm and the worm wheel

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Gears used to increase mechanical advantage

GEARS USED TO CHANGE SPEED

As youve already seen in the eggbeater, you use gears to change the speed of motion. Another example of this use of gears is in your clock or watch. The mainspring Slowly unwinds and causes the hour hand to make one revolution in 12 hours. Through a series-or train-of gears, the minute hand makes one revolution each hour, while the second hand goes around once per minute

Figure 6-9 will help you to understand how speed changes are possible. Wheel A has 10 teeth that mesh with the 40 teeth on wheel B. Wheel A will have to rotate four times to cause B to make one revolution. Wheel C is rigidly fixed on the same shaft with B. Thus, C makes the same number of revolutions as B. However, C has 20 teeth and meshes with wheel D, which has only 10 teeth. Hence, wheel D turns twice as fast as wheel C. Now, if you turn A at a speed of four revolutions per second, B will rotate at one revolution per second. Wheel C also moves at one revolution per second and causes D to turn at two revolutions per second. You get out two revolutions per second after having put in four revolutions per second. Thus, the overall speed reduction is 2/4or 1/2that means you got half the speed out of the last driven wheel you put into the first driver wheel.

You can solve any gear speed-reduction problem with this formula:

where

S₁ = speed of first shaft in train

S₂ = speed of last shaft in train

T₁ = product of teeth on all drivers

T₂ = product of teeth on all driven gears

Figure 6-9.-Gears can change speed of applied motion.

Now use the formula on the gear train of figure 6-9.

To obtain any increase or decrease in speed you, must choose the correct gears for the job. For example, the turbines on a ship have to turn at high speeds-say 5,800 rpmif they are going to be efficient. The propellers, or screws, must turn rather slowlysay 195 rpmto push the ship ahead with maximum efficiency. So, you place a set of reduction gears between the turbines and the propeller shaft.

When two external gears mesh, they rotate in opposite directions. Often youll want to avoid this. Put a third gear, called an idler, between the driver and the driven gear. Dont let this extra gear confuse you on speeds. Just neglect the idler entirely. It doesnt change the gear ratio at all, and the formula still applies. The idler merely makes the driver and its driven gear turn in the same direction. Figure 6-10 shows you how this works.

Figure 6-10.-An idler gear.

Figure 6-11.-Cable winch.