CHAPTER 6GEARSCHAPTER LEARNING OBJECTIVESUpon completion of this chapter, you should be able to do the following:lCompare the types of gears and their advantages.Did you ever take a clock apart to see what made ittick? Of course you came out with some parts left overwhen you got it back together again. And they probablyincluded a few gear wheels. We use gears in manymachines. Frequently the gears are hidden from view ina protective case filled with grease or oil, and you maynot see them.An eggbeater gives you a simple demonstration ofthe three jobs that gears do. They can change thedirection of motion, increase or decrease the speed ofthe applied motion, and magnify or reduce the force thatyou apply. Gears also give you a positive drive. Therecan be, and usually is, creep or slip in a belt drive.However, gear teeth are always in mesh, so there can beno creep and slip.Follow the directional changes in figure 6-1. Thecrank handle turns in the direction shown by thearrow—clockwise—when viewed from the right. The32 teeth on the large vertical wheel (A) mesh with the 8teeth on the right-hand horizontal wheel (B), whichrotates as shown by the arrow. Notice that as B turns ina clockwise direction, its teeth mesh with those of wheelC and cause wheel C to revolve in the opposite direction.The rotation of the crank handle has been transmitted bygears to the beater blades, which also rotate.Now figure out how the gears change the speed ofmotion. There are 32 teeth on gear A and 8 teeth on gearB. However, the gears mesh, so that one completerevolution of A results in four complete revolutions ofgear B. And since gears B and C have the same numberof teeth, one revolution of B results in one revolution ofC. Thus, the blades revolve four times as fast as the crankhandle.In chapter 1 you learned that third-class leversincrease speed at the expense of force. The samehappens with the eggbeater. The magnitude of forcechanges. The force required to turn the handle isgreater than the force applied to the frosting by theblades. This results in a mechanical advantage of lessthan one.TYPES OF GEARSWhen two shafts are not lying in the same straightline, but are parallel, you can transmit motion fromFigurc 6-1.—A simple gear arrangement.6-1
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