Figure 11-11.—How a differential works.Now let’s look at figure 11-11. In this hookup thetwo end gears are positioned by the input shafts,which represent the quantities to be added orsubtracted. The spider gears do the actual adding andsubtracting. They follow the rotation of the two endFigure 11-12.—The spider makes only half as manyrevolutions.gears, turning the spider shaft several revolutionsproportional to the sum, or difference, of therevolutions of the end gears.Suppose the left side of the differential rotateswhile the other remains stationary, as in block 2 offigure 11-11. The moving end gear will drive thespider in the same direction as the input and,through the spider shaft and output gear, the outputshaft. The output shaft will turn several revolutionsproportional to the input.If the right side is not rotated and the left side isheld stationary, as in block 3 of figure 11-11, thesame thing will happen. If both input sides of thedifferential turn in the same direction at the sametime, the spider will be turned by both at once, as inblock 4 of figure 11-11. The output will beproportional to the two inputs. Actually, the spidermakes only half as many revolutions as therevolutions of the end gears, because the spider gearsare free to roll between the end gears. To understandthis better, let’s look at figure 11-12. Here a ruler isrolled across the upper side of a cylindrical drinkingglass, pushing the glass along a table top. The glasswill roll only half as far as the ruler travels. Thespider gears in the differential roll against the endgears in exactly the same way. Of course, you cancorrect the way the gears work by using a 2:1 gearratio between the gear on the spider shaft and thegear for the output shaft. Very often, for designpurposes, this gear ratio will be found to be different.When two sides of the differential move inopposite directions, the output of the spider shaft isproportional to the difference of the revolutions of thetwo inputs. That is because the spider gears are freeto turn and the two inputs drive them in oppositedirections. If the two inputs are equal and opposite,the spider gears will turn, but the spider shaft willnot move. If the two inputs turn in opposite directionsfor an unequal number of revolutions, the spidergears roll on the end gear that makes the lessernumber of revolutions. That rotates the spider in thedirection of the input making the greater number ofrevolution. The motion of the spider shaft11-8
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