Figure 11-9.-Exploded view of differential gear system.Other Types of SpringsTorsion bars (fig. 11-7, C) are straight bars that areacted on by torsion (twisting force). The bars may becircular or rectangular in cross section. They also maybe tube shaped; other shapes are uncommon.A special type of spring is a ring spring or disc spring(not illustrated). It is made of several metal rings or discsthat overlap each other.THE GEAR DIFFERENTIALA gear differential is a mechanism that is capable ofadding and subtracting mechanically. To be moreprecise, we should say that it adds the total revolutionsof two shafts. It also subtracts the total revolutions ofone shaft from the total revolutions of anothershaft—and delivers the answer by a third shaft. The geardifferential will continuously and accurately add orsubtract any number of revolutions. It will produce acontinuous series of answers as the inputs change.Figure 11-8 is a cutaway drawing of a bevel geardifferential showing all of its parts and how they relateto each other. Grouped around the center of themechanism are four bevel gears meshed together. Thetwo bevel gears on either side are “end gears.” The twobevel gears above and below are “spider gears.” Thelong shaft running through the end gears and the threespur gears is the “spider shaft.” The short shaft runningthrough the spider gears together with the spider gearsthemselves make up the “spider.”Each spider gear and end gear is bearing-mountedon its shaft and is free to rotate. The spider shaft connectsFigure 11-10.-The differential. End gears and spiderarrangement.with the spider cross shaft at the center block where theyintersect. The ends of the spider shaft are secured inflanges or hangers. The spider cross shaft and the spidershaft are also bearing-mounted and are free to rotate ontheir axis. Therefore, since the two shafts are rigidlyconnected, the spider (consisting of the spider crossshaft and the spider gears) must tumble, or spin, on theaxis of the spider shaft.The three spur gears, shown in figure 11-8, are usedto connect the two end gears and the spider shaft to othermechanisms. They may be of any convenient size. Eachof the two input spur gears is attached to an end gear. Aninput gear and an end gear together are called a “side”of a differential. The third spur gear is the output gear,as designated in figure 11-8. This is the only gear pinnedto the spider shaft. All the other differential gears, bothbevel and spur, are bearing-mounted.Figure 11-9 is an exploded view of a geardifferential showing each of its individual parts. Figure11-10 is a schematic sketch showing the relationship ofthe principle parts. For the present we will assume thatthe two sides of the gear system are the inputs and thegear on the spider shaft is the output. Later we will showthat any of these three gears can be either an input or anoutput.11-7
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