differential gear

Figure 11-9.-Exploded view of differential gear system. Other Types of Springs Torsion bars (fig. 11-7, C) are straight bars that are acted on by torsion (twisting force). The bars may be circular or rectangular in cross section. They also may be 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 discs that overlap each other. THE  GEAR  DIFFERENTIAL A gear differential is a mechanism that is capable of adding  and  subtracting  mechanically.  To  be  more precise,  we  should  say  that  it  adds  the  total  revolutions of  two  shafts.  It  also  subtracts  the  total  revolutions  of one   shaft   from   the   total   revolutions   of   another shaft—and delivers the answer by a third shaft. The gear differential will continuously and accurately add or subtract any number of revolutions. It will produce a continuous series of answers as the inputs change. Figure 11-8 is a cutaway drawing of a bevel gear differential showing all of its parts and how they relate to  each  other.  Grouped  around  the  center  of  the mechanism are four bevel gears meshed together. The two bevel gears on either side are “end gears.” The two bevel gears above and below are “spider gears.” The long shaft running through the end gears and the three spur gears is the “spider shaft.” The short shaft running through  the  spider  gears  together  with  the  spider  gears themselves make up the “spider.” Each spider gear and end gear is bearing-mounted on its shaft and is free to rotate. The spider shaft connects Figure 11-10.-The differential. End gears and spider arrangement. with the spider cross shaft at the center block where they intersect. The ends of the spider shaft are secured in flanges or hangers. The spider cross shaft and the spider shaft are also bearing-mounted and are free to rotate on their axis. Therefore, since the two shafts are rigidly connected, the spider (consisting of the spider cross shaft and the spider gears) must tumble, or spin, on the axis of the spider shaft. The three spur gears, shown in figure 11-8, are used to connect the two end gears and the spider shaft to other mechanisms. They may be of any convenient size. Each of the two input spur gears is attached to an end gear. An input 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 pinned to the spider shaft. All the other differential gears, both bevel  and  spur,  are  bearing-mounted. Figure  11-9  is  an  exploded  view  of  a  gear differential  showing  each  of  its  individual  parts.  Figure 11-10 is a schematic sketch showing the relationship of the principle parts. For the present we will assume that the two sides of the gear system are the inputs and the gear on the spider shaft is the output. Later we will show that any of these three gears can be either an input or an output