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Show LESSON NO. 101. The Differential. One of the most important, and, without with-out question, the least understood mechanisms mecha-nisms of the automobile, is the differential. differen-tial. The purpose of this is to take care of the problems of steering and turning the car, when the two wheels are driven from a single engine. It was because the earlier inventors failed to understand the problems of steering and to solve them when understood that the automobile did not come into use sooner than It did. Let us see what these problems are. When .an auto goes around a corner or makes a turn, the outer wheels will have to make a circle which has a radius nearly near-ly five feet greater than that which the inner wheels travel. The result Is that they will have to travel considerably further, or, In other words, make a greater number of revolutions per minute. min-ute. Not only must the wheels revolve at different speeds, but, in the case of the driven or rear wheels, as they do so, the pressure of the wheels against the ground must always be exactly equal. If the pressure of ono wheel were greater than that of the other, one of them would have to slip and there also would be a "skidding" "skid-ding" of the car. As the differential permits of the above, it might be defined as a device which will maintain the tractive or cLriving moment both wheels are revolving at their proper speed, the outer wheel traveling trav-eling just the right amount faster than the inner. On the straightaway the resistance re-sistance will be equal when both wheels are revolving at the same speed. This theoretical mechanism would, therefore, solve the problem of having the wheels revolve at different speeds when turning corners, and of having the drive on them equal at all times. It, however, is not practical. For one thing, the compensating bar E would have to revolve with the wheels. This could be taken care of by having it plvotically fastened to the Inside of a ring, and making the ring revolve. Secondly, Sec-ondly, as the bar revolves on its pivots, piv-ots, it would slide off the levers C and I). This could be overcome by having many levers and many bars, all radiating from the same center, giving us a mechanism as shown in Fig. .1. If the space between the bars and levers lev-ers shown in Fig. 3 were to be filled In, so that only their ends protruded, they would really be converted into gears. Thus by fastening a gear (it would have to be a bevel gear) on the ends of each of the driving shafts A and B, as C and D, Fig. 4, and having another gear K pivotically mounted in a ring or housing F and having a means for driving the housing, we would have a practical mechanism for driving the rear wheels 0 5 Doi Z (a V f , : i -i I so as to overcome the problems of turn-j turn-j Ing corners, etc. 1 The housing F is driven by fastening-to fastening-to it a large bevel gear and driving this by means of a bevel pinion connected to the propeller shaft, as explained in the last lecture. To review the action of the differential, when the propeller shaft is revolving the ninion H will make the crear G and hous ing F revolve. When the resistance is equal on both wheels the gear K will follow C and D around without it itself resolving on its own axis pin, the entire differential and driving shafts revolve re-volve as though one solid piece. When, however, there is more resistance on one wheel, eay A, than on the other, aS is the case when turning corners, then the gear K will roll around on the gear C j and thus drive D faster, and so make B revolve faster than A. For balance and the sake of strength j there are either three or four of the j compensating gears E set in the housing. force of both the driven wheels against I the ground equal at all times, and which 1 will permit the wheels to revolve at different dif-ferent speeds when they are required to do so. When a car is turning a corner the pressure of the two wheels will be exactly exact-ly equal when both wheels are revolving at exactly he right speed to travel on their required circle. ' Thus we see that, In driving the rear wheels, two problems must be solved, namely, making the rear wheels travel at different speeds when the car Is traveling travel-ing in a circle; and having the power of the engine exerted on the rear wheels so that the pressure- will always be the same on both of them. As the rear wheels must travel at different dif-ferent speeds when turning a corner, the driving shaft must be divided. Let Fig. 1 show the wheels with their divided shafts A and B. Suppose we fastened to each of the shafts a lever, as C and D, and had a bar as E resting against these levers, with a rope F fastened to the exact mid-die mid-die of the bar. , , , u If the resistance of both wheels is the same, as the rope F is pulled both wheels will be made to move together and at the oame speed. If, however, the wheel A i be held while wheel B is free, then, as I the ropo F Is pulled, the wheel B would bo made to revolve, the bar K also sort of revolving, and taking the position , shown in Fig. 2. j Now suppose that when B has revolved re-volved a very small amount its resistance resis-tance becomes equal to that of A. Obviously Ob-viously both A and B would then revolve-together, revolve-together, and there would be no revolv- ing of the bar F. H, when both had revolved a little, the resistance of B be- , came loss than of A again, tt again j would travel while B would stop, while if when B had again moved a little tho i resistance again became equal, both would again revolve. Jf this continued ' Indefinitely, we would have A revolving , in starts and stops while B revolved con- tinuouslv and faster than A. j If A, Instead of starting and stop- ( ping, simply slowed down, botli A and B , would revolve, but B would revolve fast- I er than A, during which time the bar E would revolve un b as a pivot and C as a fulcrum. Now, when an auto is traveling in a circle, the drive, and, therefore, resistance resis-tance of both wheels will be equal, the |