00:00The differential looks complicated, but once we understand its principle, it is amazingly simple.
00:07These two wheels are mounted on separate axles and supported by a frame,
00:12so that they can revolve freely at different speeds.
00:17Let's fasten a spoke on the inner end of each axle,
00:23so that by turning the spokes, we can turn each wheel separately.
00:30With a bar or cross piece, we can turn both wheels in the same direction at the same rate of speed.
00:43Let's get something to hold this bar in place, so that it will press against the spokes.
00:50Notice that this support is not locked to the axle. It turns freely.
00:55Now, we can spin the wheels by rotating the support.
01:01This is fine, as long as both wheels are able to turn at the same speed.
01:06But, let's see what happens when we go around the corner.
01:11With this arrangement, we cannot drive one wheel faster than the other.
01:16And, if we stop one wheel, the other wheel won't budge.
01:28Let's put this bar on a pivot, so that it can swing in either direction.
01:34Now, the bar can still turn both wheels at the same speed.
01:46And, because it pivots, it lets one wheel turn even when the other is stopped.
01:54But, if turned too far, the bar will swing around until it won't drive the spokes that turn either wheel.
02:04We need another crossbar and more spokes to carry on the job.
02:10When we stop one wheel, the crossbars will continue to push the spokes of the free wheel around.
02:18As long as both wheels are free to turn, the bars do not swing on their pivot, and the wheels move at the same speed.
02:31Now, we have the working principles of a differential.
02:35To adapt the model for use in an automobile, we will have to make a few changes.
02:41In order to reduce the jerky action, caused by wide spaces between the spokes, we will put in more spokes.
02:54Further filling in the spaces between the spokes gives steadier, more continuous action.
03:01And, changing the shape gives firm, constant contact.
03:05Now, we can make the gears thicker and stronger.
03:10And, we have differential gears.
03:13The edges are cut so that they will fit together more smoothly and silently.
03:19And, another gear is added to share the work of driving the axles.
03:23The principle is the same.
03:26In order to turn the support and drive the wheels, we can fasten a large gear here.
03:33Connected by a smaller gear to a source of power.
03:45Notice that the power is connected to the differential at the center line.
03:52We can make our model more compact by moving the gears closer together.
03:57When we put our differential in an automobile, we have to leave room for the drive shaft, which carries the power from the engine.
04:07We may build the floor of the car above the drive shaft.
04:12But, if we do, we won't have much room inside unless we make the top of the car high, too.
04:20Of course, we could lower the floor and ceiling, but the drive shaft would be higher than the floor.
04:25This would have disadvantages.
04:28A shaft in the middle of the floor of an automobile would be inconvenient for passengers and would be awkward for carrying luggage.
04:37Today, engineers have found a way to make the car roomier and closer to the road without a clumsy shaft above the floor.
04:45The drive shaft from the engine to the differential is lowered out of the way, and the drive shaft is connected to the rear axle at the bottom.
04:57The new low center drive makes the rear axle quieter, stronger, and more durable, because it gives better, smoother contact between the gears.
05:08The automobile of today, with the low center drive, is stronger and more rugged.
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