How a differential gear works -- a-ha generating video

I posted this in March 2011, but as Kottke says, "it's so good, here it is again." "How a differential gear works" is a short industrial film that does a better job of explaining how differentials work than any other materials I've seen -- a real "a-ha" generator.

How a differential gear works


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  1. eat your heart out minutePhysics ;) who knew this explainer video genre wasn’t a product of the internet age…

  2. I learned that from Legos when I was a kid!  

    My sister and I had saved up all our paper route money and bought a bunch of those fancy Lego sets, one of the ones I got was a car that had a differential in the instructions, and if you screwed it up it was pretty obvious it wasn’t working properly.  It was a neat ‘learning by doing’ thing.

    I didn’t know that’s what it was until now. :)

  3. This might be one of those cases where understanding actually is effected by speaking slowly in a loud, clear voice. It might even work delivered in Klingon.

  4. What I still don’t completely understand is why, if the differential can spin around the wheels independently, doesn’t the friction of the tires against the road cause the differential to spin around the axle? How does it push the car forward?

    1. The “impeller” part of the differential is attached to a ring gear, which is driven by a pinion gear at right angles. (In an automobile, the pinion is the end of the drive shaft.) So the impeller isn’t free to spin around the shaft on its own.
      At a fixed speed of the drive shaft, the sum of the revolutions per second of the two axles is constant, though they can spin at different rates. (Note that if the drive shaft is stopped, if one axle rotates forward, the other must rotate backwards.)

        1. Basically, the way a differential works is that it allows power to be distributed over the two wheels in whatever configuration is necessary, but the wheels can’t spin on their own. So, if you have a drive-shaft speed that without a differential spins both wheels at 500 RPM (just to pick a number), then with a differential, one of the wheels can slow down to 250 RPM, but then the other has to speed up to 750 RPM. On the other hand, for the same drive-shaft speed, it is not possible to have one wheel go 250 RPM and the other 500 RPM.

          In other words, a differential does not allow wheels to spin independently of each other. If you where to raise up a car and stand under it and start rotating one of the wheels manually (assuming the parking brake isn’t on), the other wheel would spin backwards in at the same speed (so for a stopped drive shaft, if one was spinning 5 RPM, then other would have to be spinning at -5 RPM). 

  5. Saw this on a few years ago. The full 9.29 minute video can be viewed here:

  6. Clear explanations are a thing of beauty. This is the motion equivalent of Edward Tufte type graphics. Being ancient, I used to sit mesmerized by 16mm black and white scientific/educational films back in the Space Age. And it was cool that almost everyone then had appreciation and respect for science and progress, both scientific and social. Weird science deniers were just that -weird. Now they’re in charge.

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