I took one (2 actually) apart and still don't know how they
work!  Actually, I did finally get the gist of the planetary gear
system.  Somebody correct me if I get this wrong.  Remember the bicycles
with the 3 speed transmission in the rear hub?  Sturmey Archer is the
company that made those internal hub shifters.  I was always fascinated by
those too.  Never took one apart.  Apparently they also made ones that had
5 speeds and in integral drum brake!  Anyway, those use the same planetary
gear system.  Here's a website from an MIT student explaining the bicycle
transmission:

http://snipurl.com/9doy   and here's a 14 speed
model:   http://www.rohloffusa.com/frame.htm   and here's a really good one
on the AT:

http://auto.howstuffworks.com/automatic-transmission2.htm

The ratio between the shaft in the center (sun gear) and the housing on the
outside (ring gear) is always the same.  But then between them are the
planet gears on their own carrier.  By applying power to one of those 3
elements, and using one of the other elements to drive the wheels, and
either letting the third element rotate free or not, the planetary gearset
makes 3 ratios.  One choice is always 1:1, where the planetary gear system
is really just along for the ride.  Power is going straight through.  This
is 3rd gear in the vanagon, and I think 2nd on the bicycle.  For first gear
and reverse, clutches hold the planet gear carrier from turning.  (I'm
getting fuzzy here...)  For 2nd gear, that brake band holds the ring gear
from turning.  The pictures show it better than my words...

Now the valve body, that is another question.  A hydraulic computer. It
even looks like a brain inside!

Edward

At 09:28 AM 9/24/2004, you wrote:
>Thanks, Edward. I have seen exploded diagrams of automatic transmissions
>and am firmly convinced that there is no chance that I will /ever/ learn
>how they work -- in this lifetime, at least.
>
>--
>Mike "Rocket J Squirrel" Elliott
>71 Type 2: the Wonderbus
>84 Westphalia: "Mellow Yellow (The Electrical Banana)"
>KG6RCR
>
>
>
>Edward Maglott wrote:
>
>>The 2nd gear brake band is like a big Omega (like the symbol for
>>ohms.)  It's a band that fits around a spinning drum in the
>>transmission.  The 2 little "feet" of the Omega are like this: One
>>foot is
>>up against that adjusting bolt you are turning.  The other side is
>>connected to a hydraulic piston.  When those 2 feet get closer
>>together, it
>>clamps the band down on the drum and makes it stop turning.  (This
>>changes
>>the gear ratio via the magic planetary gear system.)  So when you adjust
>>the bolt inward, it causes the band to squeeze the drum sooner as the
>>piston (on the other side) actuates.  The band is metal with a friction
>>lining like brake or clutch material.  I guess the idea of the adjustment
>>is that this material may wear and affect the engagement of the band.
>>
>>The whole complicated tighten/loosen/tighten routine...  I guess it is
>>supposed to get the band fully seated as a reference or starting point
>>for
>>the adjustment.
>>
>>Edward