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I have an engineering background, I'm an EE, but I know almost nothing about mechanics and that's why I'm writing here.

This is the challenge: how to control the rotation of multiple discs along the same axis? Assume you have a sliced cylinder. The ideal scenario would be to control every slice independently, but to make things easier I think it helps to consider that you can move them in a fixed number of combinations and on the same direction (e.g. a mechanism that enables to rotate the top and bottom cylinders at the same time and same direction - combination #1 - and the inner ones in the same way - combination #2 - but not as the same time as #1).

Any idea on how to accomplish this?

I though I could use something like a "key" along the axis/shaft/rod. By moving it up/down it would lock different discs/slices. Not sure if this is a good idea. One issue I can see with this one, is that the slices that weren't locked by the "key" could be rotating freely and I want them to be steady.

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    $\begingroup$ What are your constraints ? eg are you wanting to transfer power as in a gearbox or it it just about positioning as in a cylinder lock ? $\endgroup$ – Chris Johns Mar 30 '16 at 22:20
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    $\begingroup$ What is the ultimate goal here? It makes little sense to have the non-driven disks "be steady" since the only thing you're driving is the axle. Please explain what the system is supposed to do - there may be a simpler and more reliable design. $\endgroup$ – Carl Witthoft Mar 31 '16 at 12:28
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I think you are looking for an electromagnetic clutch. There are obviously other types of clutches that would work, but electric ones are nicer when there are lots of units to control.

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As far as keeping the other devices from freewheeling you could add some acceptable amount of friction, or you could add an electromagnetic brake.

enter image description here

And like you mentioned, depending on your loads and system complexity it may be easier to directly drive each one.

There are probably other more creative ways to address your design, but you will have to share more of the details.

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A diagram would help.

A sliced cylinder suggests parallel disks on a common shaft.
"Top and bottom" suggests a vertical axle - is it?
Inner ones may also mean vertical.
How many slices?
Are they necessarily driven by the shaft?
When top and bottom turn together is it always at the same speed as each other? How many combinations are there?
If disks are ABCD and you drive AD or BC is the roration speed the same in ll cases.
Does it always go the same way ? What separation between disks?
How thick are the disks?


A basic solution:

Imagine discs are on a vertical axle. A system that would work for some combinations from above is to have a shaft which can move vertically relative to the discs. The axle has dogs/splines/cams/protrusions/thingy's which can engage with a disk when they are in the plane of the disc but a disc can be disengaged by moving the axle vertically until the disc does not engage with a drive thingy. These are at simplest "dog clutches" or splined discs and shaft but many variants are possible. Rather than the shaft moving selectors could move engagement "thingy"s or the discs could if desired by slid on the shaft.

eg dddd dddd = disc
xx = bare shaft
tttt = thingy that couples disc to shaft

Thingys tttt are fastened to the shaft and move up and down with it.
Discs dddd dddd stay at same horizontal level.
Disks are driven by shaft rotation when tttt is in line with disc.

Assembly

       xx 
  dddd xx dddd  A
      tttt
      tttt
  dddd xx dddd  B
       xx
      tttt
  dddd xx dddd  C
       xx 
       xx
  dddd xx dddd  D
      tttt
       xx
       xx

As shown rotating the shaft drives no discs.
Moving the shaft up one layer drives discs A & D
Moving the shaft down one layer drives discs B & C

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