filament pulling motor trigered by filament tension?

Hi,
i have, as a school projet, to make a working 3D printer out of a stratasys printer with dead hotend and electronic + open source 3d printer hardware/software

the stratasys printer is a dimension sst 1200es, it was using 2 filament (material and support) with 1 motor on the printing head turning one way or another to pull one of the 2 filament + 2 motors right after the spools
we would like to keep that dual filament feature
the original motors aren't stepper motors, and we aren't in an electronic class, so that might be hard to controll, thats why we will use 2 stepper motors on the head without reduction (not enough space)
but then, the motors might not have enough torque to turn the spool (+ they are laying flat, so there is more friction)

so, i though about using the 2 motors that are right after the spools, but not control them electronically, but mechanically

here are drawings to make it a bit more clear



spool is at bottom left, head is it the top
the trigger is the green thing (would be a springloaded 3D printed piece that close the circuit of the motor when pushed)
the black squares would be rings to guide the filament
here the filament is loose, so the head motors won't need much torque

but when the filament get pulled and stat to have some tension, it will push on the trigger like here



and so the motors close the the spools will give more filament




my question here is : would this work with the filament inside a PTFE tube?
or does it need to be out of the tube to bend enough?

keeping it inside the tube would be a lot easier for filament change, but it might be too rigid

thanks

Edited 2 time(s). Last edit at 12/22/2015 05:22AM by Nagby.

It would only work if one end of the PTFE tube is not fixed.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

yeah

i guess the best would be to fix it with the ring at the top, and let it slide through the bottom ring
and put the top ring as far as possible so a minimal force is requiered to bend the tube


well all of that is pretty hard to predict with calculations, so i will do that and hope it works ^^

See:

[forums.reprap.org]

I'm using 1.2A stepper with direct drive extruder and MK8 drive gears ( 7-8mm diameter ). It produces enough torque to even lift a new spool and still feed the filament. ( ask me, how I know )

I'd try it without the spool motors first.
If you still want these motors, I'd try optical switches instead of mechanical. Then no force is necessary.
I would also add a relais triggered by the switch. You never know, how much starting current the motors ask ( non stepper drivers aren't current limited )
-Olaf

Edited 2 time(s). Last edit at 12/25/2015 02:22AM by o_lampe.