This is probably an old link but here is a video of an extruder

3D Printing
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This is probably an old link but here is a video of an extruder that uses dual steppers for increased torque. Rather than a drive wheel and a bearing against it, there are two drive wheels.

It’s an interesting design
http://www.youtube.com/watch?v=gwn8_shrPVg&feature=youtu.be

Source: http://www.rockcliffmachine.com/forum/showthread.php?586-3D-Printing-Mach3-Emc-on-a-CNC-Router
http://www.youtube.com/watch?v=gwn8_shrPVg

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And a little more detail: http://www.rockcliffmachine.com/forum/attachment.php?attachmentid=664&d=1335241556

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Interesting, On the heavy side but interesting.

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I doubt weight was an issue for this guy.; He designed this as an attachment for a CNC router he built. This way, he can do subtractive machining and then additive machining on the same part.

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Wonder what kind of gains you would see using this for a bowden…

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I’m not sure the benefit to this is so much increased torque (because you can achieve that much cheaper just by down-gearing) but rather the increased grip this probably has by grabbing both sides of the filament rather than just a single side.

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Yeah that’s the gain I was seeing too.

Which makes me think you could do this with some down gearing and a pair of gears to power both hobbed bolts from a single motor.

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I did some further reading and the creator of that mechanism offered this explanation at the time:

There are a couple of reasons for the DualDrive having 2 steppers. The first is driving force - pure and simple. We are driving the filament from both sides which means you can have less compression force on the filament. This leads to less deformation of the filament(not getting squished to an oval shape) and allows for a stronger “push” which translates to less jamming. Actually, jamming is almost non-existent. This DualDrive setup is strong enough that it can actually buckle the filament before it skips steps. The second reason was to simplify the parts count as you mentioned. These are small Nema 14 motors so they fit nicely into a fairly small package.

The two motors are adjustable by the fact that there are only 2 mounting screws - one acts as a pivot point, and the other goes through a slot. You simply get one motor lined up, put in some filament and swing your other motor into position against the filament - tight but not too tight. Once it is set you can forget about it as the rollers will now be in a good position to grab the filament next time you load it.

That was an old article so I’m sure someone has done more with that design or perhaps discarded it for something more efficient.

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and here I am trying to design a duel headed single stepper extruder.
that does look elegant though. did he say anywhere what it weighed in at?

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I’ve looked at dual sided drives using one motor, and they get relatively complex very quickly. If you were able to get enough torque from smaller motors still, then it could be interesting, but this general design seems quite bulky.

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I’ve often wondered why more people aren’t using dual driven rollers, given the issues that crop up with idlers and whatnot. My printer from SeeMeCNC is the only design I’ve seen using that system.