Z axis - screw opposite linear support?

I'm trying to figure out how to build my Z axis.

A common configuration seems to be a cantilevered bed supported by two linear rods with a drive screw in between:

R--S--R
|--------|
|--------|
---------

I'm wondering why this is used instead of having the screw support the opposite side of the platform, like this:

R-----R
|-------|
|-------|
---S---

What I'd actually thinking of doing is a single linear rail instead of two rods, with a leadscrew on the opposite side. Any drawbacks to this?

Edited 2 time(s). Last edit at 02/25/2016 09:02PM by rmlrn.

Ok after reading some more I think I understand - if the platform rotates perpendicular to the linear guide, then it will bind the screw.

So the screw "support" only works if the linear guide and platform are stiff enough not to need supporting.

Cantilevered beds can bounce/sag at the side opposite the guide rails/screw. I think adding a second screw to that opposite side would make the platform much more stable, provided the second screw is driven by the same motor as the first screw. If you use two motors, all bets are off - you're liable to create a new problem.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

If you're gonna use two screws and two rails, I suspect this is the ideal arrangement:

R------S
|---------|
|---------|
S------R

This allows for the maximum distance between elements, giving better leverage. Also, the bed's weight is evenly distributed.

Any disadvantages you can see?

Edited 1 time(s). Last edit at 03/11/2016 01:14PM by epicepee.

I wouldn't build it that way. I would put the guides closer to at least one of the screws to reduce the chances of it binding.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]