Bearing for the threaded rod in the back ?
Hi there there are some mods out there to guide the rear rod with a bearing.
like this for eg.:
I would think, the front threaded rods would not need this, because of the smooth rods.
But the rod in the back might benefit from the bearing ?
If the rod itself would be bent ever so slightly, it might be better to have the trapezoid nut be able to sideways.
Otherwise the bed itself would "move" due to the wobbeling of the rod.
what are your opinions on that ?
RE: Bearing for the threaded rod in the back ?
No, an additional support is not needed and may in fact cause problems.
The lateral position of the heatbed (X, Y and rotation around the Z axis) is defined by the two smooth rods. They fully define it -- in fact they even over-constrain one degree of freedom.
The leadscrews should hence not try to define the lateral position even further. Ideally they would be fully floating laterally, and would only define the vertical position of the heatbed at three points. That's difficult to implement, so the compromise Prusa has opted for is to mount the Z leadscrews & motors at the bottom, but on flexible mounting plates which allow a bit of tilt.
So you can adjust the position of the three trapezoidal nuts at the bottom, to match the leadscrew positions. But you don't need to ensure that the leadscrews are perfectly parallel (among themselves and with the smooth rods) -- the tilting degree of freedom will take care of that and avoid jams of the Z drive. Don't take that degree of freedom away by adding another guide at the top of the leadscrew!
RE:
Just to give a second reply from someone with a bit of mechanical knowledge too: 100% what Jürgen said!
I've seen that "mod" too and I'm still thinking about approaching the author to take it offline or even reporting the model, if he won't respond.
In theory, you always want to tackle each degree of freedom only ONCE. To keep the print bed from rotating, you'd need something like the front left rod as it is and then something like a "tuning fork" on the front right.
The single rod on the left fixates the bed in X & Y, but it could rotate around on the rod. The "tuning fork" on the right would fixate the front right only in Y, keeping it from rotating, without putting any tension in X against the other rod.
The leadscrew in the rear is ONLY to determine the Z-position. If you see it moving around when moving the bed up/down: that are the inaccuries of the two front rods, which are already fighting against each other in X. If you fixate the leadscrew, that movement will go SOMEWHERE (the weakest parts) and you won't stop it.
Bending the bed, the front rods, the rear leadscrew, the rear clamp, putting strain on the front bearings, the bed getting stuck due to too much resistance etc.
In short, it's objectively bad for essential parts of the printer and not a debatable with opinions or anything.