Prusa Mendel X Idler and Motor Design Change

After building the Prusa Mendel I do find it easier than the Sells. I have one major issue that has caused me some pain. The orientation of the smooth rod groove mount walls on the x idler and motor ends.

They are currently designed to have the smooth rod on the outside of the 3 walls that the bushings are placed in. This is done with glue or epoxy. The problem is that when trying to tighten or just over time printing, the x belt becomes loose due to the glue coming separated.

At first I didn't notice the bushing pulling away from the mounts and kept tightening the belt. Rinse, repeat.

Finally one of the bushings came completely off and dropped to the bottom the z axis. After rebuilding everything and taking great care to ensure the bushings were firmly glued and after several prints, it started to happen again. This time I used a couple of zip ties to secure the bushings.

My proposed solution, rotate the 3 walled channel 180 degrees and place the bushings inside. This would work with the natural forces of the x belt being tight and prevent the previously mentioned issues. This would also have the added benefit of making the x axis easier to build. As of right now, you must:

1. Slide the x ends on the x smooth rods.
2. Insert the smooth z rods with bushings installed.
3. Add glue to bushings.
4. Slide x ends back toward the smooth z rods and hold until glue sets.

I find that last step a little cumbersome. My proposed upgrade would be implemented as follows:

1. Install z smooth rod with bushings.
2. Apply glue to the bushings.
3. Slide the x ends on to the x smooth rods into the bushings.

On another redesign note I don't see why the Z smooth rods must be on the outside of the Z threaded rods. It would be fairly simple to have the smooth rods nearest the print bed and the Z drive rods on the outboard of the printer. This also would make assembly easier.

I would love to be able to design this idea in OpenSCAD or Sketch Up if I could. But my design skills are lacking. If someone could mock this idea up I think it might make it into the next variant of the Prusa Mendel.

I just finished my glue joints yesterday and haven't tightened belts yet, so I haven't seem the detachment problem directly. But this makes sense to me: the glue joint is held in tension by the belt, and glue should always be in compression or shear.

(That said, there is something like 3 square cm of glue-able surface: that's a lot! Surely this can be made to work fine as long as you're careful to make sure the glue is coating all surfaces and filling the space.)

But if we're going to do a redesign, what's limiting us to just rotating the glue channel? Why not go farther and use a bolt-clamped design for the bushing attachment? The PLA bushings are known to be ablative and presumably need replacements, so glue just seems like the wrong choice to me. And I can attest from experience that doing the glue work is terrifying for a first-time builder who doesn't have extra bushings or the capability to produce them.

If you turn it around, it would also make sense to put the smooth rod on the inside of the threaded rod. That way you can do away with the threaded rod across the bottom of the machine and 2 bar clamps and 2 rod clamps all together.

Yes, that would be a good way to cut printing time. You may need to widen the bay area a bit to allow the x carriage room to fully print. But that is just a case of making the threaded rods longer.

Also, the top vertexes and Z motor mounts could have half circles each to clamp onto the top of the Z smooth rod. That would remove 2 bar clamps from the print too.

Using the z mount to clamp the rod was what I was thinking. This will reduce rigidity of the machine (as would widening it) so you would have to assess whether that was going to be a problem. Also if you have the rods bar clamped at the bottom and z-motor clamped at the top, you have lost the means to adjust them so they are parallel.

If someone could come up with a design for an adjustable bar clamp, that would be an awsome way to solve the problem and would also remove the need for having a sprung bed for the y carriage (you could just use the bar clamps on the Y rods to adjust the level of the bed).

Sounds like a design challenge for someone.

Edited 1 time(s). Last edit at 04/15/2011 01:04AM by Greg Frost.

What about some shaft collars on the X axis smooth rods to prevent the belt from pulling off of the bushings? Here are some: [www.mcmaster.com]