Y motor mount torsion box for Prusa simplified Mendel

by Lanthan, published

Y motor mount torsion box for Prusa simplified Mendel by Lanthan Aug 25, 2011
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update 20120103: The machine was commissioned some time ago, and this has been working great! (see picture). I'll adapt this system in all future designs. It will work with belts up to 10 mm wide.

This is part of a small audit I am doing of the current Mendel structure for the utter lack of stiffness of some key parts. The general aim is to have a superior Mendel while still keeping plastic (here: only 27 + 16 grams) & vitamins needs at moderate levels . Any help, comments, contributions and evolutions of the design most welcome!

The aims of this torsion box mount are:

  • to put a mercy end to the y axis motor sorry flexion on its mount (and the consequent lack of parallelism that has the pulley and belt doing very unwanted things)
  • to provide support to the Y axis pulley on both ends, thus reducing the forces exerted on the stepper motor bearings
  • to improve the general ability to correctly tension belts and improve stiffness, precision and fight backlash.

The design uses one 28x12x8 bearing.. Clearances are OK with or without a printed idler / bearing cover. Lately I have switched to two bare 608 bearings side by side.

There is a 16 mm wide space inside the box for the idler and all the washers that will fit.

The starting point is the dxf from Adrian's mods.

Note: a similar "treatment" will be applied to the X axis in the near future.


This is easy and straightforward to print!

I am using a vitamin aluminium machined pulley from eBay (that determined the bearing's diameter), but it should be very easy to adapt an existing printed pulley design, and also to adopt a different type of bearing.

Check the clearances with your particular setup first.
I printed this with 4 solid layers, 3 extra shells and 60% infill in hexagonal pattern.

slip in place, add washers as needed, bolt on, also secure with nuts on the M8 threaded rods, re-tension the belt. Enjoy!

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What bearing do you use in the box itself? I measure 16x27x9mm, but I cannot find any bearings of that dimension.

Can you tell me what is needed to be changed when using 608 bearings?

Lanthan, you're making me want to build a Prusa.

It is always good to know that, for the sake of fast printing and kit sales and low price, the shaving off of plastic has gone too far in a few key elements.

Not all of it is bad. First it is like "Wow! how fast I all the parts printed!". Then, after some time getting into the workings
amp; details, the agonizing "Whish I had spent my time and plastic printing a Selis Mendel" (or the variant: "I should have cut an Ultimaker just for the fun"). Finally: "well, this can be improved!"

But the beauty of Open Source is that, given enough time and patience, you can tailor your machine to your exact needs. I want my mendel to print parts (with "sufficient precision") for homegrown mech projects, this is well within reach. 8-)

Lanthan, you're making me want to buy thick printed parts from you to build a Prusa ;)

Sorry I'm not into plastic sales nor commercial operation, that one's made for my own needs.