Prusa Y-axis belt tensioner

by chrille, published

Prusa Y-axis belt tensioner by chrille Jul 23, 2011
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Prusa Y-axis belt tensioner by chrille is licensed under the Public Domain license.

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I found it a bit challenging to get the Y-axis belt tension just right by using the belt clamps, so I made this Y-belt tensioner in SketchUp. It allows for more accurate tensioning of the belt using a M3 bolt and captive nut. The design is based on the belt tensioning system used in GregFrost's LM8UU X-Carriage (http://www.thingiverse.com/thing:9869).

The holes have the same spacing as the original belt clamps, so you should be able to simply replace the existing clamps without drilling new holes.

The tensioning bolt can be reached with an allen key even with the build plate mounted on top.

Sorry for the SketchUp design. I haven't taken the plunge into parametric modelling yet...


  1. Print
  2. Clean up if necessary
  3. Insert captive M3 nut
  4. Bolt the two parts to the plate, so the bottom part has the belt slot facing down towards the plate and the captive nut is facing towards the middle of the plate. The top piece should have the teeth facing down, grabbing the teeth of the belt.
  5. Screw in the M3 tensioning bolt (I used a 25mm one)
  6. Adjust the bolt till you get the tension just right

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Planning on mounting this BENEATH the MDF bed of a Prusa Mendel Iteration 2, since it has no top bed, and the PCB is so close above the MDF that there's only space for nuts.

Has anyone else mounted these on the underside? Any problems with this approach?

(except, thinking about that now, the belt would be going in the wrong place. never mind)

You can make this work underneath. Just flip the whole assembly over. Put the small clamp piece right on (well, under) the MDF, then put the large piece beneath with the opening facing down. This aligns the belt pretty closely with the idler pulley on my machine.

Danger Will Robinson! I discovered that the screw will abrade the belt over time. This eventually caused mine to wear down to the point where I think it would have snapped after a few more hours. I printed out a half-cylinder (the "Ram") from http://www.thingiverse.com/thing:8332http://www.thingiverse.com/thi... that I screwed the screw into and it makes all the difference in the world, because then the belt is smoothly wrapped around a curve rather than impaled on a screw.

Mendel Y-belt tensioner.

Quite nice! Will it will fit in place of the X carriage belt clamp as well?

This fits my X carriage without problem.

I'm using GregFrost's LM8UU X-carriage on my prusa, and it has a system similar to this built into it, but I think these are too large to fit the original carriage. But feel free to download the sketchup file and modify it if needed :)

Thanks! This knocked my backlash right out.