Prusa i3 X axis belt tensioner

by kennethjiang, published

Prusa i3 X axis belt tensioner by kennethjiang Jul 12, 2016
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One of the first upgrades you can do on top of a vanilla Prusa i3 is to add an X axis belt tensioner.

I designed this X axis belt tensioner for a Prusa i3 variant - ANet A8. But it may very well work for other Prusa i3 variants.

You need to print the parts with support on. Feel free to make suggestions on how it can be revised to eliminate the need for support.

The design is available at OnShape here so that anyone can customize it to their own needs.

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Prusa i3


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Hi kennethjiang,
thank you for this great upgrade for the Anet.
I want to share my experience with this thing.

First it improve my builds a lot due to the better belt tension but it has also some construction improvements. The first one i printed broke where the nut is as i tighten the screw. A second part with 100% infill fixed that problem.
Now to the other two points which I would improve. The "fork"-part has no guidance so it wobble a little bit.
Furthermore if the hotend carriage comes near the tensioner it produces stress in the guide-shafts. Better would be there to relocate the pulley behind the "white" guide.

Found there a tensioner similar to yours from freemark which one I'm using now.

Anet A8 Improved X-belt Tensioner

The one you found does seem to have a better design. Will give it a shot once my hotend replacement arrives. :)

I have owned my 3d printer for a year and learned many things over that time but not printing this till now was the biggest mistake i made. This is literally the best print i have done since i got my printer.

It's so nice of you to say that! I'm glad this design helped.

I was able to tap my rods in flush with the X right side acme guide with a small dead blow hammer. My guess is if you're having issues with the bearing rods sticking out past the guide is that they were not fully seated to begin with. I measured mine as I was putting it together and they were in fact cut to the proper length, They were very tight and I figured they would be fine, which they were till I decided to put this tensioner on my A8. Brace the x motor with one hand and give one a few love taps and see if it moves.

why are there only STL's?

no source files?

This part was designed in the cloud with OnShape. Please check "Thing Details" for how to clone the design and modify it.

yep... have come a bit further.

now i just need to make space for 2 flanged 624 bearings

This broke after 5 minutes.
I can recommend 1683070 instead.

Anyone having issues with the extruder setup hitting this because it is taking away from the 220mm of travel for the x-axis? When my bed autolevels, it tires to go close to the full 220mm width of the bed and bangs up against the pulley holder. I may have to go with another solution that moves the pulley to the outside more but wanted to avoid having to buy a longer belt to do that. Anyone else having this issue?

I love this mod except for this. I just can't keep it if I'm going to lose bed space.

Same boat. Losing about 15 mm of travel. Works great otherwise. Just have to wait see if it's really an issue

I don't understand the cylinders that stick out in the cover. They make it so it doesn't fit because the x axis guides are in the way

Edit: I figured it out. I could hammer the guides further in by using the rod from the filament holder and a hammer

The idea is that this way your belt does not bend the z-axis rods as you tighten the tension but instead it tightens on the x axis rods which is no problem as it pushes on them in their "strong" direction (on their ends).

on my printer the rods stand out a little bit. i edited the model and will print it tomorrow..

I printed it and it worked fine, just had to widen the holes a little bit but that might just be my printers fault. does anyone have the exact rod diameter? unable to measure right now.

Comments deleted.

Sweet, thanks!
On my case, the bottom rod is pushed deeply but the top rod can't be pushed further into the hole. Your model should work fine for me but I guess I'll just print the original design, cut the top cylinder flat and leave only the bottom cylinder

Prints fine without support if you have your bridging dialed in!
(Printed on vanilla Anet A8)

Can you share with us your settings that made support unnecessary?

I printed with PLA. 0.2 thickness. All in all I'm printing perimeters at around 35mm/s and bridges at 50mm/s with a bridge flow ratio of 0.8. I don't know if that is the "special sauce" for it to work. Could be that you'd have to dial your extruders temp to the point where you get least sagging of bridges

Thanks! Works perfectly :)

Comments deleted.

A belt tensioner is allways good and really missing on stock A8, but you create tension by bending your Z-axis linear rods and stress the linear bearings a lot..
A much better way is to create the tension by pushing only against the X linear rods.
Maybe you have a look at that mod:
In the last pic you see the end bracket that pushes with two screws against the X rod.
I'm using that bracket without the x holder from that mod, so it is easy to install like your mod.

Easy and imo perfect would be zo combine your tensioner with that bracket ;)

Here is a pic from my A8 wher you can see this two little screws:

X Motor mount for Anet A8
by Simhopp

I love that mod! Somehow I didn't discover it otherwise I probably wouldn't have come up with my own!

Did you say that mod somehow is not easy to install? My printer is offline right now otherwise I would print that out and try it on.

Oh, it's no prob to install, only some disassamble, assamble and leveling X and bed while only the tensioner is done within 5 minutes.
But ok, I had a more simple thing for the Z-endstop, so I needed only the tensioner, if you want a nice endstop mod too, this is great for sure, the right X-holder isn't needed, cause there is enough room for a longer tensioner like your one.

Another thing is that I don't wanna do much more on my A8 cause I'm building a new printer mostly with the parts of my A8. My controller was def. from beginning and and got some money back, so I swapped to RAMPS 1.4 and be free for any modification. ;)


Redesigned K8200/3Drag V3 because why not, Build volume 300x200x300 mm

I took your advice and revised my mod to make it push agains X rods as opposed to bending Z rods. The bent Z rods did cause me problems of loosing steps on Z axis, which is no longer a problem now. Thanks FZS_Fazer!

I took your advice and revised my mod to make it push agains X rods as opposed to bending Z rods. The bent Z rods did cause me problems of loosing steps on Z axis, which is no longer a problem now. Thanks FZS_Fazer!

You mentioned that on you X axis belt tensioner that you revised your mod. Did you post it somewhere? I'd like to take a look before I use this one (that I just printed lol)

Great man, so you have now one real problem less for a few grams of plastics and only 5 minutes worktime.
Imo these are the best mods! ;)
If you wanna really pimp that think, bolt it down to a wooden plate, stiffen the towers and and the front panel and screw a thin wooden plate or something on top of the printer from side to side.
Here are some really usefull mods:

  • belt brackets, so you can work with more tension and you allready have some selfmade
  • a simple U shaped stiff anodized aluminium profile (195mm) bolted with the two idler screws, so your frontplate will never more bent under the tensin of the Y belt.
  • for stiffening the towers, one thin L-shaped aluminium sheet bolted together with the little lower and upper panels behind your Z rods and with the inner panel to create L shaped and much stiffer towers
  • now your towers are stiff, but you can twist them, for that is the top plate, bolted on each side with at least two screws
  • now your main frame is stiff, but your Y-axis isn't, I tried braces first, but they made more trouble than pros, bolt the Y axis to a wooden plate is the only thing that'll work with guarantee, for that you can find in my makes some mounting blocks, now the only weak point of your frame is the rear panel itself, that'll bent under belt tension. Just have a look in my designs and you will find a printed motormount that you can also bolt down to your wooden base.

If you want, four bearings for the Y axis are working better than the stock three (easier and better bed leveling), but mixing bearings will not work. You can use the lower X bearing for Y and print a linear bearing from ABS for using on lower X position, this printable bearing can you also find in my designs, I have done that and it works great.

Imo the easiest but fine working mod for Z endstop is to glue something printed under the left X holder, that can hold a setting screw and place the switch below. But this is only the easy way with less time to install and without the need of leveling everything after that, the better looking thing is for sure the X holder from the mod with the tension bracket I told you above.

These are the really useful mods to make the machine much better, cause they are optimizing the precision and reduce the need of leveling cause the frame can hold measures.

But however, these are only my thought of what is most needed and usefull for this china cheapy.

I've seen your new bracket, much better than the first one, but the two little screws are useful to level the bracket exact vertically and only then the power is even on both rods and everything will be parallel.

Really appreciated all these tips. Agreed that lack of stiffness is a big problem of Prusa i3 design. And using Acrylic cut-outs as frame doesn't make it better.

Another mod that I did and found helpful was similar to one of your suggestions. I bolted a 1/8" aluminum plate at the bottom of the printer, and propped it up by ~1/2" so that the front plate will never touch and get bent by a uneven table surface. Pic here: https://flic.kr/p/MvzXz1

Oh, the front panel should touch and be fixed to have it perfect leveled. It much more effective to bolt the ends of the Y threaded rods down than only the rear third. With the printed blocks I've use the rods are getting much more guidance, so the one in the middle is more an option than a must have, espacially with a stiffer mainframe. A flying front panel is no good idea, the M8 rods aren't stiff enough to hold it right in place in the air.
The aluminium Y base plate is good and vers sinful.

I see your point. However I have to move my printer to different places (for meetups etc). If the front plate touches table surface, slight unevenness of the surface will ruin the whole bed-leveling.

Letting the front part of Y axis dangling as it is in my picture actually works quite well. I have been having a lot less bed-leveling problems than before. Of course if you have the luxury of keeping your printer only in one place, fixing the whole Y-axis length to a common base sounds like a better solution.

Ah you should rotate the cover when printing. The model is upright because it was easier to design it that way.

What is the purpose of printing the cover upright? Is it also ok to rotate the part and print it flat?

I was using Cura 15.04 and it generated support for the square hole in the middle.

I printed mine with Cura at 0.2mm and no supports perfectly! Which slicer are you using?