Prusa i3 mk2S mk2.5S Y Axis Belt Tensioner no disassembly required

Made this Y Belt tensioner for 5mm or 3mm dual bearing idler pullies , after looking at a few different designs and thought they were lacking stability the way some of them tensioned with one screw gave it potential for alignment issues or belt wobble or required you to disassemble the frame to install, so had a play around with a few ideas and printed a few versions before this final model after printing a few test versions. I have now been using this for about 3 years now it really improves print quality and i think its far better then some of the other designs, I have since reprinted mine in Polycarbonate PC for heat tolerance as i now print in a heated chamber with more extensive modifications, please leave me a comment if you give it a try as i spent quite a lot of time designing these models an all i ask is for a comment or show your build .

You will need : 2x M4 bolts 30mm-40mm and hex nuts

1x dual bearing idler with a 5mm center hole or 1x dual bearing idler with a 3mm center hole

07.2026 V4 Updates : Added V4.0 3mm Idler carriage Added V4.0 3mm Idler pin Added V4.0 5mm Idler carriage Added V4.0 Main base block - little more clearance on lower threaded bar for easier fitting. Added V4.1 Main Base block - further improvement making it easier to install

Overview : This design clips in so you don't need to disassemble the frame, and yea its tight so you need to put it in pretty close to the right alignment because it does not move very easily, basically lift up your frame at the front and hook it on the bottom rod , swing it up and it locks in place , then attach the arm , you can preinstall the idler and you will need to insert pin B then push in the nut as you cant put in the nut before the pin is in the nut wont fit through the hole and serves to lock in that pin which you want to sand so that it moves in the hole but is still tight .

Make sure your print is clean and that the idler can spin with out any resistance , there is a 5mm pin for the idler which is super tight you might need to give it a light sand but you want a tight fit , if its tight its pretty unlikely to come out it has no forces on it in that direction .

Printing is relatively easy i printed in PETG with 30% infill and supports for the base and arm, the pins i printed standing up and didn't need any supports . ( used for 3 years no problems )

How it works : So if you don't understand what is going on here the adjustment bolt goes through two pins the pins actually twist a little as the angle changes rather then being sloppy holes it keeps everything tight and the bottom of the arm has a very solid pivot point , also when the arm moves in closer to the full tension it is supported either side to stop any twisting .

It has been working great for me i hope someone else finds it useful as well, leave me a comment if you have any questions or suggestions for improvements. cheers.

Updates :

02.03.24 I needed to shorten the top M4 bolt on the tensioner to M4 25mm to avoid hitting the carriage belt block still gives plenty of tension adjustment

27.07.24 V3 (beta) : Reversed the tensioner screw and added a thumbwheel solves the long M4 screw protruding and hitting the carriage belt holder , also cleaned up the arm to make it easier to move the idler pully by hand to make sure its not rubbing, modified the base for the thumb wheel and adjusted the pins for the new screw arrangement , pin C for the idler pully remains the same use version 2. May need to use a longer M4 screw for the tensioner

15.07.25 V3 installed : Have printed the revised unit and had it sitting around a long time as the previous version was working fine with a shorter bolt , but now have installed the new version and its working great the reverse screw and thumb wheel adjustment is great , really improves prints when you have good alignment and tight belts, the reversal of the tension screw also now avoids the longer thread sometimes hitting under the hotbed belt mount so i am really happy with the result . ( have added another photo of it installed ) also great you don't have to unscrew anything or do realignments when changing over to this design or changing belt parts, my little mk2.5s runs pretty much everyday printing something so I am always making mods to improve quality and speed of printing .

26.07.2026 V4 Update Installed : I have recently revisited this model to print again in polycarbonate PC carbon fibre as i now print in a high temperature enclosure so while i still have had no issues as I get over 60 degrees PETG may start getting a bit soft so I have upgraded my version to use Polycarbonate carbon fibre and changed my pins to brass which i just fabricated in a vice, but for most cases the PETG pins worked great I had no issues with them for 3 years and this printer prints 24/7 so certainly no issues with using a printed pin .

Happy printing …