Idler Pulley Mods  

Eminent Member


Have been using my MK2s > MK2.5S for some time now, and one thing always bugged me though I can't say it affected the operation much...

The Idler Pulleys for both the X and the Y Axes have tiny little 3mm shafts and bearings that "rock" a bit. As the shafts are merely threaded 3mm bolts and not sized shafts or even shoulder bolts, this adds to the effect.  

Another dislike is that locking nuts are needed because the shafts / bolts cannot be tightened much as they will easily distort the plastic idler holders.

Also, the pulleys are smooth, smooshing the belt teeth as they pass over the pulleys.  

I also find it odd that the idler pulley diameter is a fair bit larger than the drive pulley but on the other hand, this slows down the tiny idler pulley bearing quite a bit so I'm totally cool with that.

So here is my experimental upgrade:

I installed similarly sized 20T Toothed idlers with larger bearings with a 5mm bore.  The bearings run on 5mm stainless steel tubing which is cut to fit the depth of the plastic holder. A 3mm bolt is still used, and clamps against the tubing with a slight preload against the plastic.  This results in no perceptible slop or rocking.    As the original parts would have been compromised with the much larger holes, I printed them with 4.9mm holes using modifiers in PRUSASlicr and filed them larger for a tight fit.  As the genuine Gates idlers are a bit wider than the standard pulleys, I also widened the opening in the X idler bracket by 1.4mm, again using modifiers in PRUSASlicr.

A fair bit of hand fitting was needed but I like the results.   Tomorrow I'm going to put them in and try printing with it!


¡no entiendo Español!
Nein! Nicht Versteh!
Я немного говорю по-русски но не очень хорошо, и...
I'm not very good at English either! Maybe someday I'll find a language I'm good at?...
Posted : 29/07/2020 4:33 am
Karl Herbert
Famed Member

Good work !

I am curious about your feedback regarding durability in long-term tests. I also used GT20 wheels, but replaced the bearings with high quality SKF bearings. The difference to the original cheap bearings and the SKF bearings is enormous. Since 2 years I did not have to repair or replace anything.

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 29/07/2020 11:06 am
keri7240 liked
Eminent Member


It works and I somehow managed to not make things worse!

The toothed idlers are now installed along with the modified idler holders. While the X-axis idler bolt head is fairly close to the Right Z-axis leadscrew, it does clear by over 0.5mm.  Perhaps a low profile pan head bolt would be a better choice.

With the belt tensions set  1 tooth too tight the first 3D Benchy printed out very well, as good or better than its best predecessors and a whole lot better than the not-so-good ones.  Printing seemed quieter than usual even with the belts tight. There is absolutely no perceptible rocking of the idlers during printing.

After the first print to settle the belts, I set the belt tensions to about 70hz when plucked using Pro Tuner phone app.  As my tendency is to over-tighten belts, another metric for measuring tension is that the X and Y axes will move of their own weight if the printer is tilted over.

The belts are Gates Powergrip 2GT , the drive pulleys are 16T Gates Powergrip and of course the idlers are 20T toothed Gates Powergrip on 5mm custom mounts.

More Happy Printing!

¡no entiendo Español!
Nein! Nicht Versteh!
Я немного говорю по-русски но не очень хорошо, и...
I'm not very good at English either! Maybe someday I'll find a language I'm good at?...
Posted : 30/07/2020 4:01 am
Reputable Member


For belt tension for Y axes I modified the parts and made this - it is in principle similar to the solution of the Mini:


Best regards, Clemens

Posted : 30/07/2020 11:28 am

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