MMU2s Bearings not aligning to filaments  

New Member

Has anyone else had there newly assembled MMU2S unit do something similar to this?  I have yet to see this thing successfully drive the filament to the nozzle except for the one time where I calibrated the correct distance for it/reset.  The bearings on the larger drum (idler maybe? its been a bit) do not align with the selected filament extruder gear.  The unit moves to select the correct number but the bearing is not rotated correctly, in better terms, the drum appears to have either rotated slightly too far or not enough and thusly the bearings do not press onto the filament so its not driven correctly.  Additionally, what is a good way of adjusting the tension properly?  I usually go by the teeth marks in the filament but that isn't possible because of the previous problem.  Lastly, has anyone found any resource that demonstrates the proper buffer setup/installation?  I followed the online manual chapter for chapter but never found anything about the filament buffer.  

Posted : 28/05/2019 11:02 pm
Eminent Member


pls. check the two small screws on the outer, smaller,  end of the idler drum. If it is not screwed in correctly, it prevents the Idler rotating enough.
They are shown in chapter 4 - Idler body assembly, under step 5 of the assembly manual.

This post was modified 1 year ago by roy.r3
Posted : 29/05/2019 1:43 pm
Veteran Member Moderator

Be Careful with the two Drum Fixing screws!
If over tightened, they are capable of 'jacking' the plastic bush open and causing a failure of the drum model. 

regards Joan

I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK...
Posted : 29/05/2019 1:48 pm
Active Member

My idler drum screws are tight but the drum doesn't align with the selected filament except No. 1.  Any other suggestions?

During boot up there's a lot of noise as the drum beats itself up in the upper rotated position, but it doesn't seem to want to rotate to engage any filaments past No. 1.

This post was modified 3 months ago by steve.l7
Posted : 29/07/2020 6:16 am
New Member


did you ever solve this? I had similar issues with my newly built mmu2s. In my scenario when choosing "load filament" load all then when loading filament #1 it capture the filament, pull it as it should and when pulling back to be able to go to  filament load #2 then it do not manage to unload it far enough and when it moves to filament #2 it get stuck since filament is still left (1-2 mm).

I manage to make that work by releasing the screws on the idler and move idler 1-2 mm to the left and after that all was ok, except for that I get similar error on filament load #4. Wonder if its due to a print of the idler that's not perfectly aligned for the bearings in the kit from Prusa


Posted : 01/10/2020 9:40 pm
Karl Herbert
Famed Member

The MMU2 causes some people headaches. On the net there are some videos with good tips, like this one:

I would take a look at it and have some haribo bears ready.


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 : 01/10/2020 10:12 pm
Active Member


Yes it is working better now, I had to tighten the spring-loaded screws down a couple turns instead of leaving them flush with the top of the idler, and need to watch that the filament isn't balled up at the tip thus preventing retraction through the 2mm bowden tube at each location. 

There are several after-market idler components available (search Thingiverse) one has a magnet to help hold the ball bearing down, this works quite well.

Posted : 01/10/2020 10:33 pm

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