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Idler screw tension

Relevant for

CORE ONE family
MK4 family
XL family
MMU family
MINI family
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The Original Prusa FFF printers and upgrades use a screw(s) with a spring(s) to provide tension between the filament and the filament pulley. It is important to have this tension correct. If too loose the pulley will not be able to "grab" the filament, while if it is too tight the extruder motor can overexert itself and even start grinding the filament. With the exception of the MK2/S and MMU1, the correct tension remains the same across all of our printers. 

When printing flexible filaments (TPU/TPE) you want the screw even looser than what is depicted here. Please see Flexible materials for more information.

CORE One, MK4/S, MK3.9, XL

The Prusa CORE One, and the Original Prusa MK4/S, MK3.9/S, and XL have two idler screws, that are inserted into the Idler Nut. The Idler Nut keeps the Idler, the part that includes the two driven gears, in place. The tips of the screws must be aligned with the front surface of the Idler Nut

Prusa CORE One

Original Prusa MK4S, MK3.9SOriginal Prusa MK4, MK3.9

 

Original Prusa MK4 with MMU3-modified extruderOriginal Prusa XL

On the Original Prusa XL, the dwarf-cover-door prevents access to adjusting the idler tension screws. 

On single-head XL, it is possible to gain access by rotating the dwarf-cover-door. On dual-head and five-head XL, with the tool in the parked position, loosen the M3x12 bolt to remove the dwarf-cover-door. 

MK2.5S/MK3S/MK3S+/MK3.5/MK3.5S

The idler screw should be about flush to protruding 0.5 -1 mm with the plastic part when the filament is not loaded.

Original Prusa MK3SOriginal Prusa MK3S+

MMU3 unit

Both Idler tension screws should be adjusted so that the top of the screw head is flush or slightly above the top surface of the idler-body, as a default setting.

MMU2S unit

The MMU2S requires a slightly different extruder but also has 1 idler-screw on the left side of the extruder, like the picture above. Roughly the same tension should be applied but it should only be adjusted as a part of the IR filament sensor calibration.

In addition to the screw on the extruder, the MMU2S unit itself has two tension screws on top (green arrows left picture), applying pressure to the filament upon load/unload. These should be slightly below the surface of the plastic part (about 1 millimeter) when the filament is not loaded to the nozzle (right picture).

MINI/MINI+

The Original Prusa MINI/+ has its idler screw on the bottom of the extruder, located on the Z-axis extrusion. This too should be about flush with the plastic, when the filament is not loaded.

MK3/MK2.5

This extruder design has two idler screws. The head of the screw should be about flush to sticking out 1 - 1.5 mm when a filament is loaded.

MK2S

The Original Prusa MK2S had a different extruder design. It has two screws, where the length of the springs should be circa 13 mm when tightened. After some use, it is ok to tighten them a little bit more, but the two screws must be equal.

 

6 comments

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EG
I followed the cold pull steps where is asked to take out the screws for the idler door and now I can't screw in one of the screws (the one on the left). I can't find any information about how to solve this issue.
David

Hi! Just make sure the screw goesx against a nut in the idler door. Help it engage by pressing a nut towards the screw if neccessary :)

Darren
This page says the Mini idler screw should be flush when the filament is not loaded. The glossary (https://help.prusa3d.com/en/glossary/idler-screw_1592) says it should be flush when filament *is* loaded.
Giuliano - Official Prusa CS
Hello. They are both correct in a way. The screw could be flush or protrude of 1 mm when there is no filament. When filament is loaded, the screw will move inside, so if it was protruding, it will be flush, if it was already flash, it will be about 1 mm in.
poundCake
Thanks for addressing this inconsistency - perhaps the information should be added to the body of the article for those who may find this in the future?
 
>Additionally, there is no pratical way listed of "feeling for 1mm" - stop laughing - some of us don't know what that feels like.  Personally I advanced (aka tightened) the screw until I could not feel it when sweeping the area with my finger tip.  I then reveresed the screw making sure to note the position of the Allen key.  I did this procedure several times and settled on 1/2 turn to be a satisfactory 'backing off' of the screw.
>Furthermore, this information should be added or at least linked to the article on cold pulling to clear the nozzel as there is no mention of the steps to reset this component.
<https://help.prusa3d.com/article/cold-pull-mk3s-mk2-5s_2075> 
Elliot
On my prusa mini, I need to screw the idler in about a cm more than recommended to get the extruder gears to engage. The idler does not 'spring' back when pushed so I'm wondering if it is a spring problem. The idler nut has broken before for me (had to drill the screw out to replace the nut) and I think it might be related to this. Is my spring defective?
David

Hi! Is the extruder and idler somehow different to the original one? It might be necessary to see pictures to provide a valid answer. Prepare some and try live chat via shop.prusa3D.com for a diagnosis.

borisblajevici
Same problem appeared for me out of nowhere after the printer worked fine the whole time. Then suddenly inconsistent extrusion. Tightening it to the max got it working again so that filament is pushed properly into the hot end,  but I doubt this is the right way to do it. It seems that the spring lost it's force, so I needed to compensate that by over-tightening it. I feel like I'm missing something but don't see what.... All parts are original parts. Thanks for any help or suggestion
HJ_SK
I learned from my first 3d printer (anycubic) that tension can be adjusted as follows : set a point or small line with a pen from 2 cm above the ingoing point of the filament in the extruder. Then order your printer to flow exactly 2cm of filament. The difference (less or more transported) is also indication to change tension. I do not find such a function in my mk3s. I'm I missing something?
Because I find big problem to correct the first layer caliberation from the menu.  
David

Hi! Measuring the amount of the filament extruded this way is often not exactly what the printer needs to have accurate extrusion. Bondtech gears on MK3S+ should not change the extruded length according to the idler tension too much as seen on the other setups. Apart from the first layer, does the rest of the print come out as expected? 
You might want to try to print out a supplied test object with the supplied filament to confirm the printer works correctly. Check the [id=112364|title=First layer calibration article], or re-calibrate the SuperPINDA probe height if the first layer is still too high and can't be set lower anymore. If the issues persist, you can proceed to the live-chat via shop.prusa3D.com to debunk the thing.

Andres
The material printing guide for fleixble material indicates to "loosen the idler". How much should this be loosen? How do you know if it's too tight or too loose?
David

Hi! A good approach to printing flexibles is to loosen the idler completely, heat up the nozzle, go to move axis in the menu - move the E axis - so the E motor starts to spin, and then feeding the filament in while slowly tightening the idler and trying to feel with your hand for the filament being gripped by the extruder wheels. Once the filament starts being moved reliably, this might be the ideal tension. Least possible which works.

Richard Torhan
KUDOS.
Solved all my nightmares with flexible.
William H. - Official Prusa

Glad we could help. Be sure to search around our KB. Such considerations are covered in our various material guides, like https://help.prusa3d.com/en/article/flexible-materials_2057