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Adjusting belt tension

(MINI/MINI+)

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MINI
MINI+
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Loose belts would cause a printer malfunction and prevent proper printing. The easiest way to check is printing a round object - if any of the belts are not tightened properly the result is an irregular shape instead of a perfect circle. Y-axis belt is located under the heatbed, X-axis belt moves the print head.

Checking the belt tension

Belt tuner

We have our own belt tuner app. The application determines the belt tension by measuring the frequency of the vibration generated by strumming it. 

Follow the instructions in the linked belt tuner app to adjust the belt tension. 

Belt tension test

If you move each axis to the far right or left, use a finger push the belt down in the center of the axis. Some force should be needed for bending the belt. Careful when pushing so you do not overstretch the belt, as you might damage the printer. Belts should be tight enough to sound like a low bass note when plucked. 

A great way to check whether your belt is at the correct tension is to prints this Tension Meter for the GT2 belts.

Adjusting the X-axis belt

Slight adjustments

Get out your 2.5 mm Allen key:

  1. Loosen the two screws as depicted below by turning them counterclockwise (left picture).
  2. Tighten the two screws at the very end of the X-axis by turning them clockwise (right picture). These push directly onto the ends of the smooth metal rods, moving the plastic part with the belt idler, tightening the belt.
Adjusting only one of the two end screws will create a skew in the XZ-axis. However, it can also correct a skew if already present.
  1. Once the belt is tight, turn the two side screws clockwise to fix the plastic part holding the belt idler, in place.

Large adjustments

To access the X-axis belt holder you must remove the heat sink of the print head. This requires some disassembly.

  1. Loosen the belt by following the method described above.
  2. Follow the guide How to replace a heatsink (MINI/MINI+) to Step 10 to remove the heat sink.
  3. Remove the belt from the middle slot (green arrows), move it in 2 - 3 'teeth' (blue arrow), and push it back into the slot, using a 1.5 mm Allen key.
  4. Follow the guide How to replace a heatsink (MINI/MINI+) backward from Step 10 to reassemble the print head.
  5. Adjust the tension of the belt following the method above.

Adjusting the Y-belt

Slight adjustments

In the middle of the front plate of the heat bed assembly (purple arrows), you will find two screws that will adjust the tension of the Y-axis belt. Use the 2.5 mm Allen key and by turning them clockwise you will tighten the Y-axis belt.

Large adjustments

If the Y-axis belt is very loose you must move it in the Y-axis belt holder found beneath the bed carriage. 

  1. Loosen the 2 screws in the picture above by turning them counterclockwise. Do not unscrew them completely.
  2. Turn the whole printer over to its side to access the bottom of the printer (left picture).
  1. Slide one end of the belt out from the belt holder (right picture), move the belt in 2-3 'teeth'(blue arrow), and push it back into the slot using the 1.5 mm Allen key (orange arrows).
  2. Turn the printer back right side up and fine adjust the belt tension with the two front screws (as described in the previous method).
 

5 comments

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Felix91
•
Maybe to add here: make sure, the pulleys are tightly connected to the motor axis. Screw for Y-pulley is intuitive to reach. Screw for X-pulley not that intuitive but still easy. At the top side of the part where the X-axis motor is mounted there is a small hole (unfortunately ). Just insert the hex key and manually move the print head until the hex key slides into the screw.
This solved layer shifting at my printer whereas I originally thought the cause is wrong belt tension.
Reply
Tom_3D
•
For checking the belt tightness recommend using belt tuner.
Reply
rohde•labs
•
Would be nice if you embedded an audio file and/or video of the sound the belts should make instead of "sound like a low base note". Having a video and a nice audio sample would help a lot to see exactly how much the belt moves.
Reply
Jakub Dolezal
•
Hi, thanks for the suggestion. We will test it out both video and audio, however, I'm afraid, each device play sounds a little differently.
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Matt
•
Some more guidance on this would be welcome.  I can't print the guage because I don't use PETG.  Very frustrating to see layer shifting and not know which belt is responsible, if at all.  A more in-depth entry here or video would be helpful.
Reply
William H. - Official Prusa
•
You can determine which belt the shift happens by looking at the print. It would shift to one axis or the other or perhaps both. Support can help you assess if you send them some pictures of the prints, but you should check out the article on layer shifting. I would recommend to get some PETG. It is the easiest and most accessible filament for structural parts.
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PapalPotato
•
Are there any set values as to what the tension should be? 100 newtons?  130?  It would be nice to use a belt tensioner to dial it in.
Reply
Official Prusa CS
•
Hi, we do not recommend any specific values.
Reply
Mcdriver
•
I had to replace the Y-Axis belt on my mini. I installed ends of the belt with belt with belt teeth fully in the connector. could not get it on. I loosened the adjusting screws with just a few threads and still couldnt get it on. So, I finally left two notches on each end in the connector, open. now even with about 4mm gap between the adjusting block the belt is stiff tight. How far can I loosen the ends in the connector? Is it ok to have a large gap at the adjuster block and frame??
Reply
William H. - Official Prusa
•

The more teeth you leave out the bigger the chance is that the belt might slip during its lifetime. It is secured pretty well though. It sounds like you did not loosen the 2 screws on the front plate, holding the idler, enough before proceeding. 

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