Adjusting belt tension (XL)

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On the Original Prusa XL, adjusting the belt to the optimal tension is crucial to achieving good quality on the prints. A loose belt can cause Layer shifting, Ghosting, or other print abnormalities, like getting an irregular shape instead of a perfect circle. An over-tightened belt can cause an irregular movement in the X and Y axes. 

The Original Prusa XL geometry is CoreXY: it has one belt for both X and Y axes. 

Necessary tools

Prepare the belt tensioner, and a 2.5mm hexagonal Allen key.

Belt tensioner

The belt tensioner has three areas: the min area on one side, the max area on the other side, and a central area between these two. 

Checking the belt tension

Attach the belt tensioner on the top belt on the X-axis. The belt tensioner has a pointer, check its position: the ideal position of the pointer is in the center area, closer to the max mark. 

Adjusting the belt

Pre-tensioning

Pre-tension the belt by tightening the screws on both Y-axis rails. Start by tightening the screw on the right Y-axis rail, then tighten the left Y-axis rail screw by roughly the same amount. At this stage, we aim for the belt to not feel completely loose if pulled. 

After pre-tensioning the belt, move the X-axis assembly back and all the way to the front, and move the X-carriage with the extruder all the way to the left. 

Tensioning

Attach the belt tensioner on the top belt on the X-axis, and see the pointer position. 

Tighten the belt by tightening the screws on both Y-axis rails. Start with the screw on the right Y-axis rail, tighten it slightly, then tighten the left Y-axis rail screw by the same amount.

Repeat the procedure, with small, multiple adjustments, until the pointer on the belt tensioner is in the center area, close to the max line. 

If the belt tensioner pointer is on the max area, loosen the belt by loosening the screw on the right Y-axis rail, and then loosening the screw on the left Y-axis rail by the same amount. 

After each adjustment, both in tightening or loosening the belt, move the X-axis assembly back and all the way to the front. Then check the pointer on the belt tensioner, and repeat the tensioning procedure until the belt tensioner pointer is at the optimal position.

Alignment

It is crucial to have perfect alignment between the right and left sides. 

Move the X-axis all the way to the front of the printer, and move the X-carriage with the extruder all the way to the left. 

Test the alignment by squeezing the CoreXY Front Left and the X-axis on either side: there should be no play between the parts. If there is play, tighten the screw on the same side slightly, move the X-axis assembly back and all the way to the front, and test the alignment again on both sides. 

Repeat the same procedure until both sides are perfectly aligned and without any play. Once at this stage, check the belt tension, both on the upper and on the lower belt, by attaching the belt meter to either belt: the pointer has to be in the same position for both. 

In case there is a difference on the test, slightly tighten the screw on the side:

• tighten the right side to tension the upper belt
• tighten the left side to tension the lower belt