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When I pluck the belts of my CoreXY printer, I feel significantly different tension between the two idlers on the back and the tension on the sides, between the gear on the stepper shaft and the idler on the back.

Is this normal? If not, what could be the cause?

For reference, I'm using this support for the idlers:

enter image description here

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  • $\begingroup$ The belt tension can be adjusted at the carriage. $\endgroup$
    – 0scar ♦
    Commented Oct 6, 2021 at 5:57

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If you are referring to the tension in a single belt, but ar different positions, the tension is everywhere the same. It is one belt, the force/tension is the same in the belt. If the length of the belt is shorter because of a carriage to idler, the plucked sound may differ but the tension is the same.

If you are referring to different belts, e.g. the top versus the bottom belt, for CoreXY machines, the tension in the two separate belts should be equal to allow equal force pulling the carriage. If uneven, this may lead to incorrect/skew prints or binding of linear bearings (from experience). A typical layout of the mechanism is shown below. Note that there are several solutions for placing the belts; they can be in the same plane (where the belts cross in the back, as depicted below) or have the belt each in their own plane (as in the HyperCube Evolution design as depicted in the image of the question).

CoreXY reference mechanism

Drawing published by Ilan E. Moyer, taken from http://corexy.com.

For both options, incorrect belt tension causes for different forces onto the the printhead carriage and result in a torque on the carriage (either in X-Y or in X-Z). E.g. a tight blue belt with a less tight red belt cause a resulting counter-clockwise torque on the carriage in the image below:

enter image description here

I use a tool (gauge) to determine the tension to compare both belts and make sure the tension in both is the same:

enter image description here

Many CoreXY designs can adjust the tension of the belts at the carriage by screws. Some designs feature the stepper in the back and an adjustable pulley in the front.

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  • $\begingroup$ Have you calibrated it or are you just using it for comparison? $\endgroup$
    – FarO
    Commented Oct 5, 2021 at 18:02
  • $\begingroup$ @FarO For comparison, the tool has a scale on it, so you could use it for calibration as well if you measure the spring tension versus displacement. $\endgroup$
    – 0scar ♦
    Commented Oct 5, 2021 at 18:04
  • $\begingroup$ Actually in the HEVO design the motor mounts are identical left and right and the distance between belt and motor face is 2:1: 22 mm and 11 mm. If the motor shaft is designed to withstand 6.7 pounds at 17 mm distance to ensure lifespan for the bearings, the resulting belt tension should be in a ratio 2:1 as well between the belts... benchtopmachineshop.blogspot.com/2019/04/… $\endgroup$
    – FarO
    Commented Jan 18, 2022 at 22:24
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The belts of a CoreXY should be identical in length and tensioned simultaneously to ensure proper operation. Any slack or springiness difference will result in prints that are askew. In that case, where the tension or springiness cannot be removed, the Marlin firmware allows for compensations for skewing as M-Code 852

https://marlinfw.org/docs/gcode/M852.html

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  • $\begingroup$ Well better adjust the belt tension to compensate non-square frame rather than working via software. Usually small adjustments already correct the issue and do not impact the prints $\endgroup$
    – FarO
    Commented Oct 5, 2021 at 20:18
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The tension of the belts on CoreXY printers, is always the same in both belts. Since they motion system requires coordinated movement of both belts. You probably need to re tension them both, also check the length. It needs to be identical.

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  • $\begingroup$ Welcome to 3D Printing SE and thank you for your contribution. When you get a chance, please take the tour to understand how the site works and how it is different than others. $\endgroup$
    – agarza
    Commented Oct 6, 2021 at 14:17
  • $\begingroup$ Adjusting the tension of the belt is the way to correct minor skewing. It does not necessarily need to be identical, just more or less comparable $\endgroup$
    – FarO
    Commented Oct 7, 2021 at 12:36

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