Belt tension adjustments not registering.  

New Member

I've been noticing some ghosting on my i3mk3s when printing large ASA pieces, so I tried adjusting the belt tension. I read that you want the belt tension around 275 ish. The X belt registers, and is at 260ish, but the y belt, even tightned to the point where it's makes a medium-high tone sound when plucking, it still stays at 292 296. And when I loosen it extremely, still not much change in the tension. I've ran belt test many times now, still dosen't register. 

How do I fix this? The "ghosting" (if it is ghosting at all, it only seems to happen on large ASA prints and has a pattern much larger than normal ghosting) is still there, but as I said, only appears on large ASA prints, which end up getting sanded out and filler primered anyways. But the y belt tension still bugs me. 

There is a new software update for the printer that just came out and I haven't installed, but I doubt that's the issue. 

(And the filament sensor is broken, I probably touched the part too much during assembly, but that dosen't bother me because I never used auto load and running out of filament midway hasn't really been a problem, and I doubt the filament sensor is a reason why anyways)


Posted : 22/10/2020 3:59 am
Noble Member

They need to stop calling that number "Belt tension". It's actually a combined measure of the motor's load/current draw to move the axis, so things like crunchy or very freely moving bearings will affect it significantly.  All other things being equal, then perhaps, but I think there is too much variability from one build to another to really make use of that. 

How are you  re-measuring? Worth noting that in older firmware the value did *not* update unless you re-ran the wizard. In newer releases it will update if the belt test is run (or the wizard). 

TL;DR - don't pay too much attention to the numbers for the sole purpose of "belt tension". Someone did the math previously (search belt tension here on the forums to get exact numbers) based on motor spindle loads.

When you push one side of the belt with your finger towards the other (like you are going to make the teeth from each side of the loop touch), you should feel some initial slack, and then at a certain amount of deflection you should feel a step as the resistance increases and all slack is taken up to the point of stretching the rubber. 

If memory serves, a "good" amount of tension is somewhere around 1/8" to 3/16" of deflection before you feel that increase. 

Other people will say they tune the belt to a low bass note.  YMMV.

Don't get caught up chasing the "perfect" tension. Even with tensions I've actually measured with a spring scale as near perfect, there is still some ringing in the print (or, what appears to be*). The best thing for that is just slow down your external perimeters or play with the jerk value; that will reduce the velocity changes at corners and reduce the ringing. 

*There may also be other print artifacts contributing to the ringing (relating to LA value) but that's an exercise for another day. 😉


It'd help if you posted a picture of the artifact so that those with a keen eye can make a call on whether it truly is ringing or something else. 

This post was modified 4 months ago 2 times by vintagepc
Posted : 22/10/2020 1:56 pm
Illustrious Member

2 to 6 lbf tension is a good range ... you can measure it using a deflection gauge (a scale that measures ounces) - For 6 lbf, pull up on the lower belt of each pair at 7 ounces, X should deflect 6 mm, Y should deflect about 5 mm.  Belt minimum is 2 lbf, and are good for as high as 25 lbf, but 11 lbf will overload the motor bearings. Thus, 6 lbf is a fair target.

It is always wise to get more than one opinion... as for trusting Prusa? No way man....
Posted : 23/10/2020 7:31 am
vintagepc liked

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