How to tension the Ultimaker short belts once for all?
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| Upside version of the tensioner |
The usual fix is to unscrew the 4 bolts, push the motor downwards with a thumb and tighten the screws very hard while keeping the pressure...
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| Downside version of the tensioner |
There are four reasons why the "official" or "recommended" procedure fails in my opinion:
- Unlike the main belts, the short belts seem always to get sloppier with time! So I regularly had to check and re-tighten the steppers.
- (hence) the screws need to be tightened up to the point they start to dig in the plywood to stay in place, i.e. you need to damage the plywood! May be my plywood is not hard enough for the constraints, as you may see in the first picture that it started to bend on the side...
- (hence) once the washers or nut heads went deep "enough" in the plywood to stay locked for a while, moving them further becomes dirtier even, precisely because they just want to slide back to their initial holes. So you need to tighten them even more and damage further your case...
- Finally, in my opinion, no one can achieve a precise belt tension by pushing a stepper with one hand, while screwing with the other one!
Fine, robust and stable belt tension adjuster (at last) !
So I finally addressed the issue by designing my own tensioner, and stop using a procedure that damages my printer while not really fixing the issue in the first place.
Printed with 0.15mm layer height, 100% infill. With or without PTFE thermal insulators.
Finally, nylock nuts are cool but they tend to rotate freely in PLA if their slot was not exactly adjusted to their side. This is tricky because it depends much on temperature and printer settings, especially with thin walls.
The holes here are conic. It is easier to slide them in place than with a lateral/partial slot, and the more you screw, the more they are kept in place from the 6 sides. No need to finely tune your printer (which is hard if your short belts have the backslash we're just trying to fix...)
Finally, the stepper sometimes become hot enough to meld PLA. I do not fear much that the heat could propagate via the mounting screw and through the plywood layers, but I designed a special version that uses two 3mm pieces of bowden tubes as thermal insulators on the two stepper screws (see above). I used bigger washers for these to dissipate heat further.
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| I made two versions of the belt tensioner, where the bolt is facing upwards or downwards |
Pros and cons of a spring in the design
As a word of caution: do never over-tighten a belt or you will ruin the bearings!The setup I described so far lets you go much farther than what is required. After some discussion about it in the forum, I made a derivative which adds a spring between the screw head and its washer.
Keeping the stepper loose enough to slide along the wall may not be a good idea because the stepper may start resonate when direction changes (and you'll get wavy vertical textures around the corners).
Spring-loaded tensioner with a loose stepper.
Anyhow, the spring better takes care of the tension and probably dampens excessive settings. Time will tell if the revision really is worthwhile.
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| Spring-loaded version of the same tensioner |