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Belt tightness?

Posted by David J 
Belt tightness?
July 05, 2020 04:08PM
I have a CoreXY printer that's a few years old (this design: [www.thingiverse.com]). It has worked very well, but lately I'd noticed that the carriage was getting stiff to move around, especially in the Y axis, and print quality was deteriorating. So I decided to investigate. I found that the rods that run on each side for the Y axis were no longer parallel - I believe that this was due to the belt tension pulling the front-right bearing support inwards. It was only a tiny amount, I could only see a sliver of light through the gap, but it was enough to make it impossible to set the X axis rods accurately to allow a smooth movement in the Y axis. I have repositioned everything and now both X and Y move very easily... but now I have to address various design aspects, mostly to do with stopping the front pulley supports being pulled out of line by the belts. I might have to redesign a couple of parts, but that isn't too significant a problem.

What I really need to get to grips with is the correct tension for the belts - I believe that a major part of my problem was over-tightened belts. Not knowing how tight they should be I had set them up so that there was a significant note when I strummed the belts but, on reflection, I think they shouldn't have been so tight.

So, my big question is, how tight should the belts be on a CoreXY? Obviously they should be tight enough that they don't jump the teeth on the motor pulleys, but I have no idea what 'correct tightness' is.

Can anyone advise?
Re: Belt tightness?
July 05, 2020 05:42PM

Unfortunately, Blogger did something weird recently and trashed a bunch of the images that were in this and many other posts. Apparently google is aware of it and working on a fix. There are comments on belt tensioning near the end of the post.

TLDR: tight, but not too tight. If they are pulling the printer out of alignment, they are too tight! Doh! They have to be at least tight enough that they can't slip on the drive pulleys, plus a little more, but not much more.

I can't really see how the pulleys and guide rails are held in place in that printer, but it looks like thin, printed plastic, which is asking for trouble. People make the mistake of designing printed plastic parts as if they are made of steel. You need bulk to keep plastic parts from flexing. Pulleys' axles should be supported at the top and bottom by bulky, inflexible mounts. Motor mounts should likewise be bulky and inflexible.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Belt tightness?
July 06, 2020 03:03AM
This design was a 3d-printed version of an all-metal design by someone called zelogik, and it does work (mostly). Though it is worth noting that the designer of my printer released a mostly metal version a year or so later!

Most parts are extremely robust, but the flaw in the design of the part in question is that it isn't fully located in one direction, although it is in all the others. Now that I've identified the problem I intend to redesign that part - not a major problem. However, I do intend to look at making it all-metal where it matters - I have already started on this - but it will take time and a lot of thinking...

BTW: That's a very useful link - thanks. I'll be studying that with interest...

Edited 1 time(s). Last edit at 07/06/2020 03:28AM by David J.
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