GT2 Pulley < Skatebearing size mismatch?

Hi all, this might be a little out there so bear with me. Maybe this is just a problem with my mini or maybe it really is a widespread design flaw which i sort of suspect (not that i am knocking the original design team necessarily).

So here is the issue that has bugged me for quite some time and i have not seen anyone talk about it. It is the size mismatch on the mini between the very small 16 tooth GT2 pulley for the belt and the very large (in comparison) standard skate bearing on the other side therefore creating a triangle. I have run into the issue of trying to tighten my belt and sometimes have gotten it too tight so it wont go all the way across and too loose when it gets close to the motor. So basically too problems at once. I have had it okay for awhile but my belts are loose now and not making true circles anymore and thus i am nearing having to battle this issue again soon.

My main question is: Why are not the pulley and bearings the same size for even tension? On the surface this seems to just be a case of crappy engineering. But i also see that 20 tooth gears are common on other reprap style 3d printers and i have to wonder if the prototype the mini came out of originally used 20 tooth pulleys which probably would have been closer to the standard skate bearing size and probably would have had less of this issue.

To possible fixes come to mind.

  1. increase the size of lulzbot mini pulleys to 20teeth and increase print speed but loose resolution AND update the marlin firmware to match. (of which i could probably do myself but it sortof seems like a problem with ALL lulzbot minis maybe).
  2. Decrease the size of the bearing at the other end. But would require some sort of adapter or redisgn of the shaft in the far right x-axis piece. Perhaps for something like this

A separate issue is the belt tighteners themselves. I find it extremely difficult to get to the screws with a standard L shaped Allen wrench. Is it possible to come up with a better easier to access belt tightening system

I don’t know. Am i just crazy? Do these issues only affect me? Since no one is talking about these i sort of feel like i am alone here floating out on the ocean.

anyone have any thoughts?

Hi Biolumo,

no, I don’t think you are crazy, and I believe this is one detail of the Lulzbot Mini design that the designers haven’t made perfect yet. I agree that the belt that moves the platform should run parallel to the y-axis, but to accomplish this, the idler doesn’t need to have the same diameter as the pulley. It would be sufficient to offset the idler in x-direction to ensure that the belt is aligned with the y-axis. The amount of offset needed is the difference of idler radius and pulley radius.

Hey thanks!

Glad to know it really is a design issue that can be improved. Let’s find a way to keep making the mini even better!

I just got an email from It works that the new lulzbot mini 2 is coming out! I was already Hoping for a new Rambo version that had those new stepper drivers! But it is interesting that it is now z-belt driven as well. I wonder if this issue has been fixed in version 2. But I doubt it has. But does not mean we can’t fix it now!

Interesting idea about bearing offset. I could totally work on modifying the stock lulzbot mini x-axis part for that! I’ve already made mods before and in fact some of the parts on my machine right now are mods I made that are better than the stock ones. I keep updating my mini too!

I have had these same issues on my mini. I recently had the build plate seize in the middle of a print (preventing th y axis from moving, and causing the belt to jump teeth on the pulley). I have never been a fan of the IGUS plastic bushings used in the slides. When I removed the 8mm shafts, I realized they are not hardened, straight, or smooth. There are wear marks from the plastic bushings, and when you roll the shafts on a flat table you can see places wear the shafts lift off the table. Even after polishing up the shafts, pressing out the bushings and cleaning them, and reassembling, I am still having issues with the y axis belt jumping teeth on the pulley. I like having the belt tensioned well to reduce slop/play in the y-axis movement, but with such a small drive pulley there aren’t that many teeth engaged with the belt. I have also noticed the frame area around the Y axis motor flexing, causing the top end of the drive pulley to be angled toward the print bed. I’m going to stiffen up the frame, as well as replace the shafts with straight, hardened shafts and the plastic bushes with recirculating linear ball bearings.