Sick and broken fingers, belt inevitably slipping out and some logic just forced me to make a real belt pulley. The stock pulleys are not very mechanical/physical compliant…
No more hurting fingers when tensioning your belts, no more evading belts crashing your prints after 2.5 days printing!
Looks good. Just two thoughts:
° wouldn’t be M6 sufficient? M8 sounds really overdimensioned
° why not also extend it to hold the endstop?
M6 would be largely sufficient 
But, I had some M8 shaft leftovers
I wouldn’t support the endstops, as the belt tensionners aren’t fixed in Z position, the endstops have to be
Right - didn’t think about that one…
Interesting that the belts slip off on you, I’ve never had an issue myself, looking at the belts in all range of travel of the axis the belts only move about 5mm off center of the bearing idlers.
I don’t think that the bearings themselves are a bad choice, IMO it’s both cheaper then a proper idler and also it gives the belt space to align itself, a proper idler if mounted even sligtly off the belt line will cause wear on the edges of the belt, the bearings are a popular choice to avoid that.
I have to agree though that proper tightening with this method is quite a hassle, especially if you want all the belts to be similarly tight, I’ve found that a good method to do it is use a piece of GT2 belt (or any belt for that matter) put it around the bearings alongside the belt that you are tightening and pull the bearings up with this other belt. Much easier to achieve good and even tension but requires the top plate to be off.
Well, I had a belt slipping twice since a year. No catastrophic I do agree, but it happened both time after about 20 hours printing with very fast movements. Using bearings is not the point, of course it’s very good, but not the way it is by stock, if you imagine the physical forces applied and where they apply. The fact that tightening is now on a simple M8 bolt is just delighfull With these pulleys, the tension point is exactly in the middle of the bearings, so the belt doesn’t move more than its axial default divided by two
This looks pretty cool. I will try these out.
I recently built a second 8800 and had one of the belts fail. Part of the steel reinforcement cable frayed and separated from inside one of the belts. This caused a few erratic print problems and then damaged the endstop PCB before I noticed the problem. The belt failed after less than about 50 hours of operation. It is possible/likely that the belt was running over the corner of the pulley bearings to cause this.
I do wonder, though, if you may be setting your own belt tension too high? I find it is neither difficult, nor painful to set the pulley tension by sliding it upward with one hand while tightening the allen wrench with the other. You do not need to stretch the belts during this process - you want to just remove any slack.
You also should auto-calibrate your printer after any adjustment - even setting the belt tension - so there could be merit in the suggestion to incorporate the endstop mounting bracket into this part. (I usually recalibrate my printer every few days whether it needs it or not.)
Well, I know that the belts shouldn’t be tighten like violin strings, but they need some tension. The sweet spot between tension and tension deforming the teeth. A belt sings when correctly tighten, this is how we learn at least (precision mech here) vs a chainsaw chain that should be loose enough to allow a pen to go between the chain and the chain driver. Don’t take me wrong, I’m not tensioning the belts like crazy either, but this way makes it much easier for me.
Re calibrating, of course, you’re right, every single changes should lead to recal. But again, a mechanical/electrical endstop should never be mounted on a moving part and always be fixed on the machine main body. This to ensure their position cannot be changed by any changes made to hardware. Now if you guys want to blend file, gladly!