Having trouble tensioning truck bearings due to camming action of bolts

Hi all,

Been losing my mind trying to figure out why I couldn’t get the bearings on the trucks to consistently maintain their tension until I found out the bolts were having this camming behavior when rotated. The behavior is anywhere between .015" to .030" of total movement doing a full 360 degree sweep. As far as I can tell, the bolts aren’t majorly bent, but just about every bolt I’ve checked has this behavior to some extent. I’ve tried 5/16 and M8 bolts from multiple different orders and vendors, with or without the bearing, full or partial thread bolts, with or without washers, and in the trucks and on simple through holes in a test piece of metal, and all the bolts consistently have this behavior. Changing nuts and bearings didn’t have an effect. I also drilled out the truck through holes to make sure the screws didn’t bind in the holes, and was very particular about deburring them. I’m hoping I didn’t just get exceedingly unlucky after testing about 20 different bolts and had every single one of them manufactured defective. If anyone else has had this problem, what did you do to work around it? I’d appreciate your thoughts and suggestions.



Very observant… that’s the kind of attention to detail that leads to good machining!

I noticed this happens even with shoulder bolts (or rather, partially threaded). I doubt even machined bolts would be perfect in this regard, but the rolled threads we all use definitely will be way off. My workaround is to rotate them at max distance (min tension); this will help stabilize the bearing and prevent any movement after tuning. After rotating all the bearings loose this way, there should be enough play in the truck clamping bolts to get the bearings snug later on.

When it came to the core clamps though, I could not fully cure this problem without a modification. No matter what I did there was always at least one core clamp with bearings not fully contacting the rail, even with that clamp bolt insanely tight. That’s when I discovered all of the holes on MPCNC designs are intentionally oversized; this works for most folks since most printers undersize holes a bit. OTOH if your printer doesn’t undersize holes, this makes the issue with bolt camming even worse. My solution for the core clamps was to redraw them with the horizontal bearing moved inward by 0.3mm. With those modified core clamps I now get good bearing tension on all of the core bearings (with a turn or 2 leftover to handle creep later on).

You may also find it hard to snug all of the bearings on the z gantry. By snugging all of them, I mean at least the outter 8 bearings (sometimes the inner bearings won’t touch, but that doesn’t matter as the structure is fully determined with just those outter 8). If so, I found just inserting a bit of credit card plastic between the z bearing wedge and the core is all you need to finish snugging those bearings up.

Also, the use of partial threaded bolts I think helps this. The sharp edges on all threaded bolts have a much easier time deforming the plastic walls under load, where as the smoother surface spreads the load in the hole better to minimizes deformation.

I don’t think that camming behavior should matter unless the bolts are rotating during use, which should not be the case. The tolerances on those bolts are never going to be precise, not just because of the bolts but because the printed part is not a precision part to that level.

Therefore the design must not require ultra precision and it has adjustment elsewhere to get the proper fit.

What provides the adjustment for tightness are the center bolts (where the bearing is supported on both sides) that can squeeze (deflect) the plastic part slightly closed until all bearings are in good contact but not too tight.

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