RE:
Do we know the print settings that they use for the PC-CF tensioners? Should they be reprinted in 100% infill to minimize the chance of that square nut spinning?
The Printables page says "0.20mm STRUCTUAL, Filament: PETG, PC and PCCF (tested on Prusament), 15% Grid infill" for all parts. It also provides a single .bgcode file to print all PCCF parts, presumably all sliced with the same settings.
Which may be a reason why the belt tensioner pulley mounts have proven to be such a weak spot. Printing them at 100% infill instead, to prevent the nut from grinding through them, makes a lot of sense to me.
https://www.printables.com/model/1167816-core-one-printable-parts
RE: VFA Artifacts on X+Y Straight Edges
Looking at the remains of the one I had to destroy to get the seized nut and bolt out, it looks like 3 perimeters and about 20/25% grid infill.
But there's an interesting difference between the Prusa-supplied part and the downloadable model. The downloadable model has a couple of tiny voids, either side of the nut recess, that force the slicer to put more perimeters in that location, effectively making it solid infill if you use 3 perimeters or more.
Unfortunately, I don't think this would have helped in my case. My tensioner had been working perfectly up until the point yesterday where the nut seized on the bolt - I believe it was 'galling' under the belt tension. Galling creates a very strong bond, so in my opinion no amount of plastic is going to stop the nut turning in such a case.
I had to remove the tensioner mount (remove the front left upright to access the screw holding the door switch in place, which otherwise blocks access to one of the three bolts). Once I had the mount free I could work on the pulley mount. In the end I had to Dremel the head off the tension screw, because even with the pulley mount removed (destructively) I was still left with the nut 'welded' to the screw, and therefore trapped in the tensioner mount. This morning I plan look into a modification to the tensioner mount that would allow the seized nut/bolt to be removed without cutting the head off.
Luckily I had a new mount already printed in Prusa PC-CF, but it was a little oversize - the nut was very difficult to insert and the part that slides in the tensioner mount was a very tight fit. It was enough to get me up and running again temporarily though. Looking at the Prusa PC-CF setting in PS it has a default extrusion multiplier of 1.04 for some reason. Reducing that to 1.0 resulted in a reprint that was a much better fit. I've also printed a couple of spares. And I've lubricated the tensioner bolt before assembly, in an attempt to prevent a repeat of the galling.
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It’s funny you guys mention the OEM parts…
Yesterday I printed up a PC blend part for the extruder, primarily because I didn’t have PCCF on hand, and I had a similar thought to the stuff you’ve posted:
It’s super odd to me that a part that will have a lot of mechanical use wouldn’t have 100% infill.
so obviously since I didn’t have the other material, I printed the part out of 100% PC blend with 100% infill. (the part I was printing is the piece that goes behind the gold plate in the nextruder. Which apparently is a known problem: https://www.printables.com/model/1293080-nextruder-main-plate-load-cell-protection-no-suppo )
I wonder if it’s being cheap or if it’s simply concerns about parts warping ( or whatever) when they’re making parts without 100% fill. (Especially small parts where it probably matters…)
RE: VFA Artifacts on X+Y Straight Edges
Personally I think the factory tensioner pulley mount is perfectly strong enough when the nut and bolt are working properly. After all, the PC-CF is strong and rigid, and there should be very little force required to turn the screw. But when the nut seizes on the bolt it's always going to turn in a plastic part - nothing you can do to stop it.
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Yeah, unfortunately, I’m not a mechanical engineer… So it’s hard to know how to express the forces being applied to various parts. However, one might assume that a solid part would be stronger than an 10-20% infilled part; though I’m sure there are times when that won’t be the case.
RE: VFA Artifacts on X+Y Straight Edges
But there's an interesting difference between the Prusa-supplied part and the downloadable model. The downloadable model has a couple of tiny voids, either side of the nut recess, that force the slicer to put more perimeters in that location, effectively making it solid infill if you use 3 perimeters or more.
Great catch Chris. Interesting too there'd been a revision already sometime before April 11.
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Nor me - but I've encountered galling before. My camper van has a nice stainless exhaust with a stainless lambda sensor boss that is never going to come out. Effectively welded in place by galling.
Coming back on topic (even if we are straying from VFAs a bit - sorry), when that tensioner nut is seized on the bolt it needs a metal spanner on the nut, not a plastic slot, and even then you'd be as likely to shear the bolt or turn the allen key in the cap head.
RE: VFA Artifacts on X+Y Straight Edges
But there's an interesting difference between the Prusa-supplied part and the downloadable model. The downloadable model has a couple of tiny voids, either side of the nut recess, that force the slicer to put more perimeters in that location, effectively making it solid infill if you use 3 perimeters or more.
Great catch Chris. Interesting too there'd been a revision already sometime before April 11.
Ah - you downloaded them a few days before me! Yes, it's the first release of the part - R1.
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I wonder if something like this would still work properly for the tensioner mount. Should do I think, because the head of the tensioner screw is fully located by the recess when in use, but when the head is lifted out of that recess slot should allow a fused nut+screw to be removed - maybe even with the idler mount still attached.
Only one way to find out......
EDIT: Wait, no, the screw won't move in that direction with the idler mount still attached. One moment...
RE: VFA Artifacts on X+Y Straight Edges
After trying out the new instructions for the belt tensioners yesterday, I also entered the belt tensioner pulley club of the slipping nut.
The bolt has seized up and the nut is slipping.
After reaching out to support and being promised the parts would be shipped next week I had no choice to reprint the belt tensioner pulley in PETG with 100% infill (Sovol SV06).
I also used another thicker M3 nut, which has a much better fit and easier screw guidance than the one supplied by Prusa.
I also used a drop of oil 😉 .
I hope that the spare part from Prusa will be delivered soon, as I am sure that PETG will not last in the long run.
RE: VFA Artifacts on X+Y Straight Edges
This would allow the seized assembly (nut, bolt, idler mount) to be removed as one, but I'm not keen on it because I think the belt tension might tend to lift the screw into the new slot.
I'll print both and see what they're like in practice.
RE: VFA Artifacts on X+Y Straight Edges
Unfortunately, I don't think this would have helped in my case. My tensioner had been working perfectly up until the point yesterday where the nut seized on the bolt - I believe it was 'galling' under the belt tension. Galling creates a very strong bond, so in my opinion no amount of plastic is going to stop the nut turning in such a case.
Chris care to explain the cause of 'galling' to the ignorant amongst us?
I've never heard the term before but after your description I'm pretty certain this is what cause mine to fail.
RE: VFA Artifacts on X+Y Straight Edges
It's just occurred to me that some dust from my earlier failed printed idler could have got into the threads and jammed it up, and maybe it's dust that's exacerbated the problem for other people. But I have what's left of the fused nut and bolt here, and I simply cannot turn the nut with any amount of force, so it seems like more than a bit of dust.
I can't give a better explanation of galling better than wikipedia:
"Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together.[1] Galling is caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of crystal structure beneath the surface.[2] This will generally leave some material stuck or even friction welded to the adjacent surface, whereas the galled material will appear worn, chipped, or even gouged and may have balled-up or torn lumps of material stuck to its surface."
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Chris care to explain the cause of 'galling' to the ignorant amongst us?
I've never heard the term before but after your description I'm pretty certain this is what cause mine to fail.
Stainless steel fasteners as both nuts and bolt almost always will gall before you can tighten them to any appreciable amount. This is because stainless doesn't have a thick durable oxidised/passivated layer that acts as an interface. Stainless steel on stainless steel threads should be lubricated before assembly in almost all applications as the stainless steel with a bit of friction starts cold welding to itself without some interface layer. Some people will use a brass nut on a stainless bolt as well since the dissimilar alloys and self lubricating nature of brass will keep it from bonding together.
RE: VFA Artifacts on X+Y Straight Edges
Stainless steel fasteners as both nuts and bolt almost always will gall before you can tighten them to any appreciable amount. This is because stainless doesn't have a thick durable oxidised/passivated layer that acts as an interface. Stainless steel on stainless steel threads should be lubricated before assembly in almost all applications as the stainless steel with a bit of friction starts cold welding to itself without some interface layer. Some people will use a brass nut on a stainless bolt as well since the dissimilar alloys and self lubricating nature of brass will keep it from bonding together.
So maybe an idler mount with a brass heat-set thermal insert instead of the square nut could be worth exploring. It would probably be best inserted from the 'far' side, and heaven help you if it starts spinning! But in theory it might be a lot less likely to spin if it prevents galling.
RE: VFA Artifacts on X+Y Straight Edges
This seems to work quite well - the nut, the length of the bolt hole through the pulley mount, and the captive bolt head all seem to prevent any pull in the belt direction from twisting the end of the bolt out of the slot. And if they were to become fused then they can be lifted out of the main mount as one.
I may print a PC-CF version and fit it as a precautionary measure.
But first I'm going to try a brass insert for the idler mount.
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My tensioners are doing ok for now, perhaps because I've only done a single release/re-tension cycle on them since converting my printer. However, I can feel a significant difference in smoothness when turning the left and right belt tension screws. The LH tensioner screw feels scratchy/rough, while the RH turns smooth. Because of this I plan to dab some anti-seize on the screw next time I have the belts loose in hopes of preventing a problem from developing.
Additionally, because I don't have any PC on hand, last night after posting the infill % question, I printed two new spare tensioners in ASA with 0.15mm layers, 8 perimeters, and 100% infill as a fallback option to temporarily get the printer going in the case of a failure (I only have the one printer).
This led me to wonder why is ABS/ASA are not in the recommended material list for this part? Is the layer adhesion so poor with ASA that the riser tabs can fail? Or does it have to do with slow deformation of the parts under tension? Seems ASA should be as good as PETG in this latter point, but with higher temp tolerance...but I'm probably wrong in that assumption. BTW, as my very first ASA print, they printed beautifully and the nuts pressed in tight but not with difficulty.
-J
RE: VFA Artifacts on X+Y Straight Edges
My tensioners are doing ok for now, perhaps because I've only done a single release/re-tension cycle on them since converting my printer. However, I can feel a significant difference in smoothness when turning the left and right belt tension screws. The LH tensioner screw feels scratchy/rough, while the RH turns smooth. Because of this I plan to dab some anti-seize on the screw next time I have the belts loose, in hopes of preventing a problem from developing.
Yay! Something else to look forward to...
I wonder if Prusa will put out an update kit for all of their revised parts?
RE: VFA Artifacts on X+Y Straight Edges
Purple Loctite during assembly between the stainless nut and the stainless thread
RE: VFA Artifacts on X+Y Straight Edges
This led me to wonder why is ABS/ASA are not in the recommended material list for this part? Is the layer adhesion so poor with ASA that the riser tabs can fail? Or does it have to do with slow deformation of the parts under tension? Seems ASA should be as good as PETG in this latter point, but with higher temp tolerance...but I'm probably wrong in that assumption. BTW, as my very first ASA print, they printed beautifully and the nuts pressed in tight but not with difficulty.
-J
PC-CF is the only material suggested for these parts. I think Prusa's wording of the print settings merely indicates that the same settings can apply to the PETG, PC and PC-CF parts, but these pulley mounts are included in the PC-CF 3mf.
PC-CF is stiffer so should maintain the belt tension without 'relaxing'. Your settings should get the best stiffness out ASA however, and your parts may even be able to take higher stress than the originals, but you may want to keep an eye on the belt tension over time.