Tension pulley broken - Assembly and first prints troubleshooting

RE: Tension pulley broken

hyiger — Wed, 21 Jan 2026 14:54:53 +0000

There are 2 issues, galling causing binding and the tensioner bracket coming apart (in some cases). The first can be mitigated by using a brass heat set insert and the later through reprinting the part but increasing the number of walls and infill.

RE: Tension pulley broken

Augendoc — Wed, 21 Jan 2026 03:43:54 +0000

Is this an issue of the stainless steel fasteners galling and self-welding? Or is it failure of the tensioner? They are two different issues with two different solutions. Doubling the nuts is not going to do much, though I understand the desire to theoretically spread the load over more threads. Just use different material. An alloy steel M3 cap screw has much less likelihood of galling than a stainless steel one, even if the nut remains stainless steel. You can get electroplated steel square nuts, though, should you wish to swap everything out. Or use a stainless steel screw with brass heat insert and in all cases, lube lube lube.

RE: Tension pulley broken

Conrad — Wed, 21 Jan 2026 02:12:00 +0000

I will note that the square nut is 1.72 mm thick. The rule of thumb is that a nut has to be about the diameter of the screw to achieve maximum strength. With the slight chamfer we're probably getting half the strength, plus increased pressure that makes galling more likely. IMO, the moly grease is sounding better to me.

RE: Tension pulley broken

miroslav.h4 — Tue, 20 Jan 2026 20:10:28 +0000

That "certain level of skill and feeling" is evident in those who complain about the cracked tensioner pulley holders!

RE: Tension pulley broken

Conrad — Tue, 20 Jan 2026 19:16:20 +0000

A certain skill level and feel are required.

RE: Tension pulley broken

miroslav.h4 — Tue, 20 Jan 2026 18:21:50 +0000

This is really a great idea! Just turn the two nuts a little against each other and you've got them tightened like a nut and locknut, and no amount of lubrication will help!

RE: Tension pulley broken

Conrad — Tue, 20 Jan 2026 17:50:52 +0000

Yes, I figure spart PETG parts would keep the printer running for quite a while, until the proper parts could be printed or shipped. A little OT, but I had a thought on the thin nut seizing problem. If you were to modify the part to double the length of the nut slot, it would be possible to select two nuts that lined up on a screw and install the pair. Shouldn't be too difficult because you have two faces to work with and a bag of nuts is cheap enough. Then, use a plastic-safe moly grease like Tamiya and there should be near zero chance of seizing or stripping. Of course, the nuts still have to be aligned with the screw.

RE: Tension pulley broken

Paffendorf — Tue, 20 Jan 2026 13:47:21 +0000

Posted by: @hyiger

Posted by: @paffendorf

Hello,

I have an update regarding my issue. I spoke with Prusa's technical support and they are going to send me the part again. I asked them if they could send it to me in PCCF and they told me that the part is now manufactured in PCCF.

However, looking at the part in detail, it does not have 100% infill, which may be one of the issues.
See for yourselves.
As far as I'm aware, this part has always been printed in Prusament PC-CF. I reprinted mine in PC Blend which is stronger than PC-CF. A common misconception is that carbon fiber adds strength in the Z direction, in fact it does the opposite. Layer adhesion is weaker with carbon fiber.

However, due to the position of the part and how the toothed belt enters the pulley, most of the effort is exerted on the Y-axis in both positive and negative forces, with the Z-axis (vertical) contributing little or nothing.

RE: Tension pulley broken

David Holland — Tue, 20 Jan 2026 13:29:03 +0000

Posted by: @conrad-2
IDK, but the part is pretty chunky, and I can't imagine it creeping enough to matter. FWIW, the "pull" on the part will be twice the belt tension, which works out to about 4.3 kg. That's a bit more than a gallon of water, if somebody wanted to load one for a test.

I wasn't thinking quickly.... More like "how much does it creep over the 3? 4? 5? 10? year lifespan of the printer?" (Sorry, that's not a load test I'm willing to wait for. The water will likely evaporate in that time. 😀 )

I think (based on what you've said) sounds like PETG would probably be a pretty reasonable temporary fix till you could get a new PCCF one from Prusa/PrintedSolid/Partsbuilt/Friend/other supplier of choice.

Speaking of, I should probably make sure I've got my spares "somewhere findable".

Cheers..

David

RE: Tension pulley broken

Paffendorf — Tue, 20 Jan 2026 10:48:16 +0000

Posted by: @conrad-2
IDK, but the part is pretty chunky, and I can't imagine it creeping enough to matter. FWIW, the "pull" on the part will be twice the belt tension, which works out to about 4.3 kg. That's a bit more than a gallon of water, if somebody wanted to load one for a test.

I think putting 4.3 kg on the pulley is a basic test, since you have to take into account the stress it is subjected to during sudden changes in direction.

You just have to note that the printer's greatest movement is in short, very fast changes of direction.