Belt and pulley not meshing

I recently printed a 66T pulley out of PLA just to see if it works. And it did. The belt and pulley engaged perfectly. However for my drive train I need a larger gearing ratio so I changed the groove count to 72T and printed my design in PETG. Now… I have a very bad engadement between the belt and the pulley… Could it be the 3D printer or something else? The groove design of the pulleys stayed the same only the PD, OD and groove count changed between the designs.

Some parameters of the pulleys: 66T (perfect belt engagement) Printed in PLA OD: 103.9mm

72T ( horrible engagement) Printed in PETG OD: 113.45mm

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@Fosterqc or @ShutterShock can yous guys assist homeslice?

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It seems as though you did not have a good model.

If it doesn’t work, idk what you can do besides reprint.

guessing you used a pulley generator, not sure if you could have messed that up or when slicing it got scaled.

So hard to know really.

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how did you do this? manually?

or you have some special algorithm in your 3d program that can recalculate the teeth number & diameter etc?

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Okay, attached is the design of the pulley. It was a manual design from pulley profile information I have researched over a few days to get the correct design. I did not use a generator so if I need to make some changes to the design, I will be able to do it. The thing is, nothing has changed on the 2 designs I’ve made accept the number of grooves (and the obviously the OD).

I’ve done some meteorology of my own in the meantime and the OD of the 2 different pulleys matches the CAD model. I have measured the OD of the 72T pulley to be Ø113.5mm. Which is quite accurate as the theoretical OD for a HTD-05 72T is Ø113.45mm more accurate than that is impossible… hahahaha!

The only other variable I can think of messing my pulley up is the printers. The 72T PETG pulley was printed with 0.4mm nozzle and the 66T was printed with 0.2mm nozzle. But that also does not really make sense…

When you change pulley diameters some of the other numbers have to change as well, did you do that properly? How did you come with the correct numbers?

I use this site and it worked fine

Thanks. Yes l’m quite aware of the variables that need to change with the increase in groove number. I’ve checked that over a few times. Anyway, that site is very nice, I’ll go through that as well now. Maybe something in my model is off… I don’t know.

Okay, so I constructed a new model based on the information from Capolight and got this… The blue one is the old 72T pulley and the red one is the new pulley… The OD’s are the same, only difference is the size of the grooves. The old pulley has slightly larger grooves to create space for the belt’s teeth and make up for the tollerances etc… basically the same as I did for the 66T pulley that worked… So the only thing I can think of is 2 different printers and 2 different materials PETG and PLA.

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I mean I personally believe that the size of the grooves can make a difference. Why don’t you take a small slice and try printing it to see if it meshes properly?

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You can also download a reference part to triple check,

But kinda sounds like a material / printing / shrinkage issue.


Take a damn 1/8" rattail file and do it to it sis. Damn computers made everyone forget their roots. Juss kiddin kitten, carry on.

Very nice site… thanks! Usefull info, going to print a slice of that pulley tomorrow and have a check on the grooviness… :laughing:

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You joke…? First thing I did was picking up one of those jewelry round files to try remedy this pulley.


I wasn’t joking but I have to say I am so nobody gets offended. I could have broke out of prison 10 times as much as I’ve filed.

There is shrink of matl. Then, there is the FACT, that the belt is measured while flat. BUT, when you curve the belt, the distance shrinks, according to the amount of bend, due to diameter of pulley.

So, you need to modify the tooth geometry. To account for the full flexure.