HTD Timing belt Issue

[holewizard]

Hey all, I am working on a two stage gear reduction for my bike and was hoping to model and then machine a custom 106 tooth 5m pitch HTD pulley. The thing I am struggling with is that the two belts I have (and their associate pinions) both seem to claim to be 5mm pitch HTD belts but seem to have different pitches. I feel like I am missing something obvious but here is where I'm at:



In the "Description.jpg", the top belt seems to be a 5mm pitch 15mm wide 285mm long HTD belt.
http://beltpalace.com/2855m15.html?viewfullsite=1

The bottom belt which is the one I would like to cut the pinion for seems to be identical width but I am not sure what the "74" or 23 24 25 numbers are. From what I understand this belt should also have a 5mm pitch denoted by the 5M, but from the backlit photos you can see that each belt does not mesh with its alternative's pinions.




Anyone know what I am missing? Thanks in advance!

 

[Addy]

Are they both new belts? Perhaps one is used and has stretched a bit?

I use a single stage HTD5 belt reduction to drive the bike that I'm working on. I've got a 128 tooth pulley that I generated with openSCAD for 3D printing. I had a lot of trouble getting the teeth to mesh properly. The 3D printer had to be calibrated quite well so that the 200mm diameter pulley was the right size for the belt. After all the fine tuning, you might pick up my pulley and belt and notice that they don't quite mesh together fully. The belt has to be under some tension before it lines up properly in my case.

I ordered 800T belts from two different sources, one in the USA and one in china. The chinese belt was cheaper, and looks to be poorer quality. It's thinner and it seems to stretch more.

When I was trying to get all this to work, I looked around online to find the full belt specifications. From what I remember, HTD5 belts should all have the same pitch, so I'm not sure what those other numbers mean.

 

[holewizard]

I thought stretch might be the issue at first but if you look closely at the back lit photos you can see that there seems to be a difference in the peak of each tooth of the pinions themselves (where the smaller diameter pulley has smaller/tighter peaks).

For this reason I am thinking that the two belts really are spec'd differently and there's something in in the specs that I am missing which is driving me crazy. May come down to just some trial and error of machining a few different pitches until something lines up...

 

[holewizard]

Well after 3 machining trials I was able to cut something that fit. Turns out this belt had a (roughly) 4.92mm pitch not a 5mm pitch... If anyone is having trouble with a similar cheap timing belt feel free to get in touch to troubleshoot it. Here are some pictures with captions:

https://imgur.com/a/mPmFI