# REV rt25 pulley help

I need some help designing custom pulleys. I used the REV belt generator and I think I misunderstood something with it. I had the 16 tooth pulleys on our gripper, and followed the steps in the generator. the belts it gives me look to fit great in cad, but are mostly too tight in real life. so I need to make custom pulleys for this. and the mkcad app has custom pulley maker, but not being able to trust the belts in cad, I need to know how to go about figuring out the tooth count I need without just 3d printing a whole butt ton of pulleys and hoping to find the right one. is there some math I dont know about that I can do, or is there a better feature script for the rt25 belts?

I figured out I can do custom pulley sizes with a specific belt, but no matter what I do, it tells me the belt doesnt fit the distance in the model. and with that im not sure how to make sure im good either

I have the length between the axisâ€™s, the belt length I want to use, just need to know how to calculate the pulley diameter and tooth count.

If i recall correctly, RT25 is designed to be on pitch with the ION build system and is interchangeable with #25 roller chain.

This is correct. It uses a 0.25" pitch just like the rest of the ION system (well, multiples of that at least). If using identical sized pulleys on each end, you can use any multiple of 1/4" for center-to-center. However, if the pulleys are different sizes or you use more than 2, thatâ€™s all out the window and youll need to calculate things.

what I mean is, I know how to use the mkcad app to make the custom pulleys I need, but I need to figure out which size pulleys I need. I have the center distance between the axisâ€™s, and the belt I intend to use, but I dont know what pulleys I need.

I have axis lengths of 5.22" and 5" using the 56 tooth belt, with has a pitch length of 14"
and one at 3.224" using the 40t belt, pitch length of 10"
now I need to figure out what pulleys to put in there without being way too tight

Belt pitch length is just number of teeth * pitch (0.25" in this case). Total belt length using 2 of the same pulley can be calculated as `2 * C2C + Pcirc`, where `Pcirc` is the circumference of the pulley at its pitch diameter.

So if you know 2 of those 3 values, you can calculate the third. Just be consistent in units (tooth count vs inches)

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Some rough math suggests youâ€™d need a pair of 16 tooth pulleys for the 56 tooth belt, and a theoretical center to center (ctc) distance of 5".

To calculate this I took my ctc distance of 5", and doubled it (since the belt has to go from one pulley to the next and back again), so that accounts for 10" of belt. Taking that away from my total belt length of 14", Iâ€™m left with 4" for pulley engagement, or 2" per pulley. This means, since the belt has a pitch of 0.25" (or 4 teeth for every inch), each end of the belt engages 8 pulley teeth. If the pulleys are the same size, the belt should only engage half of each pulley, meaning that I have double the number of engages teeth on each pulley (so (8 teeth)*2), giving me 16 teeth per pulley.

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Calculating center distances is pretty simple, IF both pulleys are the same. The math gets complicated with different size pulleys.

I made a couple timing belt configurators in OnShape:

.25" Pitch

5mm Pitch

Just change three variable to get the correct center distance: Belt Length and the two pulley diameters.

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You can use the Chain/Belt page in my AMB calculator to find the right C-C distance. First, change the type of belt/chain to RT25. If both of your pulleys are 16t, youâ€™ll put 16 for both pulley tooth counts. Then you can either enter the approximate center-to-center distance that you want and it will show you the nearest â€śroundâ€ť distance. If you want to use REVâ€™s belts which come in multiples of 8, put that for the â€śRound Toâ€ť field. Or you can also switch to â€śBy # Teethâ€ť mode and enter the belt length you have to get the proper C-C distance.

For most belts I recommend using the exact C-C distance without any adjustment (hence the zero C-C adder). But you may find if you canâ€™t hold such precise tolerances that you want to shoot a bit under because itâ€™s easier to tension a loose belt than lengthen a tight one.

https://ambcalc.com/chain_belt?=&type=RT25&t1=16&t2=16&mod=8

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