Calculating C-C distance for GT2 3mm belts

I am hoping to use WCP’s GT2 pinions and pulleys with a 775Pro, but I’m unable to find the correct formulas (or online calculators) to get the center - center distance between the motor and the driven shaft. SDP-SI’s calculator only goes down to 16T for GT2 3mm pulleys. It goes down to 12T with GT2 2mm pulleys, but it looks like the pitch affects the needed C-C distance.

In this thread Austin Schuh recommends modeling the belt in SolidWorks, but I don’t entirely follow all the details. (For instance, do all of the arcs just butt up against each other? What is the diameter of those arcs?)

Various other formulas I have found don’t take into account pitch and tooth profile, so I’m not sure which ones to trust.

I would greatly appreciate any information on calculating distances for GT2 3mm belts. Links to the correct formulas, explanations of the post I liked, etc., would all be great. For reference, I am hoping to use the 12T pinion with a 36T pulley and a C-C distance around 4".

WCP has a fantastic video on this.

I can answer your specific questions if you don’t have 15 minutes to watch the entire video. All the arcs and lines are tangent to each other. The pitch diameter is circumference/pi, or pitchnumber of teeth/pi. Constrain everything in CAD, set the total length of the belt’s path to the belt’s length (pitchteeth), and the computer yield the center-center distance automatically.

I also prefer the CAD method to calculators, mostly because I can change one of the variables and have everything update.

The video, along with the other educational videos WCP has on their YT channel is very helpful. All your questions should be covered by watch them…

I strongly recommend using CAD to model your system.

Essentially:

  1. You have an initial hole, this is where pulley A is located or mounted on axle. Create a circle concentric with this hole and use the following formula
    to model the size for your pulley:

= Pitch Pulley Tooth Count/Pi*

  1. Make a second circle horizontally aligned with the first circle, and Smart Dimension that using the formula.

  2. Create two tangent lines connecting the circles, and then trim out the inside lines so all you’re left with is a loop.

https://i.imgur.com/J4Xn1Ya.png

  1. Use Smart Dimension Path, select all of the components of the loop, adjust the value using the following:

=Pitch*Tooth Count of Belt

  1. The path length will automatically adjust to match the constraints. Now you can simply measure the C-C distances.

  2. Create a new hole, concentric to the center of the location of your second pulley.

  3. Extrude a belt using the same path length as the one you previously modeled.

  4. In assembly mode, make sure your pulleys and belt properly mate…

I just recently spent quite a bit of time perfecting our 2017 Drive Train on CAD, and using the WCP videos… I saved myself lots of time.

If you have any questions, please let me know. I can upload more example photos too.

Good luck!

Clem1640 posted a very useful spreadsheet calculator a number of years ago here. I have used this a number of times and it has yet to fail me. No idea how it works, but it does :smiley:

I’m a little skeptical of the accuracy of this method, because it doesn’t seem to take the teeth geometry into account much. Though I guess pitch is included in the radius and length calculations.
Have you run a belt in competition after using this calculation method before? If so, what kind of issues did you have with it?

Tooth geometry is irrelevant in pitch circle and pitch length calculations. A single pitch length of 3mm GT2 and 3mm HTD is still 3mm, however way you put it.

You can actually get the SDP-SI calculator to go down to 12T with GT2 3mm, you need to select “Timing Pulley Stock” as the material option. This is what I use for doing the C-C distances.

If you select GT2 3mm the “Timing Pulley Stock” option disappears.

Edit: Ah, if you set your measurements to ‘metric’, then it reappears. Thanks!

Welp, I was able to use the SDP calculator and ended up getting a different value (+0.008). Not a big difference. Any suggestions of trusting their calculator vs my own drawing? I’m tending towards trusting their calculator.

I’ve got the exact same problem. I’ve got a desired center distance of 1.775 inches on a 12T GT2 pinion and a 36T Hex GT2 Pulley.

I’ve used several calculators but have gotten numbers ranging from 56 to 70 teeth for the belt.

Edit: I used the CAD method outlined in the WCP video and in ImMoMo’s post and I got 70 teeth. This is the same value I got from this https://www.chiefdelphi.com/media/papers/2715 belt calculator. Can anyone verify that I got the correct amount?

Edit 2: I figured it out, I accidentally used a pitch of 2Mm rather than 3

Gates also has design software for gears.

Link: http://www.gates.com/catalogs-and-resources/resources/repository/engineering-business-applications/design-iq

We’ve run a bunch of belts this way before successfully. For short runs, sometimes it is a couple thou too tight.

For applications involving adjustable tensions, like a WCD style system… this method works great! We’re using the VEX Cam tensioner with this setup, looks good so far!