# Timing Belt Center Distance with Idlers

I am currenly working on a new swerve design and am having some packaging issues. The solution I have come up with is to add some idler bearings to the belt and use a double sided belt. The new issue at hand however is I don’t know how to calculate my center distances properly with the idlers. How would I go about calculating the center distances for a belt and ensure proper belt tension in a setup like below?

There may be a solution for this in CAD, but I don’t know it. The naive (and annoying) solution is to do the geometry by hand.

It may not actually be that annoying, though, because the geometry immediately around the motor pulley is not going to change - it looks like the only nontrivial step is solving for the degrees of wrap around each idler as a function of distance (assuming center-to-center distance between the motor pulley and the wheel-side pulley is the only parameter you’re going to vary).

Depending on the type of belt you are using, the length of the belt is simply the pitch diameter times the number of teeth. You can sketch the belt path and then “Path Length Dimension” it to pitch diameter * tooth count. I would reference the WCP GT2 belt tutorial.

It looks like you’re using SolidWorks, but I can’t quite tell. If so, are you familiar with the Path tool in the Sketch tab?

This is exactly the kind of thing I was looking for, thanks. I also didn’t realize that WCP had these design tutorials up, something I will be for sure looking at over the next few days.

I am using Solidworks (forgot to mention that origonally), and am not (rather was not) familiar with the Path tool, however after watching the tutorial from WCP I am beginning to tinker with it.

Awesome! Just as a tip, I model the pulleys as construction circles at their pitch diameter, then draw the path of the belt’s pitch line and make the path out of that. Since you’re using a two sided belt, you can either make two parallel paths corresponding to the two pitch lines of the sides, orrr you can stick with one path and adjust some of the pulleys’ pitch diameters to do the same trick. If you increase or decrease all the pitch diameters that engage with one of the sides of the belt, you’ll end up with the correct center to center distances.

Double sided timing belts have only one pitch line, at the center of the belt.

+1 to this. It makes designing with them very easy!
bbmann is a good source for belts I’ve heard. I’ve used SDP-SI a few times but their shipping is terrible.

Might check out the Gates Design IQ program. It can handle idlers and multiple pulleys, not positive about double sided belts.

+1 to this. We used BBMan for all our belts this year (after trying someone else who was terrible) and the belts were high quality and shipped fast.

For more complicated belt paths, I’ve used Gates DesignIQ software: http://www.gates.com/catalogs-and-resources/resources/repository/engineering-business-applications/design-iq

It lets you layout all your pulled and idlers (fixed, sliding, or pivoting) and ensures that you have an integer number of teeth somewhere within the range of the idlers.

I’ve had very responsive ordering from V-Belt Supply as well. They carry a full line of Single and Double Sided 5M HTD belts at very reasonable prices.

When you need belts during build season, dealing with slow or unreliable suppliers is beyond aggravating.

Not to rain on the parade, but why not just run larger pulleys? The pros is maybe you need that area around the swerve mount or pulley availability. Some of the cons to the idlers is weight and drag, possible pulley engagement issues and belt availability. Your setup is similar to ours which, of course, is cool.

https://www.chiefdelphi.com/media/papers/3375

The desired ratio from the CIM to the module is 2:1. I’m having trouble finding pulleys any larger that would work. I’m currently running 24:50 in the second stage (well third if you include the 1:1 bevels). Any larger than a 50 tooth gear on my 3in wheel and I start to get worried about how close the gear is to the wheels perimeter. I tried moving up to 24:36 pulleys for the first stage (also adjusted the last stage to compensate) but I still needed idlers to clear the VP. Any pulleys larger than that from vexpro (most preferable supplier for us) require hubs which again adds weight like the idlers and also costs more than the idlers. This to me was the cleanest/easiest solution at the time.