Hi! I was wondering whether there is a calculator to calculate center distances for a belt running on 3 pullies instead of 2 (like 2910’s flywheel in 2022 IIRC)?
Also, are there any drawbacks to this method (for example, driving a third pulley with two driving pulleys instead of gearing the motors together into a single pulley)? I can imagine that the belt will have less contact with each pulley, but considering there are more pullies I’m not sure how it would affect things. Thanks in advance!
One of the bigger drawbacks you’ll run into is that it becomes harder to get pulley wrap. The belt having less contact with each pulley means that it can’t handle quite as much loading, unless you add at least one idler pulley that can be used for both tensioning and enforcing wrap. If you get particularly creative, you can do some interesting tricks with the belt and pulley arrangements, though–like running two shafts in opposite directions at the same or different speeds.
The big thing about pulley wrap: It’s hard to have too much, but if you have too little, you’ll know pretty quickly when the belt starts skipping on that pulley.
Some CAD packages may have a belt calculator built-in; I have a love-hate relationship with Inventor’s version of same (because it has quite limited belt selection).
The easiest method that I’ve found is to set up a 2d sketch of the belt path you want where the arc lengths representing the belt wrap around the pulley and the straight segments all add up to the length of the belt you are using. Difficulty depends on what cad you use, I use Autodesk inventory where it’s a little guess and check to get it right but SOLIDWORKS allows you to dimension the length of a closed loop sketch.
This method can be used for as many pulleys and idlers as you want and works with fairly complex belt paths.
As for pulley wrap, Gates provides recommendations in their documentation. I don’t recall it off hand but we’ve ran some belts with 90° of wrap with no problem. But if you get creative with idlers and tensioners you can increase the wrap where needed.
You can use standoffs and some loose plastic bushings as idlers to increase pulley wrap really easily, but that makes it slightly more complex. Luckily belts are all closed arcs and lines so the arc length isn’t hard to calculate especially if your cad program will do it for you.
This is a great way to do it, just keep in mind the belt thickness: the length that wraps around the pulley is at the pitch diameter, while the back side of the belt goes around the idler. Different kinds of tooth profiles have their “pitch line” at different points along the belt thickness, and usually this info can be found online.
The videos linked on this page are excellent. Resources – WestCoast Products
Not that hard to set up an offset to handle this with little to no math, or just build in enough adjustment the approximation is fine. But you are right, in tight spaces and depending on where you can take up slack this can be a big deal.
A lot of planning/foresight skill comes into effect when doing pulley layout and knowing you may change pulley ratios in the future.
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