WCD CAD Practice

[Chris is me]

Originally Posted by AdamHeard

Exact C-C actually guarantees all the belt runs will be at uneven tension.

When you do exact C-C the belt tension is set by the c-c distance plus the sum of all your manufacturing tolerances (which includes the belt! and that can be more than a thou on drive length belts!).

Gates has a lot of great documentation out there that explains this and many other factors with their belts. It's a great read for any team who runs belts.

That makes sense. I was mentally comparing my team's exact-center set up to our previous belt drive, which used plastic tensioners with a lot of adjustability. In this case, we could easily set up our drivetrain where one belt is much more tense than the other, causing ratcheting. I do think that a WCD bearing block setup, relying on a small cam that's easy to make fine adjustment with, could result in more optimal tension than our drive. I don't know how necessary that is, but I'm sure it would help with efficiency.

It's worth noting that for an exact center belt drive made from a single piece of tubing, the spacing tolerance is just determined by how precise your mill is. For many teams this precision is really "good enough" and better than they could get with a tensioning system.

Gates's documentation on belts is very useful, and something that should be read by anyone designing a belt drive, but at times the guide's advice and specs are quite conservative. The Gates manual recommends never using an exact-center belt drive, but for FRC drivetrain purposes (at *most* a few hundred hours run time on a well used practice robot) the factors they cite that suggest a tensioning system don't really apply.