Chain length calculator

[Rob Stehlik]

I'm wondering what teams use to calculate the correct center spacing for sprockets when an idler isn't used. Obviously this is trivial if the sprockets are the same size. It gets more difficult if the sprockets are different sizes, and when you realize that a sprocket is not a circle, but a polygon. For several of our mechanisms last year we used short runs of 25 chain, and tried to design in the correct spacing with Dr Joes chain path spreadsheet. Unfortunately, the chain was loose in every case.
I recently compared Dr Joes spreadsheet result with several online calculators, and they all came up with a center spacing that was larger. On one of the pages, it explained that a simple calculation assumes the chain follows a circular path around the sprocket, but this introduces some error. A better method assumes the chain rollers will be at the sprocket pitch diameter, but they are connected with straight lines, not arcs. For simple chain runs, I'll try this method in the future, but the great thing about Dr Joes tool is you can use more than two sprockets, and it shows a graphic representation of the chain path. Does anyone know of other tools out there that can do this? Should I ask Dr Joe for an upgrade?