Why the 28 tooth (35mm) sprocket design?

I assume that the answer to this question may be a simple one but I was curious why the 3D printed sprocket design uses a 28 tooth sprocket design for the 35mm film?

I was looking at actual simplex projectors for movie theater film and the standard seems to be 32 tooth. I’m all for convenience but if the kinograph was able to borrow from existing industry metrics it would enable us to at least source metal parts that could last longer.

Just a thought.

Gonna answer my own question (I think) but I’d still like to confirm my thinking.

For 35mm (@24fps) the gearing required for 28 teeth would be a 7:1 ratio and 32 teeth a 8:1 ratio. Hence either are viable options but your sprocket simply makes another 1/2 turn or so per second.

Hi there. I don’t have a good answer for you, unfortunately. I kinda just took my best guess at what a safe radius would be and that turned out to have 28 teeth.

Fortunately, the new version does not use teeth at all so this won’t be an issue moving forward.