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.

Great topic. I would like to fully explore having a 3d printed download for a 32 teeth sprocket, I will additionally post my findings with regard to metal fabrication of the part. Matthew, is there any way that I could get a link or information on your 28 teeth sprocket? Love this site BTW. I’m currently using the AACA library model (original) as a guide and your teethed roller, which I additionally need for a developing apparatus, would help guide.

@WirelessGuy_NY curious if you have come across anything regarding ordering or making a sprocket of this nature?

I appreciate all of your time.

Here are all the downloadable files for v1. In there you will see a couple of roller-related files. One is “roller_rhino_python.” It has the script I used to generate the sprockets in Rhino. If you download Rhino on a free trial you should be able to do the same. Here is a video I made demonstrating how to use the files provided in Rhino: - YouTube.

I hope that helps!