Choosing shaft diameter for machine

Found this documentation on linear bearings and choosing shaft diameter. LINK

Not sure if anyone else has suggestions of choosing the correct size shafts for a printer. Think the calculation in this document was helpful, so a shaft start to droop as it gets longer and the smaller the shaft the shorter distance it can span without getting a curve. The math they have is 24 X shaft diameter. So 8mm shafts are good for about 192mm so the normal 200mm build plate is right at max or just over so may have a slight amount a droop or curve in the center. So weight on the head would increase this but having two shafts fixes that issue.

Just thought it was nice to find some math behind the design of the machine out there. Are there any other documents that explain it better? It seems a bit high to use 16mm rods for a 400mm gap.

Well, I could not explain 5 years studies in 10 sentences on a forum. Lenght/24 is a bit short. It depends of the load and of the final expectations. Linear guides selection is a complex process which is shortly described here. Read here too. Try to find a manufacturer's linear guide catalogue, the maths and process is well described inside.

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Collective intelligence emerges when a group of people work together effectively. Prusa i3 Folger (A lot of the parts are wrong, boring !)

Yeah, guess the only way to really figure it out is measure, and trial and error. I do not see a clear way to calculate the length for droop. The manufactures really are not being very helpful in there descriptions, because the material used in the rod really has allot to do with it as well.

Only reason why I started looking into it is I have some stainless rods 8mm x 850mm and had them on a rack above my bench and noticed that there was a little gap between it and some aluminum square tube in the center they are spanning a 24" gap, so that is definitely too far of a distance.

Once the diameter is figured out then moving on to bearings is another subject, just want to make sure I am getting a large enough diameter before I figure out the load and torsional effects on the bearings.

You could always use fully supported guide rails...

Thanks, I was looking at your post with the pictures using supported rails sounds like a good solution for larger gaps.

Edited 1 time(s). Last edit at 03/13/2015 08:26PM by AquaticsLive.