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At present I am designing a machine but at present I'm not sure which way to go for the longitudinal supports.

Do I use a single shaft 1000mm each side (at present 20mm dia,) or would dual bars at 20mm dia, with a spacing of 100mm give a more ridgid setup

Or do I go for bigger single bar say 30mm dia ?

The spacing of the carriage bearings will be 200mm apart

Paul

Keep it simple: use 30mm rails or if you’re concerned about the weight of your gantry, use 40mm...

Using dual rails for this application is unusual, as there are issues with alignment and one of the rails would be redundant due to the bearing tolerances (I have tested the setup) using laser alignment and blue marker the results showed only 1 of the rails was used predominantly.

Question: How many gantry machines are in the market place with dual rails for there longest axis?

Thanks for the advice I am trying to keep it simple it was that I had already got a set of 20mm x 1000mm rails from your good self and will build the prototype using them as for some of the applications I'm sure 20mm will be ok The question about using tandem rails was not only about ridigitiy but also keeping everything the same size for ease of stock of components, hence the question about using twin rails as this sounded a feasable solution.

But I will take your advice against using twin rails and see how the 20mm perform then if needed will upgrade to 30 / 40mm

Regards

Paul