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I have a long 8 mm smooth steel rod of about 55 cm long. This rods bend easily due to the length. If I replace them with 8mm solid carbon fiber rods, will the bending reduce? Will the bearings wear off the carbon fiber rod? I couldn't find too much information about this.

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Steel is the best material for a linear rod when you have a fixed cross-section. It will have the least flex of any rod (aside from some exotic metals) of the same size.

Carbon fiber's material properties might seem superior at first sight, but the stiffness is very anisotropic -- it's very stiff along the grain and not very stiff across the grain. So multi-axis stresses like bending aren't necessarily going to perform up to the theoretical specs. Carbon fiber has exceptional stiffness-to-weight ratio, but the stiffness-per-area isn't necessarily superior in this application.

People do occasionally use carbon fiber for linear rods/rails, but only in much larger sizes than 8 mm. Think >25 mm.

And that's really the problem here. 8 mm diameter at 550 mm long is well outside what's reasonable for bending stiffness. Bending deflection increases with the CUBE of length, and this is simply far too long for the size of rod. The general rule of thumb for precision motion applications is length < 25*diameter. That's a conservative rule, but it's the right ballpark. You really shouldn't be going over 200-250 mm or so with an 8 mm rod.

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  • $\begingroup$ Thank you very much for the info! 2020 V-Slot extrusion should behave way better (that is, the bending should be almost negligible) right? $\endgroup$
    – Andres
    Apr 3 '16 at 20:18
  • $\begingroup$ I could only add that you can try to use kinda print leveling feature. That's not the solution for bending rods of course but you can decrease the influence of it. $\endgroup$ Apr 3 '16 at 20:31
  • $\begingroup$ Yeah, 2020 is fine for ~500mm spans. You might want to upsize a bit for longer than that. The 25:1 rule works pretty well for dual steel rods (diameter) OR single aluminum extrusions (nominal thickness). The deflection math just works out convenient that way. $\endgroup$ Apr 3 '16 at 22:20
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I agree that steel is the best material for rails but the info provided on carbon fiber is just wrong.

When making a carbon fiber part, whether a tube or any other shape, you lay the fabric in the orientation you need for the direction of the forces. If multi-directional strength and stiffness is needed then the fibers are laid in multiple directions. Nobody who knows what they are doing lays all the cf in one direction...

In general, carbon fiber is stronger than steel by both the strength to weight ratio and by volume. I say "in general" because parts need to be designed in the right way. A poorly designed carbon fiber part or a low quality steel can easily be weak.

There is a great video on YouTube showing a strength and stiffness comparison of a carbon fiber drive shaft vs the same part made in steel. The steel drive shaft bent and snapped with 3 times less force while the cf drive shaft had no visible bending until it snapped.

The main reason why steel is a better linear rail material is surface hardness. Linear rails have bearings traveling up and down continuously. The rails need to be hardened to prevent them wearing out too quickly. The surface hardness (how easy it is to scratch or penetrate) of cf is far lower than steel. There is no comparison.

Cf is great material for the frame but not the rails. For stiffer rails, I suggest buying some square THK rails. 15mm the rails will be plenty stiff enough for most 3d printers.

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