You will need to slide rods through a clamp, a bearing and into another clamp. The whole rod must be grease-free for the first clamp but you could just degrease the far-end after passing the bearing but before entering the second clamp.
I just degreased the whole rod and lubricated after final assembly.
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Printing with Mendel90 from fedora 25 using Cura, FreeCAD, MeshLab, OpenSCAD, Skeinforge and Slic3r tools.