Rod flex under carriage weight

[Sebastian Finke]

So a downside of direct drive extruders is the added mass will cause the X-axis rods to flex. This is particularly apparent when the carriage is in the centre position as the carriage is now the furthest distance from a fixed support point. This can lead to tramming issues as the the nozzle-to-bed distance is smaller in the centre of the bed than the outside of the bed and people often mistakenly diagnose a warped bed to be the issue. In reality the cause is simple physics... the carriage is heavy and this added weight is causing the carriage to sag.

For interests sake I thought I'd run a little analysis to determine the amount of 'sag'. My printer has standard 8mm rods that are 400mm long. Many printers will fall in line with this. Total sag is 0.1mm in the centre position. In the analysis the drop distance is along the Y-axis, not the Z-axis.

First, the setup. Mesh and constraint/load placement:
01.JPG

Next the result. Deviation under load listed in the table:
02.JPG

Form the analysis its easy to see why many people, beginners in particular, struggling with levelling their beds and have adhesions issues with larger prints. If the desired 0.1mm gap is achieved in the bed centre this will create a 0.2mm gap on the outskirts of the bed... and suddenly prints don't adhere and start curling or coming loose. One solution is to go Bowden. Alternatively a gap compromise must be reached.

Hope this helps.



EDIT: the force exerted here is that of a standard FFCX dual extruder i.e 2 steppers with heater blocks, etc, heat spreader bar, carriage and 4 bearings. Total downward mass is approx. 720g or a downward force of 7.056N. This spread over 4 points representing the linear bearings.