X-Axis Gantry

The toolhead (which holds the extruder E-axis and hotend) rides on the X gantry. Similar to the Y-axis bed, the most common form of maintenance to be performed on the gantry is checking if the belt tension is right and tightening/loosening as necessary. You can read more about it here:

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Tensioning belts

X carriage. Image from Prusa.

Assembly and disassembly of the X-Axis also requires assembly/disassembly of the Z-axis as well. You can read more about this here:

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X-Axis Assembly

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Z-Axis Assembly