I noticed that the bar changed position (horizontally) with each turn of the Z-axis motors, and also had play by manually swinging the bar, caused the hotend to move a bit, and consequent bad quality problems.

This cap serves to keep the Z-axis threaded bar centered, helping to prevent noises and extrusion offset, reducing wobble and misalignment between layers.

This is way more simple than the versions with bearings, and works nice.