A bushing is a plain bearing (no rolling elements) that only provides a bearing surface. They are less noisy than linear ball bearings, but not always practical to integrate. In addition to commercial available bushings, there are a slew of printable bushing designs available on Thingiverse. These include self-centering bushings, LM8UU bearing style bushings, and clip on bushings.
|3DMakerWorld||US||8mm bushing and 10mm bushing||8 and 10 mm||bronze|
|Alfa-Tech3D||DK||Self-lubricating Bearing||8 mm||bronze||Delivered Worldwide|
|RobotDigg||CN||Self-lubricating Bearing||8 and 10 mm||bronze|
|Goldmine||US||Bushing with flange||8 mm||bronze||Don't use on 5/16" rod.|
|Paoparts||FR||Iglidur D6||6 mm||plastic|
|Paoparts||FR||Iglidur D8||8 mm||plastic|
|TechPaladin||US||Sleeve Bushing||8 mm||bronze||Self-aligning.|
One RepRapper has used Ultra High Molecular Weight Polyethylene to create self-lubricating bushings that are less noisy, but deform under load and have increated wear and play over time compared to linear bearings. More information in DIY linear bearings (no PLA) on the forum.