Short answer: most hobby 3D printers use four kinds of bearing. LM8UU linear bearings (8 x 15 x 24 mm) slide along 8 mm smooth rods. 623ZZ (3 x 10 x 4 mm) and flanged F623ZZ bearings carry the GT2 belt idlers and extruder arms. 608 bearings (8 x 22 x 7 mm) turn spool holders. Small MR and 68x/69x bearings appear in extruders and mods. Always check the bill of materials of your printer, because designs differ.
Which bearings does a 3D printer use?
A printer needs bearings in two places: where something slides in a straight line (the axes) and where something turns (belt idlers, pulleys, extruder idlers and the spool). Sliding is done by linear bearings on rods or by wheels and rails; turning is done by ordinary radial ball bearings. The table gives the usual pairings; your printer may differ.
| Where | Typical bearing | Size (mm) |
|---|---|---|
| X, Y and Z axes on 8 mm rods | LM8UU or LM8LUU | 8 x 15 x 24 / 8 x 15 x 45 |
| Belt idler (GT2 belt) | 623ZZ or F623ZZ | 3 x 10 x 4 |
| Extruder idler arm | 623ZZ, MR105ZZ or 693ZZ | 3 x 10 x 4 / 5 x 10 x 4 / 3 x 8 x 4 |
| Spool holder | 608ZZ | 8 x 22 x 7 |
| Larger rods | LM10UU, LM12UU | 10 x 19 x 29 / 12 x 21 x 30 |
3D printer bearings: LM8UU linear bearings
The LM8UU is a linear ball bearing: a steel sleeve with recirculating balls that roll along an 8 mm hardened rod. "8" is the rod diameter and "UU" means it has rubber seals at both ends. It is 8 mm inside, 15 mm outside and 24 mm long. Many Cartesian printers use LM8UU bearings on their 8 mm rods.
The LM8LUU is the long version, 45 mm, roughly two LM8UU bearings in one sleeve. It holds its alignment better on a carriage and is a common upgrade for the Y-axis bed carriage. If a design calls for two LM8UU bearings in a line, an LM8LUU often fits the same space.
LM8UU bearing dimensions
Inner diameter 8 mm, outer diameter 15 mm, length 24 mm. The bearing needs a precise, hardened, smooth rod: on soft or rough rods the balls wear grooves. It is also a good idea to clamp an LM8UU along its body rather than squeezing it hard, because a tight clamp can distort the sleeve and make it bind. The other sizes, LM6UU (6 x 12 x 19), LM10UU and LM12UU, are in our linear bearings collection.
623ZZ and F623ZZ: belt idlers
The 623ZZ is a 3 x 10 x 4 mm deep groove ball bearing with two metal shields. It fits on an M3 screw, which is why it is the default idler for GT2 timing belts. Printed idlers often stack two 623ZZ bearings to match the belt width.
The F623ZZ is the same bearing with a flange on the outer ring (11.5 mm across). Two F623ZZ bearings placed flange-out make a simple belt idler with built-in guides that stop the belt walking off. If you are replacing an idler, check whether the original had flanges.
608 bearings for spool holders
The 608 (8 x 22 x 7 mm) is the classic skate bearing. On a printer it shows up in spool holders and filament guides, because it is cheap, robust and takes an M8 rod or a printed 8 mm axle. The shielded 608ZZ spins most freely; the 608-2RS is fine too and better in a dusty workshop. The flanged F608ZZ stops it pushing through a printed part. Search terms like "608 bearing 3d print" lead to many printable spool holders and projects that use exactly this size.
ZZ or 2RS for a 3D printer?
Inside a printer, ZZ shielded bearings are usually the right choice: the environment is clean and dry, and the low friction of a shield helps idlers and spool holders turn freely. 2RS sealed bearings add drag and are mainly worth it in a dusty enclosure or a shared workshop. The full comparison is in our ZZ vs 2RS guide.
3D printer bearing size: how to find the right one
- Look in the printer's bill of materials or assembly manual first; it will name the bearing.
- If the old bearing is in your hand, read the code on the shield. Our guide to bearing codes explains what it means.
- If there is no code, measure bore, outer diameter and width and compare with our bearing size chart.
- For a printed part, model the pocket to the nominal outer diameter plus a small clearance that suits your printer, and test-print a single pocket first.
3D printer bearing grease and lube
Shielded and sealed radial bearings such as 623ZZ and 608ZZ come greased and do not need lubrication. Linear bearings are different: LM8UU bearings benefit from a little light grease or oil on the rod and from keeping the rods clean, because dust and old lubricant mixed together make them noisy. Follow the printer maker's maintenance advice for the type of lubricant; some plastics in printed parts do not tolerate every oil.
3D printer bearing wheels and other mods
Printers with aluminium V-slot extrusions use wheels instead of LM8UU bearings. The wheels themselves are plastic, but they run on small ball bearings inside, often of the 625 or MR size type; check the wheel's specification. Other printed mods, such as filament runout sensors and smooth spool rollers, frequently use MR105ZZ, 688ZZ or 625ZZ bearings. Steel balls also turn up in printed projects as detents and ball joints; see our precision balls.
Where to find them
All of the bearings above are grouped in our 3D printer bearings collection. For other sizes, browse miniature ball bearings, flanged bearings and 608 bearings. Each product page shows the bore, outer diameter and width in millimetres.