Designing bearings, shafts and rotating fits for FDM

Rotating fits are one of the more demanding applications in printed parts, because they combine two things FDM is not naturally good at: fine dimensional control on curved surfaces and long-term wear resistance. That does not rule them out. A large share of gearboxes, knobs, wheels and pivoting mechanisms we produce use some form of printed bearing or shaft seat successfully, but only when the design accounts for how FDM actually forms round features.

This guide covers plastic-on-plastic sliding fits, printed shaft seats, and when it is worth designing in a standard rolling element bearing instead.

Why round features need extra clearance in FDM

A printed circle is not a smooth circle at the microscopic level. It is a polygon approximation built from discrete extrusion moves, and the outer wall shows visible layer lines and the characteristic slight ovality that comes from acceleration changes as the nozzle changes direction around the curve. On top of that, horizontal holes printed without support tend to shrink at the top where the walls bridge over themselves, an effect sometimes called hole sag.

• Print shaft holes with the axis vertical whenever the assembly allows it, for the roundest possible bore
• If the hole must be horizontal, undersize it slightly and ream or drill to final size
• Add 0.2 to 0.4 mm diametral clearance for a sliding fit between two printed parts
• Add 0.1 to 0.2 mm for a closer running fit that still needs to rotate freely
• Print test rings at the target diameter before committing a full assembly to one clearance value

Material pairing for plastic-on-plastic bearings

Two identical materials sliding against each other tend to gall and wear faster than two dissimilar materials with different friction coefficients. PA12 against PA12 works acceptably at low speed because nylon has reasonably low intrinsic friction, but PETG against PETG tends to squeak and wear faster. A common and effective approach is to print the shaft in one material and the bushing in another, for example a PA12 shaft running in a PETG or ABS bushing, or to add a thin film of PTFE-based lubricant or grease at assembly, which is a standard and low-cost way to extend service life.

Designing a seat for a standard rolling element bearing

For anything beyond light, occasional rotation, a standard deep groove ball bearing pressed into a printed housing outperforms any plastic-on-plastic solution and is not expensive. Design the housing bore about 0.05 to 0.1 mm smaller than the bearing outer diameter for a light press fit, and print the bore with the axis vertical for roundness. Because FDM parts creep under sustained press-fit stress more than machined plastic, add three or four small retaining ribs or a shoulder to carry the axial load instead of relying on friction alone to keep the bearing seated over time.

1. Model the bearing bore 0.05 to 0.1 mm undersize on diameter for a light press fit
2. Add an axial shoulder or retaining lip so the bearing cannot creep out under load
3. Print the bore vertically wherever the part geometry allows
4. Where the mating shaft is metal, size the shaft seat to the bearing bore tolerance, not the printed part tolerance
5. Test fit on a sample print before committing to a production batch, since printer-to-printer variation of a few hundredths of a millimeter is normal

Shaft design and torque transmission

A printed shaft that also has to transmit torque, for example a shaft carrying a printed gear, is weaker than the same shape machined from bar stock, because torque loading puts shear across the layer bonds around the shaft's circumference. Keep printed shafts short and thick where possible, avoid small diameter shafts under continuous torque, and where torque is significant, use a metal shaft through a printed hub rather than an all-printed shaft. A common and reliable combination is a printed gear or pulley with a metal shaft running through a keyed or D-shaped bore, which transmits torque through a positive mechanical feature instead of relying on friction or the printed material's shear strength.

Post-processing for rotating fits

A quick pass with a hand reamer or a drill bit run at low speed through a printed bore removes the small ridges left by layer lines and brings a horizontal hole much closer to round than as-printed. For sliding surfaces that will see continuous motion, a light sanding of the shaft, followed by a thin coat of PTFE or silicone-based grease, measurably reduces friction and initial wear compared to running two as-printed surfaces against each other dry.