🔧 PaMiLoCELL #11 | J4 Timing Hub – Alignment & Bearing Resistance

Can a 0.15 mm deviation cause a robotic joint to bind?

During assembly of the Annin Robotics AR4-MK5, I noticed resistance over ~30° of rotation at J4. Loosening the J4 Timing Hub made it disappear.

📏 1. What I measured

• Required hub offset: 25.30 mm

• Measured: 25.15–25.21 mm

• Hub difficult to slide onto the shaft while keeping the pulley face square (90°).

A 0.15 mm offset alone is unlikely to explain the issue. More likely causes: an out-of-square hub, a tight/uneven bore, or excessive axial bearing preload.

🔍 2. Technical feedback from Community

• Keep the bearing contact face flat and parallel: avoid aggressive Dremel machining.

• If the bore is tight, lightly finish it without introducing play.

• Check printed hub thickness: ≤15 mm at the flat face.

• Tighten preload screws only until bearing play is removed and rotation remains smooth.

• Reprint the hub if its bore or face is distorted.

⚠️ 3. A disassembly mistake

I overlooked the different set-screw lengths:

• 1 × M4×10 engages the shaft alignment hole.

• 3 × M4×5 secure the hub around its perimeter.

• 4 × M4×5 face screws adjust bearing preload.

During removal, the longer screw was still engaged, damaging both the printed hub and the aluminum shaft.

🧱 4. Printed polymer vs. aluminum

The hub is designed to be 3D-printed (PETG or equivalent; 50% infill, 4 walls).

Aluminum adds machining cost without a demonstrated performance benefit for this application.

✅ 5. Next steps

Inspect the shaft; reprint the hub if needed; check the 25.3 mm position and squareness; set light preload; verify smooth, play-free rotation.

💡 LESSON LEARNED

A dimensional deviation is not automatically the root cause.

Check alignment, bore fit, bearing preload and screw lengths before modifying a component.

Thanks to the Community for the technical exchange!

❓ Repair or reprint: what criteria guide your decision?

🔧 6. Additional Assembly Issues Resolved

✅ Corrected blind or incompletely drilled holes.

✅ Eliminated minor belt and pulley interference points, ensuring adequate mechanical clearance for safe operation.