Anycubic Kobra ACE Idler Gear Replacement

AnyCubic K3 Combo Idler Gear Replacement Guide

A few weeks ago, I ordered two AnyCubic K3 Combos. Unfortunately, both arrived with broken gears in the idler mechanism. The issue presented as excessive clicking and popping noises when the ACE attempted to retract filament. Additionally, the filament bunched up around the spool because it couldn't rewind, leading to potential tangles. After contacting AnyCubic, they agreed to send me two replacement mechanisms rather than just the gears I initially expected. While waiting for the replacements to arrive from China, I decided to model and print my own replacement parts.

Gear Failure Analysis

Of the eight gearboxes across both printers, three contained defective gears. These gears failed immediately when attempting to print a single-color object (Slot 1 in both ACEs). Upon closer inspection, it appeared that the injection molding process was flawed, resulting in a thin, skin-like layer running from top to bottom inside the gears. Instead of showing traditional breakage like fracturing, tearing, stretching, or crushing - the gears simply separated along this weak layer, leaving behind smooth surfaces.

Steps to Replace the Idler Gear

1. Remove the Idler Assembly

Follow AnyCubic’s official guide to remove the assembly:
Idler Pulley Assembly Replacement Guide

2. Disassemble the Gearbox

• Remove the screw that mounts the gearbox onto the 5mm rod.
• If the broken gear is in Slot 2 or 3, you will need to remove both gearboxes on that end of the assembly.
• Open the correct end of the gearbox. Avoid opening the end with springs, as reassembly can be difficult.

3. Inspect the Gearbox

• Some gears may be in pieces, while others may appear intact but have hidden cracks.
• Examine closely to determine if replacement is needed.
• You can try to turn the gear by hand, even if there are small cracks and the gear is in place, you should still hear the clicking and popping sounds.

4. Print a Replacement Gear

• I measured and modeled the gear in Blender.
• Due to the fine teeth on the friction drive gear, I used a fuzzy surface texture to improve grip.
• My printed replacement has worked for over 100 hours without issue.
• The STL file included in the 3MF file has fine teeth, which can be printed with a 0.2mm nozzle. To test this option, the height modifier on the model must be removed.
  

5. Install the New Gear

• Press the bearings into the new gear. It’s a tight fit, but a small amount of pressure will seat them properly.

• Insert the gear friction driver end up, as shown, into the gearbox.

• Reassemble the gearbox using the screws removed earlier.

6. Reinstall the Gearbox

• Slide the gearbox back onto the 5mm rod.
• Orientation Guide:
  • Slots #1 & #4: The smooth end of the gearbox should face away from the center.
  • Slots #2 & #3: The smooth end should face towards the center.
• Align the gearboxes correctly using the matching pin-and-receiver system.
• Ensure the gears rotate counterclockwise around the rod.
  

7. Secure the Gearbox

• Be careful when inserting the screw into the rod—misalignment can damage the gearbox. The rod itself is not threaded, so the screw must pass through the gearbox sidewall cleanly.
• For Slots 2 & 3, use an Allen wrench from the backside to align the hole before inserting the screw from the front. Once it starts threading into the rod, alignment should fall into place naturally.

8. Final Assembly

Follow the steps in AnyCubic’s wiki to reinstall the idler assembly and complete the process.

With this replacement gear, my printers are now running smoothly again. If you’re experiencing similar issues, this method should get your K3 Combo back in action without waiting for factory replacements!