Retractable keychain with planetary gears

I’ve been wanting to make a retractable keychain that would be a fun topic of conversation in the office for a while now. I found a model by pyromaniak. I didn’t like the hardware choice or the shape of the gear. I later learned that it was quite difficult to assemble. I started reworking it to fit my needs. I ended up scrapping 3 iterations of remixing and decided to redraw everything. I ended up with a pretty nice design, with the M2 hardware and one bearing.

I gave a few of these to my colleagues at work. So far (6+ months of daily use) everything is working out well.

This isn’t the easiest project to print and build — the STLs have areas of little to no adhesion to the build plate, you need the M2 hardware, and the assembly is a bit tricky. Keep that in mind!

Bill of Materials

• 1x set of printed parts - you can choose from one to four planetary gears, print the appropriate roller of your choice

• 1x retractable keychain - I tried two cheapest options from aliexpress and both work well:

• 4-7x M2x6 DIN 912 screws (depending on the number of planetary gears)

• 2x M2x8 DIN 912 screws

• 4x M2 heating inserts (length 3mm, outer diameter 3.2mm)
  I bought from this link (NOT an affiliate link):

• 1x MR85 bearing (outer diameter = 8mm, inner diameter = 5mm, height = 2.5mm)

• tools needed: knife or flat spatula, pliers, allen hex key, scissors, twizzers and lighter

Print Settings

• all parts are already in proper printing orientation

• 0.10 mm layer height for all layers (first and all later) - this one is VERY important as all bridges are designed for 0.10 mm layer height

• 0.4-0.45 mm layer width

• 3 outer walls

• 8 solid top & bottom layers

• no supports

• PLA and ABS are fine (PETG might leave too much retraction strings)

I printed everything on both Prusa MK3S+ (PLA parts) and Voron 0.2 (ABS parts) with a standard 0.4mm nozzle and everything works fine. A smaller nozzle might work better, but 0.4 will still do the trick.

Assembly

I recommand watching YT video and reading the steps. 

Parts preparation

1. Disassembly keychain (you will not be able to use it again, so use one you are not afraid destroying). Use flat spatula to split plastic covers or just crack the shell. Carefully extract spring, nylon string (try to cut it right before ties to keep it as long as you can), small rubber ring (if present) and card attachments, keyrings.

2. Clear all parts form strings, check through holes for 2 mm screws in gears and body_bottom and hook.

3. Press heat inserts into the case_top.stl

Assembly (video starts here)

1. Make a knot at the end of the string and push it through the reel. Put small rubber ring on the nylon string. Push nylon string through the hole in dongle. Make a knot at the end, use lighter to melt it a bit. Attach the kering to the printed dongle.

2. Use M2x6 to screw gears into the reel. Start with planetary gears. Don't overtighten them. The gears should spin freely and had little-to-no play in vertical axis. When you are satisfied with them, screw in sun gear. Again, don't overtighten it, check if assembled gears can rotate freely with little to no resistance.

3. Insert bearing into the reel

4. Using pliers shorten the 50-100 mm of spring. Shape the other end into “C” (it will need to fit the slit in bottom_case). Again using pliers, bend the 1-1.5 mm of the spring in 90 degree angle. The end of the spring should fit into the reel. If you are carefull you should be able to get the exact amount after 3-4 tries. Don't worry about shortening the spring too much. Fit the end of the spring into the reel and then fit the whole spring into the reel. From now on be carefull not to loose the spring from the reel!

5. Take body_bottom and match it center slit into the spring other end. You might need to use a bit of force.

6. Reel and body_bottom install into body_top. Don't worry about the nylon string, leave it fully retracted. Match planetary gears with outer gear. Try to rotate the reel a bit, everything should move smoothly.

7. Connect both body parts. On the body_bottom you can find small arrow which tells you direction you should rotate the part to wind up the spring. Start rotating the part in that direction. You should see the nylon string beeing pulled into the keychain. If you have issue with gears not moving you can give them a little nudge. When whole nylon string will be wound up on a reel, check if the spring tension is correct by pulling the dongle. Keep rotating the body_bottom untill you are satisfied with the result. I make 4.5 of full rotation to tension the spring. If you want to loosen the spring just rotate body_bottom clockwise.
   If the whole assembly is not moving freely then just hide the nylon string and move to next step. You will fix that in a moment.

8. Screw in two M2x6 from the dongle side. Don't crack the plastic. Now you should be able to check the gears in action. Pull the dongle and run the mechanism a few times. If needed you can use grease to smooth out the action.

9. If the mechanism does not run smooth disassembly everything back to step 8. Remember about the spring! 

10. Attach the hook and screw other M2x8 screws. Don't crack the plastic.

DONE!

Additional Notes

• use different colours for gears and dongle vs rest of the parts - I use filament samples to print gears and dongles, and other colour for the case.

• use oil o grease for smoother action - I use cosmetic vaseline 

• do you want to sell this print? Let me know, I am open to share my design 

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TODO

• ✅ add version without hook (as fidget toy)

• design another bottom case with room for buckle

Changelog

• 02.08.2026 - public release