I'm happy to report the 3D printed gears are working and my lumbar support has been repaired!
I'm unable to attach the file I 3D printed as it is not supported. I am also unable to link externally to the file as it triggers a filter. Basically, go to Thingverse and search for “Lumbar Support Gears, Chevrolet Tahoe” and you will find the files. I wish STL files were supported on the forum, but not sure if that is a feasible change. Hopefully the Thingverse link stays where it is, but I have also backed up the files myself.
Use 100% infill when printing this part. Mine were done in PLA. I printed at the local library, 7 gears, enough for 2 repairs with a spare gear, cost $1.
After I got the gears, I worked them over with a 320 grit sandpaper, working dry. The main thing is to smooth the faces by sanding them flat, just a few seconds on each side. Then roll the sandpaper into a tube shape and twist it on the inside of the gears to smooth them where they mount on the posts. I cleaned the dust off with some rubbing alcohol afterwards.
To install these in the mechanism you will need to get the gear mechanism open, which has square security screws. I bought a set of square bits at harbor freight for less than $3 which removed these.
Once inside the mechanism there are 2 pieces, one that attaches to the cable and has 3 posts on its underside for the gears, and another piece below that with an inward facing gear which interfaces with the 3 we are replacing.
Start by putting the gears on the posts. It is important is to put the gears on with the side that was the bottom of the print towards the bottom of the post, and the top of the print towards the top of the post (away from where it attaches to the rest of the piece). There is a groove on the gears for the posts to spread a retaining clip into and the top of the gears will have a clearer groove for this than the bottoms.
When putting the posts on the gears, use some silicone based grease to lubricate the new gears. Do not use petroleum based grease as it breaks down plastic and can lead to failure.
Now fit the piece with the gears down into the mating piece. You may need to spin the gears slightly to get teeth aligned. Use more grease if needed.
Rotate the pieces relative to each other so that the cable attachment point is at the 11 o'clock position as shown below. This is important since the cable will be fully relaxed when re-installed. The underlying piece has a portion (shown at the 12 o'clock position) which blocks the top piece from rotating. The top piece, with the gear posts, must be oriented such that it is immediately to the right of this blockage so it can spin clockwise and take up the cable from the relaxed potion.
View attachment 17162
Finally, wiggle the two pieces into the mechanism and reattach the cable. Replace the front cover and tighten screws. The mechanism should be functional now, but before closing up the seat back it is worth running it both ways to be sure. Also note that the button on the passenger side seems reversed, ie. the bump reduces lumbar and the hollow raises the lumbar, which I am pretty sure is the opposite of my driver side.
On a final note, I'm not sure I would use brass gears in one of these. As it is, these gears seem like a "designated point of failure" to protect other components. They are also the simplest components, and cheap and easy to replace with 3D printed versions. Putting a brass gear in might result in damage to other significantly more complex components for which no easy replacement is available.
