How to fix a plastic gear
[pictures to be posted "soon"]
Introduction:
The Saab 9-3 is a great car, but there are a few issues that are common to them. One of them is the recirculation motor/actuator fails when a plastic gear strips and prevents movement of the lever arm. This actuator normally switches the A/C system between using outside/inside air. Typically it's fine to always use outside air, but during the dead of winter, you need to recirculate the air to warm the cabin and prevent fogging.
My local Saab dealer wanted $440 to replace this motor with a new one! This is one expensive plastic gear, and no, there isn’t a lot of labor involved (it took me about a half hour to replace the motor, not including gear-repair time). I figured this was a great opportunity to solve a common problem and save some real money.
I experimented with a few different methods. I'll explain the successful method here, which took about a half hour to do, and consumed maybe $0.50 worth of supplies (clay and plastic).
Summary:
- Imprint a mold of good teeth.
- Use the clay mold, along with original gear, as a 2-part mold to create the tooth.
- Use Smooth-Cast polyurethane for the cast.
- CAREFULLY trim excess polyurethane from gear.
- Super glue the new tooth onto the original gear.
Part 1: Create mold of good teeth
You do not need to make a mold of the whole gear. You need to create an imprint of a small fraction of the teeth on the gear. This will enable you to simply slide the gear in and out of the clay, because the gear and clay, together, will be the mold for the new tooth.
Modeling clay works very well, since it is more durable, and less likely to break apart than Play-doh. Feel free to expirement. Bake-able modeling clay would probably work even better, as long as there is minimal shrinking of the mold as you bake it. Then you don’t have to worry about disturbing the mold when casting the new tooth.
Part 2: Cast new tooth
I created the new tooth using “Smooth Cast 300”, which is a 2-part polyurethane plastic kit from Smooth-On: http://www.smooth-on.com/Urethane-Plastic-a/c5_1120_1209/index.html
This kit is easy to work with, and lasts a long time in the bottles. It's been over 2 years since I used these bottles, and they're still good. Stronger plastics are available on their website, but this is the kind I had.
To cast the tooth, mix a small amount of "Part A" and "Part B" together. I used about 5 drops from each bottle. Drip the mixture near the clay mold, and slide the original gear section with the missing tooth, into the clay mold. The resin will be forced into the void for the new tooth.
Wait for the plastic to cure, this kit took about 15minutes.
Part 3: Trim/Superglue new tooth
First of all, it is unlikely that the polyurethane gear you created will unlikely adhere to your original gear. Keep this in mind so you’re careful while trimming excess plastic from the gear.
At some point, the new tooth will start to flake off the original gear. At this time, apply the superglue between the new tooth and old gear, and wait for it to cure.
Trim more plastic with a razor as needed, until it properly mates with the other gear(s). Then test it :)