scanlime029 – Bad Gear

A winch gear split, taking a closer look and failing to repair it. How would you fix this nylon helical gear?

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Mmm, crispy.

I hate power electronics.

This was my first attempt at fixing a broken Cuisinart cordless kettle.

The original symptoms: I notice the kettle occasionally turns itself off. If I turn it back on, it works again for a little while. This kept up for a day or so, then it refused to turn on at all. I open it up, and one pole of the dual-throw relay (really, a combination on/off switch and thermal switch) had melted its plastic supports. The contacts no longer touched, so the kettle got no power.

The first fix: That pole of the relay was shot, but luckily there was another identical mechanism on the other pole. I could replace the broken pole with some other type of safety cutoff device, and it would be good as new. Ideally I’d use a thermal fuse for this purpose. The local Fry’s didn’t have any, so I used two 7 amp pico-fuses in parallel. Not ideal, but better than nothing.

So.. my fuses did their job, and my new wiring was fine. When I measured the resistances during reassembly, everything seemed fine. The first few pots of water after this fix are uneventful. Then I notice the bottom of the kettle (just under where my new wiring was installed) started getting really hot. Like, 300 degrees. This was starting to melt the plastic a bit. I open it up, and my wiring harness is what you see above.

So, what happened? I failed to fix the reason why the relay failed in the first place. I was assuming that the relay just used shoddy plastic, and it overheated because it was so close to the heating element. I assumed it was a design flaw. But, the other (presumably identical) relay pole was totally fine. It turns out that the original failure was a loose crimp in the joint that appears rightmost in the photo. The crimp wasn’t quite tight, so the wire rattled around in there a little. This added about 1 ohm of resistance when it was in just the wrong position. A manufacturing flaw.

This kettle nominally runs at about 12 amps. At 12 A and 120 V, a 1 ohm resistor will dissipate 144 watts. Yow.

So, I just replaced that entire section of the wiring harness with two new fuses and two brand new crimps. First pot of water was a success, and I rewarded myself with some irish coffee. We’ll see how well it holds up. I might find myself ordering some slightly better quality crimps and some actual thermal fuses if this fix also has problems.

(Buy a new kettle? Nevar!)