Repairing A Gear With A Candle (and Some Epoxy)

You have a broken gear you need to fix, but there’s no equivalent part available. That’s the issue [Well Done Tips] faced with a plastic gear from a lawnmower. While we’d be tempted to scan the gear, repair the damage in CAD and then 3D print a new one, we enjoyed hearing about his low-tech solution. In addition to the write up, there’s a video showing the process you can watch below.

The idea is pretty simple. Using a piece of pipe and melted candle wax, he prepared a mold of an undamaged section of the gear. Then he cast epoxy resin in place to recreate the missing pieces. There are a few tricks, like putting holes in the remaining part of the gear so the epoxy flows into the existing part. Depending on the gear’s purpose and original material, you might be able to just use it as-is. However, you could also use the repaired gear as a template to create another mold and then cast an entire gear from resin or even metal if you can cast metal.

You can argue whether resin is better or worse than PLA, but of course, it depends on the kind of resin—photopolymers are different from epoxy resins you’d use for this sort of thing. If you think you might like to make your new gear out of aluminum, you might find some inspiration in a previous post.

19 thoughts on “Repairing A Gear With A Candle (and Some Epoxy)

    1. what they always leave out is that the reaction is really exothermic. i burned myself a little trying to use the technique. i mean it works, just be careful when you do it.

      1. And probably filler. I’ve added some dried sand for filling voids with epoxy, but for gears you would probably want to minimise size of grains acting like a dressing wheel, so cement it is.

    1. Adding glass fibers to plastic is a common practice with injection molded parts. It was not clear from the video but I don’t think this part was reinforced. Glass spheres are added to filler epoxy used on aircraft to reduce weight. Adding fiberglass or carbon fibers to the epoxy will increase strength and durability (wear resistance). I think the material design is flawed and the repaired gear will fall. Use the repaired gear to make a mold for an aluminum casting will provide for a better part.

    1. Even of it won’t hold (and i tend to agree) it’s inspirational because it shows a technique that you can easily do yourself. Perhaps he did a second try where the epoxy is allowed more material on the inside to get more grip, then would improve the chances of surviving over time.

    2. With the right epoxy the gear is more likely to fail on some of its original teeth again than the epoxy – it looks like there is more than enough surface area to bond replacement to old gear and the right epoxy blend can have far better material properties than the cheap injection moulded gear likely has…

      And even if there isn’t enough bonding area or that epoxy does fail (which is unlikely) its pretty easy to add some wire pegs, deeper keying marks, and switch to a better epoxy mix. Or simply use this style of repair to create the mould for the new casting.

  1. Whitnessed live repair of a car gearbox in a bush workshop with a very limited set of tools but resourceful, skilled people:
    – gearbox was teared apart and faulty gears isolated
    – sandy ground of the workshop was used for casting
    – gears were the same way as here rotated to imprint the missing teeth in the sand
    – molten steel was poured in the cavity
    – lots of hand filing afterward

    The whole repair took about one day and costed about 20 USD. The fixed gear was a bit noisy, but the fix held without problem the rest of the 6 weeks / 1000+ miles trip.

  2. “You can argue whether resin is better or worse than PLA”

    Ugh. PLA? Sure you can argue if you want to but where is this lawn mower? Northern Alaska?

    Put PLA in a lawn mower part and it’s going to get hot and go as soft as DJ Drumpf at an educational event.

    PLA is a great beginner material but it’s a stepping stone, like kindergarten. Once you are good with it do yourself a favor and move up.

    1. Forget PLA. Use nylon.

      A 3d printed nylon gear can often last longer than an original plastic gear. Especially since the above gear looks like it was designed to minimize the amount of plastic, giving it less than the bare minimum of strength. This failure is probably a result of cost cutting and/or planned obsolescence.

  3. This was really interesting but after watching it, it seems with some play dough or something you could just press the broken piece in, rotate it so all the teeth line up and the missing part is filled and push it down again and than just cast an entire new piece. No screwing around with the old one.

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