Sourcing a plastic/nylon cog

[Dil]

Hi all, hoping someone might be able to help with this. I'm trying to repair a battery powered articulated model with small electric motors and discovered two (identical) cogs have broken across where the shaft goes (probably due to mishandling when it was running). I'm probably not searching properly, but all I get are mixed bags with no details of size, number of teeth, shaft size,etc. I'll try to include a photo of one I glued (I tried glueing the other one, but it didn't work - as expected.) There are 10 teeth, across a diameter of 10mm; the teeth are 3.5mm across; there is a shoulder to one side of the teeth, 4mm across and the shaft hole is 2.5mm square. The words "horse", "flog" and "dead" spring to mind, but I thought I'd give it a shot. TIA.

 

[gcusick]

Try bearingboys.co.uk, who offer a range of plastic gears. Otherwise search online for plastic gears - throws up loads of potential sources.

 

[Jameshow]

Have you looked at Lego technics cogs, mechano cogs or scaletric ones???

 

[ChaiLatte]

How accurate are your measurements?

Metric gears are specified by Module. To find the module divide the outside diameter by (number of teeth + 2). For the measurements you give, this works out as 10/12 or 0.83, which is not a standard value.

Imperial gears are specified by Diametral Pitch. To find the DP, divide (number of teeth + 2) by diameter in inches. For the measurements you give, this works out as 12/0.393 = 30.5 DP, again not a standard value.

The gear you show will mesh with another gear, hopefully much bigger. Do the calculations for the meshing gear, measuring as accurately as you can and see what numbers come out*.

Last resort if they are non-standard is to use the number you calculate to 3D model the item for someone to 3D print.

* There are two reasons it is better to measure the bigger gear: any error will be less significant and small gears (i.e. the 10t you have) often use something called profile shifting to make them stronger, which is not possible to detect using a simple OD measurement.

 

[guineafowl21]

You_engraving on ebay is worth a try. He makes gears up to order.

 

[sploo]

If you struggle to find a replacement I could probably 3D print you some - just let me know.

 

[Fergie 307]

I would try a visit to your nearest decent model shop, somewhere that does radio controlled racing cars etc, they usuaĺly have a huge selection. Or try repairing it by slipping a suitably sized piece of metal tube over it to hold it together.

 

[TPK]

I saw a video on YouTube of someone making mold using old cog then making new cog with baking powder and superglue. Don't know the chemistry behind this but finished result was impressive. No idea how stong it would be though.

 

[GweithdyDU]

I was in a similar situation to yourself but managed to make a new plastic cog by using the good one to make mould and mixed up a 2-pack poured it in and flippin hec, it worked! This was to repair one of those working educational cut-away models of a simple internal combustion engine for one of my local schools. I repaired it about 9-yrs ago and it is still being used in the classroom today. I can't remember what the liquid plastic was called, but if I hunt around, I may be able to find the pots it came in. Just let me know if you can't find anything, but this seems similar. Polycraft SG2000 - 2kg Fast Cast Polyurethane Liquid Plastic Casting Resin Kit 5055844701809 | eBay

 

[Dil]

Many thanks to all who took the trouble to advise me. I think I'll have a go at making them, as suggested. What did you use for the mold, GweithdyDU? I was thinking plasticine? Thanks also to sploo and pops92 for the kind offers of 3D printing - I may yet take you up on that!

 

[GweithdyDU]

It gets pretty warm as it is curing but it is likely I used plasticine as my kids were living at home at the time so silicone might be better I think as long as you cover the cog you use as a former with a release agent. Thick washing-up liquid seems to work for that purpose. The modeling product called fymo might do good job as well. It is a polymer clay that doesn't need firing and can be purchased from good toy supplies or of course the internet

 

[Ttrees]

Looks sorta like guitar tuning machines, but with a differing ratio.
I wonder if you could find something off the shelf this way, using the same terminology / keywords to find the correct one.
I briefly looked at bass guitars with open gears without a possible match,
Surely there's something out there.
Good luck

 

[Insanity]

Many thanks to all who took the trouble to advise me. I think I'll have a go at making them, as suggested. What did you use for the mold, GweithdyDU? I was thinking plasticine? Thanks also to sploo and pops92 for the kind offers of 3D printing - I may yet take you up on that!

They probably need to be printed with a resin printer, so take that in mind.

 

[clogs]

GweithdyDU.
do want some of my empty tins.....? ...sounds a bit like me......lol....

found some plastic gears available from china, even weired ones...via ebay.....
quite like the idea of resin casting.....might come in handy......

 

[sploo]

They probably need to be printed with a resin printer, so take that in mind.

Most resins would likely be a bit too brittle. As long as the gear teeth aren't too small an FDM printer can produce pretty usable gears.

 

[clogs]

this got me interested........found this on utuube.....



anyone interested or tried it.....?

 

[profchris]

CA glue and baking soda is often used to fill the nut slots on guitars if they are too low. It wears well, but I suspect it might be rather brittle for a cog.

CA and sawdust works as a wood filler (but is darker than the wood), so I think almost any filler could produce a solid item. I'd suggest experimenting with different fillers (talc, maybe flour) to see what might work.

 

[pops92]

Many thanks to all who took the trouble to advise me. I think I'll have a go at making them, as suggested. What did you use for the mold, GweithdyDU? I was thinking plasticine? Thanks also to sploo and pops92 for the kind offers of 3D printing - I may yet take you up on that!

No problem ust let me know it's no trouble and good experience on FUusion 360.