Alloy idlers worn... look like they've been sharpened!

[Monkeysmate]

I noticed one of my lads cars sounding slightly rough, but still running OK.
I noticed that the alloy idlers appear worn and look like they've been sharpened...
Can anyone tell me why this may have happened?
I've gone back to the white plastic ones for now...

Also, can anyone tell me what the black plastic idlers are made of? Are they any good? I found some in some spares...
Thank you,
Tim

[neallewis]

check the outer layshaft bearing. its likely to have worn resulting in layshaft movement and additional wear to the idler.

[Ashlandchris]

When I ran team c I was advised against the alloy idlers as this is apparently a common problem. I was told the white polyamide idler was better and didn't damage other gears.
Advice came from pbm models in Derbyshire

[Clive Loynes]

I noticed one of my lads cars sounding slightly rough, but still running OK.
I noticed that the alloy idlers appear worn and look like they've been sharpened...
Can anyone tell me why this may have happened?
I've gone back to the white plastic ones for now...

Also, can anyone tell me what the black plastic idlers are made of? Are they any good? I found some in some spares...
Thank you,
Tim

The white, POM, gears are stronger than the black ones but I don't know what the original black ones were made from.

If two involute gears are running in perfect mesh then the teeth should roll across each other. But that isn't going to happen. Also, the aluminium gears are fine when they are running on the hard anodising but once that wears, then the aluminium underneath will vanish like mist in the sun!

If you can track down a professor of tribology he would no doubt be able to talk to you on this subject for a day or two! But in a nutshell, there is wear because the teeth are sliding over each other to some extent, due to incorrect mesh. Also, when there is any form of abrasive grit to hand, and there is always something, then this tends to embed itself in the surface of the softer material and thus it becomes an abrasive "pad" rubbing against the harder material. I believe that the ancient Egyptians were able to engrave glass in this manner using a copper wheel and an abrasive slurry.

Once wear starts to take place, this provides more "grit" for this proccess, mixed in with the grease to form a nice grinding paste.

It would probably start by the original wear from the hard anodising embedding itself on the diff gear, from where it would be able to mount an attack on the first idler.

I would always try to avoid having two aluminium gears running together.

[Pitman Ed]

I tried several combinations. The best is use two white plastic gears. I've had no issues since doing that.