The Cogs & Their Function on a Bicycle

Video Transcript

Charles McMahon for Expert Village. We’re talking about the bicycle. If you want to get more information you can look at my college level textbook on merionmedia.com. We’re still talking about wear resistant materials. Now at this time we are going to talk about the cogwheels, you can see there are a whole bunch of them in the back, because this bike has 12 different gears so that you have actually 14 here, seven cogwheels back here. Now obviously the chain is rubbing against the teeth on the cogwheels. So the teeth have to be hardened for wear resistance. However, you do not want to have the whole cogwheel made of hardened steel, because when the steel was really hard it tends to be brittle, so you don’t want to snapping these things off, so you don’t want to harden the whole cogwheel, but you do want the surface to be hardened that where the wear would occur. So what they will do is carburize these cogwheels, in other words they take one of these cogwheels and you put in a furnace and you let, say methane into the furnace, a mixture of methane and hydrogen, the surface of the cogwheels picks up extra carbon and then you quench that and temper it just like a normal hardened but the inside is low carbon and therefore it is tougher, so you want a tough low carbon that means softish core, but on the outside you want the surface to be hardened and that is why you carburize it and this is a common way of having a very hard wear resistant surface on a tough material, the tough material is a material that resists cracking. So you want to resist the possibility of one of these little points cracking off for whatever reason.

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