Wear in a Roller Chain Transmission.

[rc690]

Hello, I'm new here, so I'm not sure whether I'm breaking forum rules or etiquette by posting outside my own field, but I'm sure someone will let me know.

It's commonly argued among cyclists that it is possible to prolong sprocket and chainring life by buying a set of three chains, and rotating them from one to another regularly throughout their life. Does anyone know if there's any merit in this practice, and if so why? I can't get any explanations much beyond "everybody knows" or "stands to reason" or "reduces wear" etc. I'm sceptical, I think it's going to reduce the life of the gears if anything. Here's my argument:

Rotating three chains is roughly equivalent to using one chain that wears at a third of the rate, with a threefold increase in lifespan, so imagine what would happen if we had an everlasting chain that never wears out. If it never got any longer it would always sit snugly at the base of the teeth and so all the wear would be concentrated at the same point low on the teeth. Eventually the base of the teeth become undermined, resulting in the familiar hook pattern, and ultimately the chain gets drawn up the back of the ring because it can't disengage from the hooked teeth.

Compare this with what would happen if chains wore out very quickly: they would rapidly rise up the teeth, wearing a thin skim of material off as they go, before being replaced with a succession of others which all do the same. The eventual effect would be to distribute the wear over a larger area of the tooth face, whereby the depth of wear will be reduced, but more importantly the shape of the wear pattern will be altered so that hooking is not so pronounced, and less likely to prevent the chain from disengaging. The "hole" worn in the teeth will be shallower and wider.

My case is that rotating chains shifts the bias toward chains wearing at a slower rate relative to the chainring, thereby exaggerating hooking of the teeth and reducing chainring lifespan.

So far, the main objections I've heard are that chains don't ride higher on the teeth as they become more worn, and that wear increases as chain rides higher because the area of contact between the tooth and roller gets smaller. Firstly, the chain has to ride higher so that the ever increasing chain pitch always matches the tooth pitch, and secondly, according to Archard wear rate is not dependent on contact area.

Opinions welcome........

 

[MikeHalloran]

My take:
Cleaning the sprockets and chain regularly will extend their life, vs. just letting the road grit do them in.

Rotating three chains over the sprockets' lives should work even better if the idle chains are stored in a bucket of oil.

Since sprockets are typically case-hardened, any noticeable 'hooking' basically defines 'end of life'.

Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

What is the difference in wear? Three chains, one at a time for the life of three chains or three chains changed out at 1/3 intervals over the life of three chains. 6 of one, half a dozen of the other; or three of one, quarter of a dozen of the other.

Ted

 

[rc690]

Mike: I'd not noticed that the teeth are case hardened, that makes my argument all the more relevant, a narrow deep wear pattern will break through the case hardening sooner than a wide shallow one.

hydtools: Rotating three chains is not the same as using one at a time. For example, after the first 3000m the you're comparing a chain that's done 3000m with one of a set of three that have only done 1000m each. At any point in the process the set of three are exhibiting a third the wear of the single on average, because the same amount of wear has been shared between three chains.

My case is that slower chain wear creates a deeper narrower wear pattern in the face of the tooth, so the chain will become unable to disengage sooner.