Picking Out Roller Chain

[Raul]

OK Matt, I'll bite! We have been using #25 chain for many years on our drive system. Ask anyone if we ever broke down because a chain broke. Last year we even used #25 chain for our wings and that never broke even though I thought I had designed it "on the edge."

Here is my real point: Noteworthy engineering is about making the proper trade offs in an intelligent way to maximize the efficiency of your design. To flat out say that one should never use #25 chain on your final drive is not appropriate. It is no different than saying that you should only use steel for your frame because aluminum bends too easily.
If you are careful to design your system such that it will not see significant dynamic loads, you can use #25 chain and not ever see a problem. Without getting into too many details, that loading depends on your gear ratio, coefficient of friction and a few other things.
Besides, in some cases, you prefer to have this "mechanical fuse" as your controled breakage point. As an example, depending on your design, you may prefer to fix a broken (or stretched) chain rather than a transtorque coupler or a keyway.

To answer Adam's original question about what happens if you run chain above it max working load but below it break load - it stretches gradually.

Raul