Originally Posted by old's'cool

iab, I'm not sure if you're familiar with the concept of "fatigue limit". It is complicated to describe so rather than attempt it here I suggest those unfamiliar look it up to their satisfaction.

In a nutshell, some materials, e.g. steel have a fatigue limit that allows them to be designed into an application such that they have infinite life, as long as the design is manufactured, assembled, and used according to the design specificiation.

By the same token, with an educated guess as to the realistic maximum number of cycles a component may be subjected to in its expected lifetime, a component can be engineered to have a fatigue limit that is close enough to infinite that nobody will know the difference, unless something goes wrong like the designer screwed up or it wasn't applied according to the design specification. I suspect garage door springs and bicycle spokes fall into this category.