So how does the jump hour in the 85/8900 work?

So @Archer inevitably wear will happen even without manipulation of the hands, because of the architecture?

I would say no. One of those two wheels on the jump hour wheel has its teeth in constant contact with the pinion on the minute wheel — so essentially it functions like the “regular” hour wheel, whose teeth typically do not break. The second wheel on the jump hour wheel has detents or “catches” that get whacked by some other intermediate thing-a-ma-jig every time you put the crown in position two and use the jumping hour feature (e.g. to set the date). This whacking can cause the detents to break and the jump hour wheel won’t function properly. So if you never use position 2 to set the date, or if you only use it sparingly, you should be good.

Of course, a “regular” wheel has a wheel and pipe that are one piece. The hour hand is pressed on to the pipe and they all move together. For the jump hour wheel, each wheel moves independently of the other and each wheel must be capable of moving the hour hand. I’m not sure how this is done but I suspect that the second wheel with the detents has its own pipe, and this second wheel and its pipe are friction fitted over the “regular” wheel and its pipe - sort of like a canon pinion is friction fitted over the stem of another wheel. So this could cause some additional issues with wear, but again if you don’t hammer the second wheel in the second Crown position everything seems to run just like a regular watch and there should be no problems.



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