On 2/23/2013 2:09 PM, Jim Ialeggio wrote: > Ron wrote: > > <Sorry, no. Neither is in any danger of walking out, whatever the glue > line configuration. People envision fence posts walking out of the > ground with frost heave and yes, this certainly happens. > > > I was thinking more along different lines, walking-wise. Nails in wood > walk out, sometimes all the way out in exterior applications. It happens > all the time with natural wood siding on the east and south side of a > building, unless something like a ringed shank nail is there to restrain > the walking. Granted these situations experience temperature and > humidity cycling and presumably much more aggressively than a piano > sees. I think water soaking and freezing is a factor too. In any case, bridge pins don't. > However, seeing is believing, and I've been considering running this > relative pin movement test for a while anyway. Movement of the pin is > presumably pretty small. Can you describe your procedure...see if I can > replicate your results. It wasn't very sexy or sophisticated. I made a couple of different configurations of about 30-32mm tall bridges, one unison each, pinned with #7 x 3/4". I dried them out in a mailing tube with a Dampp-Chaser heater bar inside and towels over the ends so moisture could easily pass. I measured height until they quit shrinking and then recorded base to top of pins, base to top of cap, and base to top of root. Then I sealed them in a Tupperware with a dish of saturated table salt. This makes a pretty good controlled 70%RH. Again, I measured every few days until they quit growing and wrote down the measurements. Some subtraction gave me cap thicknesses, and pin height above the cap. I found pin height changed as much as 0.011" on about a 9mm (? memory) solid cap. Zero relative movement between the pin and cap happened at the root/cap glue line. I tried to get somewhere near the actual MC% swings we get here, and I think I got reasonably close. The experiment wasn't for exact climate numbers, but rather for expected relative dimensional changes. The cap riding a string (missing here) up and down opposing slanted pins repeatedly is, I concluded, what scrubs those pear shaped wear marks in old bridge pins and accounts for a lot of the pitch changes with humidity swings. Incidentally, those old tracks are pear shaped, wider at the bottom because with repeated cycles, the notch edge crushes and the string goes a bit farther down the pin until the notch is crushed about as far as it's going to go, where the string scrubs the track deeper because it's not going down further but just cycling in place. That is, unless some tech comes along and hammers the string down, further crushing the notch. I wasn't using the laminated veneer caps yet when I did this, but I suspect the pin height will change very little with this cap. Anyway, that's more or less it. Ron N
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