Ron commenting on Phils reply to Rics Post: My comments in red >/ There is also downbearing. If the bridge cap is />/ going to move up relative to the pin, and the string is going to go with />/ it, then these forces have to be overcome. By my calculation in a previous />/ post the stress induced in the cap was 4080 PSI. This is considerably />/ higher than the 1500 PSI allowable. / And is considerably higher than downbearing induced loads, which would make this the primary mechanism for crushed notch edges. That, and the fact that the angle of the crush track at the notch edge far exceeds any downbearing angle. --It is also based on the erroneous assumption that sidebearing and hence friction between pin and string increases as the bridge takes on humidity. Since this is not the case, the numbers need to be re-run. Downbearing will increase, sidebearing will decrease and so its effect on said friction. >/ Also, you bring up another good point which I don't think I've seen />/ mentioned. As the bridge moves relative to the pin, the pin hole is />/ probably not going to want to stay parallel to the pin. So, the bridge is />/ probably trying to tweak the pin, as you say. This may put considerable />/ stress on the edge of the hole at the cap surface, which would tend to />/ enlarge the hole, which would tend to make the pin loose, which would tend />/ to exacerbate pin flagpoling, which some think is the source of false />/ beats. The more you look at it, the more this pin termination seems to be />/ a bad idea. / Friction going up also will put more side bearing stress on the pin by leverage from the pin angle, crushing wood in the hole behind the pin. --The Pin is just going to follow the line that the hole defines. As the bridge expands this line will tend towards more perpendicular to the surface of the bridge, The only possible reaction the pin can have to this change in angle is to simply comply. No bending, no resistance at the surface of the bridge. The pin might have to slide upwards abit to accomadate the newly defined bottom of the hole, but thats about it. So where is all this extra friction coming from. The reduction of pin angle can only serve to reduce friction between pin and string. There is no crushing of the wood in the hole behind the pin in this scenario at all. >/ I don't want to put words in his mouth, but I believe />/ that Ron is saying that strings don't climb pins - period. The notch edge />/ is receding from the string (at least in dry weather) because it has been />/ crushed. This gives the appearance that the string has climbed the pin />/ because you can tap it down at the pin into the crushed portion of the />/ notch. But the string hasn't lifted itself completely clear of the cap />/ across the entire width of the bridge. / I am. --I think most of us know what Ron is saying, and he is wrong. That is to say it is wrong to say this is the one and only, or even primary reason for the strings finding themselves detached from any portion of the bridge. Indeed, the condition Ron describes can occur. But there are other conditions that can occur simultanously with strings not fully in contact with the bridge. >>/Pictures... grin... yes this would do a lot of good. I'd get accused />>/of faking them by those who have already decided whats going on. />/ />/ />/ Perhaps. But it might convince others on the fence, or it might convince />/ the skeptics to get out their feeler gages and see for themselves. / Pictures are much easier to fake than math and logic, and so are going to be of no use. --I would say its a toss up there myself. Something this supposedly common and obvious should be reproducible. --agreed. If strings get up and stay up pins against positive downbearing, offset angle, and pin angle while people are playing the piano continually until the tuner can come and tap them back down, it ought to be dead easy to pull them up the pin and make them stay there any time you like. --you can I don't see how they could be kept down. The feeler gage can prove it happened, and playing the thing for a while and trying the gage again will prove that strings stay up pins. I'll most definitely attend the convention class where that is demonstrated. --I would greatly enjoy seeing that as well. >>/One final note. />>/ />>/The whole line of reasoning Ron N lays out lives and dies upon the />>/existance of />>/negative bearing when the string is off the cap. Alls one has to do is find a />>/case of strings off the surface of the bridge while at the same time />>/finding plenty />>/of positive bearing. />/ />/ />/ I believe he did say that he doesn't believe any piano that has positive />/ bearing could have strings off the bridge. He also seemed receptive to />/ being proved wrong. Anyone with a feeler gage (and a downbearing gage, I />/ might add) can do so. No one has spoken up yet. / As I continue to point out, even under positive bearing, the notch edge will still be below the string after it is sufficiently crushed by cyclic bridge movement. --no one has ever contested this. What is contested is that this and negative bearing are the only conditions which can see the string up the pins and away from the bridge. Clearly many many many technicians have found unmarked bridges on brand new instruments with plenty of downbearing whilst still finding strings in need of being tapped down. And just as clearly many many many technicians can find this exact same situation in pianos that have been used for a few years. >/ I also don't think he said that string seating was useless. I think he />/ said that it was temporary. / I said it was temporary, and I said it didn't fix the problem, because the resulting tonal problems are almost entirely from loose bridge pins.. -- I also am aware of Rons position here, and disagree here as well. Number one the only tonal problems that can be directly associated with loose pins is one particular kind of false beat. There are other improvements in tonal clarity that are affected by tapping. Number two, the type of false beat we do associate with loose pins is not fully understood either. One simply observes that CA clue, or an applied screwdriver can lessen the beat. What the exact cause of the beat is we dont know. Not that this really matters given the first point. >/ 3. To investigate the effects of string vibration alone is a little />/ trickier. Perhaps the setup in number 2 but with no downbearing. Subject />/ this to string vibration. One potential problem here - is the />/ arrangement of two bridge pins having typical angles, but no side to side />/ offset, clamping the string down in a realistic enough way for this test to />/ be meaningful. Thoughts on this? />/ />/ Phil Ford / It isn't conclusive, since I can't know the piano's entire service history, but de-stringing a bridge, I typically see more pin and notch damage on the speaking side. I have no way to determine whether this is from play, front bearing angles, or seating of strings. Ron N --As I have said time and time again. Of course notch damage can develope over time to fit the bill Ron draws out. And just of course is the likelihood that assuring good string contact with the bridge is a near impossible task once this developes. But you simply can not turn that around and state that the only reason strings ever find themselves off some portion or all of the bridge cap is because of this kind of damage. No amount of manipulating numbers and numbers perspectives to attempt to show this is the only thing possible can change what is clearly observable. Namely that positive bearing over the entire surface of the bridge surface can exist at the same time as strings are in need of seating. Cheers RicB
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