>Hi Phil, and others.. > >Since your last reply included so much of my previous one... and it gets >like reaalllly long :)... I'll just snip out a few important points to >answer and/or get clarifications on. > >----------- >First ..... Calculating the pressure on the bridge > >In order to get a realistic picture of how the bridge gets indented from >the clamping of the pins in the face of the bridge surface wanting to >rise there are several points you are going to have to get cleared up, >strikes me as quite a bit more complicated then what has been figured so >far. A few points are : > >1: The intial compressive stress is 1500 psi. But as the wood starts to >compress its resistance to further compression is going to increase yes >? It may take 1500 psi to start the indentation, but it will take more >to deepen it. So you need to know what that gradient is and figure it in I'm not sure I've ever seen anything describing this phenomenon. Do you have a reference that would provide some compression allowables due to this? >2: I dont think discounting any lessening in the pin angle due to the >hole straightening a bit is a good idea myself. At least not until that >is first quantified adequately. >3: You need to know to what degree the pins are actually being pushed >upwards, and to what degree they will drag the strings up with them. For what reason? Because this is going to be lessening the load on the bridge cap? I think this points out that details of whatever experiment gets done have to be carefully recorded and reported, as some of these details may be variables that affect the results. What were the pin materials, were the pins polished, how much of an interference did the pins have in their holes, and were the pins bottomed in the holes. All of these may have an effect on the phenomenon that you're talking about. >4: The maximum initial pulse a string has for any blow hits the bridge >in an upwards motion. Very much of the strings energy is transfered >then. How much upwards force does this supply. It is my contention that >this will tend towards an equalization of whatever situation is at the >bridge notch. So if there is more pressure on the bridge then usual due >to the wood taking on humidity, playing the piano will tend to equalize >that by pulling up at the string. That's possible. But there's also the situation where the piano doesn't get played during a humidity increase. Perhaps that's a more severe situation. >5: I'm not so sure ignoring the downbearing is such a good idea either. >The downbearing force itself may be neglegible, but perhaps the angles >themselves are more important. I'm not sure what you mean by this. >That said, an un pinned bridge with >positive downbearing will experience indentation also... so downbearing >is in the picture me thinks. I believe that it's part of the picture. But I would like, at least at first, to attempt to isolate it. >6: The general affect of the soundboard / bridge vibrating with play on >all strings/notches. What amplitude of vibration does the soundboard >have, to what degree is it always in phase with string vibrations, what >happens to the unplayed string/notch condtion when the soundboard is put >into near maximum amplitude ? What happens when the soundboard starts to >move downwards in its first pulse, pulling the strings afterwards ? >7: How does the longitudinal vibration of the string effect all this ? >The bridge will be pulled and pushed at as well as yanked upwards and >then to a lesser degree downwards. If the string breaks the friction of >the pin in one direction, it is fairly free to move slightly in any >direction. That first upward yank will also be a forward yank. > >Some of this might sound a bit fetched.. but I doubt seriously anyone >has looked closely enough at these along with probably a lot of other >things that should nee factoring in if one is to arrive at a figure of >just how much friction must be overcome and by what component at any >given moment for the string to climb up the pin a tad.. All of this sounds OK as far as trying to get a more theoretically realistic picture of string - bridge interaction. But I'm not a full time researcher. I need something that I can accomplish with a small time investment that will still have some practical applicability. I talked about this in my post to Ron. >As I've said several times now, strings not fully in contact with the >bridge can occure in the face of positve bearing at all points on the >bridge. I don't think anyone is disputing this. The behavior that Ron is postulating is completely consistent with this statement, at least if I understand what you means by 'positive bearing at all points', which I'm not sure I do. To expand on this a bit - as I see it, the only way for the string not to be in contact with the cap at the edge of the notch, and yet to be in contact with the rest of the cap, is for a small portion of the cap to be lower, or angled down if you will, right near the edge of the notch. If this were not the case - if the cap face were perfectly flat - then for the string to not be in contact with the cap at the notch edge it would have to be up the pin slightly. This is turn would necessitate the string forming a straight light between this point and the back bridge pin, meaning that the string would have to be completely off the face of the cap, except right at the back bridge pin, and you should be able to stick a feeler gage under the string anywhere along the cap except right near the back bridge pin. I don't see how it's physically possible with a flat, undamaged cap, for a portion of the string to be off the cap but not other portions of the string. So, the cap right at the edge of the notch has to dive down a little. This could be because the planing of the cap caused a slight downward dip right near the notch edge (possible, but it doesn't seem likely) or because the cap has been crushed down at the notch edge by the string. >Your own limited experiment which you just shared served to show >at least that much. A more thorough testing of many pianos will show >that upon pulling up the strings and playing afterwards you will still >be able to get your feeler gauge under the string on some. And some of >these will measure positive bearing even at the notch. Yes, as did the piano that I checked. But pulling up a string and seeing that it stays there is not the same thing as observing that a seated string has over time levitated up off the cap. >..... >(Phil querries..) > >I'm not sure what you mean by positive bearing over the entire >surface of the bridge surface. Do you think there could be positive >bearing on some points of the bridge surface and negative on other >points? I look on downbearing as a yes/no thing. Either this string >has positive downbearing on the bridge or it doesn't. > >Phil F > > >This is central to Rons argumentation... that tho there can be overall >positive bearing, the indentation at the notch is below the string line >(from top of bridge to front termination), hence negative bearing at >only part of the bridge. In the case where this actually is the >situation, then of course any string seating will be a very temporary >affair. No argument there. > >Cheers >RicB I think we may have a semantic difference here. When I say that there is positive bearing at the front side of the bridge what I mean by that is that the string takes a downward angle away from the bridge. Even if a small portion of the cap near the notch edge is down below the string (is not in contact with the string) I would still say that there is positive downbearing on the front of the bridge if the string is taking a downward angle from the contact point on the front side of the bridge. I would say that there is a portion of the cap that is not in contact with the string, not that there is negative bearing there. Phil F
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