>>And if a string is found above the bridge cap I'd like some assurance >>that it hasn't been like that all along, but was in fact seated on the >>cap at some time and with the passage of time levitated. > >Circumstantial evidence is probably the best you'll do here, depending on >whether the monitored intentionally levitated string stays up the pins, >and if it is found naturally occurring in the wild. The first step is to actually find one occurring in the wild. With a few people armed with feeler gages we should quickly be able to build up a large number of checked pianos. The more pianos that are checked that have no strings levitated above the bridge cap, the harder it's going to be to believe that this happens. If we do find some (and the strings are checked to see that there is positive downbearing) then I'll have to devise Plan B for determining if they can levitate. > > 1. Isolating the string's resistance to a rising bridge cap to attempt to >>demonstrate that this mechanism does in fact exist and can damage a cap >>of traditional materials. > >That shouldn't be too tough. > > >>2. Isolating the downbearing. I think we both believe that this load by >>itself is not enough to damage a hard maple cap. > >Not exactly. It's enough to damage the cap, but not nearly to the extent >and angular deformation of notch edges we see in older bridges. If it is, then the bearing areas I'm assuming are too large or there's something else going on that I don't understand, perhaps involving the bridge pins. Using the bearing area that I used in the early days of this thread (.005 Sq. In.) for getting a feel for whether the string could indent a rising bridge cap, the bearing stress from downbearing is far below the allowable of the maple. >>But I wonder if there's some mechanism involving the notch edge and >>angled pin that would cause the downbearing to indent the cap. Or if I'm >>assuming too large a bearing area and that the effective bearing area >>resisting downbearing is small enough that the cap would be indented. A >>simple experiment might demonstrate whether my belief is true or false. > >Not that I fell asleep and broke a string pulling it up to pitch, or >anything like that, but I have had occasion to look at a bridge cap that >has been under bearing and tension for mere moments before de-stringing. >The (quartersawn maple) cap was visibly dented, more at the speaking side >where the front bearing was slightly higher. But that isn't entirely >downbearing. The clamping effect of the pin and offset angles was there >too. Bearing alone would have to be without bridge pins. Without bridge pins would be instructive I think. But more importantly without the side bearing and the clamping effect at the pin that goes with it. A few posts ago when I originally proposed these tests I had suggested having the string pass through angled pins, but have them situated such that there was no side to side offset. I was concerned that perhaps the pins were forcing the string down against the bridge in some way that I don't really understand and can't therefore describe, but which might be increasing bearing stress at the notch edge. Anyway, it might be instructive to do it both ways, with and without bridge pins, just to see if there's any difference, especially since it wouldn't be hard to do. Another comment here - I originally proposed doing this and humidity cycling it. As I think about it I don't see why. Humidity cycling shouldn't really have any significant effect on what's happening due to downbearing. If the bridge height increases, and the soundboard underneath it rises, the downbearing shouldn't change significantly. It probably would be easier and just as instructive to choose a downbearing angle at the high end of the range and forget the humidity cycling for the downbearing part of the test. Also you mentioned the clamping effect from the angled bridge pins and the sidebearing. I hadn't really been thinking about that. But because of the pinching effect at the pins, the sidebearing is also going to be pushing the string down into the cap at the pin. I don't know how significant that load is. Time to run another number. Also, I'll note again here, that I don't think we'll ever get to the point where there's zero cap indentation. Because the contact between a cylinder (the string) and a plane (the bridge cap) is a line, the theoretical contact area, or bearing area, is zero, and the bearing stress is infinite. With real world strings and caps I think there's going to be some tiny bearing area with very high stress. The cap will indent slightly until the contact area is great enough to lower the stress down below the bearing stress allowable of the cap material. So, even for a very hard cap material such as you're making, you might still see a little indentation. What I don't know is what an acceptable upper limit is. What would be the deepest indentation at which we wouldn't hear string anomalies from the string not being perfectly seated? >..... >>So, perhaps I'll descope this. I would be happy to accomplish numbers 1 >>and 2 strictly in a static situation. I think this would be educational >>and I don't think it would take that long to do, if you have some way of >>accomplishing humidity cycling.... >I'd want one additional measurement of pin height above the cap surface >through the fluctuations. >... >Ron N Why? Phil F
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