What at about a capo surface inset at the notch. Something round like a bridge pin but on it's side...possibly the vertical bridge pin and horizontal pin would buzz David I. ----- Original message ---------------------------------------- From: Phillip Ford <fordpiano@earthlink.net> To: Pianotech <pianotech@ptg.org> Received: Mon, 18 Apr 2005 08:52:00 -0700 Subject: Re: Seating strings >> >>>I think somewhere I have a sample of the Hydulignum made by this >>>company. Maybe I could whack a piece off for you to experiment >>>with, assuming I have anything that will cut it (which isn't a >>>trivial assumption - I think I got this sample when visiting a >>>waterjet cutting operation - I think the point of it was something >>>like, we can even cut this stuff). >> >>Spitfire propeller blades! I'd love a sample if the hacksaw survives. >OK. I'll give it a shot, assuming I can locate the sample. If the >hacksaw doesn't work I'll fire up the laser cutter. >> >> >>>I wouldn't think that maple would absorb the epoxy well since it's >>>not porous. >> >>This is 0.6mm sliced veneer. By the time a blade is pushed through a >>saturated flitch to shave it off, there's not much structural >>integrity left cross grain. The epoxy presses clear through the >>stuff in the clamps. >I see. >> >> >>> >>>If you're essentially making a cap out of epoxy, why not make it a >>>carbon fiber composite, rather than a wood fiber composite? It >>>seems that it would be stronger and harder. >> >>Functionally, there's no reason not to other than notching >>difficulty (and maybe carbon fiber dust). Psychologically, might be >>another problem. The appearance of normalcy carries more weight than >>it probably should. Eventually... >Yes, I see your point. >> >> >>>While we're on that subject, I don't know how much of the >>>resistance to rendering comes from the string friction on the >>>bridge cap. I think the resistance of the cap to down bearing (and >>>to humidity movement) takes place mostly near the bridge pins. So >>>the area of the cap between the pins is really doing nothing but >>>increasing the resistance to rendering. Perhaps the cap should >>>have a relief between the pins. >> >>Given the same friction coefficient between the string and cap for >>all forces, the down vector force alone, 5.4lb for each pin, sort of >>overpowers the 2.7lb from downbearing - and it's at the pin, not in >>the middle. Where does the curved bridge top to take downbearing >>force off of the notch edge fit in here? >It wouldn't be helping things in that regard. You can't have everything. >> >> >> > OK. But the crushing seems to be more severe at the notch edge. Because >>>of the path that the string is taking it has a fulcrum point, if >>>you will, at the notch edge (which I believe Ric mentioned). So, >>>the bridge is trying to lift the string up at the edges of the two >>>notches and they're being crushed. >> >>Look at your own PSI figures and decide how much of the damage is >>done by downbearing and how much by pin friction and bridge >>dimensional changes. Then look at the photo I posted, and tell me >>how a string on that bridge crushed the notch edge at that angle >>from downbearing. I don't believe that piano ever had a 20° front >>bearing angle. In the world of science, how could downbearing alone >>EVER crush a notch edge past the point where the string can touch >>it????? I don't see this as being geometrically possible. >I agree that downbearing couldn't crush the notch edge past the point >where the string is making contact with it. I also agree that the >damage is (mostly) not being done by downbearing. It's being done as >the cap moves up and the string doesn't. Let's remove the pins and >the notch. The bridge is now a square block of wood. Run a stiff >string over it so that it's coming off the square block at a 1.5 >degree angle on both sides. What are you going to see? The string >will be contacting the block at the edges and up in the air in >between. The bearing stress at those contact points is going to be >very high. If you try to lift the block up it will concentrate even >more load right at the edges of the block. If you curve the top of >the block so that the string is coming off tangent to that curve on >both sides of the block and is in contact over the whole upper >surface of the block the bearing stress is going to be a lot lower. >Try to raise it up a little bit and the stress wouldn't go up much. >This was my thinking. Now put the pins back in and things get more >complicated. The pin is providing a concentrated down force, because >of the friction, against the string's upward movement. Even if the >top of the bridge was curved the string may get crushed down into the >cap in the vicinity of the pin, as you say. I'm inclined to think >that moving the notch back away from the pin might make this crushing >a little less. >> >> >>>I'm not sure that I agree. Because of the present geometry, as the >>>bridge moves up, it is essentially lifting the string up at two >>>points - the notch edges. If the top of the bridge described the >>>curve that the string naturally wanted to take, as the bridge tried >>>to lift the string up, it would be lifting it along the entire >>>length of contact with the bridge, which would lower the bearing >>>stress considerably and perhaps prevent indenting of the cap. >>> >>>Phil F >> >>The friction is still there, and the PSI load is still there. The >>cap would still crush at the edges as the bridge cap expanded, and >>the string would lose contact with the notch edge in dry seasons >>even sooner than with a flat cap. >> >>Ron N >I'll agree with that. >Phil F >>_______________________________________________ >>pianotech list info: https://www.moypiano.com/resources/#archives >_______________________________________________ >pianotech list info: https://www.moypiano.com/resources/#archives
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