Seating strings

[Phillip Ford]

>
>>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

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