Bridge pin angles

[Phillip Ford]

>
> > The conclusion from the limited sample was that it was possible for the
> > string to be above the bridge, but it seemed to be for situations where
> > the bridge pin angle was low enough.
>
>Excellent! Your calculations were correct. I stand corrected.
>Strings can be made to stay above the bridge within the range of
>normal and functional pin, offset, and downbearing angles. So be it.
>Now, the obvious question. Why weren't any up the pins when you
>checked them all before starting the experiment?

Because strings don't climb pins? <G>  To state the obvious, this little 
experiment shows that for some pin configurations it's possible to get a 
string to stay up the pins, at least temporarily, were it somehow to get up 
there. It doesn't say anything about strings climbing pins.  Perhaps it's 
possible when stringing to end up with some strings above the bridge 
cap.  Checking some unprepped new pianos and freshly strung pianos should 
give some indication of this.  I'm still intending to go check some 
unprepped new pianos.  As far as freshly strung rebuilds, my shop isn't 
high volume enough to collect much data, and my next restringing isn't 
coming up soon.  Perhaps some higher volume rebuilder would take on the 
task of checking under strings with a feeler gage on freshly strung pianos 
that have been pulled up to pitch but not had the strings tapped down.  Any 
volunteers care to check and share the information with the list?

Also, this indicates that I may have to rethink what I was saying earlier 
about decreasing pin angles to attempt to lower the load from the string on 
a rising bridge cap.  I don't think you'd want to get the angles low enough 
so that the strings wouldn't tend to seat themselves on the cap should they 
somehow get above it.

>  If you haven't
>already knocked down the string that stayed up under playing, could
>you check it daily for the next several days and see if it stays up
>there? I'm curious to see how permanently it's un-seated, or if
>overnight temperature changes will overcome the friction enough to
>let it slip back down.

I was curious about the same thing, but the piano was going out right away, 
so I reseated the string.  I'll have to look for another example situation 
to monitor.

>  Since play alone knocked most of them down
>(so much for the pianist knocking them UP the pins), I wonder how
>secure the levitator is. I still hope people will take their feeler
>gages out into the field and check, particularly those pianos making
>the sort of nasty noises that earn them a good seating. I'd still
>like to see some evidence that this occurs naturally.

So would I.  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.



> >> 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
> >
> >
> > I've noticed the same thing.  If we want to try to establish which of
> > these factors is contributing to bridge or pin damage then I think we
> > need to come up with some experiments which attempt to isolate the
> > various factors.
> >
> > Phil F
>
>These could be very long running experiments.
>
>Ron N

Not as I had envisioned them, but perhaps I'm oversimplifying.  As I 
mentioned in a previous post, I had envisioned breaking this down into 
three categories.

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.

2.  Isolating the downbearing.  I think we both believe that this load by 
itself is not enough to damage a hard maple cap.  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.

3.  Calling everything else 'string vibration' and seeing if the cap could 
be indented because of it.  Based on comments on this thread I can see that 
this may be opening up a can of worms.  I didn't consider the right kind of 
vibration, I didn't properly consider the interaction of string vibration 
with the other mechanisms, etc. which resulted in less than definitive results.

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.  Make a small section of bridge, put a 
frame around it on which you can stretch a string and subject this setup to 
humidity cycling.  If you have some way of cycling I would think that you 
could quickly put on enough cycles to see cap indentation if some was going 
to occur.  As a result of this you would hopefully determine how hard the 
cap material needed to be to prevent this damage.  If you can make a cap 
that hard, then put that material on a piano and put it into service and 
see if cap indentation occurs.  If it doesn't then you're done.  If it 
does, then I suppose that you assume that indentation is occurring because 
of some sort of string vibration, or interaction of string vibration with 
the bearing forces, and your experiments have to get much more elaborate if 
you want to get a better definition of what's going on.

On the subject of humidity cycling - you mentioned cycling a bridge and 
seeing an .011 inch differential movement between cap and pin.  Did you 
just wait for humidity cycles to naturally occur or did you have some sort 
of humidity cycling setup.  If so, would you mind describing it?

Phil Ford