Seating/false beats

[Ron Nossaman]

Hi Richard,

I got it this time, a very intriguing point. Allow me, with your indulgence, to mess with it a little.


The deepening of the string grooves over time looks like a dead on explanation for the pear shaped track observed on the bridge pin. It also fits cleanly into my proposed humidity cycle model without serious conflict. I've interspersed comments in the tex
t for (hopefully) clarity. Here goes.








I can't tell if it's a swipe, or individual indentations. The hammer marks would be so many thousand to the inch, I'm not sure it's distinguishable. Ever see silver work that was so finely planished that it looked buffed? It's possible.

Now I've got to go out and measure the track height against the string diameter and the depth of the groove in the bridge top and see if it all fits. More later (if I don't hurt myself).

Ron Nossaman





At 09:22 AM 4/24/97 +0100, you wrote:
>Ron and list,
>
>Thanks for the reply. I'm glad you had a close look at some pins. This
>is the type of applied theory I like to see. I've often said that a good
>thinker is one who can figure out why nine out of ten of his bright
>ideas won't work before he even tries them.



If as many as one in ten of MY bright ideas worked, I'd be making a lot more money. I've always said "don't design in hardware", means the same thing.



>In my first post I stated that the strings don't move on the pin. Let me
>clarify. Certainly they move, after all there's a whole lota' shaking
>goin' on when the piano is played, but I don't believe there is enough
>friction between the string and pin for the string to hang up on the
>pin, hence my original statement. The string will always return to its
>point of least deflection quickly.
>
>The marks on the pin do indeed tell us that the string is moving on the
>pin somehow. I think these marks are an indication of the changing of
>the string's point of least deflection as the bridge becomes grooved.
>The string starts at the high end of the mark when the bridge is
>fresh/ungrooved. It won't stay there long though because this is maximum
>deflection where the most/quickest grooving happens. As the grooving
>progresses over time the string will find its point of least deflection
>lower and lower on the pin. The string then moves down the pin forming
>the mark. The travel of the string and formation of the mark on the
>string happens more quickly at first, then more slowly as the the
>deflection becomes less and less. This is why the mark is wider at the
>bottom. The string has spent more time at the bottom as it nears zero
>deflection.
>


This looks to me to be an excellent explanation for the observed pear shape of the track. With high humidity cycles the bridge pushing the string up the pin crushes wood, deepens the grooves in the cap, and scrubs metal off the bridge pin, widening the sk
id track. With low humidity, the string follows the bridge back down (generally, more later). Less damage is done on the dry cycle because the string isn't being forced against tension and bearing. The damage to the bridge pin in the low RH cycle is at th
e bottom of the track, because the bridge cap groove (early in the piano's life) has been crushed deeper by the last expansion cycle and the string pounds the ever widening skid track farther down (during play) until it seats again on the cap.

When a piano is new, the string and bridge pin are tangent to one another and the contact point between the two is minimal. As the pin wears  The friction between the pin and string increases as the track wears wider and the contact area increases. As the
 piano ages, the possibility of strings hanging up bridge pins should ingrease accordingly. In some pianos, this will happen. Others never seem to reach the critical threshhold. Some pianos don't have enough false beats to be a real problem, some do. I th
ink we can pin it on the tension/sidebearing/downbearing ratios designed into individual instruments.

Late in a piano's life, as the cumulative effect of wood crush and wear loosens bridge pins, it's entirely possible that the expand cycle would pull the pins out of the bridge slightly. Tapping the string here may be moving the pin down too. I'd think it
would have to be pretty loose to respond to the gentle taps indicated here.




>Would you agree that the mark appears to be a swipe mark and not
>multiple individual indentations? If so, that would confirm both the
>lack of friction to hold the string up and the non-linear decent of the
>string over time.
>



The pin scrubbing up and down with the repeated humidity cycles would erase the signs of any past high or low travel marks (as the bridge groove deepened) and blend together to look like a continuous swipe. Our outlooks aren't mutually exclusive here. The
y seem to mesh logically. I think we're both right here.




>Gotta' go, I'm off to Wisconsin Days!
>
>Richard Anderson
>
>


Well, what do you all think? I've got some measurements that fit the model pretty well and if I can figure out how to organize the logic without four pages of text, I'll post it. Another picture would probably be the way to go. I'll work on it.

 Ron Nossaman