Down bearing

[Keith Roberts]

It would seem to me the height of the bridge would float to a position in
between the high and the low point of the bridge. This didn't account for
bridge roll. The string is obviously trying to get to a straight line from
the hitch pin to the tuning pin. If the bridge rolled forward till the
unison patch was parallel to that line, .... I must say in my limited
experience I haven't seen deterioration that bad to consider that a
possiblity.       But,   I was cutting wood before I became the Kimball
Expert.
Keith
----- Original Message -----
From: "Isaac OLEG SIMANOT" <oleg-i@wanadoo.fr>
To: <pianotech@ptg.org>
Sent: Wednesday, April 03, 2002 10:50 AM
Subject: RE: RE: Re: Down bearing


> Keith,
>
> In my understanding, when the board goes flat, if the stings are pulling
the
> bridge up, by reaction, the front of the bridge will go down (bridge roll)
.
> I hardly imagine how it can go high enough to add visible DB at the front
> pin.
>
> What I have seen is that the pressure place moves slowly from the first
> front third of the bridge when in a normal configuration, to the rear of
the
> bridge, to the point apparent DB remains only visible a t the rear pins,
> giving that particular sound without much density and clearness.
>
> Any further explanation I will appreciate.
>
> Regards.
>
> Isaac OLEG
>
> > -----Message d'origine-----
> > De : owner-pianotech@ptg.org [mailto:owner-pianotech@ptg.org]De la part
> > de Keith Roberts
> > Envoyé : mercredi 3 avril 2002 20:01
> > À : pianotech@ptg.org
> > Objet : Re: RE: Re: Down bearing
> >
> >
> > Daniel,
> > No, I don't think we misunderstood you. You said the board pulled
> > away from
> > the strings as you destrung the piano. The bridge has to be
> > thicker than the
> > distance between the string and the sound board to have
> > downbearing. In your
> > case, because of the slant(downward slope towards the hitch pin) of the
> > bridge, the rear bridge pin pulls the bridge upwards causing the front
pin
> > to rise higher than the string plane. Yes it appears normal to the
gauge.
> > Pay attention to Dale Erwins post on stringing the board from underneath
> > before and after.
> > No, the plate does not flex enough to change the crown of the
> > soundboard or
> > downbearing. Nose bolts are supposed to stop the plate from
> > flexing to much
> > and cracking. Sometimes they are needed but are never used to distort
the
> > plate to affect bearing.
> > Boards are crowned by arcing (cutting not bending) the ribs to a
> > 40 to 60 ft
> > radius or drying the board more than normal for gluing and the swelling
of
> > the board after makes the crown. Or both.
> > Check the list archives for discussions on soundboards.
> > Keith R
> > ----- Original Message -----
> > From: "Daniel Lindholm" <mailinglists@home.se>
> > To: <pianotech@ptg.org>
> > Sent: Sunday, March 31, 2002 2:40 PM
> > Subject: Re: RE: Re: Down bearing
> >
> >
> > > Hi!
> > >
> > > It seems like (because of my not-so-good english) that my
> > > total post have been misunderstood.
> > >
> > > The problem isnt to measure the downbearing. There is
> > > really not any problem at all. I'm just wondering if the
> > > stringtension can make the soundboard compress through the
> > > lenght of the piano increasing its crown when strunged.
> > >
> > > Just to give you an answer about the downbearing. We
> > > noticed about 4mm difference when the strings were up to
> > > tension and when the strings were taken away. We usually
> > > notice this kind of behavior when we remove the strings.
> > > Its not a question about IF the downbearing decreases when
> > > the strings are removed. Its rather a question about how it
> > > is possible.
> > >
> > > So, could it be that the string tension actually do
> > > compress the cast iron, pushing on the soundboard and thus
> > > increasing the crown when strung?
> > >
> > > When I get back home I'll take some photos where I can draw
> > > some arrows etc, just to make sure that there are no
> > > missunderstandings because of my way of explaining (in
> > > other words my bad english ;)
> > >
> > > Best regards,
> > > Daniel Lindholm / Sweden
> > >
> > > > David,
> > > >
> > > > When apparent downbearing is small it is very easy to be
> > > fooled by these
> > > > commercial 3 feet gauges sold. They will show you a gap
> > > behind the bridge,
> > > > where it goes in a slant, and you can take that for down
> > > bearing showing.
> > > > After that, if you use the string method You will see
> > > that the sting goes in
> > > > a straight line from one end to the other.
> > > > Is it what gives your input unclear ?
> > > > If you move the gauge at every possible place before and
> > > after the bridge,
> > > > you will have a better figure and see bridge roll, the
> > > slant on the top of
> > > > the bridge and how it is related to "measured" down
> > > bearing.
> > > >
> > > > It is way more precise to have some home made gauges of
> > > different lengths,
> > > > that can help you to ascertain the existing bridge and
> > > strings segment
> > > > geometry.
> > > > BTW if your gauge have only 2 feet  I can't see how it
> > > works
> > > > The measuring gauge with a bubble, while not so accurate
> > > as we can think,
> > > > can add some information too.
> > > >
> > > > I feel it is necessary :
> > > > - to learn to use the sound as a DB appreciation (but
> > > this seems to me a
> > > > long process to learn for a beginner (?))
> > > > - To check crown BEFORE & AFTER unstringing.
> > > > - To use the same method of measure before and after if
> > > you want to compare
> > > > things and try to deduct something.
> > > >
> > > > I remember reading some very instructive lectures on
> > > these subjects in old
> > > > (! 8 years) PUT journal issues, and these are now
> > > available on the CD ROM
> > > > reprints. at these times it helped me to understand why
> > > what different
> > > > gauges where showing was different of the real picture.
> > > >
> > > > Illustrations are needed there.
> > > >
> > > > Hope and so on .
> > > >
> > > > There is an old adage that says, the less you know about
> > > DB and crown, more
> > > > happy you live !
> > > >
> > > > Much respected by many "restorers" there.
> > > >
> > > > Another says " In the factory, it worked to put 1mm
> > > there, 2 there and 3
> > > > there, so it may work the same 40 years after that "
> > > >
> > > >
> > > > Regards
> > > >
> > > > Isaac OLEG
> > > >
> > > > Francium
> > > >
> > > >
> > > >
> > > >
> > > > > -----Message d'origine-----
> > > > > De : owner-pianotech@ptg.org [mailto:owner-
> > > pianotech@ptg.org]De la part
> > > > > de Daniel Lindholm
> > > > > Envoyé : samedi 30 mars 2002 23:11
> > > > > À : pianotech@ptg.org
> > > > > Objet : Re: Re: Downbearing
> > > > >
> > > > >
> > > > > Hi David
> > > > >
> > > > > When the strings where on we used one of those metal-
> > > > > thingys with two leveled 'legs'. One leg on the sounding
> > > > > part of the string and one on the part of the string
> > > that
> > > > > rested on the bridge. If you can wip it, it tells you
> > > that
> > > > > you got positive downbearing. If it stands firmly on the
> > > > > string, it tells you that you either have zero or
> > > negative
> > > > > downbearing.
> > > > >
> > > > > As for asking our instructor, we did. He couldnt come up
> > > > > with any better theory than we did with the cast iron
> > > and
> > > > > soundboard actually getting compressed between the cast
> > > > > iron pins and the tuning pins, increasing the positive
> > > or
> > > > > negative crown of the soundboard, altering the
> > > downbearing.
> > > > >
> > > > > I suspect that even though it has alot of negative
> > > > > downbearing right now, when the strings are off, it
> > > would
> > > > > get back to what it was (positive) before removing the
> > > > > strings, if we restrung it and tuned it.
> > > > >
> > > > > > Hello Daniel -
> > > > > >
> > > > > > At this point I would ask you how you went about
> > > taking
> > > > > downbearing
> > > > > > measurements.  The rest of your question seems to
> > > reflect
> > > > > some basic
> > > > > > confusion which might be best addressed by your
> > > > > instructors.
> > > > >
> > > > > Thanks,
> > > > > Daniel Lindholm / Sweden
> > > > >
> > > > >
> > > >
> > >
> > >
> > >
> >
> >
>
>