Bearing

[Delwin D Fandrich]

Oleg Isaac wrote:

> Newton, Frans,
>
> I appreciate a lot all of your mail.
>
> Regarding bearing, I wonder if there is a relatively accurate method to
> mesure the crown of the sounboard ?

If you're more limber than I am you can crawl under the piano and stretch a line (a string, or a thread) from one edge of the
soundboard to the other between ribs. With the line held tight, the amount of crown, if any, is shown by the gap between the
line and the surface of the soundboard. Repeat this between each pair of ribs that you can reach. Comparing all of these
measurements will give you a rough idea of how much crown there is in the soundboard assembly and what shape it is in.

With the plate removed from the piano there is a much easier way. Set a straightedge across the rim of the piano
perpendicular to the grain of the soundboard. Measure from the bottom of the straightedge to the top of the soundboard at
three points: at each edge of the soundboard and right beside the bridge in the areas you'd like to check. You can calculate
the actual soundboard crown from these measurements.

I have built a special tool using two identical "feet" that are movable along a straightedge so that I can set one on each
edge of the soundboard. There is a third adjustable rod that is extended down to the top of the soundboard next to the
bridge. When the length of this third rod is measured it gives us the distance from the top of the soundboard to the bottom
of the straightedge. This also gives us an indication of soundboard crown. (If this explanation is confusing to you, send me
your mailing address and I'll send you a sketch of the tool.)

Soundboard crown is not the same as downbearing. Crown is a measurement of the curvature in the soundboard assembly (if any).
Usually, this is indicated as a radius measurement. That is we might say that a certain soundboard had a crown radius of 18 M
(or approximately 60 feet). Unfortunately, understanding soundboard crown is never quite that simple because the loads on the
soundboard are not uniform. This is compounded by the fact that soundboard crown changes with time and with the environment
in which the piano resides.



> I've learned to put a typical 3 mm in the bass , going to 1.2 mm in the
> extreme treble on the pianos whe where re-stringing 18 years ago.
>
> The two last stringing job i've done (last year in fact, I computed the
> bearing to have a 2° angle in the center, going to 1,5) at the extremes.
>
> Is an excessive downbearing in the extreme bass and treble (without a
> suspended bridge for the bass) susceptible of blocking a bit the ability
> of the sounboard to amplify nicely the sound.

Soundboard are not amplifiers -- but that's another story...


> I have seen some grand pianos with a full 3 to 5 mm bearing on the
> treble bridge STRUNG , and a harsh , non-singing sound in this section.
> How can the sounboard get this big crown in the treble (there was) ,
> only humidity ?

Just because there is 3 to 5 mm of string bearing on the treble strings does not mean that there is that much crown. In fact,
it is highly unlikely that there could be that much actual "crown" in the board at that point without the soundboard breaking
away from the rim and/or belly rail.Now we're talking about downbearing. And this is not simple either. Usually it is given
as the distance between a straight line extending from the front termination point (whether that point is an agraffe or a
V-bar), across the top of the bridge (just touching it) and the back bearing bar. If that distance is 3 mm, we usually say
that we have 3 mm of downbearing. Unfortunately, this doesn't tell us much. So, a more accurate method has been developed
using the string deflection angle to determine the actual string load on the bridge. This is best done with the piano strung
and at pitch. A "bubble gage" is placed on the speaking length of the string and set to "zero." It is then moved to the back
scale and checked for deflection angle. This can then be translated into string downforce against the bridge.

The most convenient gage to use is the one made by Baldwin. It is calibrated in units that indicate 17 minutes of arc. Thus
each calibration mark that the bubble moves indicates a string load of 0.5% of the string tension against the bridge.

Another, much more readily available, gage is the one made by Tom Lowell. You'll have to check with others on the list for
the correct method of reading this gage.



> Something comes to my mind just now . I wander if the CAPO BAR on
> BOSENDORFER, is regulation-nable, (strung) beacause of the many screws
> wich seems to hold it ?

I don't know. You'll have to check with someone having much more experience with Bosendorfer's than I have.



> For the matching of the crown and downbearing, I always had put the
> bearing approximately, try not to stress too much the sounboard, (of
> corse if collapsed, or if too much crown) I heat the table a bit before
> any stringing , trying to stabilize it.
>
> Back to the first question. Is there a method to match
> crown/downbearing. ?

New soundboards and old soundboards are treated differently. I'm assuming that you're interested in older boards. A good rule
of thumb is to set the string bearing so that it forces the soundboard down by a distance of approximately 1/2 of the
soundboard crown distance (as measured above). In other words, if you measured 3 mm of crown at a certain point along the
bridge, you would set string bearing so that it would press the soundboard down approximately 1.5 mm when the piano is strung
and at pitch.


> If 3mm to 2,5 mm on the bass bridge, and 2,5 mm to 1,5 mm mm on the
> large bridge, are not the bass strings taking too much pressure off the
> middle strings when strung ? (causing a lack of power in the middle? )

These numbers are arbitrary. The actual amount of string bearing you set should be determined by the age and condition of the
soundboard, the length of the back scale, and the point along the scale. And, yes, too much string bearing can easily cause a
lack of power anywhere along the scale. Not just in the middle.

> Best regards, I hope all my questions are not too much...
>
> Isaac

If the above has just caused more confusion, please accept my apology. I have a fairly lengthy technical specification that
we supply to our soundboard and rib customers. If you would like a copy, I can attempt to sent it to you privately via
E.mail. It's a bit long to post to pianotech -- I'd probably just be told to shut up and stop wasting everyone's hard drive
space. Let me know if you want it sent to you.

Good luck,

-- ddf