Downbearing

[David Skolnik]

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.

Regards

David Skolnik


At 12:02 AM 03/30/2002 +0000, you wrote:
>Hi!
>
>I'm attending at a instrumentmaking school in Sweden as a
>pianotech (three year course). As you probably all have
>noticed the downbearing seem to get lower/totally dissapear
>when lowering the string tension/taking away the strings
>completly.
>
>How come? At first I thought it all had something to do
>with when you dry the soundboard to open the cracks when
>repairing it. When we took apart a 185 grand at the school
>we measured the downbearing all the way of the piano and
>noticed some positive downbearing all the way through the
>piano (+1mm at least). When we had lowered the tension of
>the middle strings and started to lower the tension of the
>left strings in the middle the metal wedges between the
>cast iron and the 'front' of the soundboard (just where the
>list wich the damperwires go through) fell off. That
>indicated that the cast iron actually is compressed alot
>under the string tension. I got surprised, even though I
>can understand that is compressed somewhat under that
>enormous tension the strings have.
>
>Then I formed a second theory about the dissapearing
>downbearing. Could it be that the strings actually
>compresses the cast iron and the cast iron compresses the
>whole soundboard giving it a part of its crown in that way?
>I cant think of any other way, but I'd like to hear what
>you all have to say about this since I got little
>experience in this area. Do you always measure the crown
>before and after the strings are taken away to make a more
>accurate calculation of the downbearing when rebuilding
>instruments? Any tips, theories etc are welcome. I'll be
>back on thuesday to reply to any comments to this post.
>
>Note that the downbearing was measured just before and
>after the strings were taken away on that grand mentioned
>above and the difference was very big. In some parts we had
>about 2mm positive downbearing before removing the strings.
>After we removed the strings we had 1-2mm negative
>downbearing in some places.
>
>Thanks,
>Daniel Lindholm / Sweden