[pianotech] Bending bars

[John Delacour]

[ was Re: [pianotech] Soundboard Analysis ]


At 11:21 -0700 24/06/2011, Albert Picknell wrote:

>Bows also don't have forty thousand pounds of tension on them.  Even 
>with nosebolts in place the forces which are of sufficient magnitude 
>to bow the plate struts upward are able also to pull up the 
>backposts with them.

Sure, but not very much at all.  20 tons is the total tension of a 
highly-strung piano.  The maximum possible tension in the area of a 
single bar is, say 4 tons, but the line of force is only about 7 mm. 
below the bottom of the bar, so the force tending to bend that bar 
would be in the order of hundreds of pounds.   But the hitch-plate 
and the wrest-plate are also contributing to oppose that force, not 
to speak of the rim and the framing.  I'll leave it to Nick Gravagne 
to quantify the forces, but suffice it to say that cast iron frames 
are not designed to bend much before they break and any good piano 
maker will not allow the bars to bow more than a very slight amount.

JD