Killer Octave Question

[Ron Overs]

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>----- Original Message -----
>From: "Ron Overs" <<mailto:sec@overspianos.com.au>sec@overspianos.com.au>
>Subject: Re: Killer Octave Question
>
>SNIP
>
>  > Sure the long bridge will rotate backwards
>>  on its axis (from end to end) just slightly to accommodate the crown
>>  of the board, but it will be so minimal as to be insignificant.
>
>Please try and clarify the above statement. Are you defining an axis 
>going from the extreme treble end to the low tenor end of the long 
>bridge? Then are you speaking of the two ends rotating backwards to 
>meet the crowned board? I guess then one could speak of the killer 
>octave area (or thereabouts) rotating forward if the the two ends 
>are considered as reference points?

If you imagine a line which passes through each end of the long 
bridge, the bridge rotates around the axis of that line such that it 
rises in the middle to accommodate the rising centre of the sound 
board. When we started installing entire long bridges, we had it in 
mind that we would crown the underside of the bridge to fit the crown 
of the board, similar to John's suggestion. But when we built the 
first basic bridge blank, we noticed that it fitted the board's 
crowned profile almost perfectly as is. Consequently, we haven't 
bothered crowning the underside of the bridge to this day.

>
>  > The
>>  rib profile,  'stress in extreme fibre' and paring profile is where
>>  its at!
>
>I'm trying to understand this statement. Rib profile - OK, the 
>shape/curve of the rib.

Yup.

>Stress in extreme fibre - what is that?

Look up your engineering handbook. The chapter on beams as load 
bearing structures has the info. The maximum stress in extreme fibre 
is the loading within the beam which results from a given load (this 
is always at a maximum at the top and bottom of the beam, and 
virtually zero in the middle). After reading about the behaviour of 
beams, take a look at any 1900 Bl=FCthner or Bechstein board with their 
low 18 mm high sound board ribs. No surprises as to why they've sunk 
out in the middle right under the long bridge. Its all about 
excessive 'stress in extreme' fibre.

>Paring profile - totally lost on this one - what is that?

The paring (you might have a different term for it) at the ends of 
the ribs have a marked influence over the tonal outcome. Have a look 
under a few different pianos and think about what you see and how 
they sound.

By the way Terry, congratulations on your new venture into belly 
work. Looking good.

Ron O.
-- 
_______________________

OVERS PIANOS - SYDNEY
Grand Piano Manufacturers

Web: http://overspianos.com.au
mailto:info@overspianos.com.au
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