MH BB Pinblock

[Nick Gravagne]

Hi All,

For what it is worth it is interesting to consider the forces acting on the
pinblock by the immense pull of the strings. Steinways and M&Hs do not come
equipped with plate bushings, thus the rearward pull of the aggregate
tension is uncompromisingly transferred via the tuning pins directly to the
pinblock wood, and thence to the plate flange for resistance. 

But that is only part of the story; we also have a significant downward
force acting on the block as well. Assuming a 5 degree lean-back of the
tuning pin and a 160 pound pull (and much higher in the bass) of a single
piano string, the axial load down the center of a single pin would be about
14 pounds (a 4 degree lean would cause an 11 pound axial force). The tuning
pins are thus charged or "energized", and the force is directed downward to
the pinblock. The 5 degree lean-back on all the pins would cause a combined
downward force of about 3000 pounds. With zero degree lean-back there is no
such direct downward force at all.

The pinblock is also acted on by a significant rotational force or moment as
well. The force to rotate the block down at the plate flange and up at the
stretcher amounts to about 1500 foot pounds (depending on where we assign
the actual point of rotation), and much of this is resisted also by the bass
and treble supports and fit of the block all around. I have not double
checked my math and trig, but it seems correct to me.

The factors acting to keep all in equilibrium are several: a firm fit of the
block to the plate flange (most critical), the bass and treble rim supports,
a firm fit to the stretcher, the strength of the plate webbing and bracing
there, and lastly, our lowly plate screws. Given a good pinblock fit all
around, and a reasonable amount of cast iron and bracing, a higher tension
scale should require more plate screws - and that is what we see in the
higher-tensioned and larger pianos in general. 

Still, having said all that, it comes down to a matter of choice. Plenty of
examples of well built and fine sounding pianos do not have blocks rigidly
attached to the stretcher, and some cannot, like the Chickering I am
currently working on. However, I think there is something to be said for
full-fit treatment where the original block was so installed. Regarding the
Mason in question here, we all know that most original Mason grands come
equipped with full-fit blocks. That would guide me if nothing else.  

Nick Gravagne, RPT
Piano Technicians Guild
Member Society Manufacturing Engineers
Voice Mail 928-476-4143
 
-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of A440A at aol.com
Sent: Saturday, August 02, 2008 7:31 AM
To: pianotech at ptg.org
Subject: Re: MH BB Pinblock

 David writes:

<< pinblock is screwed to the inner rim via a couple of plate screws that
penetrate the block down to the rim) all the way to the stretcher why would
you need to attach the pinblock to the stretcher to create case stability.

>>

I don't think it is a question of case stability, but rather, stiffness in 
the plate/block assembly.  
    The attachment of the block to the stretcher assists the block in 
resisting  movment of the strings in the vertical plane.  These vibrations
are quite 
small, but I consider them important, and without some vertical component to

the "beam" that resists them, more energy is lost to heat than otherwise
would 
be.   
   It could be that the increased stiffness afforded by attaching the block 
to the stretcher would also increase the entrainment of the strings'
vibration 
to the case.  
Regards,
 
Ed Foote RPT 
http://www.uk-piano.org/edfoote/index.html
www.uk-piano.org/edfoote/well_tempered_piano.html
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