----- Original Message -----
From: <ANRPiano@aol.com>
To: <caut@ptg.org>
Sent: March 22, 2000 4:35 AM
Subject: Re: Frame Cracked Twice, Engineering help?
> In a message dated 3/21/00 8:33:47 PM Central Standard Time,
> rchayden2@juno.com writes:
>
> << You make me think that I have stressed the frame by
> perhaps leaving the bolt too high, then leaving out the shims, forcing
> the frame unnaturally. >>
>
> This can certainly lead to cracks in the plate. But I just took a Kawai
> apart that the nose bolts were holding the plate at least 1/4 off the
dowels.
> It seems the factory worker just screwed the nose bolts until they
bottomed
> out. Sometimes plates will just bend around our general stupidity and
> sometimes they wont.
>
> Just my 1 cent worth.
>
> Andrew Remillard
----------------------------------------------------------------
Yes, well... Most factories deny that they ever use the nosebolts to
"adjust downbearing," but they nearly all do it. On a tour of one factory
by the tech service folks some time back, I was being assured that this
factory never used the nosebolts to adjust for proper downbearing while we
were standing there watching a worker adjust downbearing with the nosebolts.
I'm still not quite sure just what all of the ramifications of this incident
were/are.
As may be, much is said about the rigidity and brittleness of cast iron-more
properly 'grey iron.' Grey iron is commonly thought to be somehow stiffer
and to have less give-that is, to bend somewhat less under a given
stress-than other metals, particularly steel, because of this brittleness.
So much so, in fact, that this quality, along with its so-called damping
capacity, are the most often quoted advantages when various metals for piano
plate fabrication are being discussed. Rigidity (i.e., "not flexible or
pliant; stiff") is, perhaps, not the best word to describe the stiffness or
bending characteristic of cast iron. In fact, cast iron is not nearly as
stiff as even the mildest steel plate. Its tensile strength is typically
one-third that of even the mildest steel.
Brittleness is not a particularly good word to use, either. What is really
meant by all of this is that cast iron exhibits very low-extremely
low-plastic deformation. When an increasing force is applied to a bar of
steel it will bend elastically as the force exceeds the resistive strength
of the bar. The bar will bend elastically at a rate determined by its
modulus of elasticity up to the proportional limit of the material. That
is, up to the proportional limit if the force is removed the bar will return
to its original shape. If the bar started out straight, it will become
straight once again. If the force is made great enough to bend the bar
beyond its proportional limit and is then removed, the bar will only
straighten partially and will retain some amount of permanent bend. If, on
the other hand, this same increasing force is applied to a bar of gray iron
of similar size and shape it will also bend elastically, again at a rate
determined by it modulus of elasticity, but only up to a certain point.
Then, instead of deforming plastically as the force increases beyond some
critical point, the gray iron fractures and the bar breaks catastrophically.
Like Andrew, I would normally be hesitant to suggest that bending a plate
this amount would result in a crack. I have encountered plates installed
with much more bending than is being described by Roger. However, here we
have a plate that has a crack originating at a nosebolt hole and extending
into the plate panel. It has been described as a particularly thin spot in
the plate. We know that the plate was indeed 'bent' across this nosebolt
creating a high stress point. I will stick by my assessment that the
bending stress across the nosebolt, coupled with an original weakness in the
plate casting at that point resulted in this crack. Given what I know of
this pianos history, were the piano in my shop we would have the crack
welded once again and would reassemble the piano as described earlier.
I have been involved in many cracked plate situations over the years. To
the best of my knowledge they have all been successful. The criteria I go
by is to discern why the crack appeared in the first place. The reasons
fall into several broad categories:
1) Weak metal/poor casting. Don't bother with these. No amount of welding
will make them whole again. Some early German plates ended up with small
mazes of cracks throughout the plate. Sometimes a crack will originate from
a gas hole buried inside the casting, invisible until the crack shows up.
(These can often be fixed, but there may be more gas holes that can not be
seen that will cause problems later.)
2) Poor plate design. Especially at the strut between the bass & tenor of
grand pianos. This is a point of fairly high stress on the grand plate and
often the strut cross-section is quite thin. In time, a crack will develop.
Unless the piano has sufficient value to warrant an investment in some
re-engineering, forget it. The only thing that will salvage one of these
plates on a permanent basis is to design and install some type of 'horn' at
the break in the style of the Steinway type plate. (Don't forget to ensure
that the bellyrail structure is strong enough to hold the additional load
you're going to place on it.) We have encountered plates with horns that
have cracked because the wedge had been left out for some reason. These
also can be fixed and will hold nicely once a new wedge has been fitted to
the horn.
Other design related problems can be traced to areas with abrupt variations
in mass, such as the point at which the longitudinal plate strut encounters
a particularly massive capo d'astro bar. Or sections of the plate that end
up being cast particularly thin. Or areas within the tuning pin field where
the holes drilled for the tuning pins are particularly close together.
3) Improper plate installation. This, in my opinion, is what we have here.
Once the problems with the installation are fixed, there should be no
further problems.
4) Over-stressed plates. Some pianos with quite normal looking plates
ended up with quite abnormal stringing scales. We've measured string scales
with up to 64,000 lbs (29,000 kgs) of string tension. Sohmer and a couple
of others come to mind. Sometimes a piano with an acceptable original scale
has been 'rescaled' improperly and ends up with string tensions -- and the
related plate stress -- that is far higher than the plate was originally
intended to support.
Plates that have been converted from the conventional hitchpin arrangement
to vertical hitches have broken when the strings have been allowed to ride
too high on the hitch. The plate stresses in this situation can also get
quite high. I also encountered one hitchpin panel that broke because a
rebuilder has placed a much higher than normal back counterbearing bar under
the strings. From a plate load perspective this is nearly the same as
installing vertical hitches and setting the strings high on the hitch.
5) Physical damage. The piano has been dropped off the stage. The truck
tipped over. The roof caved in during an earthquake. Etc.
OK. I'm sure I've forgotten and/or left out a number of reasons for plate
failure. These are the ones that come to mind. My point is simply that if
I can convince myself that I understand the principle reason why the plate
broke, and if I can correct that, I have no reservations in going ahead and
repairing, i.e., welding, the plate and putting the piano back in service.
Sorry for getting so long winded...
Regards,
Del
PS. Parts of the above were originally written for future publication,
hence the following:
Copyright (C) 2000 Delwin D Fandrich
No part of the above may be copied or reproduced in any form without
permission from the author.
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