Open face vs. closed face pin blocks

[Delwin D Fandrich]

----- Original Message -----
From: "Phillip L Ford" <fordpiano@lycos.com>
To: <pianotech@ptg.org>
Sent: August 13, 2001 6:04 PM
Subject: Re: Open face vs. closed face pin blocks


>      As I see it, with a closed face pinblock, the plate in the pinblock
area
> is acting as a beam - a Z section beam if you will (like an I beam with
> a couple of pieces of flange missing).  The plate flange against which
> the pinblock bears is one flange (or cap) of the Z and the upstanding
> flange at the front end of the plate (along the stretcher) is the other
> flange (or cap) of the Z with the portion of the plate over the pinblock
> acting as the web of the Z beam.  The presence of this web allows
> the entire section to work as one beam.  If this web is not there (as
> in an open face pinblock) the two flanges work independently as separate
> beams.  The bending stiffness of these two independent beams is
> substantially less than the bending stiffness of the entire section
> working as one beam.  In practice, on an open face pinblock, the
> flange against which the pinblock bears will be the only flange
> working (resisting bending) with the front flange in effect doing nothing.
> So, there should be less deflection of the pinblock in the direction of
> the string load with a closed face pinblock than with an open face
> pinblock.

While I agree with your analysis in theory, it doesn't seem to matter all
that much in practice. Steinway plates are designed with a "horn" to couple
some of the stress from string tension down to the belly. Baldwin, using a
similar--though somewhat heavier--plate design, does not. Yes, the Baldwin
plates deflect a bit as they are pulled to pitch, but it doesn't seem to
matter much. Once at pitch they are quite stable.

To carry this further we have to go back a bit. Several U.S. grand piano
builders through the late 1800s and very early 1900s used what we would
today probably call an "open-face pinblock coupled to a three-quarter
plate." In other words, the plate butts up to the pinblock but does not
overlap it. These plates have no front flange and there are no pinblock
overbars. In general they do have relatively massive pinblock flanges. Even
here, though, I've seen 'conventional' plates with more iron used through
the pinblock flange section. We've had several of these pianos in our shop
and after well over 100 years these arrangements seem to have stood up well.
There has been little evidence of structural failure in the original
pinblocks and once remanufactured and tuned to pitch (A=440) they do not
show any more stress deflection than we would expect to see in a typical
Baldwin-style plate. Obviously, the designers of these pianos understood
something about stress loads and about supporting them. Some of these pianos
have massive wood structures forming what we would probably now call
stretchers but others do not, the latter being a bit more substantial than
we would expect today, but not by much.



> What effect this has on the tone and on tuning stability
> would probably depend on just how massive the pinblock and plate
> structure of the open face pinblock in question are.

While I personally don't have enough experience tuning pianos with open-face
pinblocks to give any kind of definitive answer on their long-term
stability, I can say that they are certainly easier to tune. They
pitch-raise easily and stabilize quickly.



> I can imagine
> it could have an affect on power, as some of the string energy
> would be used in flexing the pinblock and plate.

Again, this analysis sounds reasonable in theory and is one I have generally
subscribed to for some time, but it doesn't seem to hold up in practice. To
the best of my knowledge there has been only very limited comparison testing
done to prove this notion. And when the two different types are compared it
is with instruments that are quite dissimilar in many other areas of their
design as well. It would be most interesting to be able to compare two
pianos having identical designs except for this one area.

I will say that my recent experience with several pianos using open-face
pinblocks and several other pianos with all-agraffe string terminations has
caused me to rethink much of what I thought I knew about both of these
designs. We have recently remanufactured pianos of both types in which we
installed my now-standard scaling and soundboard designs that were most
satisfactory. I'd have been quite happy with the results regardless of their
plate configurations. As to whether their open-face pinblock designs or
their all-agraffe designs made them any better than their more common
counterparts, I cannot say.

Given this experience and my observations about the performance
characteristics of both designs, for me, at least, the jury is back out
reconsidering its earlier verdict. And I am definitely looking forward to
working with both designs in the future.

Del