Open face vs. closed face pin blocks

[larudee@pacbell.net]

Phil,

Sorry you had to miss "Friends" to work on this, but love your response.  Couple of comments:

Phillip L Ford wrote:

> My thinking was that when a string is struck and vibrates then there must
> be an periodic increase in tension.  When a string of fixed length is
> displaced then it seems to me that there must be an increase in tension.
> For the fundamental, as the string swings up the tension increases, comes
> back to nominal as the string comes back level, and increases again as
> the string swings down.  This is happening several times a second.  Also,
> this is happening for all of the partials, just at different frequencies.  This
> increase in tension must be sustained by the plate.  Admittedly, the loads
> in question are low and the deflections must be small.  But, cast iron has
> high damping (by metal standards) and the vibrations are occurring several
> times per second, so even a small amount of deflection could perhaps
> dissipate consequential amounts of energy.  The amount of this deflection
> will depend on how stiff the plate and pinblock arrangement are.

One thing to consider is that it is never just one string that is vibrating, and that they
are not going to be vibrating in phase.

> >Even if it does work as intended, we need to remember that the tuning pins are 3 or 4
> >times farther above the pin block in a closed design and that string tension is
> >therefore exerting that much more leverage upon them.  To the extent that string energy
> >might be absorbed by the pinblock/plate in an open face design, wouldn't it be at least
> >as likely to be absorbed by the flex of the tuning pins in a closed design?
> >
> >I am to some extent playing devil's advocate here, because I'm sort of half convinced of
> >your argument, but would like someone to remove my remaining doubts.
> >
> >Paul Larudee
>
> ---
> Good point about the tuning pins.  I hadn't considered that.  Do you think
> that the string is 3 or 4 times further from the pinblock in a closed design?
> I had thought perhaps twice.  Maybe .25 inch for an open face and an
> additional .25 inch for the thickness of the plate in a closed face.  To get
> some idea of relative deflections I did a few calculations.  These are based
> on some assumptions about the structure of the open face design and the
> structure of the closed face design so they would vary for any specific
> pianos in question but hopefully are in the ballpark.

I string so that the height is 1/8 inch above the plate (or pin block in the few cases I have
restrung open systems).  1/4 inch is perhaps the thinnest plate, but by no means the
thickest.  That puts the string 3-4 times higher in a closed face.  I think 1/4 inch string
height is way too high, and I want to discourage future techs from curing a loose pin by
driving it further.  The closer the string tension is to the fulcrum (i.e. the top of the pin
block) the better.

> At this point I also need more convincing to believe that the closed
> face pinblock arrangement really is superior to the open face pinblock
> arrangement in terms of power.

Yup.

Paul