Open face vs. closed face pin blocks

[Phillip L Ford]

Paul,
    I agree about various strings vibrating simultaneously and not in phase.  But
I still feel that there is a net increase and decrease of load on the plate
while strings are vibrating.  The question is, is the tiny plate movement as
as result of this enough to cause significant loss of energy.  I don't know
the answer.
    In my previous post I showed some calculations for deflection for tuning
pins and for open and closed face pinblock arrangements.  One case I
neglected to examine was the case of a closed face pinblock with tuning
pin bushings.  Assuming that the bushings are truly effective, then the pin
bending in this case should be no more than for an open face pinblock
arrangement.  In which case, the closed face design with tuning pin bushings
would have the lowest deflection (would be the stiffest) of all.
     I agree about the benefit of keeping the string as close to the surface
of the pinblock as possible.  The pin sees a shear load equal to the string
tension.  But it also sees a bending moment equal to the string tension
times the distance to the surface of the pinblock.  The shear load does
not change with string location but the bending moment goes up proportional
to the distance of the string from the surface of the pinblock.  The shear
load will be reacted as a bearing load of the pin against the wood in front
of the pin.  The moment will be reacted as a bearing load that is proportional
to the distance from the center of rotation (somewhere down in the block), in front of the pin above the center of rotation and behind the pin below
the center of rotation.  So the portion of the block in front of the pin
near the top surface of the block is getting the worst of it.  Also, the
stress here should be higher on a closed face block than for an open face block since the string is higher above the pinblock.  This might lead you
to believe that an open face pinblock would be longer lasting.  I suppose
it depends on the actual stress levels vs. allowable stress levels for maple
as to whether or not this is a factor in pinblock life.
    Why do you want to discourage tightening a pin by driving it further
into the block?

Phil

---
Phillip Ford
Piano Service & Restoration
1777 Yosemite Ave
San Francisco, CA  94124

On Wed, 15 Aug 2001 19:12:38  
 larudee wrote:
>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
>
>


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