Plate Bushings - was Re: Bushing vs. bigger pin?

[Richard Brekne]

So... you are gonna make me take a stance eh ??? I gotta bad feeling about
this...grin.. but here goes..

First let me say I have yet to be able to find a grand that was difficult
to tune that I could without any doubt attribute to a poor pinblock to
plate flange fit, and I believe the "problem" is probably overstated. That
is just a belief and I really dont have any evidence to back it up outside
of my own tuning experience. That being said I have heard tell of folks
demonstrating movement in a pinblock by inserting a feeler gauge between
the flange and pinblock on grands without pin bushings, raising pitch on
the opposite end, and finding the feeler gauge is impossible to pull out
without breaking it. This is supposed to establish poor a poor pinblock to
flange fit. I have not tried this myself, nor have I ever personally seen
it done.

As for how this relates to pin bushings.... while it is perhaps true that
unrestricted movement away from the flange would no doubt result in crushed
bushings, this is hardly the case. Movement is restricted (in both cases)
by all the screws tightened down,the friction of the face of the block and
the underside of the plate, and in some cases by the attachment of the
pinblock to the casing (as in Bechstein). Indeed, there seems to be good
reason to question whether or not the block moves significantly enough to
cause any stability problems to begin with, unless perhaps it is very loose
and very poorly fit. 

Still, if we assume for the moment that the block is able to move enough
for there to be a tuning problem, it certainly does not have to move more
then a fraction of a milimeter in any direction to do so. This amount of
movement would certainly not be enough to cause plate bushings to crush.
Further, and movement by the pinblock would be reactive in nature. The
pinblock is not exerting any force on the bushings, only the strings
pulling on the pins do. The block itself acts as a resistor in this regard.
Under any circumstances something would indeed be have to be wrong in
Smallville for the pinblock to be able to move so much that the bushings
would be crushed by the force pulling on the pins from the strings.  So it
seems to me that the "crushed bushing" argument kinda dies on its own
unreasonablitiy. If one was to argue along the lines of the effects of
pinblock movement away from the flange (in the case of bushed plates) it
would seem to me better to point out that since there is no flange on the
back edge one would expect even greater instability. But I dont believe
pinblock "movement" in grands with these bushings is really the point.

Rather I believe the point is more one of the direction force exerted on
the block, by the pins. In the case of non bushed plates, there is a force
exerted at the top of the pinblock pulling it towards the hitch pins by the
strings, and tilting the flange edge downwards by pins (slightly flexed
towards the hitch pins) wanting to straighten out. In the case of the
bushed plate, the pulling force exerted by the strings is on the bushings
and plate itself, leaving only the tilting force from pin left to affect
the pinblock. True enough in moving the fulcrum at which the pin flexes
some milimeters higher then the top surface of the pinblock, this tilting
force would also tend to push at the pinblock in a direction away from the
flange, but I would think the tendency to tilt would be the stronger of the
two. I suppose some kind force vector math would be able to establish the
exact condition. In anycase, its not so much that the pinblock wants to
move backwards, as it is that it no longer wants to move forwards.

My own feeling, and why I share your skeptism towards the flange fit
"problem" is that this tilting force is present in both cases and in effect
serves as a restricting force for movement frontwards or backwards. The
flange end of the block will stress the holding screws more (in a downwards
direction) in effect increasing their "hold" on the block, and the opposite
edge will press more into the underside of the plate increasing the already
substantial friction present. It would seem to me that if the pinblock ever
really "moved" under the stress of tuning, that you would actually (at
least some of the time anyways) be able to hear it break free of all that
friction, much as you hear sticky tuning pins break clean of the friction
holding them. The only time I ever hear anything that makes me suspect this
at all is when the piano has plate-to-block screws that are noticbly loose,
and even then this is the exception and not the rule.

Grin... so there you have it.. my humble musings on the matter for what
they are worth. Now go ahead... :) tear me apart.. hehe..



Ron Nossaman wrote:
> 
> >As for your comparison of bushed pins to pins riding on the plate... hmm I
> >am not so sure.. tho I see where you are coming from for sure. As to the
> >force exerted on the pinblock in general.. 250 some pins with all that
> >string weight on them gotta exert some force for sure, and its easy enough
> >to imagine the circumstance as these fellows describe.. seems to make
> >sense, tho as I stated in the last post I am not sure just how big a deal
> >it all is. I suppose one could easily enough put together a makeshift
> >simulation that would show actually what goes on ..
> 
> >Richard Brekne
> 
> Alright, let's try it another way. Instead of comparing what faction "A"
> says, compared to what faction "B" says, let's hear what you think, and
> your supportive reasoning.
> 
> Ron N

-- 
Richard Brekne
I.C.P.T.G.  N.P.T.F.
Associate, PTG
Bergen, Norway