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

[Ron Nossaman]

> I have just started on the partial rebuild of an Old Knabe grand. The pin 
>block is made up of 4 layers of quarter sawn maple which are layered, from 
>the top:
>thick-thin-thick-thin. In contrast a "Hex-a-Grip" is layered, from the top,
>thin-thick-thin-thick-thin. This thingee has plate bushings in it and was 
>still tunable although the pin torque was highly erratic..........when I got 
>the pinblock out I could see that NONE of the laminations were still glued 
>together except for a very few spots. actually just enough to tell it was supp
>osed to be a pin block. :-)

If 'tweren't for the bushings, the pins would have been spinning on the
plate long ago. I've been thinking a little more about how pinblocks work
too, and have some random observations and speculative ruminations. 

Hard maple has an end grain crush limit of 7830 psi, and a side grain limit
of 1470 psi. That being the case, it seems desirable to try to get as much
end grain between the top half of the pin and the string tension as is
reasonably possible. The closer to the top of the block a lamination is,
the more compression it's subjected to. Making that top lamination thick,
with the grain running parallel to the string would seem like a good enough
idea except that the short length of fibers under the compression applied
would tend to split along the unison pin line. You about need a full length
thin lamination on top, with the grain perpendicular to the strings, to
hold the load bearing lamination together. Better yet, maybe three veneer
layers on top (long,short,long), then a thicker short grain, thinner long,
etc., at least for the top half of the block. The bottom half doesn't have
the structural requirements that the top half has to deal with, so it
shouldn't much matter what the lamination configuration is except that it
should have about the same resilience and composition as the top half to
supply similar torque resistance. It would need to be cross plied to some
extent to stay together through humidity swings, and should end up with a
long grain lamination on the bottom. I suppose it would be best if it was
symmetrical top to bottom, from the centerline, so it wouldn't try to warp
through humidity swings.

The thicker the laminations, the less crush resistance, but the more spring
back resilience. I would think crush resistance would be more desirable
where plate bushings are absent, making it less likely that the pin ever
contacts the plate. Appropriately sized plate holes and some attention to
centering during drilling couldn't hurt either. 

With bushings, figuring about 1.3 of the pin circumference being load
bearing, the bushing alone would support 170 lbs of the 200+ lb average
from string tension through the bushing height. That will take a lot of
pressure off of the block, and allow a more resilient, less compression
resistant block without shortening it's longevity potential. 

Just a little more smoke from the fire, and a couple more ants for the picnic.


>  It is/was a "floating" block and the crushing of the bushings was  what I 
>would describe as "normal" with just slight forward crushing evident. I 
>wonder if the first thick and first thin layers were enough to hold "tunable 
>torque"? Or if possibly some 'shearing' effect was responsible?

I think it's that shearing effect, and the "holding" is coming more from
the pin bending along the curved wall of the hole.  



> BTW this thingee is built like a truck! It has both rims made of laminated 
>Oak with the inner rim containing six 1/4" laminations and the outer rim four 
>1/4" laminations. (how'd they bend this sucker? with steam?)  

I don't know, but it sounds reasonable. That or just glue and heat, but I
would think heat would be necessary. There are a heck of a lot of bent rim
oak chairs out there that were done that way.


>The sound board still has gracious plenty 
>crown throughout the scale and does not touch the outer rim anywhere. 

Tada! Case closed? Point made? Good job!



>  As for the Research Grant minus the "suits" and "paperwork"....sorry Ron it 
>ain't gonna happen. However perhaps you could pad the Grant application 
>enough to be able to add an AMS-P to the faculty. :-)
>Jim Bryant (FL)

Yea, I know. Guess I'll have to go out and see if I can buy a GOOD lottery
ticket. Not one of those scratch and sniff throwaways, mind you... a GOOD
one. Dumb luck looks like my only hope.

Ron N