Plate Bushings, an experiment

[Richard Brekne]

Ok Ron, and other interested parties.. this is what I did.

I took a piece of alumimum about a yard and a half long, roughly 2 inches
wide, and 1/4 inch thick and attached a piece of wood to simulate a flange.
I cinched this down to a couple of saw horses with a 2 x 4 under it to hold
it solid and keep this length from buckling (just in case..grin). Then I
bored a couple holes about an inch apart to insert tuning pins through.
These were 10 mm holes as I wanted to keep the pins from riding on the
"plate". I then took a piece of pinblock leftover and attached it up tight
against the "flange" securing it with four screws bored in so that two were
just in front of the two tuning pins and two were just behind. Then marked
and bored in the holes in the block for the pins. I screwed down the
"pinblock so that it had a bit of play. I wanted to see how it would move
if it was allowed to move.

Ok having set this up I screwed in a couple tuning pins and put on some
piano wire and hooked each end to a couple hooks I had attached at the end
to act as hitch pins. There were three directions of motion very easily
noticible as I started tightening the strings. Upon tightening the first
string a bit, (the right one... treble side if you will) the block rotated
counter clockwise and tilted downwards at the right forward (flangeside)
corner. When I tightened the other pin to roughly the same tension the
rotation was canceled and the block was pulled tight up to the "flange"
with the forward edge evenly tilted downwards. This downward tilt was
stopped by the underside of the "plate" so I tightened the front screws a
bit to see what would happen. The tilt was reduced and the block pulled
tighter against the "flange". Ok then I decided to put a bit of pressure on
this little system so I started tightening the two strings up a bit, evenly
so as to keep tension pretty much equal on both pins. The tilting and
forward movement of the block stressed just a bit more then stopped, and
then after increasing tension just a bit more the whole area of the "plate"
flexed upwards and towards the "hitch pins". I repeated this wice. The
first time with screws just barely loose and the next time with the "plate
screws" as tight as I could screw them in with the same end result, but
there was no noticable movement of the "block" in relation to the "plate"
or "flange" with screws dead tight, and just a very little move with the
screws barely loose. It seemed to "lock" itself into place frankly and
didnt move again even as the "plate" began to bend upwards.

This got me to thinking.. of course this would happen.. the pins and block
left completely free to move would align themselves in such a way that the
pins would assume approximately the same line as the strings. The movement
forwards (or backwards) of the block itself are partially the result of
the  fulcrum point and partially the result of the condition of the
constraining factors.. the screws, the underside of the "plate" and the
flange. In this case there was no fulcrum on the pin itself. The block
tilted on an axis determined by the constrainments.

Rotational movement seemed to be non existant with strings at nearly equal
tension.

Ok.. this is probably a shaky experiment at best, and more probably a
poor description of it, but it clearly shows what makes me think it is this
"tilting" force on the block which is the predominant one. Anyways.. it was
a nice way to kill the time in between coats of lacquer on my latest
project.

I would welcome your comments on this.. that is if you can make any sense
out of my description.. :)


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