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<DIV><FONT face=Arial size=2>Don,</FONT></DIV>
<DIV><FONT face=Arial size=2>indeed you are right, i did a confusion =
with what
is also called the "sweet spot". The sweet spot (no reaction
forces) coincide with the pivot when the center of =
percussion is
at the point where the hammer hits the string.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>regards</FONT></DIV>
<DIV><FONT face=Arial size=2>Bernhard</FONT></DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>
<A title=eromlignod@kc.rr.com =
href="mailto:eromlignod@kc.rr.com">Don A.
Gilmore</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A =
title=pianotech@ptg.org
href="mailto:pianotech@ptg.org">Pianotech</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Saturday, December 20, =
2003 12:22
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Cockeyed hammers / =
Don
Gilmore</DIV>
<DIV><BR></DIV>
<DIV><FONT face=Arial size=2>Bernhard:</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>You don't want your pivot point to be =
at the
center of percussion. Actually that's physically impossible =
since your
radius of gyration would have to be zero. The whole point of =
using the
center of percussion is that you want it to be where you strike the =
rotating
object (hammer) so that it results in zero force on the =
pivot.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Don A. Gilmore<BR>Mechanical =
Engineer<BR>Kansas
City</FONT></DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>
<A title=b98tu@t-online.de =
href="mailto:b98tu@t-online.de">Bernhard
Stopper</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A =
title=pianotech@ptg.org
href="mailto:pianotech@ptg.org">Pianotech</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Thursday, December 18, =
2003 6:57
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Cockeyed hammers =
/ Don
Gilmore</DIV>
<DIV><BR></DIV>
<DIV><FONT face=Arial size=2>Don, Sarah</FONT></DIV>
<DIV><FONT face=Arial size=2>my last post has to be =
corrected(fat
cursive):</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV>
<DIV><FONT face=Arial size=2>To design a hammer so that its =
pivot
coincides with the center of percussion is THE way to get a free =
sound and
the maximum of energy transfer to the string. it also allows the =
hammer to
reject the fastest way possible because oscillations in the =
pivot
produce friction and slow down hammer movement at contact =
point.
</FONT></DIV>
<DIV><FONT face=Arial size=2>i did several simulations with =
software called
pro/mechanica and reshaped hammers after results found. one could =
say that
actual hammer with heavy felts have their center of percussion much =
far away
from the pivot than lighter hammers have. putting a small lead in =
the tail
of the hammer can make them come closer to that point. center of =
hammer mass
should ly on a line along the center of the hammer =
shank and
<STRONG><EM>the center of percussion coincides with the pivot, when =
hammer
shank length x is</EM></STRONG> calculated by the formula
x=I/(m*y) with I= moment of inertia, m=mass, y=distance =
pivot - center
of mass</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>regards,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Bernhard</FONT></DIV></DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- =
</DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>
<A title=eromlignod@kc.rr.com =
href="mailto:eromlignod@kc.rr.com">Don A.
Gilmore</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A =
title=pianotech@ptg.org
href="mailto:pianotech@ptg.org">Pianotech</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Friday, December 19, =
2003 12:22
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Cockeyed =
hammers / Don
Gilmore</DIV>
<DIV><BR></DIV>
<DIV><FONT face=Arial size=2>
<DIV><FONT face=Arial size=2>Well, I didn't mean to imply =
that the
impulse at the pivot is all absorbed; but it is all lost as far as =
the
string is concerned.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>The center of percussion is one =
of those
counter-intuitive phenomena in dynamics. For the multitude =
of
college students that only take a semester or two of physics, the =
concept
of forces on a free body acting at the center of gravity is a =
paradigm
that is hard to dislodge. For an object in free space =
or
translating in a straight line, which comprises most =
situations, the
center of gravity is always used. But for an object pivoted =
at some
location other than its c.g., all bets are off. The most =
publicized
version of this is a baseball bat's "sweet spot" as you
described.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Think of a wooden board hung from =
a pivot
point at its top. If you strike the board up near the hinge, =
the pin
will experience a force coming from the direction of the =
strike. But
if you strike the board at the bottom, the pin will experience a =
force in
the opposite direction as the board tries to spin about its =
center.
There is a point in between these two where the pin force is =
neither
positive nor negative and the pin will actually see <EM>no</EM>
force. This is the center of percussion and is <EM>not</EM> =
located
at the centroid of the board. It's not usually even =
particularly
close.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>It would seem to be a good =
idea to
design the hammer so that the string contact point of =
the head passes
through the center of percussion. That way the maximum =
amount of
energy would be transmitted to the string...like dropping a =
ball on
it. But I'm not sure if they actually design them that
way.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV>Don A. Gilmore<BR>Mechanical Engineer<BR>Kansas City</DIV>
<DIV> </DIV>
<DIV></FONT>----- Original Message ----- </DIV></DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>
<A title=sarah@gendernet.org =
href="mailto:sarah@gendernet.org">Sarah
Fox</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A =
title=pianotech@ptg.org
href="mailto:pianotech@ptg.org">Pianotech</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Thursday, December =
18, 2003
2:38 AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Cockeyed hammers =
/ Don
Gilmore</DIV>
<DIV><BR></DIV>
<DIV><FONT face="Courier New" size=2>Probably any baseball =
player would
know that it's not smart to hit the ball with the very tip of =
the
bat. It makes for a nasty shock to the hands (and to the =
extent
that the hands are not rigid in space, it takes away from the =
power
transferred to the ball). Why do they do this with piano
hammers? More to the point, is this not considered an =
unneccessary
demand to put on the hammer flanges and bushings? Perhaps =
it's
also a drain on efficiency? Wouldn't it make sense to have =
some
sort of asymmetrical molding whose center of mass is on the far =
side of
the action center, so as to put the center of mass of the hammer =
assembly closer to the line of strike?</FONT></DIV>
<DIV><FONT face="Courier New" size=2></FONT> </DIV>
<DIV><FONT face="Courier New" size=2>Also, wouldn't it at =
least make
sense to angle the head of the hammer inwards slightly, so that =
the
radius to the head's center of mass is perpendicular to the =
strike
axis of the head? It seems to me that this would help to =
control
wobble in the hammer</FONT></DIV>
<DIV><FONT face="Courier New" size=2></FONT> </DIV>
<DIV><FONT face="Courier New" size=2>I'm not sure I agree =
about part of
the impact being "absorbed" by the pivot. Sure, there =
would be
force against the pivot, but for impact to be absorbed, the =
pivot would
need to be compliant and inelastic. I realize that's true =
to an
extent, as no system is ideal. However, do you think this =
would be
a substantial drain of energy? It seems to me that the =
hammer felt
and the strings are far more compliant and are closer to the =
center of
mass, such that almost all of the energy would be dissipated =
there
(rather than at the comparatively rigid center). Have you =
ever
measured this?</FONT></DIV>
<DIV><FONT face="Courier New" size=2></FONT> </DIV>
<DIV><FONT face="Courier New" size=2>Just curious on your =
take. I
can't do much more than scratch my head about this one. If =
the
hammer were perfectly rigid and the center bullet proof and
noncompliant, there'd be nothing to worry about. But =
considering
that it does the hokey pokey and shimmies all about... =
Well, I
don't know.</FONT></DIV>
<DIV><FONT face="Courier New" size=2></FONT> </DIV>
<DIV><FONT face="Courier New" size=2>Peace,</FONT></DIV>
<DIV><FONT face="Courier New"
=
size=2>Sarah</FONT></DIV></BLOCKQUOTE></BLOCKQUOTE></BLOCKQUOTE></BLOCK=
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