<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; =
charset=iso-8859-1">
<META content="MSHTML 6.00.2800.1276" name=GENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff background="">
<DIV><FONT face=Arial size=2>Don/Sarah,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </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 the =
center of
percussion is at distance x from pivot that can be calulated by the =
formula
x=I/(m*y) with I= moment of inertia, m=mass, y=distance to =
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>
<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></BODY></HTML>