Cockeyed hammers / Don Gilmore

[Isaac sur Noos]

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HelloGetting back at that old post, I noticed that Yamaha hammers that have
more wood on the tails side than many, seem to be better balanced and more
stable in time, we also feel something while playing like if the hammer
where thrower more naturally than on other kind with thinner keys.

Is it the percussion point that is involved (I still did not get it really)
or does it have to do with the actual center of gravity of the hammer/shank
system ?

Thanks in advance

Isaac OLEG

  -----Message d'origine-----
  De : pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]De la
part de Bernhard Stopper
  Envoyé : vendredi 19 décembre 2003 01:02
  À : Pianotech
  Objet : Re: Cockeyed hammers / Don Gilmore


  Don/Sarah,

  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.
  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

  regards,

  Bernhard
    ----- Original Message -----
    From: Don A. Gilmore
    To: Pianotech
    Sent: Friday, December 19, 2003 12:22 AM
    Subject: Re: Cockeyed hammers / Don Gilmore


    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.

    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.

    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 no force.  This is the center of percussion and is not located
at the centroid of the board.  It's not usually even particularly close.

    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.

    Don A. Gilmore
    Mechanical Engineer
    Kansas City

    ----- Original Message -----

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