Shanks parallel to strings

[Dean May]

Good points, David.

Pardon me while I dust off some of my long unused engineering background.
The force involved here is inertial force, which is a function of the
hammer's velocity at the instant of impact and the time it takes for its
velocity to go to zero (this is in turn a function of hammer resilience,
string movement, etc). The angle it makes with the shank has nothing to do
with it. The equation is F=ma, where F is the strike force, m is the mass of
the hammer, and a is the deceleration of the hammer.

 What we want is for all of the momentum of the hammer, all of its kinetic
energy, to be transferred to the string, or at least maximize it. Same is
true with a bat hitting a baseball. Now if you've played baseball you know
that there is a "sweet spot" on the bat. If the ball hits that sweet spot,
it maximizes the transfer of the bat's kinetic energy to the ball and you
feel very little of the impact force in your hands. On the other hand, if
you hit the ball on the very end of the bat, or close to the handle, it can
cause quite a stinging effect. When that happens, some of the energy of your
swing is transferring to your hands, not to the ball.

The sweet spot is called the center of percussion: it is that point on
radius of the bat's swing where all of inertia of the bat's mass is
effectively acting. Now in a hammer and shank assembly, the mass of the
rotating shank is negligible compared to the hammer, so the center of mass
of the hammer is effectively the sweet spot, or center of percussion. If we
wanted to be really anal, we could extend the shank out a little ways
further to compensate for the shank's mass. (That would involve a
differential equation and I don't do those anymore.) Since we take the mass
of the shank to be negligible here, it really does not matter what angle it
is on, or even where it is located when the hammer hits the string. What
matters is that the hammer is oriented so that all of its inertial momentum
is focused on the strike point, and we get there by making it perpendicular
to the strings

Dean

Dean May             cell 812.239.3359
PianoRebuilders.com   812.235.5272
Terre Haute IN  47802

-----Original Message-----
From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]On Behalf
Of David Love
Sent: Wednesday, June 16, 2004 10:13 PM
To: Pianotech
Subject: RE: Shanks parallel to strings

How unparallel are you talking about?  If the hammer is radically non
perpendicular to the string then it will be striking askew and not on the
strike point.  That might stress the glue joint as the force would be taken
at the joint rather than straight through the hammer moulding.  The voicing
would be another issue.  But if the hammer is perpendicular to the string
at contact then I don't think it makes much difference as to whether that
shank is parallel to the string or not.  There are many examples of pianos
(older Bechsteins come to mind) that are short bored and the hammers are
raked out.  It may cause a slight raising of the let-off button and drop
screw.  But I haven't run into one that couldn't be regulated.   You can
even argue that with the hammer shank starting in a higher position it
minimizes the change from a horizontal to a vertical vector (do I have that
right) and the action will actually weigh off slightly lighter due to a
reduction in friction.  You can even make the argument that since shanks
tend to flex on the way up, that slight short boring without the rake is
justified since that will likely result in the hammer hitting perpendicular
to the string.  For those reasons, if you are going to err, err on the side
of short bore as opposed to long bore.  Though I always aim for the hammers
perpendicular to the string at impact, minor variations are not likely to
cause a great deal of trouble.  As far as shank parallel to strings, it's a
reasonable standard that usually allows everything else to fall in line.
But in and of itself, much ado about nothing.

David Love
davidlovepianos@earthlink.net


> [Original Message]
> From: Phillip Ford <fordpiano@earthlink.net>
> To: <pianotech@ptg.org>
> Date: 6/16/2004 2:44:20 PM
> Subject: Shanks parallel to strings
>
> I would like to revisit the idea that shanks should be parallel to
strings at hammer contact.  I know we've had some discussion of this
before.  I looked back through the archives and also in the back issues of
the Journal and couldn't really find what I wanted.  I would like to have
some theoretical reasons why having the shank parallel to the string gives
better action performance than not having it parallel.  The standard
thinking seems to be that the hammer should be perpendicular to the string
line at contact and the hammer should be perpendicular to the shank, which
results in shanks parallel to strings.  I can see reasons for having the
hammer strike perpendicular to the string line.  But I don't see any good
reason for having the hammer perpendicular to the shank or the shank
parallel to the string.  Reasons given in the archives or the journal for
not permitting non-parallel shanks are along the lines of:
>
> 1.  It's bad practice.
> 2.  It reduces power delivery.
> 3.  It won't give as strong a joint at the hammer to shank interface.
> 4.  The regulation will get screwed up.
>
> As to these reasons:
>
> 1.  If it's bad practice, is there some explanation on offer?
> 2.  Why would it reduce power delivery?  The hammer is still traveling in
the same path as it was before - the angle of shank to hammer doesn't
affect that.  If the hammer is still striking perpendicular to the string
why would any reduction in power have occurred?
> 3.  I don't see that the joint should be weaker.  If anything, having the
shank not perpendicular to the hammer would mean that the hole through the
hammer has to be a little longer, which would seem to result in a stronger
glue joint.
> 4.  I can see that this would be true if the action was designed to work
with the shank parallel at contact, and that drastically changing this
would cause various things like rest rails and letoff buttons to be in the
wrong places for the new shank position.  But I'm talking about an action
that was specifically designed to have the shank non-parallel at contact.
In this case everything could be positioned to work properly so that
regulation would not be compromised.
>
> Any thoughts?
>
> Phil
>
>
>
> Phillip Ford
> Piano Service and Restoration
> San Francisco, CA
> _______________________________________________
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