Bechstein B hammer rake / more thoughts

[Dean May]

>>When I think of it, I can see only one ideal situation, that is when the
hammer is perpendicular to the string at impact time, and the shank is
closest to parallel to the string at that time.  In this condition, the
energy transfer from the hammer to the string seems maximized to me.  


>From an engineering perspective (at least, this engineer's perspective),
maximum energy transfer of the rotating mass of the hammer would dictate
that the hammer be perpendicular to a line from the tip of the hammer to the
center pin. That's not likely to happen given the physical constraints we
find ourselves with. If the shank were pushing the hammer against the string
then the hammer would need to be perpendicular to the shank. But it isn't.
It is the kinetic energy of the entire rotating mass that is slamming the
hammer against the string. So the maximum force line will be perpendicular
from the point of impact to the center of rotation. 

The other thing to maximize energy transfer is to make the above
perpendicular line to be in line with the center of percussion,
http://en.wikipedia.org/wiki/Center_of_percussion. This is the sweet spot on
a bat when playing baseball. If you hit the ball on this spot all of the
bat's rotating energy goes into ball. If you don't hit the sweet spot, some
of the bat's rotating energy comes back into your hands and you feel the
sting. 

The woody sound one hears in the treble is a result of the hammer not being
in line with the sweet spot. Some of the rotational energy is being absorbed
by the shank and flange causing the wood to vibrate. Thinning the shanks
moves the center of percussion further out towards the hammer which reduces
the vibrational woody sound. One could toy with adding weights to the back
side of the hammer to move the cp further out to get an even better
alignment of the hammer with the cp. 

I would think it also be important for the hammer to be perpendicular to the
strings at impact. Now, if we can get Del to design a piano to make all that
happen at once, we'd have a harmonic convergence and world peace. ;-)



Dean

Dean May             cell 812.239.3359 

PianoRebuilders.com   812.235.5272 

Terre Haute IN  47802

 

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Stéphane Collin
Sent: Friday, June 20, 2008 11:14 PM
To: 'Pianotech List'
Subject: RE: Bechstein B hammer rake / more thoughts

Hi Ric.

When I think of it, I can see only one ideal situation, that is when the
hammer is perpendicular to the string at impact time, and the shank is
closest to parallel to the string at that time.  In this condition, the
energy transfer from the hammer to the string seems maximized to me.  If the
shank is not parallel to the string, there is a portion of the hammer
movement that will not be normal to the string plane (but parallel instead),
and that movement will spend energy in friction (of the hammer against the
string), and the rebound of the hammer will also be slightly impeded because
the hammer will have to reverse the direction of that portion of movement
too.
I fail to understand the importance of the shank being horizontal.  What are
the ideas of Bob Hohf about that ?

The trigonometry is fine, but you could add an analysis of what happens with
the measurement errors that you do when measuring
- String height over keybed (I don't seem to be able to do this measure with
less than 0.5 mm error)
- Rounding this value for all notes between two samples you measured
(typically at each side of each section), so if for example you measure 231
mm at the right of the treble section and 230 mm at the left of the alto
section, all notes in between will have an additional error of 0 to 0.5 mm
- Center pin over keybed : here you have two measures and one approximation
: the approximation is that when you measure any point on the stack over the
keybed, you do this with the action out of the piano (I suppose, or do you
have a better way ?) and so the action rests on another surface against
which you do the measure, but is the action frame sitting exactly the same
way ?  I find it difficult to avoid another 0.3 mm error here.  Also you
don't measure directly the center pin height (or do you ? but how ?) you
measure one accessible point at the top of the stack, then measure the
vertical distance between that point and the center pin, and here again
errors sum up
- yet another rounding error for all notes between the extremes, if there
happens to be a slight difference in the measurement of the first bass
center pin height and the last treble one
- angle between string plane and keybed (I can't even figure out how to
measure this accurately.  My best take up till now was to throw a ping pong
ball from 20 cm over the strike line and measure where it falls down after
its first rebound, but this assumes that the keybed is horizontal, which I
can grosso figure out with a bubble gage)
- again approximations across the scale
- and finally the error in hammer bore itself

This is where the sample hammer with known bore distance trial in situ
shortcuts a whole bunch of accumulated errors (in the trigonometry formulas,
any measurement you have done will multiply even further all the errors), as
there is only one approximation, that is the perpendicular condition of the
hammer with the string plane, which is quite easy to measure with a piece of
straight wood with a nail put in it at straight angle, which you put on the
strings with the nail facing the ground.  And if the hammer is perpendicular
to the shank, then necessarily, the shank will be parallel to the string
plane.  But this tells nothing about the horizontality of the shank, and I
fail to understand why I should care. 
So I don't understand either the rake thing, short of trying to float for
all the said approximations when it is time to have the actual end user
hammer as perpendicular to the actual string as practically possible.

What do you think ?

Best regards.

Stéphane Collin.



-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Richard Brekne
Sent: samedi 21 juin 2008 0:38
To: pianotech at ptg.org
Subject: Bechstein B hammer rake / more thoughts

Hi agin


    "The most clever comment about that came from John Delacour, who
    considered that the distance between shank center pin and hammer
    molding center was fixed by design, which makes sense in the way the
    back checks for example are also at a fixed place offering the exact
    catch angle if there is such a thing"

Been thinking a bit more about all this and have the following 
observations. I've assumed its generally accepted that the shank should 
be in horizontal position when the hammer hits the string.  If this is 
so, and if one insists that the hammer should be perpendicular to the 
string at impact as well,  it seems to me that it follows that both the 
actual hammer bore length and the distance out on the shank the hammer 
(center of molding) is given by the position of the strike line and the 
angle the string is off horizontal. Actually boils down to 8th-9th grade 
basic triangle trig. The horizontal  line out to the normal up to the 
strike point becomes the adjacent line to the angle between the string 
plane and the horizontal, and the line parallel to the string plane out 
to this same normal becomes the hypotenuse. The rest is then given.

If one on the other hand accepts the design parameter the factory gives 
for hammer shank center to center molding distance and insist on a 
hammer to string perpendicular relationship, then one accepts the 
eventuality that the shank could be off horizontal.... perhaps 
significantly sometimes at impact.  Bob Hohf's article on action 
elevations comes to mind when it comes to the desirability of a 
horizontal shank at impact. 

Does anyone have any comments about just how horizontal the shank should 
be at impact ?

Cheers
RicB