Bechstein B hammer rake / more thoughts

[Dean May]

>>... I obviously stumbled on a rather large hole in what I thought I had
already clear in my mind

 

Precisely why we don’t have harmonic convergence and world peace. :-)

 

I really don’t mean to presume to answer the questions you two were raising,
only to comment on the transfer of the rotational energy. 

Dean

Dean May             cell 812.239.3359 

PianoRebuilders.com   812.235.5272 

Terre Haute IN  47802

 

  _____  

From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Richard Brekne
Sent: Saturday, June 21, 2008 12:03 PM
To: pianotech at ptg.org
Subject: Bechstein B hammer rake / more thoughts

 

Hi Stéphane / Dean

I'm going to drop the horizontal shank bit for a while, because any
justification there is for this takes the discussion off in a direction all
its own, which for the moment hasnt my attention.

Lets just start with this basic formula for bore length thats been tossed
around here forever namely String height - Hammershank center pin height. If
that formula is valid, then any hammer bored to the resulatant length is
going to leave the shank fairly close to horizontal unless the rake of the
hammer is quite large.  Do we agree on that much ? Yet the Bechstein I'm
doing now has its shanks at the hammer molding about 6 mm higher then
horizontal at impact. Ok they are old... but no way can they have lost 6 mm
of total length or anything close to it. Lowest bass hammer maybe has lost 1
mm and the highest treble perhaps 2 mm. This ends up just conflicting too
much with the bore length formula given. 

Indeed.. if you stop to think about it for a second,  any requirement for
the hammer to be perpendicular to both hammer shank AND string at impact
puts the hammershank on the string plane, i.e. significantly higher then
String height - Hammershank center pin would yeild.

So where does that formula come from and why hasnt this been tossed out
before and replaced with something more close to whats needed ?

We are using the term rake in the same way so thats not a problem Stéphane.
And yes I understand a hammer that is glued perpendicular to the shank and
is of appropriate length so that when it touches the string the shank is
parallel to the string means the hammer is also perpendicular to the string.
Its just that doing this conflicts with the string height formula I've been
working with for like ages... and thats whats got me scratching my head.

On this Bechstein.. the difference between the two protocols yeilds around a
4 mm difference in bore length as far as I can tell.  Thats a lot I'm sure
you'll agree.  Yes ?

Keep bouncing thoughts here you two... I obviously stumbled on a rather
large hole in what I thought I had already clear in my mind.

Cheers
RicB



RicB  wrote :
I'm not quite sure how you could use that trick to get an accurate shank
parallel to string plane at impact measurement.  At least not one that is
any more accurate then the measurement above.  One way or another some small
degree of error seems inevitable I suppose. My main querrie is really this
shank at horizontal bit. If I drop that requirement, then I'd be able to get
the hammer perpendicular to both the string and shank at impact if the
action cavity allows for it I susppose... or at least pretty close to it.
And that would account for a backwards rake yes?


Here I don't understand.  If the hammer core is perpendicular to the string
plane and perpendicular to the shank, then the shank is obviously parallel
to the string plane, not ?  and per definition, the hammer perpendicular to
the shank means that the rake is 90°, not ?  Or do we use different meanings
for the word rake ?  I don't call this a backwards rake, but rather
improperly no rake.  If the string plane angles up a little, then the shank
will be overcentering a little bit, if we agree that overcentering means
that the shank will be positively angled with the horizontal plane.  Did I
mix up things ?

Best regards.

Stéphane Collin.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/pianotech.php/attachments/20080621/94a53912/attachment.html