overcentering justified?

[David Love]

There are a couple of things to consider when deciding about the hazards of
overcentering (short boring).  I personally do not consider it such a
problem for a variety of reasons.  First, you are only talking about a
difference of 1/16".  The standard bore on a Model D is 1 15/16"  in the
treble and you report a measured bore of 2".  Second, the strings are
generally not parallel to the keybed.  They run slightly uphill to the
bridge.  Thus, the strike angle of the hammer, if calculated based on the
strings running parallel to the keybed, will not be precisely 90 degrees
anyway.  In addition, shank deflection will also influence whether the
shank strikes at 90 degrees.  The harder the blow, the wider the angle. 
There are many makers (Bechstein comes to mind) who have routinely short
bored their hammers and raked the angle out slightly to compensate.  These
pianos do not seem to suffer from repetition problems.  For these reasons,
undercentering (boring longer than the measured difference between center
and string height) is more likely to produce a hammer that does not strike
at 90 degrees.  Ironically, many hammer makers who bore for you add an
extra 1/16" for wear.  This extra amount is more likely to cause problems
with non 90 degree striking and weight than erring on the other side.   

The repetition issues have been outlined well by other contributors to this
thread and I won't address them.  One additional point, though, repetition
can be influenced by the weight of the hammer and its influence on movement
of the balancier beyond the effects of the repetition spring.  The new
hammer you are putting on is probably much heavier than the old one.  This
can cause the balancier to deflect more on unchecked blows such as are
found in the Ravel piece you mention.  I have found repetition benefits
generally from repinning the balancier so that there is more resistance (I
haven't really measured exactly how much friction, but more than comes out
of the factory).  This is not an area where excess friction will be felt in
the normal operation of the key.  Some additional resistance in this area
can prevent excess deflection of the balancier which allows the hammer to
reset a bit more quickly.  I perform this operation on performance pianos
routinely even when putting in new Renner wippens.  Shimming the rest
cushion with additional felt to insure that there is minimal (no more than
one hammer shank) distance between the bottom of the shank and the top of
the rest cushion can also help if the slight short boring raises the shank
too high.  Keeping the angle of the shank from dipping to low is important
as is making sure that your bore distance doesn't force you to have to set
the angle too low.  Try this experiment: weigh off a note at 1 3/4" blow. 
Now lower the shank so that the blow is 1 7/8" and reweigh.  You'll be
surprised at the difference.  

All in all, I would prefer a slight overcentering to undercentering, though
I usually bore exactly at the difference between shank center and string
height.  I can't think of an instance where boring longer than this would
be desirable.  I can think of an instance where short boring is necessary. 
That is when the length of the measured bore will not allow you to get the
hammers under the pin block when inserting the action.  This can happen on
some older Bechsteins and others.  Before removing original hammers it is
wise to measure the rake angle (as you have done).  If it is not 90
degrees, there is usually a reason.  Awhile back, I rehammered a Bechstein
in which the hammer height in each section was all over the place.  The
hammer bore, if taken from measurements, would have ranged from 1 15/16" to
2 3/16".  That was just in the treble section.  The bass was
proportionately longer.  The original treble hammers were bored
consistently at 1 15/16" while the bass hammers were bored at 2 1/8" and
the rake angle changed in each section to compensate for string height
differences.  Even with wear on the hammers, the action functioned
extremely well.  Boring the treble and bass hammers at the measured
distances would not have allowed the action to slide in under the block.  I
decided to duplicate the original bore and rake and found no problems with
getting the action to regulate or function properly.  

Just a few additional thoughts.

David Love
davidlovepianos@earthlink.net


----- Original Message ----- 
From: Bob Hull 
To: pianotech@ptg.org
Sent: 8/15/2003 3:57:53 PM 
Subject: overcentering justified?


List, 
I have a question about appropriate bore distance to see  if overcentering
is justified, even designed into the action from the first.  Here's the
background for the question(s): 
In determining the bore distance for a new set of hammers on a Hamburg D, I
have done the usual math to determine a perfectly level shank when the
hammer meets the string.
According to this the new hammers should be bored at 2" for the treble
hammers which is a considerably longer distance than the old hammers are
bored at and also longer than the specs I have from Steinway ( 48mm).
The old hammers overcenter due to their bore and even more so of course due
to their wear.  Was this intended by Hamburg in their design?
After boring at the longer distance I thought would be better than their
old short bore distance,  I see that of course I have to lower the capstan
to have a blow distance like the old and of course the let off.  The result
is that 1. The shank is only 1/8 or less of the cushion and 2. The
repetition speed is very mediocre.  (I can improve the repetition speed on
the long  bore distance hammer by decreasing blow distance to about 1 5/8".)
The old hammers repeated very easily and as fast as you could want.  (The
old blow distance is about 1 3/4".  

I searched the archives and read Ron Overs, Richard Davenport, Newton Hunt
and Dave Love about the benefits of a higher shank.  Also, Sam Powell's
article in the Sept. '93 Journal about the reduced friction that comes from
the knuckle not being too far below the line. (Effects of Hammer Bore on
Escapement Friction)  These seem to give some approval to overcentering.  
But, will there be an unacceptable loss of power?  Won't the hammer be
sliding into the string rather than striking it at 90 degrees?

If I resort to overcentering like the old hammers on this piano, would it
be advisable to hang them with a positive rake angle to compensate? 
Hamburg Steinway specs call for 0 rake.

I realize some of this may be a rehash of an old subject, but any good
guidelines would be appreciated.  
Thanks,

Bob Hull



Do you Yahoo!?
Yahoo! SiteBuilder - Free, easy-to-use web site design software