Fw: bore distance: was Re: hammer replacement

[David Love]

Ron, Dale, Newton:

It's not just the friction reduction that changes the resisitance as the 
shank rises.  The function that describes the change in force required as 
the hammer rises is exponential (y/cos0--don't have the symbol for theta).  
So the rate of reduction of resisitance as the hammer goes through it's arc 
is not constant.  If by raising the shank 2mm a reduction of 4 grams in the 
force required to get the hammer moving is achieved, the next 2mm will not 
achieve the same amount.  (I'll have to review my trigonometry to describe 
it mathmatically.)  Anyway, if you graph out the change in the required 
force to move the hammer through the stroke, you will see that there is a 
point (approximately 1/2-2/3 through the stroke) where the change in force 
for the duration of the stroke becomes nominal.  Let's say, for arguments 
sake, that at that point the force required to further raise the hammer is 
40 grams.  If you then move the shank closer to the string at the start (as 
a result of short-boring or over-centering by 2 mm, then the range of total 
change in weight becomes 46 - 40 or 6 grams rather than 50 - 40 or 10 grams. 
  The action is not just lighter, but the stroke should feel more consistent 
from beginning to end.

David Love

>From: Ron Overs <sec@overspianos.com.au>
>Reply-To: pianotech@ptg.org
>To: pianotech@ptg.org
>Subject: Re: Fw: bore distance: was Re: hammer replacement
>Date: Sun, 1 Jul 2001 02:32:18 +1000
>
>David Love wrote:
>
>>In a class given by Richard Davenport he demonstrated that over
>>centering the hammer by 2 mm resulted in a reduced DW of 4 grams and
>>an increased UW of 1 gram.  The balance weight is reduced by 2.5g as
>>is the friction.  Let's assume this is true.  Of additional interest
>>to me is this:  The change in downweight is measured from the rest
>>position.  But the downweight can be measured from any point in the
>>stroke.  If you measure the downweight from a point, say, when the
>>hammer is halfway to the string, you will get a reading that is
>>something less than the downweight measured from rest position.  But
>>the downweight measured from the halfway point should not vary as
>>much between the over centered and normally centered hammers.  If we
>>hypothesize, for the moment, that measured starting from the halfway
>>point, the downweights are equal.  Then in the normally centered
>>hammer there would be a 4 grams greater difference between the
>>downweight measured at rest and the downweight measured halfway
>>through the stroke.  Another way of saying it is that the rate of
>>change of resistance through the key stroke would be greater in the
>>normally centered hammer than in the overcentered hammer.  If that
>>is true, it suggests to me that the overcentered hammer would have a
>>"smoother" feel.  The key would not feel as though it is falling
>>away from you once you got it moving or that the resistance goes
>>through a more sudden change.
>
>This all sounds like excellent thinking to me David. I believe the
>overcentered hammer does have a better feel (ie. in a standard action
>- with our action it feels like its overcentre and more with standard
>boring). The further away from the rest position that the downweight
>is measured, the less becomes the jack/roller contact offset from the
>line of centres. Hence the downweight reduces as the hammer rises in
>response to the reduced friction and the lowering hammer leverage
>ratio. The action we are bringing to Reno in our piano has a
>hammer/key ratio of 6.0:1 at the line of centres (ie. when measured
>at half key stroke), and yet it still feels light and fast with a
>full concert grand set of Abel hammers installed. Because the
>hammer/key ratio is more linear in our action throughout the key
>stroke, we have to set the ratio at 6.0:1 to get proper aftertouch
>(where the jack clears the roller in check) with a 10mm + a fraction
>key dip. You can't beat getting rid of friction to make an action
>'fly'.
>
>>On another matter, the loss of power from an overcentered hammer I
>>believe comes from the hammer delivering a glancing blow to the
>>string (if it is not perpendicular on contact).  Raking the hammer
>>out should solve that problem.
>
>You may be correct here. Although, even if the hammer is angled back
>with overcentre boring, the hammer mass will still tend to travel in
>a direction which will stroke the string rather than strike it. I
>have suspected that the closer the hammer centre is located to the
>string plane, the greater the power, for the same reason.
>
>Greetings to you too Newton, I'm looking forward to meeting you also
>at Reno. Our piano left Sydney today, bound for LA. Enjoying a
>weekend with the family for the fist time in some months.
>
>Regards to all,
>
>Ron O.
>--
>Overs Pianos
>Sydney Australia
>________________________
>
>Web site: http://www.overspianos.com.au
>Email:     mailto:ron@overspianos.com.au
>________________________

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