[pianotech] Increasing bridge height

[Will Truitt]

Hi Jude:

Did I just hear the monkey wrench being dropped into the whirring gears?
:-)

Let me start by saying that I lack the knowledge and understanding of action
relationships and geometry that you and others on the forum have.  So if my
questions lack sophistication, please forgive me.  Still, I would like to
have a better practical understanding of what is going on here, and how
changes in bridge height will reverberate throughout the action train.  

Where do we have an ideal string plane -within the designer's CAD program on
the screen?  Or does it exist in real production pianos, other than by
occasional dumb luck?  Given the vagaries of the warpage of plate castings,
workmanship of varying accuracy, combined with Steinway's ridiculous method
of top fitting and face fitting the pinblock in the piano (where we end up
with widely varying string heights from piano to piano, as well as similar
discrepencies for and aft) I do wonder how often we see the ideal,
particularly in Steinways. 

I think that leveling the plate out from bass to treble is what Del is
talking about, by increasing both the thickness of the pinblock and the
thickness of the bridge, at least at the treble end. How much of each I do
not know.  

One question I would want to ask is this:  If I make a change to the bridge
height and possibly the pinblock thickness (which would then definitely
raise the string plane at the strike point), and I accommodate that by
boring my hammers to the new string height values (even to the point of
tapering the bore as needed, and further customize the back check - hammer
tail relationship to insure proper checking, how would this change
reverberate through the rest of the action train?  What else would be
changed in a significant way, and how would I address those changes in
re-setting the action?

Will 

 
 
-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Jude Reveley/Absolute Piano
Sent: Monday, March 23, 2009 10:29 PM
To: pianotech at ptg.org
Subject: Re: [pianotech] Increasing bridge height

Hi Will,

The way I see it, you have an ideal string plane that is level and parallel 
to the keybed across the compass and down the string longitudinally (prior 
to being offset by the bridge). The design spec for string height is 
directly related to the ideal action geometry via the strike point, hammer 
bore, center coordinates, parts' geometry, key proportions, entrance height 
etc. ie change one, change all.

So now it depends where you want to start redesigning this thing.

First, we know Steinways are designed with a string height of 7-1/2" or 
191mm. It is typical to see them dip down in the high treble to 189 or 188. 
I would correct this if I could to level the plate out from bass to treble 
allowing for some upward bowing in the tenor section and minor discrepencies

that cannot be corrected without refitting the capos.

Now imagine your ideally level longitudinal plane being parallel to the top 
of your bridge. Insert the bridge and you have two angles formed that 
together make up your downbearing. This composite angle of 0.5 to 1.5 degree

angle is barely perceptible and changes ever so slightly, depending on the 
ratio of the speaking length to the backscale and changes to the bridge 
height and counter bearing height. I'm not BTW taking into account any 
changes this will introduce to the load. The further the bridge is from 
strikepoint, the less any minor changes in bridge height will have on the 
string height. In addition, the shorter the backscale relative to the 
speaking length, the greater that rear bearing angle will be. Thus you may 
be able to make your plate height adjustment at the back to accommodate a 
taller cap without affecting the string height.

If you're going to change the string height, do so purposefully, knowing all

resultant changes.

As for spruce roots, I'm not sure speed of sound is a factor in range of 
human hearing as far as I know; although I don't doubt they sound different.

Not to mention that any inherent stiffness in additional height is easily 
mitigated by the material.

Jude Reveley, RPT
Absolute Piano Restoration, LLC
Lowell, Massachusetts
(978) 323-4545