[pianotech] key position at rest

[David Love]

Is there an established general guideline with respect to the height of the
damper flange center such that with a given damper flange center height the
height of the key end felt off the key bed should be x.  My own approach,
like many, has been to simply to set the action and back action on the
bench, pull a string, depress the key, twist my neck into some position that
might see where things should go and make a best guess as to what should be
changed and how much, do it again.  But with a given back action whose
damper flange downward angle is too acute it seems that there are two
approaches.  You can either lower the height of the back action or increase
the thickness of the key end felt.  Increasing the height of the key end
felt is easier and on the surface has fewer related problems (dag
interference, hole relocations, etc.).  While the distance from the damper
flange center to the balance rail changes from action to action and with it
the slope of the line drawn between them, it seems it would have to change a
lot before the any significant change in the height of that line off the key
bed at the flange/key end felt interface would change.  Just thinking out
loud here.   Nevertheless, it shouldn't be that hard to develop a plug in
formula that measures the flange center height, the distance from the flange
center to the balance pin, the height of the key/balance rail contact point
(off the key bed) and the distance from the balance pin to the key end felt
and come up with the optimum height of the key end felt (off the key bed).
Has anyone done that (before I take it on)?  With overly acute angles of the
underlevers being a common problem it would be an easier approach (I hate
trying to pull all those strings in line and get my eyeballs on some plane
that hopefully sees what's really happening).  Am I missing some downside to
doing it that way?  I prefer math to mock up and best guess when I can get
it.  

 

David Love

www.davidlovepianos.com

 

From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Jon Page
Sent: Sunday, March 15, 2009 6:54 AM
To: pianotech at ptg.org
Subject: Re: [pianotech] key position at rest

 

Sorry I don't quite get it ... but  1 1/2 inch from where ?  From exactly
what point

on the whippen to exactly what point elsewhere does this 1 1/2 inch come
from ?

 

>From the key bed. Generally a S&S will have the key end lifter felt at rest
at ~1 3/8"

from the key bed and the underlever at rest is around 1 1/2" from the key
bed

which is lifted by the key to around 1 3/4" from the key bed.

 

As an aside, knowing the height to which the underlever is lifted allows

you to determine if the underlever is situated at an optimum angle.

Measure the height of the end felt with the key depressed. Lift the front

of the underlever to that measurement and notice the lever's angle relative

to the key bed. If it is parallel, that is ideal. If it is still angled down
then that

indicates that the lever is scrubbing too much on the felt (increased
friction)

and there is probably a trough worn into the felt.

Lifting beyond parallel is counter productive as well.

 

I think it was in the same class (involute gear) with Chris Robinson

that he drew a chart with an x and y axis. Friction is minimal along each
axis

but as you migrate further into each quadrant, the friction increases.

 

So looking at the underlever as being dropped down from the horizontal x
axis

with the center pin on the y axis, it makes sense to lift it to a point of
least friction

(x axis) which also happens to be the point of Let Off at the end of the key
stroke.

 

This is why I reset the tray pivot such that the underlever is lifted to
parallel

to the key bed or to the x axis. Lifting beyond parallel crushes the corner

of the felt and reintroduces sliding friction (getting into the upper
quadrant).

 

 

>...capstan angle...why is the standard today then perpendicular to the key
?

 

Ease of installation. Mark a point under the cushion, drill them straight
in;

no need to waste time fiddling around with an angle.

-- 


Regards,

Jon Page

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