[pianotech] Underlever Spring Tension Measurement

[Dean May]

I'm trying real hard, but I can't visualize your method of adjustment
 

Dean

Dean W May                (812) 235-5272

PianoRebuilders.com    (888) DEAN-MAY

Terre Haute IN 47802

 

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From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Ed Foote
Sent: Wednesday, July 28, 2010 2:10 PM
To: pianotech at ptg.org
Subject: Re: [pianotech] Underlever Spring Tension Measurement







 Greetings, I wrote: 

   I set the underlever springs by measuring several along the length and
marking them with a spot of chalk after setting them to the appropriate
tension. Turning the whole assembly upside down, I give it a slight up and
down shake.  It is easy to then set the entire rack to the index underlevers
by simply pushing the levers down a little until they also move the same
amount as my guides. 
Regards,



David asks: 

What tension do you like that balances effective shut off and minimal
touchweight differences with and without the pedal. 

    I like the least amount of tension possible.  Usually that is the
minimum recommended by the string kit,(can't off hand remember what). It
also means on a  the upside-down damper tray, with flanges and springs
attached, the underlevers will all bounce together, with the more treble
ones showing a little more motion, tapering to virtually no spring at the
last sprung lever. . The heavier underlevers will thus have heavier springs.
I do taper the spring force so that there is  no discernable "step" between
the last spring and the first unsprung underlever.   I think the work is
actually being done by the weight, and while the springs contribute  to the
down force,(of course), their pressure doesn't  allow the string movement to
throw the dampers back upwards and off the string under heavy oscillation.  
     The slow motion pictures of dampers reseating have amazed me in that
the upward string excursion can send the dampers, (and underlevers), upwards
strongly enough so that the damper leaves the string after contact.  I think
the springs do a lot of work simply preventing the underlever's momentum
from causing the assembly to leave the string.  
      Also, as Kent mentions, it is very easy to weaken a spring too far, so
one has to learn how little depression of the upside down lever it takes to
bring its spring into agreement with the guides. This is not a scientific
approach,but rather, one that I have empirically arrived at.  It takes into
account the resistance provided by the friction of the spring as it slides. 
Regards, 
Ed Foote RPT 

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