Key Height

[Farrell]

Please forgive me, some of your terminology is unfamiliar to me. I assume the soldier is what others might call the capstan? And the backtouch is the back rail cloth/key interface? Your 3.75" figure would be for an upright with tall capstans. Your figures are OK for that case, I was envisioning a more common configuration where the capstan was more like 1.5 inches above the back rail felt with key at rest. In that case the movement would be about 0.0117 inches - well about my estimate. Sorry, next time I'll do my math before inserting foot into mouth!

Terry Farrell
  
----- Original Message ----- 
From: "John Delacour" <JD@Pianomaker.co.uk>
To: <pianotech@ptg.org>
Sent: Sunday, May 05, 2002 4:26 PM
Subject: Re: Key Height


> At 9:33 am -0600 5/5/02, Joe And Penny Goss wrote:
> 
> | Even though the distance might be slight, if there is a dimple in the felt
> | cushion, moving the capstan slightly will make a rather significant
> | reduction in friction at the start of the key movement. Sort of not having
> | to climb out of a hole.
> 
> Dimple or no dimple, the soldier top should be positioned directly under the centre of the lever heel.  The presence of a dimple (and there always is one) will make precisely nil difference to the friction, since friction is subject to the following basic laws:
> 
> (i) The frictional force is independent of the apparent area of contact,
> (ii) The frictional force is proportional to the normal reaction between the surfaces
> 
> In other words it will take the same number of mice to drag a packet of tuning pins along if it is stood on end as if it stands on its base.
> 
> What will make a difference in friction is if there is more than the ideal sliding motion at the contacting surfaces, and this is minimized by proper alignment of the three points mentioned in my previous message so that the movement is a rolling motion of two dissimilar arcs entailing a _very_slight_ amount of sliding owing to their different radiuses.  In a properly designed and regulated piano, whether grand or upright, wear at this point is negligible.
> 
> The alignment at half-blow I have described is traditional and accepted practice.  It has often occurred to me that it might be better to have the points aligned not at half-blow but at key rest, since the frictional forces are generally many times greater when the key is first struck than at half-blow.  According to the accepted practice, the sliding motion is greatest when the frictional force is at its maximum.
> 
> The moving of the soldier top towards the back will (however slightly) increase the leverage, add weight to the touch and require a shallower touch depth without affecting the friction except in so far as the correct alignment is disturbed.
> 
> At 10:28 am -0400 5/5/02, Farrell wrote:
> | Let's say you put a 0.050" punching under all the keys at the center rail.
> 
> That would be a very unusual thing to do on an upright when the same effect can be achieved with card or veneer under the _balance_ rail.
> 
> | a good 1/8". That's a fair bit of a raise. How far would the capstan tops
> | rotate back? Hmmmmm. I calculate..... It just couldn't be more than 0.005".
> 
> Well it will move  hs / lk * 0.125" where hs is the height of the soldier top from the backtouch and lk is the distance from the backtouch to the key front, so for hs = 3.75" and lk = 14.00", the horizontal displacement will be over six times your figure at about 0.033" and even on a grand it must be more than 1/100".  Not a huge amount in either case, but if we're calculating, let's calculate!
> 
> 
> JD
> 
> 
> 
>