Hi JD:
>"The key pivot centre is not precisely at the balance pin centre;
>rather it pivots about a centre which moves towards the front edge
>of the key balance felt punching as the key is depressed. This
>causes a change on the ratio of the key-capstan pair.
Not on a piano with a properly designed balance rail. Some balance
rails are quite obviously properly designed, and others are better
designed than they appear at first glance, for example the Steinway."
Could you expand on this, please? As in how you go about determining how it
is properly designed and avoids the movement of the pivot point towards the
front of the balance rail punching. Any other comments in addition would be
appreciated also. Thanks.
Will Truitt
-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of John Delacour
Sent: Monday, January 18, 2010 8:49 PM
To: pianotech at ptg.org
Subject: Re: [pianotech] Action Ratios Recap - non-ideal approach
At 16:22 -0800 18/1/10, peter sharp wrote:
>I have been reading, with interest, some of the PTG email regarding
>deriving the action ratios.
One or two comments on your posting:
>In order to set up the piano action correctly, for touch, the hammer
>travel distance, as hammer blow, is regulated to fit the given dip
>at the front of the key. For any given piano, there is little choice
>about how much blow to set, once the dip is determined.
This assumes that you have a "given dip". I always regulate the blow
and set-off first, so that is the given. I then regulate the key dip
to achieve the proper escapement.
>There is some choice with aftertouch, and the technician looks for
>this in ensuring the jack has sufficient clearance from the knuckle.
>Pianists may also ask for a "softer" and "closer" aftertouch.
Pianists should be discouraged!
>Secondly, the hammer letoff must allow for escapement whilst at the
>same time give control at all dynamics; for the pianist, a close
>letoff will give nice control for pianissimo, but for the
>technician, it may lead to double-striking or bobbing of the hammer.
>Again, there is some small choice with let-off.
Close set-off will not lead to burbling if the escapement, the spring
tension, the check angle, the hammer tail etc. are properly regulated.
>The response of the hammer to the key travel is non-linear; that is
>to say, the ratio of displacements (also called the velocity ratio)
>varies as the key is depressed. This is because the internal ratios
>of the action components vary as they move.
Certainly, but the precise ratio can be calculated at any point.
>The key pivot centre is not precisely at the balance pin centre;
>rather it pivots about a centre which moves towards the front edge
>of the key balance felt punching as the key is depressed. This
>causes a change on the ratio of the key-capstan pair.
Not on a piano with a properly designed balance rail. Some balance
rails are quite obviously properly designed, and others are better
designed than they appear at first glance, for example the Steinway.
>The capstan, in turn, slides across the whippen heel, so that the
>velocity ratio varies for this capstan-jack pair. We can regard the
>jack to whippen pair as non-variable, and does not need to be
>considered as an additional pair here.
In a properly designed and set up piano the capstan does _not_
"slide" across the lever heel; they roll round each other as two
pinions. The slippage is so slight as to be almost completely
negligible over the whole range of the motion, even if there is a
slight deviation from the designed set-up. The velocity ratio does
indeed vary, but not for this reason. The capstan is on a path of
decreasing ascent while the lever heel is on a path of increasing
ascent for a given angular velocity, and the arcs are of different
radii. That is the reason.
>The jack-to-knuckle contact point varies as the jack slides (rolls)
>across the knuckle as the hammer lifts, and changes the velocity
>ratio across this pair.
Here, yes. But the same considerations apply in addition.
>With these three pairs of varying component ratios within the
>action, a simple multiplication of the three ratios to give a number
>consistent with the overall velocity ratio is not so easy; this is
>due to the difficulty of determining exactly which ratio we should
>use for each pair.
It is also difficult to eat soup with a fork. Use a spoon and the
difficulty vanishes.
>...Using dial gauges is useful, but becomes tedious when working in
>the real world.
The measurements necessary for proper and precise action set-up are
simple and can be made with a millimetre rule. Precision
measurements might tell you something is wrong but they won't tell
you how to put it right.
>The point here is that to try to derive this result theoretically,
>by assuming certain positions for lever centres and points of
>contact, is fraught with frustration, unless you recognise that the
>ideal calculations must be modified by the non-ideal behaviour of
>components turning on wool and leather about movable rotation
>centres. And theoretical ratios can only be taken at one position,
>being at letoff.
As I've said above, the precise ratios can be calculated at any
position of the key. There is nothing "ideal" or "theoretical" about
these calculations and the wool and leather have nothing to do with
it. If the "moveable rotation centres" were random points, then sure
it would be tricky, but they are precisely defined at every moment.
>All of this recognises that the action component ratios, within each
>of the three pairs, are themselves able to be modified to give
>different responses within the same overall action ratio. But that's
>another story.
No, that is THE story.
JD
--
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