Hammer Shank Ratio

[Ron Overs]

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Phil, Richard and all,

A couple of good posts from you today Phil.

>Ron Overs replied:
>
>When our piano was exhibited at Reno, David
>Stanwood made a calculation of the leverage ratio of the action 
>using his weighing method. He got a figure of 5.5:1. Now I designed 
>the action for
>this piano on CAD to have a ratio of 5.8 (at the strike point).

At 6:21 PM -0800 13/2/03, Phillip Ford wrote:
>
>What do you mean by a ratio of 5.8?  Hammer strike point moving 
>5.8mm for 1mm movement of the key?

Precisely, as calculated on the CAD drawing with the key at half dip 
(in the case of our action with the jack/roller contact on the line 
of centers). As I have mentioned on previous occasions, the real 
ratio will deviate somewhat throughout the key stroke. However, as 
you said in your post of today, if we work with the same set of 
parameters when measuring different actions, it is certainly possible 
to work out appropriate settings for various different installations.

>
>>   If you take the 5.8 ratio and multiply it by 130/138, you get 5.46. I
>>  believe this may explain the different figures arrived at via the two
>>  different measurement systems.
>
>Where do the 130 and 138 figures come from?  Are these actual 
>numbers from your action?  On your attached sketch you showed 136 
>for hammer location.

Yes they are the numbers from our action installation into our 225 
piano, 130 mm is the distance from the hammer center to the center of 
the hammer moulding, while 138 mm is the distance from the hammer 
center to the strike point of the treble section hammers. The bass 
section strike points will obviously have a higher ratio than the 
treble since the distance to the strike tip is greater. Furthermore, 
the bass section hammers will be slightly heavier at the cross since 
the hammer tail is longer. This is a believe a causal factor of a 
tone break at the crossover, although the relative sound board 
activity with respect to the two bridge ends is the more influential 
factor. Nevertheless, in order to minimise the effect of the 
differing hammer weights between the bass and treble hammer across 
the break, we leave the treble hammer tails with more wood thickness 
that the bass tails to minimise any hammer weight transition across 
the break (I've digressed again!).

>On your attached sketch you showed 136 for hammer location.

The sketch was copied straight from one of my CAD drawings so it is 
not scaled to reflect the numbers shown, which were taken straight 
from Richard's example.

Richard's posts have been interesting, his posts of a few weeks ago 
got me thinking about this again (although I am currently very busy 
on other aspects of piano making at present, so I'll need to do some 
more work later on to test a few things). The understanding of 
actions will always be a work in progress.

>Another problem is that ratios derived from travel measurements that 
>I have seen are based on traveled lengths.  Neither the hammer nor 
>the key are traveling in a straight line so using lengths (such as 
>45mm of upward hammer movement happens in 8mm of downward key 
>movement and therefore the ratio is 5.6) seems erroneous to me.  It 
>make more sense to me to talk about traveled angles or arcs (such as 
>3 degrees of key movement results in 16.5 degrees of hammer and 
>shank movement and therefore the ratio is
>5.5).  Perhaps measuring in this way would give better 
>correspondence between travel and weight.
>
>Phil F

I agree entirely with you Phil. The arc of rotation of the hammer tip 
relative to the arc of rotation of the key front is perhaps the 
significant factor. Of course it gets messy when we consider that 
there are so many different key stick lengths in use. So in practice, 
it is generally easier to measure the linear distance of travel of 
the arcs we are investigating. If you work out the error between the 
circumference of an and the linear distance it insignificant. We are 
more likely to make larger errors in the translation of the CAD data 
to the working action. I practice, I try to maintain dimensional 
accuracy to 0.5 mm or less.

Best,
Ron O.
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_______________________

OVERS PIANOS - SYDNEY
Grand Piano Manufacturers

Web: http://overspianos.com.au
mailto:info@overspianos.com.au
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