Key Ratio ?

[John Hartman]

Richard Brekne wrote:

> That seems clear enough, and I find it curious that the quantity of Balance
> Weight so well conforms to measureing all three levers very much  in this
> fashion. In fact, when you think of it,  that the Stanwood measurements yeild
> the correct quantify for BW so well is kind of odd given the fact that the
> weight measuments taken have all the parts in an orientation they never all
> three find themselves in at the same time in the action itself.

The Stanwood method works out like this because it is based on weight 
measurements not lever arm measurements. The two are related but not the 
same.



> Dont you also have to take into consideration the orientation of the parts at
> the their starting point. In the example I gave, it seems clear that the
> amount of purely vertical movement in the back of the <<key>> compared to the
> front is going to vary dependent on where in their respective arcs each point
> is at the start of motion. If the <<key was tilted forward enough... grin...
> the back end would actually pass through the top of its arc,... or if it was
> tilted back from horizontal it could rise more with downward motion at the
> front. In that sense it almost seems meaningless to talk about a ratio which
> mixes change in vertical movement with a lever that moves in an angular
> fashion. Yet this is exactly what we do all the time.

I am not clear about what you are saying. Are you saying that the key 
ratio changes depending on its angle of inclination?. You should sketch 
this for us.


> What do you mean by the back length of the shank ?

 From the center pin to the hammer.


> What do you mean by the "pitch point" ?

The pitch point defines the ratio of motion of contacting profiles like 
the rep lever/knuckle or the capstan/wip heel. It is the virtual lever 
arm lengths. You need this if you are interested in the relative motion 
of action parts. For convenience the action's half stroke is used to 
figure action transmission ratios but in fact the pitch points move 
during the key stroke so relative motion is changing during the key stroke.


John Hartman RPT

John Hartman Pianos
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Rebuilding Steinway and Mason & Hamlin
Grand Pianos Since 1979

Piano Technicians Journal
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