Moment of Inertia of grand action parts.

[Erwinspiano@aol.com]

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            John
            Thanks for the non math clarification. My comments by the way 
should not be taken as a complaint. Your desription of MOI is useful & I see more 
clearly where this discusion is trying to go. Intuitively many of us realized 
long ago that measuring static weights could not predict what an action does 
in a dynamic sense we just didn't have a tool for analyzing it. Methods 
popularized & developed by D. Stanwood has helped enomous. This to me is the next 
step. Yes it will take time.
    Dale
Dale wrote:
>  Ron
>   I just wanted to say publicly--Thanks . This is a great clarifying 
> summary. Clarification is always enlightening. Your post also points out 
> that "the confusion that remains is still how all this stuff 
> mechanically relates" which is true. At the end of the day we have to 
> put all this into a useful sytematic practice in rebuilding the actions 
> we all work on.These discussion  increase our confidence secure better 
> results for us & perfromance benifits for our clients.


Dale,

Several people have complained that this material has just brought more 
  confusion to our understanding of the grand action. That's 
understandable since the study of the dynamic action is at least ten 
time more involved that studying the static action. It took many years 
for the simple static principles Stanwood has developed to be accepted 
and understood. I would expect these dynamic principles to take a lot 
longer. The task is especially daunting since getting a mental grasp of 
how it works requires familiarity with math and physics. I expect that 
most piano technician's would need to bone up on high school level 
algebra and physics to gain access to this knowledge.

Even though this is going to be a lot of work, as you said, it can be 
accomplished one step at a time. The first step is getting the notion of 
moment of inertia clear in ones mind. Envision a cylinder attached to an 
axle with a rope wrapped around it. We pull on the rope and the cylinder 
rotates around its axle. If the cylinder is made from a light material 
like wood, it is easy to pull the rope. If it is made from a heavy 
material like steal, it will be hard to pull the rope. The moment of 
inertia describes how hard it is to pull the rope and get the cylinder 
to move. Knowing the MOI of the cylinder and the radius we can predict 
how hard it is to move. We can also know how much tension there is in 
the rope. With a high MOI the tension is high and with a low MOI the 
tension is low.

"What the heck does this have to do with pianos John?". Well, if the MOI 
of the action is high it will feel harder to play. I am not sure if the 
player could feel this with playing cords slowly at various dynamic 
levels but it will certainly feel hard to play scales and fast passages. 
Let's imagine a room full of our cylinders each with a rope attached. 
Your job is to pull each of these ropes in fast order. It will be a lot 
easier if they are made of wood rather than steel.

But what about balance weight? Use the cylinder again, but this time 
wrap another rope around the back side and attach a weight. have a shelf 
for the weight to rest on. Now when you pull the rope you have a weight 
to lift along with the inertia of the cylinder. The cylinder doesn't 
move until the weight is lifted. Go back to the room of cylinders. Would 
you like to have the wood cylinders with a heavy weight attached or the 
steel ones with lighter weights?

One of the things learned from studying MOI is just what Balance weight 
does. It determines (along with friction and let off resistants) the 
minimum force to move the action. The total force after that is 
determined by the force required to accelerated the action minus the 
force of the BW. At very soft levels of playing the BW will be a 
significant part of the total force while at forceful levels the balance 
weight is insignificant. Through the dynamic range of playing the force 
needed to accelerate the action increases while force to overcome the BW 
stays the same.

Hope this attempt at a non math explanation helps.


John Hartman RPT

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