Key Inertia

[John Hartman]

I have a simple way to visualize what happens to the kinetic energy in a 
grand action.

Assume for a moment that the action has no friction and all the parts 
are perfectly rigid, there are plenty of losses due to these factors but 
it helps the thought process if you get them out of the way temporarily. 
Set the let off so the hammer blocks on the string and remove the front 
rail punchings. Press the right pedal down so the dampers are disengaged.

When you play a note set up like this almost all of the force applied to 
the key is transferred to the string through the hammer. The only loss 
is due to the compression of the rep spring. The force applied to the 
key accelerates all three components, the key, wippen and hammer. They 
all come to a stop when the hammer strikes the string and all of the 
energy in all the parts gets applied to the string.

Now regulate the note with proper let off and dip. Again the force 
applied to the key accelerates all the parts. When the hammer disengages 
from the rest of the action at let off it strikes the string delivering 
it's kinetic energy. The kinetic energy of the wippen goes into the let 
off punching, drop screw and rep spring. The energy of the moving key 
gets dissipated into the key bed.

The points are:

1) The only force that gets transferred to the string is that portion of 
the force applied at the key that works to accelerate the hammer and shank.

2) The portion of the force applied to the key used to accelerate the 
key and wippen do not go into into the string but are wasted.

3) Removing mass from the key and wippen will make the action more 
efficient.

4) Given our ideal action with no bending and friction losses changing 
the hammer mass does not effect the efficiency of the action.

Note:

Efficiency is not the whole story. The action needs to be designed to 
work within the range of force that is comfortable to the player. The 
force applied by the hammer to the string needs also to be controllable 
by the player so that dynamic shading is possible.  Furthermore, 
changing the efficiency could adversely effect the tactile feed back the 
player gets from the front rail. I can envision an action that is very 
efficient, that plays loudly with ease but the player is hindered by a 
lack of control and confused by unexpected tactile feed back.

John Hartman Pianos
[link redacted at request of site owner - Jul 25, 2015]
Rebuilding Steinway and Mason & Hamlin
Grand Pianos Since 1979

Piano Technicians Journal
Journal Illustrator/Contributing Editor
[link redacted at request of site owner - Jul 25, 2015]