The mechanics of a grand action are brilliant, a series of levers that reaches inside the piano while converting finger force into a hammer force that can play strings with musical expression. Such a feat has many complicated details, but observed individually, they are logical, pragmatic – mechanical. Geometry is the design engine for this collaboration. And its leverages both transfer these forces and transform the distances they travel. Each finger’s mechanical advantage (2 kilograms to play a 10-or-less gram hammer) is divided by 5 in a tradeoff multiplying finger travel distance by 5 into a hammer travel distance that accomplishes fff. Or ppp.
Levers: Hammer weights and travel parameters get amplified or de-emphasized depending on the geometry between keytop and hammer crown. Generally, weight reduction at the key equals loss of hammer rise, and each of the three main levers contributes to this as a give and take. Their relative impact varies, of course, so we benefit by having a well-observed hierarchy in mind as we decide what to try.