strike weight

[Stephen Birkett]

David Stnawood wrote:
> [....]
> To measure shank strike weight use the above method but with the
> hammer removed and the end of the shank resting on the felt wedge.
>
Your measurement of `shank strike weight' will lead to inaccuracies
if it's used for dynamic analysis. Consider just the shank
temporarily. Assume this is a cylindrical rod (for simplicity). The
shank strike weight measured as you describe will be 1/2 the weight
of the shank, since it is resting horizontally on two pivots. For
static analysis this is fine (first moment about the pivot). Using
the same `weight' in dynamic analysis will give inaccuracies,
because you need to use the moment of inertia (second moment about
the pivot) in this case to find the `effective mass of the shank'.
For a cylindrical rod the moment of inertia when pivotted at the end
is 1/3ml^2, thus the rod striking another mass (e.g.the string) at
its endpoint will behave as a mass of only 1/3 the total shank
mass...rather than 1/2 as you get with the static shank weight. The
same comments also apply to the hammerheads because their mass is
distributed about the strike line. Unless all the mass of the
hammerhead were located at the strike line there  is difference
between static (first moment) and dynamic (second moment)
`effective mass'. These differences may turn out to be not too
significant, but they should be checked....bigger hammerheads and/or
larger shank/hammerhead mass ratios would increase the significance
of the effect.


Stephen Birkett (Fortepianos)
Authentic Reproductions of 18th and 19th Century Pianos
Waterloo, Ontario, Canada
tel: 519-885-2228
fax: 519-763-4686