Keyweights and moment of inertia

[Doug Richards]

Rob, Bob and all,

You can measure the moment of inertia on parts using a few long strings.
Hang the part by two or three strings so that the inertia axis you want to
measure is perpendicular to the ground and the "pivot" is adjusted by where
the strings are attached.  Then give it a twist.  The frequency of the
oscillation is related to the inertia.  You  have to measure the string
length and the distance from the "pivot", but there is a formula that will
relate these to the inertia of the part (I'll have to dig it up if you are
interested).

Otherwise, we have a machine that was designed (using a torsion spring) that
we attach parts to that we want to find the inertia of when designing disk
drive components.  I think I've got some inertia numbers for a friends
Yamaha upright (key, wippen, and some hammers).

doug richards
San Jose, CA

> -----Original Message-----
> From:	Richard Moody [SMTP:remoody@easnet.net]
> Sent:	Wednesday, October 07, 1998 11:34 PM
> To:	pianotech@ptg.org
> Subject:	Re: Keyweights and moment of inertia
> 
> Rob, Robert, List
> 	 
> 
> > Also, does anyone
> > make a device that measures total moment of inertia?
> > 
> > Bob Scott
> 
> The question is can moment of intertia be measured? If so, so what?  This
> is where I get stalled in physics. Newton says that force equals mass
> times acceleration. Now  another law Newton came up with says that a
> body (mass) in motion will keep that same motion unless acted on by other
> forces. 
> But here acceleration is zero, so does that mean a body in motion has no
> force?  
> 	But where I flunk out in physics is asking the question "if a body
> in
> motion has no acceleration, then what happens to the formula f=ma ?"  (a =
> 0 )  From that I suppose there is a formula that should read x=mv.  here m
> = mass and v = velocity, but what then is x  ? 
> 
> 	So to put it in perspective,  I would suggest to forget about the
> "laws"
> of "moment of interia" as far as piano technology is concerned. Where ever
> the weight is placed on the key, I suspect these "laws" would predict an 
> effect close to zero.   As far as the player is concerned and how the key
> pressing down to him feels...what counts more, the praise of the player,
> or the plaudits of physisysts? 
> 
> Ricisyst
> .. 
> 
> 
> ----------
> > From: Robert Scott <rscott@wwnet.net>
> > To: pianotech@ptg.org
> > Subject: Keyweights and moment of inertia
> > Date: Wednesday, October 07, 1998 9:17 AM
> > 
> > Rob Kiddell wrote:
> > 
> > >    Recently, I published  an article on grand piano action 
> > > diagnosis, and one of the conclusions I came to was re-balancing 
> > > keys with many 1/2" leads per key. Basically I remove larger leads 
> > > near the balance rail and insert smaller leads near the end of the 
> > > key. This seems to make the actions feel inertially lighter, and 
> > > there is less 'thumping' from heavy keys on balance rails. However, 
> > > it was brought to my attention that (I quote here)
> > 
> > >   When the key is moving the "moment" of inertia becomes much 
> > >   more important. Unfortunately it is affected by the distance
> > >   from the balance rail by a factor of R squared.
> >