Hammer assist spring ?

[Mark Davidson]

You can minimize this effect by using a weaker spring and winding it up
more.
A spring that generates less force per distance change will have less total
percent
distance change for a given movement.

For example if you have a spring that generate 15 grams when compressed 90
degrees,
there is an approximate spring equation that says force = K * degrees.  K in
this case would
be 15/90 = .1666.  A 10 degree change results in a change of 10 * .1666 =
1.666 grams.

A weaker spring with twice as many windings will require 180 degress
compression to get the
same force.  This gives K = 15/180 = .0833.  For this spring a 10 degree
change results in only
a .833 g change in force.

So you can have the same force but less change in force and less of a
"springy" effect
by using more windings (or smaller gauge wire).  Of course there's only so
much
space for that little spring.

-Mark

----- Original Message -----
From: "David C. Stanwood" <stanwood@tiac.net>
To: <pianotech@ptg.org>
Sent: Saturday, June 07, 2003 4:19 PM
Subject: Re: Hammer assist spring ?


> >This progressively reducing assist force is one of the disadvantages
> >of assist springs. The wippen, by virtue of it small degree of
> >rotation, is the more practical of the two levers to assist. We have
> >reduced the lack of linearity by including an extra coil in the
> >spring. And yes I agree with you Bill (Ballard), when there is no
> >adjustment screw, setting the assist spring pressure is nothing short
> >of a pain.
> >
> >Ron O.
>
> Hi Ron,
>
> I have never considered it to be a problem because the motion of the
hammer
> when measuring up and down weight without springs is not significantly
> different with them.  You roused my curiosity..... (Thank You!) so I just
> did this little experiment:
>
> I set up a key on the digital scale to measure Front Weight.  I attached a
> rail to the metrology table and hung a wippen, (with very low friction in
> the flange), on the rail so that it sits nicely on the capstan.   I loaded
> jiffy leads on the wippen to simulate the force of the
hammer-shank-knuckle
> that would normally sit on the wippen.  I then held the front of the key
> down on the scale by loading the front of the key with a 100 gram weight.
> Then I tared the scale to zero and hooked up the spring.  The scale
reading
> indicated the spring was working 17 grams, a figure considered by many,
> including myself, to be a safe maximum for the amount that a wippen spring
> should be designed to reduce the touch weight by.   Then I lowered the
> scale by 10.5mm simulating a maximum keydip.  The reading changed by 1.5
> grams.  When the spring was working 12 grams the reading was closer to
> gram.  So it looks, according to this sample, like about an 8% reduction
in
> the working effect of the spring at the front of the key from top of
stroke
> to bottom...
>
> Hmmmm... Interesting.....
>
> Regards,
>
> David Stanwood
>
>
>
>
>
>
>
>
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