torque/ bobbling thread.

[Richard Brekne]

Ok... so I see I should have avoided bring <<grams of friction>> into 
this picture.  That said, it was accompanied by quite a bit of 
descriptive text so as to point more in the direction of being 
interpreted in a more general sense.  The same basic amount of 
<<tightness>> (if I dare use such an unqualified term) in the jack 
center pin  should exist as is reflected by the Steinway tap test for 
hammer flanges... which works out to be about 3-4 grams of friction when 
measured as hammer shank flanges are with the relevant gauge tool 
available at supply houses for that measurement.

All this, to simply make the point that the jack center needs to be 
tight enough to keep the jack firmly in place and on path, whilst loose 
enough to provide for quick and easy return with  moderate return spring 
tension applied.

Remembering the origional context of this thread should, along with the 
comment made in my last that numbers are more handy as a general 
reference then as some exacting critera to meet, should keep us on track 
with jist of the origiona thread... ... namely one of the causes of 
hammer bobbling in some newer pianos... in particular some of the Yamaha 
uprights.

Cheers
RicB

>
> I think there a couple of things askew here.  One is that some people 
> are talking about hammer flange center pins and others are talking 
> about jack center pins.  Two is that, with regard to center pin 
> friction, a gram reading by itself is meaningless.  Action centers 
> rotate, and that rotation is caused by a torque, not a force.  And the 
> friction associated with them is measured by a torque, not a force.  A 
> gram reading is a measure of force.  To get a torque you have to know 
> the distance from the center that the force is applied.  On a hammer 
> flange, 4 grams applied next to the screw hole gives a completely 
> different torque than 4 grams applied at the tip of the flange.  If 
> you're going to specify a gram reading for flange friction then you 
> also need to define the point at which you're taking that gram reading.
>
> Phil Ford
> San Francisco, CA, USA, Earth
>  
>