[CAUT] restrung D

[John Delacour]

At 2:35 pm -0600 18/4/07, Jim Busby wrote:

>I’m coming in late on this but Jim Ellis speaks of the “friction 
>coefficient” (I hope that is the term he used) between different 
>types of metals. In Europe, especially with historical instruments 
>and such, they use brass bridge pins. Jurgen Guering sells them. 
>Juan Mas Cabre swears by them for his Pure Sound Wire (Stainless 
>steel). Maybe Jim could enlighten us? Or at least someone smarter 
>than me could better understand this metallurgic relationship as Jim 
>explains it.

The page below gives quite an interesting table:

<http://www.school-for-champions.com/science/frictioncoeff.htm>

(cf <http://www.engineershandbook.com/Tables/frictioncoefficients.htm>

Indeed I have a bridge here from a late 19th century Ibach grand 
which has brass pins, and I have come across them fairly often, 
though steel pins seem to be the norm even on pianos of high quality 
from, say 1860 at least.

One thing this table shows is that the coefficient of friction 
between two like materials, eg. steel on steel, cast iron on cast 
iron, is relatively high.  I would guess that the CoF between cast 
steel wire and a mild steel pin would not be as high as that between 
steels of identical composition but that it would be much higher than 
the 0.35 of hard steel against brass or phosphor bronze) or the 0.4 
of steel against cast-iron.

An interesting quote from the above link is:

"For example, clean dry steel sliding on steel has a coefficient of 
friction of mu  = 0.78, but if the surface has oxidized, the 
coefficient changes to mu  = 0.27."

... which brings mu down almost to the level of steel/copper-lead 
alloy.  This might suggest that blued steel pins might, at least when 
new, produce less friction even than brass or bronze.

Supposing, just supposing, minimal friction at the bridge is 
desirable, then a graphited bridge (mu=0.1) and pins of a material 
that against cast steel will produce the lowest coefficient of 
friction would be the way to go.  The question in my mind is how much 
friction is desirable _in_practice_.

I notice that Ron Overs' bridge, posted the other day, is not 
graphited and appears to be lacquered all over, though this may be a 
false impression.  I have no figure for the coefficient of friction 
between hard nickel and cast steel, but I imagine Ron has done 
experiments to establish this.

JD