capstan

[V T]

Hello Don, Mike, and List,

Mike, you understand exactly the point I was trying to
bring up.  In fact, when I inspect a well used S&S
angled capstan and the matching heel, I can always see
how much dragging the capstan did along the felt. 
there is an elongated track in the felt.  This track
is longer than the one you see on the heel that goes
with a 90 degree capstan.

Now, there are some who believe that an angled capstan
lowers the action ratio, but I do not see how that can
be if the contact point with the heel is kept fixed. 
The location where the capstan enters into the key
stick is not important.  In any event, S&S could have
placed the capstan wherever they wanted to in order to
get the action ratio they wanted, so that doesn't seem
like a plausible explanation.  Did they want a
progressive change in action ratio during the course
of the key travel???

So, I wonder ... why did they do that?  I suspect they
had an interesting reason ...

Vladan

================================

Don wrote:

>
> The capstan is supposed to be "involute". i.e. it
> doesn't slide at all--it rolls or rocks on the
> whippen.
>
A common misperception.  It does slide.  The involute
curve is the standard
gear tooth profile.  The important quality of the
involute curve, is that
it transmits uniform angular velocity.  So that as the
driving gear rotates
at constant RPM, the driven gear also rotates at
constant RPM, with minimum
noise and vibration.  But they do slide.  Make your
self a paper cutout and
try it.

It's also interesting to consider the angle of gear
teeth.  Standard angles
for spur gears are 14 and 20 degrees.  14 degree teeth
are smoother and
quieter, and have a longer wear life because at a
given load they have
lower contact stress.  In fact, zero angle or negative
angle would be even
better, from a contact stress point of view, but with
small or negative
angles there is a geometry problem known as
"undercutting",  where the base
of the tooth ends up narrower than the contact area,
and is prone to
fracture.  20 degree teeth are a compromise, used for
pinions with very few
teeth to prevent undercutting.

So does a capstain slide on the wippen cushion felt,
except for the instant
that the contact point passes through the "magic
line".  Before and afer
that point, the Overs capstain slides the least,
standard vertical capstan
slides a little more, and the Steinway backwards
leaning capstan slides a
little more than that.

more than you wanted to know, and it still does not
explain why S&S might
have done that.

Mike



		
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