capstan

[Michael Spalding]

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