[CAUT] moving capstans question

[Keith Roberts]

Thank Ed. That is where this action ended up at and that is probably why it
feels so right. Of course if the alignment of the capstan to the heel is
such that the wippen rolls from the front of the capstan to the back, the
action ratio will increase and counter act the decrease of going past the
magic line. In my opinion, major scrubbing should be avoided. With only a
6mm movement through the arc of the smaller radius of the wippen, 3mm past
the magic line results in not much change in the leverage of the wippen but
by changing the distance of the effort arm of the wippen by the width of the
capstan head and you have a major change. A slight angle of the capstan to
the heel will start the up push near the front of the capstan and roll to
near the back. When the capstan is lined up square to the heel it starts in
the center and rolls to the back. As the divot forms in the felt, I think
the machining on the edges of the capstan becomes a factor in this
configuration and as you say, the forward scrub of the capstan when it
reaches past the magic line does increase and the back edge of the capstan,
being a point, no longer increases the action ratio due to the roll and it
suddenly goes from an increasing action ratio to a decreasing one.

Keith Roberts


On 9/22/07, A440A at aol.com <A440A at aol.com> wrote:
>
> Keith writes:
>
>
> Do I care if the the action ratio was 5.52 or 5.59? No. I'm sure I could
> make an arguement that shows that the BW is not exactly halfway between the
> UW and the DW. Do I care? The instantaneous action ratio can change through
> the movement of the action. It can have one action ratio at the start and
> another at the finish as the parts move. Do we use the max ratio or the
> average or a section of movement that we consider to be representative? The
> reason there is a difference in these ratios is the criteria set upon the
> way it is measured and not because the action puts out three different
> amounts of work.
>
> Greetings,
>    It may be of interest that check the EAR with 3 succesive 2mm key
> depressions, to see if the ratio stays the same. It will not!  As the
> capstan approaches the axis 'tween centers (the"magic line"),  the ratio
> begins to steepen, as it pass the line, it begins to lessen. If the capstan
> contact point arrives at the line at let-off, you will notice that the ratio
> increases up to that point.  In effect, the action starts in low gear and is
> shifted to high by the end of the keystroke.
>      Given that the highest ratio is found at the line, as is the minimum
> friction,(which is dependant on relative speed of the capstan to the heel ),
> an action  that has the capstan on the line at the half-way point is an
> action that begins to have its ratio decreased as the hammer is approaching
> maximum velocity. This is counter-productive, in that after the inertia has
> been overcome, and the hammer speed is increasing along with the ratio, a
> decrease in the ratio halfway through the travel will cost power, unless the
> finger powering the action can somehow increase its speed through the last
> half of the key travel.
>    If the capstan reaches the line at let-off, the point  of maximum
> friction has been placed at the very beginning of travel,where the speed of
> the relative motion is at its minimum and the ratio is at its most
> advantageous.  Through the stroke, the ratio is steadily increasing as the
> friction is either decreasing or staying the same, due to increased speed of
> relative motion, ultimately reaching the point of zero friction and maximum
> ratio at escapment.  I have found this set-up to give a measurable increase
> in power and control.  I think the idea of setting the "magic line" may be
> the factory production attempt to minimize the variability in the production
> line, itself.
>     This is why I prefer the capstan/heel contact point reach the line at
> let-off, not before; it creates a "magic point" right where the pianist
> looks for maximum control.
> Regards,
> Ed Foote RPT
>
>
>
> **************************************
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