Hi All,  Over the past week or so, I've been communicating with Doug
Vensel about the 6-point air motor.  During the process of rebuilding
the motor, I collected all of the various measurements from Doug in an
attempt to more clearly understand what Jeff Davis said in his posting
(180223 MMDigest).  What I ended up creating in the eMachineShop CAD
program show how the motor moves (see attached video at link below).

While I don't understand the geometry well enough to explain it,
the other picture shows two circles where the radius is equal.  This
occurred when the bellows were open an equal amount.  Naturally, as
one bellows closes and the other one opens, the radius of the circles
and their orientation to each other changes.  As the bellows on one
side opens, the diameter of the circle decreases and the center of
rotation gets closer to the fulcrum.

Meanwhile, as the bellows on the other side is closing, the diameter
of the circle is increasing and the center of rotation is moving away
from the fulcrum.  When the boards of the bellows that's closing are
parallel to each other, the diameter of the circle is infinite and the
diameter of the circle on the other side is at its minimum.

Maybe an engineer in the group can explain all of this better...

Musically,
John A Tuttle
Player-Care.com
Brick, New Jersey, USA

 [   Movement of the 6-point Pratt-Read air motor
 [ http://www.mmdigest.com/Attachments/18/03/01/180301_052723_motor-movement-1.mp4 
 [   Two circles of rotation at the balance point
 [ http://www.mmdigest.com/Attachments/18/03/01/180301_052723_circles-1.jpg 
 [   One circle of rotation at the extreme
 [ http://www.mmdigest.com/Attachments/18/03/01/180301_052723_circles-2.jpg