[ Doug Vensel wrote in 180221 MMDigest:

> You can imagine that this center point acts as the center of
> the teeter-totter.

You're on the right track.  One of each pair of pneumatics (possibly
both) needs what I call a "floating" hinge.  It needs to space the
pair of boards for each pneumatic about 1/4-inch apart instead of
being tightly held face to face.

The reason for this is geometry.  You have two pneumatics that move
in an arc, as part of a circle.  If you have two pneumatics with two
tight hinges that share a board, it wants two centers of rotation --
not possible.  The cover board only wants one center, so binding
results.

Think of it this way.  You have two circles with two centers, some
distance apart, trying to operate in the exact same area.  Geometry
says it won't work.  All motors that share a center have this problem.
One or both common pneumatics will need a floating hinge.

Be careful covering the pneumatic.  I use Popsicle sticks as spacers
to keep the boards apart as I recover them.  Also, make sure the
boards are centered to each other before you do the sides.  You should
see an isosceles triangle when you look through the edges.  Neither
side should be higher than the other.

When you glue the pneumatics on the deck, don't glue them tight to the
center but give them room to move.

Good luck with fitting the cover board to each pair.  A floating hinge
should make this a little easier.

Good luck.

Jeff Davis
Seattle