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MMD > Archives > December 1999 > 1999.12.29 > 11Prev  Next

Flexible Joints in Gulbransen Player Action
By Richard Vance

The flexures used to connect the linkages between the pneumatics and
the whippen fingers of the Gulbransen-Dickinson late model players
are made of thick buckskin (deer skin or elk skin).  This is the only
material that has the necessary density, as well as flexibility, to
work properly.

It is vitally important to attach the replacement pieces to the
wood with hot glue only.  Hot glue, when used to attach any flexible
materials to anything else, will not soak into the pores of the
flexible part, but only grip its surface with a strength that can not
be compared with any other adhesive.  Any modern adhesive will retain
its liquid properties until it fully dries, saturating and stiffening
the flexible core of the material that is supposed to remain soft.

The central part of the Gulbransen flexures, between the two planes
where they are glued to the wooden pieces, must remain fully flexible
to allow the joint to rotate slightly.

In spite of all the trouble that rebuilding a Gulbransen glued-stack
player involves, the design of that unit amazes me.  As an engineer,
the elegance and simplicity of its design, from a production
engineering point of view, is impressive.  The simple buckskin flexures
is only one example.  Also note that there are hardly any gaskets in
the system.

Another neat trick they used was to make the lower valve seats from
eyelets driven from below.  After the eyelets were installed, the top
edges, forming the seat, were milled down, using the upper seat surface
as a guide-stop, thus insuring the correct valve gap by a simple,
repetitive industrial process, without the usual trial-and-error
involved with setting the gaps at assembly.

The roll motor regulator was a simple device and cheap to make, with
only one moving part: the lid of the pneumatic which uncovers a tube
from the high vacuum supply as it moves.  Such a regulator is difficult
to adjust properly, because the valve movement, from closed to fully
open, is so short.  The spring has to have a very high spring-constant,
as well as the proper force, so that the feedback control loop retains
the correct feedback signal to insure stability.

Gulbransen used a bow shaped spring, attached to the regulator pneu-
matic at one end and the fixed panel at the other, with long screws.
By tightening or loosening these screws, one can adjust both the force
of the spring and its rocking point, and thus the length and spring
constant of the active end.  Thereby one can easily get this regulator
to behave correctly.

Richard Vance

(Message sent Wed 29 Dec 1999, 12:03:51 GMT, from time zone GMT-0500.)

Key Words in Subject:  Action, Flexible, Gulbransen, Joints, Player

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