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Hinge Material & Rebuilding Pneumatics
By Larry Broadmoore

[ Ref. 030418 MMDigest, Material for Pneumatic Hinges ]

Regarding the use of so-called "pillow ticking" for pneumatic hinges:
pure white canvas "duck" is available at some awning stores, made to
military specifications, which can be depended upon for enormous
strength per thickness and also for great longevity, which is not
always true of canvas not carrying the mil-spec rating.  This stuff
has for many years been used by the armed forces for parachutes, tents,
boat rigging, backpacks, etc., and in certain cases human lives depend
upon its reliability.

I have never seen this material with any kind of striped pattern, but
have often dismantled pneumatics whose hinges were made of the familiar
striped stuff, which crumbled away with rot, although only a few years
old.  Not so of the government material.

Incidentally, a piece of trivia: I recall the striped canvas being
used in the spines of books we made in high-school bookbinding class.
It was folded in half and glued neatly into the ends of the spines, and
as far as I know was only a traditional aesthetic touch, a tiny striped
edge showing at each end.

Over the past 37 years I have spent countless hours experimenting
with methods of producing hinges for use with striking pneumatics,
as free from friction as possible.  Noticing the carefully worked out
Simplex system of pneumatic covering, it came to my attention that no
need was seen by the inventor for any kind of internal hinge (it should
also be observed that, partially due to gasketing considerations,
perhaps, there is only a scant 1/16" overlap of the cloth, on one side
of the Simplex pneumatic -- no overlap whatsoever on the hinge end!)

This makes sense to me, because where an internal folded hinge is
used, the pneumatic has in fact two separate hinges, including that
formed by the cloth covering the outside. These are, it seems to me,
in mechanical conflict with one another, and act to stiffen and resist
the free movement of the pneumatics movable board, a condition which
wastes energy and promotes variation of the friction coefficient from
one pneumatic to another, which is ruinous to attempts at achievement
of a fine quadruple-pianissimo setting across the keyboard.

A few years ago I developed, after many iterations, a pneumatic
covering "machine" or jig, which accommodates virtually any size or type
of striking pneumatic.  Machined of brass, this complex little gadget,
ripping with adjustments in every parameter, enables even unskilled
workers to rapidly produce perfect pneumatics with little instruction.
When covered, these "box out" and do not retain a crease unless
compressed slightly, but the tension of the fabric alone is sufficient
to cause the cloth to return to its original flat shape.  We use
neoprene-coated cloth applied with fish glue.

The pneumatics are tested under pressure, not vacuum, and must
withstand a substantial internal force without closing, when their
inlet is closed.  One feature of this method is that it is possible to
cover the pneumatics without first hinging the boards, if so desired.

We find the fabric more than sufficient to act as a hinge and therefore
only use internal hinges if the customer insists upon this.  In most
cases I provide a 1/16" gap between the boards to reduce the tension
near the hinge at the sides, even if a relief was provided by the
manufacturer by chamfering or milling the corners at the hinge end.

Incidentally, I feel that it is highly desirable to use a flexible
adhesive in at least one place on a striking pneumatic: the overlap.
Otherwise, one is in reality applying a chemical "spring" across the
width of the hinge -- just what we don't want!  It is true that if hot
animal glue is used, however, once the glue cracks at the hinge, it
remains flexible.  But is it still 100% airtight?

An adhesive which should be in every player technician's tool kit is
3M-847 Gasket Adhesive.  This is a brown, viscous substance which does
everything the mythical "burnt shellac" is supposed to do, but usually
doesn't.  It remains flexible and strong, looks exactly like burnt
shellac, sticks to virtually everything including glass and metal,
sands clean when dry, is compatible with traditional adhesives and
sealants, and tacks quickly.  This is only the beginning of its uses.

It is available in tubes or quart cans, and there is a version 847L
which has a longer drying time.  This product should not be confused
with any of the other 5,000 excellent adhesives 3M makes -- accept no
substitutes!  It must be #847, for our purposes. I discovered it by
accident when it was included, under another name, in a kit I bought
for replacing the felt seal on the drum on my gas dryer, with which
it was used due to its resistance to high temperatures.

Another, highly unique means of hinging, having many potential
advantages, is under gradual experimentation as time permits, and
I hope will prove suitable someday for widespread use.

All best wishes,

Larry Broadmoore

(Message sent Sun 20 Apr 2003, 05:50:41 GMT, from time zone GMT-0700.)

Key Words in Subject:  Hinge, Material, Pneumatics, Rebuilding