The Wurlitzer variable-speed roll mechanism originally used a leather
friction drive wheel running at right angles to the cast iron
driven wheel (sometimes a die-cast wheel in pianos).  The problem is
that shoe sole leather isn't as durable as it was in 1915 or 1920.  Or,
perhaps the old leather was treated with something that made it last
longer.

When we restored the "Monster" in Burlington in 1976, there was enough
of the original leather wheel left to re-face the surface in a lathe and
reuse it.  After serving full-time duty in the 1920s, it lasted for
seventeen more years until 1993.  Then I bought the best leather I
could find, made a new wheel just like the old one, and it lasted for
three years.

Two wheels running at right angles to each other have an inherent
conflict between "good" and "bad" friction.  In theory there is a
specific radius, or distance from the center of the driven wheel, where
the leather driving wheel should run, to produce any given speed.

In theory, the leather should touch it only at this radius.  In
actuality, the leather has a certain amount of thickness -- typically
about 1/4" -- so the contact surface is spread out over a _range_ of
speeds at any given time.  The theoretical center line of the leather
wheel is at the desired tempo.  Leather outboard of that line is trying
to drive the iron wheel slower, and inboard, faster.

We need friction -- and a lot of it in the huge late-style Wurlitzer
tempo mechanism with its heavy coil pressure spring -- to drive the roll
mechanism.  But the friction between the two surfaces outside of the
theoretically infinitely small contact point is wasted on a rubbing
action that leaves the growing pile of leather (or neoprene, or
whatever else you substitute) particles on the shelf below.

In lighter-duty applications, like a Seeburg piano, an O-ring usually
makes a good drive tire because the contact point is narrower than the
edge of a leather disc, and because the spring tension isn't as great
as it is in a Wurlitzer.  (This works in most Seeburgs, but a few will
squeak due to the alignment of the shaft, which not only rotates but
pivots too).

Wurlitzer used many different friction wheels in different pianos and
organs.  In instruments with small music rolls, the leather works just
fine.  In the larger, heavier-duty Wurlitzer applications, including the
late duplex mechanism, the contact spring tension will squash any O-
ring that is soft enough to provide the necessary "good friction,"
spreading out its contact point, increasing the "bad" friction, and
leaving powdered neoprene just as fast as it does powdered leather.

My answer to the problem in several commercially-used band organs that
we maintain (yeah, commercial use is alive and well) is to use leather
because it works just as well as the original did; it just doesn't
last as long today.  I keep looking for the best available leather.
When I need to make a new friction wheel, it doesn't take that long to
make a few spares.  I keep them with the organ, and it's a simple
matter to install a new one when necessary.

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