I've found that modern round leather belting works amazingly well on
instruments used in indoor settings with reasonable humidity control,
if the groove in the motor pulley matches the one in the pump pulley,
if the belt is the right cross-sectional diameter to fit well in
both grooves, and if the pulleys are aligned properly.  We've used
it on dozens of European orchestrions with very few problems.  It's
aesthetically pleasing and conforms to the originality of the instrument.
(I haven't used it in American instruments that originally had v-belts.)

If a round leather belt is the correct size for the groove in the large
pulley, but of too small cross-sectional diameter for the groove in the
motor pulley, it will slip.  If it's too large for the groove in the
large pulley, it won't stay on.  If it slips on the motor pulley and
you have to use a larger diameter belt that runs off the large pulley,
then the pulleys don't match or they're not aligned.  If it keeps
stretching and has to be tightened regularly, then it's too small in
cross-sectional diameter to handle the power needed to drive the pump.

If both pulleys have the same groove and the belt fits in that groove,
it doesn't have to be extremely tight.  I've found that orchestrion
pump and motor pulleys have large enough grooves to accept belting that
won't stretch unduly.  Excess tension on too-small belting is what
makes it stretch continually until it breaks.

When making a new round leather belt, I cut a piece a little longer
than needed, put one end in a vise, and pull on the other end as hard
as I can to pre-stretch the leather, if only a little.

The clip for round leather belting will only click if it sticks up from
the surface of the belt.  In the shop I like to drill the holes on a
drill press, but it's possible to drill them on location with a battery
drill.  Start the hole first with a very sharp awl and aim the drill
carefully so the hole doesn't veer off to one side.  Of course, lay the
belt on a scrap piece of wood when drilling; don't try to drill through
the leather into the air.

Before drilling, make sure each end of the belt is cut perfectly
square.  I prefer to use a single-edge razor blade, but the special
belting tool does a pretty good job.  Then use a new single-edged razor
blade to cut a small groove in each side, straight from the hole to the
end of the belt.

With each hole in the correct place, with the grooves the right depth,
and with the correct size belt clip made for that purpose, you can
press the clip down into the grooves so it's below the surface of the
leather and it will be silent.  Don't cut the grooves so deep that they
weaken the belt.

Keys to success include having a clip of the right size for the size
of belting, and drilling the holes in exactly the right places so there
won't be a gap between the ends of the leather.  Six sizes of belting
and clips, from 1/8" to 3/8" diameter, are still available from
McMaster-Carr.  It can be found under the heading "Leather Round
Belting".

The clip wire is soft enough to bend with a tool, but hard enough not
to bend under belt tension.  Forget about trying to make a belt clip
out of a nail, which is too hard to shape correctly.  McMaster-Carr
also sells the tool that cuts the leather, punches the holes, and
crimps the clips, but it doesn't form the grooves.

The new round leather motor belt on the Style 4 Welte Concert
Orchestrion that was in the Sanfilippo Collection from 1984 through
1997 never came off and never needed tightening.  I attribute that to
the belt having large enough diameter that the relatively low torque
of the low-speed Welte pump didn't make it stretch, and the fact that
the belt fit well in both pulleys.  Even the belts on the large German
piano orchestrions, including the Hupfeld Style 3 Super Pan and Style
III/39 Helios, which run on much higher vacuum and have much higher
crankshaft torque, only rarely need to be tightened, even with seasonal
humidity changes.

Round leather belts in band organs in unheated buildings generally have
to be tightened when the humidity goes up significantly after a very
dry season.  We've taken care of the Wurlitzer Style 155 "Monster"
organ in the unheated carousel building in Burlington, Colorado since
1976.  It has five belts and a chain.

Three of the belts are round leather belts:

  A) The belt from the crankshaft to the 4-beater mechanical snare drum
almost never needs to be tightened because the pulleys are big enough
to provide sufficient contact area and friction with the belt, and the
drum mechanism takes little power.  I've probably tightened it once in
37 years.

  B) The long belt running across the back of the organ (over 15' of
belting) runs on pulleys of about 5" diameter.  They have plenty of
contact area so the belt doesn't have to be extremely tight, it doesn't
stretch much, and it only has to be tightened once every 5 years or so.

  C) The belt that runs the music roll mechanism has the original
double-pulley Wurlitzer adjustable tensioner found in most Wurlitzer
band organs with belt-driven spoolboxes.  It has to be tightened every
May when it rains for about a week after having about 25% humidity in
the building for most of the summer, fall, and winter.  Once every few
years I back off the tensioner and shorten the belt.  If it were
loosened after the rain stopped in early summer, even that shortening
wouldn't be necessary.  Once it's tightened when the rain starts, no
one ever goes back and loosens it when it gets dry again.

Art Reblitz - Reblitz Restorations
http://www.reblitzrestorations.com/