Polyurethane Drive Belts
Round Polyurethane drive belts (sometimes called 'Polyamide belting') are
used extensively in hand-turned organs, usually to transmit the drive to
the music spools. These are very convenient as they can be easily obtained
in various thicknesses, and can be made up to any length in the home workshop.
Most organ builders use the 4mm or 5mm diameter belting. The
belting is cut to the required length and the ends are welded together, to
form a belt loop.
Sounds simple, but I must have wasted many metres of belting trying to get
the welding right. The usual faults are that the weld isn't strong
enough and the joint pulls apart under tension; the ends are not properly
aligned with each other; or the belt is either too loose or too tight.
To weld the ends together, all you have to do is to cut the belt to length,
heat the two ends until the plastic melts, then press the ends together tightly
to form a belt. The manufacturers of the belting sell a tool to assist
in this operation but it costs over £600 !! You clamp the two
ends of the belt in the tool, melt the ends of the belt, then press them
together. Even this isn't foolproof, as the melted ends of the belt
often deform and slide against each other instead of being pressed together.
You can repeat the operation, but each attempt makes the belt shorter
until, when you do finally succeed, the belt is then too short and you have
to start all over again!
Some common faults
Ends not welded properly
Ends not properly aligned
There are various home-made contraptions shown on Youtube for doing this
job (and avoiding the £600 rip-off) but most of them are little better
than holding the belt by hand and trying to align the ends by sight. This
doesn't work well, as the ends are very slippery when hot, and will slide
about all over the place when they are pressed together.
Well, I have come up with the simplest solution imaginable, and it works
every time. All you need is a small scrap piece of MDF or plywood about
1/2" thick, and a soldering iron.
Drill a hole through the MDF right next to the edge. Make the
hole about 1mm larger than the diameter of the belting, so that the belting
is a comfortable fit in the hole. Clamp this piece in the vice to hold
Now get a soldering iron, switch it on and rest it next to the vice. Wait
for about 10 minutes for it to get nice and hot and then press the two ends
of the belt on the barrel of the soldering iron. Don't hold them on
the tip of the iron otherwise you will have to clean the molten plastic off
before you can use it again for soldering! Wait for a few seconds until
the ends of the belt have thoroughly melted and then quickly insert both
ends into the hole in the MDF. Press them together very tightly for
about 30 seconds and then wait for another 30 seconds for them to cool down.
The hole in the MDF will make sure that the joint is straight and correctly
aligned. Now pull the joint through to one side and try and pull it
apart. You shouldn't be able to no matter how hard you try.
Very good, the joint is finished, but how do you get the loop out of the
MDF? You just cut away a slot from the edge of the MDF into the hole
and the belt will come out through the slot. Take care not to saw too
far and damage the belt. Now it is usually necessary to have to cut
away a surplus ring of plastic around the joint, but using my method this
ring is contained within the hole in the MDF, so doesn't usually need much
trimming. You might think that you can simplify the removal of the
belt by cutting the slot beforehand, but the hole needs to completely encircle
the joint for it to work properly.
Heat the ends up and push them into the hole in the MDF.
Press them together tightly. (Vice not shown!)
Cut a slot in the MDF to release the finished belt.
Any molten residue can be trimmed or sanded off
The key to the success is heat and speed. Make sure the ends of the
belt are completely melted, and remain so until they are pressed together.
You could try a few cold practice runs from the soldering iron to the
MDF to make sure you can insert the ends and press them together as quickly
as possible. It will help if you can get somebody to hold the soldering
iron right next to the hole in the MDF. Then you can insert one end
of the belt through the hole before applying it to the soldering iron. The
assistant then pulls the iron away and you just insert the other end.
Manufacturers of this belting often specify a tension in the form of the
percentage to reduce the belt length by to obtain the correct tension, but
I have found this to be much too tight for our purposes. I simply wrap
the belt around the pulleys, stretch it a bit until the tension seems about
right, and then cut the belt at that point. If the drive slips, it
is easy to cut the belt and shorten it, but not so easy to lengthen it!
Wooden pulleys generally need less tension than metal ones, due to the increased
friction of the rough wooden surface. Wooden pulleys can easily be
made up using my Circle Cutter (This
could form the basis of another article!)
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