Making Polyurethane Drive Belts

by Melvyn Wright

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Making 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 it steady.

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 where it has oozed out around the joint, either with a pair of cutters or a knife. This itself can be a very fiddly and time-consuming job, but using my method this ring is contained within the hole in the MDF, so hardly any trimming is necessary.  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 MDF residue can be wiped 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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