Belts & pulleys,

[Jonathan]

The distance between the pulleys for a given belt length is easy to calculate, or you can use an online calculator such as this one. However slight manufacturing variations in the belt length and possibly the need to put the belt on without stretching means you always need some adjustment to ensure the correct tension can be obtained in the belt. If you have a few mm adjustment of the centre distance, either side of the calculated value, that should be plenty. I like to incorporate much more adjustment so that you can change the ratio, e.g. from 1:1 to 1:2, without having to change the belt.

Since you seem to have opted for 1:1 the centre distance doesn't make a lot of difference as the belt will wrap round 180° of the pulley regardless. I'd just use whatever fits nicely with the rest of the design, which will probably end up being around 100-150mm.

Bearing station are the cheapest place to get the timing belts and HTD pulleys and second cheapest are belting online by the time you include their VAT and higher postage charge. I did just recently find a local shop who are cheaper than both of these companies, plus of course no postage and waiting around, so might be worth checking if there's somewhere near you?