Calculating timing belt lengths

[Robin Hewitt]

I tried a bunch of pulley separation calculators a couple of years ago then checked the result by drawing and listing and found they were all a bit naff.

Then I tried writing my own and found the maths was quite beyond my poor old brain.

Have they improved? I get the separation by drawing, measuring, redrawing, measuring... etc., until I get my tolerance.

[birchy]

Calculating the exact centres is never possible because belts may wear and/or stretch (particularly in hot environments) and I've often found that even brand-new belt sizes can vary between different manufacturers and materials. The longer the belt, the bigger the difference can be unless you're paying premium prices for matched sets. This is why adjusters were invented.

[Neale]

If I am aiming to use a couple of 20T HTD5 pulleys, and not allowing for potential pulley change later, what kind of adjustment should I allow for belt tolerance, stretch, fitting, etc? The steppers will be mounted on plates with slotted holes for fixing and with due allowance for the central boss. Nominal pulley centre-centre distance is 115mm. I've never used these things before so not sure what a sensible range of adjustment should be.

Posted in this thread as it might be useful information for others.

[Clive S]

If I am aiming to use a couple of 20T HTD5 pulleys, and not allowing for potential pulley change later, what kind of adjustment should I allow for belt tolerance, stretch, fitting, etc? The steppers will be mounted on plates with slotted holes for fixing and with due allowance for the central boss. Nominal pulley centre-centre distance is 115mm. I've never used these things before so not sure what a sensible range of adjustment should be.

Posted in this thread as it might be useful information for others.

Neale slots of about 40-50mm would be fine and certainly make it easier to put the belts on.

[Neale]

Thanks, Clive, and you have reminded me that I need to allow for the flanges on the pulleys so a bit more adjustment needed than I was thinking - but 40-50mm seems fairly generous? I was thinking of more like +-5mm?

[Clive S]

Thanks, Clive, and you have reminded me that I need to allow for the flanges on the pulleys so a bit more adjustment needed than I was thinking - but 40-50mm seems fairly generous? I was thinking of more like +-5mm?

The longer slots are to be able to change pulley size if needed and also so than it will fit a small range of belts sizes.

[Robin Hewitt]

I don't allow for any adjustment. That's the wonder of CNC, you cut it right and everything fits