Calculating timing belt lengths

[Jonathan]

Here's a spreadsheet to help when working out timing belt lengths. There are plenty of these calculators online, but I made this as I find it makes it easier to compare options when you can list them.
I've put 5mm in the belt pitch to use it with 5M HTD pulleys, which are often recommended here for driving ballscrews, but if you want to use it for a different belt series just enter the pitch. So for instance to use it with XL timing belts, put 5.08mm.

8374

As for buying the belts and pulleys, Bearing Station (http://bearingstation.co.uk/) tend to be the cheapest, but sometimes a bit slow to deliver. There's also Belting online (http://www.beltingonline.com/) who offer a service to bore and tap the pulleys, however they are more expensive.

[Ross77]

Thanks Jonathan, I was trying to find pulley sets that had the same centre distance so I could make my motor mount future proof and be able to change the ball screw ratios by just using different pulleys.

Hope you don't mind but used yours as a base to do a comparison tab? I've attached my addition in case it helps anyone else but please feel free to remove it there's a problem.

Cheers88118811

[Jonathan]

I'm glad you found it useful. I tend to make motor mounts with about 20mm of center distance adjustment, as that's generally enough to allow changing the ratio whilst only changing one pulley. That might cost you less than buying (or making) 4 or more pulleys per axis.

[birchy]

How about this online version? SDP/SI (http://www.sdp-si.com/Cd/default.htm)

[Ross77]

How about this online version? SDP/SI

Yeah found that but you cant quickly and easily compare different pulley combos and belt lengths.


I tend to make motor mounts with about 20mm of center distance adjustment, as that's generally enough to allow changing the ratio whilst only changing one pulley. That might cost you less than buying (or making) 4 or more pulleys per axis.

good point but I was thinking along the lines of a quick change system with the pulleys all fixed permanently and just move the belt over like on lathes or mill spindles. anyway going a bit off topic.

[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.16759

[Robin Hewitt]

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