[taiden]

So I have a ballscrew and a stepper motor that would like to interact with each other with a 2:1 gear ratio. I thought to myself, no problem! My shaft diameters are 10mm and 3/8". My stepper motor is rated for 320oz-in at about 300 RPM.

The largest pulley can have a total OD of 2.125". Cool! Great!

So I look around and lo and behold, I can't find anything larger than a 3/8" wide XL pulley that will fit a 10mm or 3/8" shaft. Come to find out, according to SDP/SI whitepapers, a trapezoidal timing belt in this size is really only good for... 90 oz-in with a 16 tooth minor pulley. What the fart? Alright, let's try a 5mm pitch x 9mm width HTD. Great! Oh no. 280 oz-in. Damn.

I can size up the belts but then I need a shaft larger than 10mm or 3/8".

Am I missing something?

[harryn]

You aren't missing anything, except maybe you are looking at the wrong supplier for that power range of belt and pulleys. Brecoflex has more options. Maybe consider to use the AT profile x 25mm - 50mm wide range.

I know that sounds really wide, but the belt mfg rate the power load on tooth strength, not stretch. For cnc use, it is easy to underestimate belt stretch / rebound effects, which show up as backlash in the system.

Don't be surprised if you need to have the pilot hole in the pulleys bored to match your shafts - that is really what they have in mind and why it is called a "pilot bore".

[ger21]

You don't need a belt anywhere near 25mm wide. Our machine at work uses a 32mm belt to turn the ballscrew with a >1Kw AC servo moving over 1000lbs at 1500ipm

Alright, let's try a 5mm pitch x 9mm width HTD. Great! Oh no. 280 oz-in. Damn.

Where did you get this info? That seems awfully low. The breaking strength of a 9mm belt is about 675 lbs-ft. I saw a note that say belt should be specified using a maximum of 1/15 of their ultimate strength.

[bhurts]

I used xl timing pulleys on my router for both a rack and pinion drive and screw drive. In several years time I have never replaced a belt or had any issues with accuracy. You should be fine with xl or htd. Remember also that your motors may be capable of 300+ oz but very rarely will it see that kind of load.

Ben

[RCaffin]

The older profiles (XL, HTD, etc) were for power transmission. They have backlash too. You want accuracy.
Change to a more modern profile such as GT3 or AT3.

Width: belts do have elasticity. 3/8" or 10 mm might be OK with a steel core. You can go wider for little extra $.

Cheers