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manofgresley
15-09-2013, 04:36 PM
Hi

I should know the answer to this, and i think i do, i just need reassuring.

I have a 3Nm, Nema 23 motor, i am going to use to power my Z axis on my milling machine, i intend to use a toothed belt & pulleys, my question is this, should the DRIVER pully (ie, attached to motor shaft) be LARGER or SMALLER the the pulley being DRIVEN (ie attached to Z axis Leadscrew)

Regards

Ray

JAZZCNC
15-09-2013, 05:18 PM
Well really that depends on what your after.? If your wanting higher speeds then larger pulley on driver, this will come at the cost of torque.
If have a fast motor and are wanting slower speeds with more torque then smaller on the driver.

I think you'll be looking to increase torque so go small on driver.

birchy
15-09-2013, 05:18 PM
It all depends what gear ratios you want to achieve. Generally speaking, a direct drive system would use either Oldham Couplings (http://uk.farnell.com/oldham-couplings) or drive pulleys (http://www.beltingonline.com/timing-pulleys-bars-272/htd5-pulleys-285/to-suit-15mm-wide-belt-482/pilot-bore-493/) of the same diameter. For increased torque and slower speed, you need to use a smaller pulley on the motor (or a larger pulley on the leadscrew).

Incidentally, notice I linked to HTD5 pulleys above? I bought T5 pulleys and then discovered the belts are pretty expensive. The HTD5 pulleys are (apparently) a better choice as they will have less backlash and are easier/cheaper to get hold of.

manofgresley
15-09-2013, 05:30 PM
Hi again

My X & Y axis are direct drive, using Oldham Couplings, the Milling head, gearbox & motor are quiet heavy, i am not to bothered how fast the Z axis travels, i am more interested in relieving some work on the motor, thats why i thought of using timing belt instead of direct drive, should i use 2:1 or 3:1 Should this be Driver to Driven or Driven to Driver?

Regards

Ray

JAZZCNC
15-09-2013, 05:34 PM
I'd go 3:1 with small pulley on stepper.

If you want to relieve some stress on the motor then use some form of counter balance, either weights or gas struts.

manofgresley
15-09-2013, 06:00 PM
Hi All

Again many thanks, believe it or not, that's just what i was thinking of, honest, just had one of those age moments that lasted a couple of hours.

Ray

birchy
15-09-2013, 06:11 PM
I have pulleys for 3:1 on my Z-axis as well!

However, this interesting response was a reply when I asked the very same question: http://www.mycncuk.com/forums/faqs-problems-solutions/6360-do-i-need-keyway.html#post47446

manofgresley
16-09-2013, 04:43 PM
Thanks Birchy.

I had already decided on a key way driven pulley on the Z axis Leadscrew, has the existing shaft has one in, so i will utilise the existing keyway.

Regards

Ray