Closed Loop belt for Z-Axis Question

[Mcmitterson]

A new member here and I apologize if this post is in the wrong section. If it is please direct me to the correct one.
I am building a three axis machine out of steel using Arduino and 4 large high torque Nema 23 stepper motors (425). I am running 2 half inch 5 start lead screws for the X-axis, one for the Y, and one 8mm 4 start screw for the z axis. I would like to center the weight over the gantry by mounting the heavy Nema 23 vertically along the back of the gantry trolley and running a closed loop belt to a pulley rotating the 8mm lead screw. The belt would span about 4 inches over the top of the gantry. My idea was to run a 20 tooth 6.35mm bore pulley off of the motor shaft and connect it to the lead screw using a 50 tooth 8mm bore pulley using said closed loop belt. I am estimating that the weight to be lifted (router/z axis components) will be about 13 pounds. Does this sound like a feasible option? This is my first cnc build... i suppose i should mention that...

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[wizard]

If I understand your intended arrangement there should be little in the way of problems. The span is really the center to center distance and if that becomes excessive you should ad idlers mid span. 4" isn't a problem and frankly Ive seen as much as 8 inches between the leadscrew and the driver. This on a high precision lathe two. The important thing is to select high quality pulleys and belts that run smoothly together and make sure your alignmentt of the parts is accurate. The fact is a belt drive between a motor and a lead screw is common practice in automation and machine tools. It is the cheapest way to implement a mild ratio between the leadscrew and driver.

However with gear ratios come other issues, make sure you need a ratio over a 1:1 drive.

[ger21]

You may find that 1:1 might work better, but it depends on how fast you want it to travel.

[skrubol]

What's the pitch of the Z screw (I assume the 8mm is the diameter?)
With a multi-start acme screw I would think your pulley ratio might be in the right range. XL and MXL type belts are old tech and have some backlash to them even when properly tensioned. Newer styles like GT2 can have zero backlash, though given you're running acme screws and rollers, a few mils of backlash from the belt may not outweigh the price difference.