ballscrew pitch vs. gear/pulley reduction

[Hans_G]

I'm retrofitting an XY table with ballscrews (most likely 5/8" - 0.2" or 16mm x 5mm screws from Roton or LinearMotionBearings on eBay... so yes, budget is important) and servo motors (rated 6krpm, already have them). Top speed is not a concern, so I am considering a 2:1 reduction via gears or pulleys. Then I noticed Isel makes 16mm screws with a 2.5mm pitch, low profile, and priced similarly to Roton. What disadvantages would there be to using a finer lead on the screw vs. timing pulleys? Pulleys/gears may add backlash, and are slightly more complex mechanically than a direct drive from motor to ballscrew. Finer lead screws cannot handle the same loads. What else should I be thinking about? Thanks!

[Matt Adams]

Watch out for whip speed.

With a smaller pitch screw you'll be spinning the screw much faster to get the same travel speed, and may approach the screw's whip speed.

How long are you thinking for screw lengths?

Matt

[Hans_G]

good point! The X axis screw will be approximately 20" (it's a palmgren 18.5"x6" XY table). Looking at Roton's site, a 5/8" screw should be able to handle 5krpm. If Isel's 16x2.5mm screws can only handle 1/2 that (2500rpm), I would still be looking at 250ipm rapids. 25ipm feeds would have the screw turning at 250rpm. Seems like plenty of margin for critical speed.

So, other than reasons like what is available on eBay or already sitting in the parts bin, would one ever pick a 10mm pitch screw and a 4:1 gear box over a 2.5mm pitch screw in the application I'm describing?

[diycnc]

if you want 250ipm with .1inch lead you have to turn 2500 rpm. if your motors are rated at 6k i would think you would still need belt reduction. probably 2 or 3 to 1.

on my machine with servos rated at 3000rpm i am going to run 4:1 belt reduction with 5mm pitch screws off ebay. the belts i ordered from www.bbman.com as they had all the cad files and were the most cost effective

[Mike Everman]

A couple of things to consider:
6krpm motors needn't be run at that speed, in fact the fat part of the torque-speed curve is always toward the low rpm end. It's more about your bus voltage available as to how fast your motor is going to go.

Second,
When worrying about whip, consider that it takes some distance from one end (the worst case) to get up to some speed, in which case the unsupported screw length has gotten shorter, whip critical speed allowed goes up, so depending on your acceleration ramp, you can easily exceed some worst case static interpretation of the critical speed.