Quote Originally Posted by GEOFFREY View Post
One of the reasons that I wanted to use a "beefy" leadscrew is because I am hoping to be able to use a ball screw mounted on the top of my Z slide as the top bearing. I don't know if this will work, but intend to drive the screw via a servo/encoder fixed parallel to the spindle and using toothed belt and pulleys.
Hi Geoffrey

Ah! Didn't realise you had a "real" machine. It looks gorgeous

There are three considerations. How far does the pulley have to move sideways before the teeth can slip, what force is trying to bend it and how concentric is the nut to the bottom bearing.

Let me grab some numbers out of the air and try it.

Assuming half a fat bloke up down force to move it, 50mm diameter pulley, XL belt.

A 4.5 lb side loading has to bend the screw 2mm before it slips. Doesn't sound like it will be a problem.

But the shorter the distance from nut to bottom bearing, the more concentric the nut has to be. Without the top constraint the shaft will tilt as the nut moves up and down. Concentricity decides how much it tilts.

Good luck

Robin


Here's the boring bit in case someone wants to check it...

To slip the belt has to fit around a circle pulley radius + belt tooth height
Assume a 25mm radius pulley and a 1.27mm XL tooth.
157mm circumference has to become 165mm, a difference of 8mm
But we lose half on the semicircular contact and another half because the slack happens on both sides.
2mm of bend to slip.

Next the bending force. Assume half a fat bloke on a 5mm screw so 0.5Nm motor.
Our pulley is .025m radius so the side load on the shaft is 0.5 * 1/0.025 = 20 Newtons = 4.5 lbf