I am still cad-ing up my ideas and have been reading a lot on here, my question has to do with nuts for lead screws. I understand the need for antibacklash on single nut systems, and am aware of the ability to elimminate that with double nut systems, but my questions is:

By using a double nut system to prevent backlash you are inherently putting load on both the nuts and the screws. I am not to worried about wear on the screws, but on the nuts. Anyone have any idea of lifetimes?, Sugestions on improving it?

My CNC is probably going to be mounted only a few inches off the floor and the thought of having to dissassemble some very large parts to change nuts every year or so (some random timeframe i have come up with in my head) doesn't appeal to me much because the table will be very much crane worthy.

Thanx in advance
Todd

There are going to be several factors that will determine how long it takes to wear out a nut. Lubrication is going to help a great deal and try not let too much dust onto the lead screw. The amount of force on the nut itself can also lead to wear. I doubt that you would have to replace the nuts once a year depending on design. I'd just use good materials and try and reduce friction anyway possible. If it really bothers you go with ballscrews. Are you planning on using a self adjusting (spring loaded) antibacklash nut or one that can be adjusted by hand? Shouldn't really matter but it's nice to see what other people are doing.

Check the forums there's an article written not too long ago about leadscrews.

Use ballscrews :)

But, since you have anticipated the problem, you could engineer "split halfnuts" (axial split plane), which you could remove from the machine without diassembly of the screw. As a matter of fact, I have never seen anyone describe this type of antibacklash system, but you could likely rig up a split halfnut with some kind of a push-pull setscrew system that would slide one half of the nut against the other half, to reduce backlash. Because this would not be spring loaded, you would not really be creating an unneccessary wear load on the nut.

Thats along the lines of what I was thinkin Huflung, does anyone have experience using split nuts? are they as dimensionally stable as normal ones? Also does anyone have any experience as to etter approximate nut life?

how about copying the split nuts from
the lead screw mech of a lathe.

Yes, the lathe halfnuts was what I was thinking. Here is a crude model of the method. The vertical bolt holes in the lower half of the nut block would be slotted, so that the push pull setscrews on the ends could slide that half of the nut slightly, and then be clamped by tightening the 4 vertical bolts. This would be the anti-backlash device

This assembly would be bolted to a baseplate on the machine, and hopefully, by sliding the top half of the nut along the screw, someplace you would find enough clearance to be able to rotate it one half a revolution so it would drop off the screw. Either that, or provide an additional spacer block above the top side, that would give you extra disassembly height.

I like the idea Jan and Huflung...i'll kinda work it over and see about making it fit....muy problem is that I am limited by a low ceiling in the area and I really wanna push the Z axis heigth because of the need to do for the uses this is going to be intended for.