Preloading

[haku]

I am a total newbie at CNC. What does it mean when people are talking about preloading nuts, and bearings etc??

I have the bigbear CNC plans, which I like alot, however I am planning on modifying it a bit, well just the linear bearings and lead rods.

Thanks
Haku

[Eurisko]

Haku,

Imagine a single nut on a threaded rod. There will always be some clearance between the threads on the nut and the threads on the rod. Otherwise, the nut wouldn't turn at all.

This clearance is taken up as the nut rotates in one direction. If the nut is slowly rotated in the opposite direction, you will notice that the nut is turning, but it doesn't immediately move in the opposite direction. It doesn't begin to move until the clearance is taken up again.

If this nut were driving your CNC's X axis, you would notice this lost motion ( backlash) every time the X reversed direction.

Now imagine two nuts on the same threaded rod. The 1st nut has been turned clockwise to take up the slop in that direction. The 2nd nut has been turned counter-clockwise to take up the slop in the other direction. Now we join the 2 nuts together. We may need a bit more pressure on each nut ( preload) but our two nut arrangement can be tweaked to (almost) eliminate backlash.

This also applies to ballnuts and ballscrews. A single nut will always have clearance. That is why 2 nuts are preloaded to eliminate backlash in both directions.

[haku]

Thanks you.

Can a person buy any special lead nuts to compensate for this??

Thanks
Haku

[Cartierusm]

Yes, if you are using Acme Lead Screws you can use an anti backlash nut, seen here:

http://www.dumpstercnc.com/leadnuts.html

I used these in my original design but did get some backlash off one of them and eventually they will wear out or deform, in my opinion, the stresses they have to undure.

If you are using Ball screws, which I recommend, you can go either of two ways. You can buy one ball nut and replace the balls with larger balls that take up the space so you get very little backlash or a measurable consistent back lash. The other option, which I use and think is best, is to buy 2 ball nuts, to me they aren't that expensive for somthing you have to buy once, and place a spring in between both nuts and it pushes them away creating almost no backlash, none that I could measure. And as the balls wear the spring continuously takes up the backlash. Most people will tell you that NO backlash is almost impossible, especially building home made machines with inexpensive parts, but you can come dam close.

If you are designing a machine in my opinion don't use acme screws go for ballscrews from Roton.com. I didn't know about them when I built my machine and when I found them they were just about $20 more expensive per axis for ball screws as opposed to acme screws.

[BobF]

I just looked at Roton. Looks like the 5/8 .200 screws are pretty inexpensive, but the 1/2 .500 is about 3 times as much. (my machine has 1/2" bearings) Why would that be?

[degreaser]

Originally Posted by BobF I just looked at Roton. Looks like the 5/8 .200 screws are pretty inexpensive, but the 1/2 .500 is about 3 times as much. (my machine has 1/2" bearings) Why would that be?

Supply and demand?

Here's an excellent ballscrew primer:

http://cnczone.com/forums/showthread.php?t=8813

BTW - I received an email from Bill at Roton last week. He indicated that the lead error is .003" to .006" per foot and axial backlash is approximately .006". Preloading would help greatly with these ballscrews.