Ballscrew/Ballnut question.

[alexccmeister]

Hi all,

I just bought a length of rolled ballscrew and a couple of nook's ballnut. I would like to install the ballnut in pairs to eliminate backlash. How do I install the ballnut? A or B or either way will work? Thanks.



Alex

[acondit]

Jeff Davis sells a cheap set of plans for converting two single ballnuts to a "professional" double ballnut. Here is his URL. I bought a set of the plans and it looks like a fairly simple task to do the conversion. There may also be some sources of free information, I'm not sure.

http://www.homecnc.info/ballnuts.htm

Good luck,
Alan

[fatal-exception]

Basically it dosen't matter, they will work either way.

You can check out this thread to see what I tried this weekend: http://www.cnczone.com/forums/showthread.php?t=46579

Take care,
Paul

[alexccmeister]

Thanks guys. My only concern is with the direction of the turn when I put the ballnuts in opposite direction to each other. But thinking about a nut which can be turn either way to a bolt made sense after reading the replies. My plan is the same as Paul's. Thanks again.

Alex

[acondit]

Originally Posted by alexccmeister Thanks guys. My only concern is with the direction of the turn when I put the ballnuts in opposite direction to each other. But thinking about a nut which can be turn either way to a bolt made sense after reading the replies. My plan is the same as Paul's. Thanks again. Alex

Alex,

The problem with that approach is that if you screw the two nuts towards each other with no spring takeup, you will either bind at some points or have backlash at other points. It would work if the threads were perfect along the length of the screw, but they aren't. In fact if the threads were perfect you could just adjust the size of the balls in the ballnuts for a very precise fit.

The idea of the spring loading is to allow the nuts to be pushed close to each other at the points where the threads allow them to be close and at points where the threads diverge the nuts can push back against the spring and separate slightly rather than binding. This may be clear as mud but I can't think how to state it more clearly at the moment.

Alan

[alexccmeister]

Hi Alan,

No problem there. I am with you in your explanation. But I do have a question though. I have seen ballnuts with spring, but how stiff is the spring? Can I fabricate something similar? Could I not space the ballnuts enough to not cause binding? But then again I would have to compromise on the fact that I may experience a small amount of backlash. How small is yet to be seen.

Thanks.

[acondit]

The commercial double nuts use belleville washers (or disc springs) as the springs. I am not sure of the stiffness in foot pounds but I would characterize them as very stiff. One thing I saw said 125 to 500 pounds compression.

Alan

[fatal-exception]

Alan is right, in theory, about the binding/backlash if not using springs to hold the ballnuts apart, but I've installed the opposing ballnuts on the x-axis of my machine. I've adjusted them to be zero backlash, and I can easily turn the screw with my fingers from one end of the table to the other end, with no noticible change in torque. (30" long)

I think that since the nuts are close to each other, the difference in pitch of the screw within a 4" span is very small, small enough that they don't tend to bind.

I still agree that they should be sprung apart rather than fixxed, but this is a very simple way to eliminate backlash, 1 large tapped hole.

Paul

[ironDigit]

My first thaught with double nuts is springs since these will compensate any wear automatically meaning lower maintenance then two set nuts having to check on the preload every once a while.

my second but i might be wrong is that the two nuts should be preloaded in opposite directions or maybe even towards eachother,making every nut taking up the forces for the movement in its own direction in wich it is preloaded and staying idle when moving in the opposite direction.You can see that this has potential in extending maintenance periods.


i hope i was clear on this one.

[LeeWay]

When you screw the two nuts together on spring washers, do they stay locked the way you tension them? Like a jam nut?
Just curious. I plan on adding a second nut to my X and Y axis. After three years, I am starting to see some back lash. I will replace the balls in the first ones and add a second nut. Room isn't an issue on this machine. Plenty of it with no loss of travel.



Anyone have any nice designs they could share? I'll check out the above link as well.
Thanks.