Linear Motion Double Ball Nuts

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TomS

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Anyone have experience with Linear Motion Technologies double ball nuts? I can't find any technical info online except for dimensional data. What I'm looking for is the nut preload and feedback on backlash and long term accuracy.

I've got about .0018" of Y axis backlash and .0034" of X axis backlash with single ball nuts. I thought about going with larger balls but that may be a hit or miss option until I find the correct size balls. Don't want to spend days disassembling and assembling my mill and changing out different size balls then deal with the wear issue at a later time. I want to do it once and be done.

My X and Y axis screws are 1605 and the Z axis is 2005. Haven't checked the Z axis backlash but if changing out the table axis screws the incremental cost for doing the Z is small. Also going to adjust the AC bearing preload from .002" to .004". I think jbolt did this with good results.


Tom S.
 
Hi Tom,

We are using them on the CNC router at the high school. If I recall the backlash was under a thou. I did have Chai replace one because the nut had no pre-load when installed. I have no idea what the preload is supposed to be but you can feel it when installing the nut. These are 2005 on the X & Y and 1605 on the Z. I am also using them on my smaller CNC router at home but I have never checked it because for what I use it for +- a few thou is not an issue. On my mill I went with two ball nuts on each axis and use Bellville washers to set the preload. It takes a little trial and error to get it right but I like that it is adjustable. After two years I am starting to get some excess backlash in the X axis. At this point I'm not sure if it is the nuts, AC bearings or wear on the screw.

Jay
 
jbolt said:
Hi Tom,

We are using them on the CNC router at the high school. If I recall the backlash was under a thou. I did have Chai replace one because the nut had no pre-load when installed. I have no idea what the preload is supposed to be but you can feel it when installing the nut. These are 2005 on the X & Y and 1605 on the Z. I am also using them on my smaller CNC router at home but I have never checked it because for what I use it for +- a few thou is not an issue. On my mill I went with two ball nuts on each axis and use Bellville washers to set the preload. It takes a little trial and error to get it right but I like that it is adjustable. After two years I am starting to get some excess backlash in the X axis. At this point I'm not sure if it is the nuts, AC bearings or wear on the screw.

Jay
Click to expand...

Jay - thanks for your comments. I ordered the nuts this morning. Any tricks for installing the nuts on the screws without losing any of the balls? Someone wrote on this forum that they went from .002" AC preload to .004" and that improved/decreased backlash. Was that you?

Thanks again,

Tom S.
 
TomS said:
Jay - thanks for your comments. I ordered the nuts this morning. Any tricks for installing the nuts on the screws without losing any of the balls? Someone wrote on this forum that they went from .002" AC preload to .004" and that improved/decreased backlash. Was that you?

Thanks again,

Tom S.
Click to expand...
Yes I have added more shims to up the preload on the AC bearings. For getting the nuts off and on I turn shafts to the minor diameter of the screw. Leave a shoulder on the shaft so it cant slide through. If both ends of the screw are turned put a pocket in he end of the shaft.
IMG_0103.png IMG_0104.png
 
Are you sure it's backlash and not just lost motion somewhere else? Could be bearings, nut mount amongst other things. I would verify that it's actually that before doing anything else
 
chevydyl said:
Are you sure it's backlash and not just lost motion somewhere else? Could be bearings, nut mount amongst other things. I would verify that it's actually that before doing anything else
Click to expand...

There could be other contributing factors but I won't know until I tear it apart. I don't want to disassemble my mill then find out the source of backlash is the nuts then wait 2 to 3 weeks to get them. Three double ball nuts delivered to my door for $109 is relatively cheap insurance.

Your point is well taken. I certainly will look at the sources of backlash before putting it back together.

Tom S.
 
Reason is I had this same problem, except I had brand new quality screws and still had about .003 or so lost motion in Y, it's not the screw or nut, I put new oversize balls in it and no change, it's probably the yoke or the bearings. I need to tear it down again and change the spacer for more preload. I even bought fancy AC bearings that prototrak specifies for their mills. The X uses a pair of good deep groove bearings for thrust with no lost motion, that's how I am determining it's the yoke most likely.
 
chevydyl said:
Reason is I had this same problem, except I had brand new quality screws and still had about .003 or so lost motion in Y, it's not the screw or nut, I put new oversize balls in it and no change, it's probably the yoke or the bearings. I need to tear it down again and change the spacer for more preload. I even bought fancy AC bearings that prototrak specifies for their mills. The X uses a pair of good deep groove bearings for thrust with no lost motion, that's how I am determining it's the yoke most likely.
Click to expand...

I appreciate your insight. It could be a multitude of things but until I get through the offset issue as mentioned in another thread I can't tear into it. Hopefully soon though.

Tom S.
 
If you have (or know someone who does have) a 3d printer, it's pretty quick to print a sleeve for a removal/install tool. I have a set of the double ballnuts and my backlash on a small mill is just under and over a thou. I can't comment on long term because I've only had them a few months. I bought mine through Automation Technologies but Chai machines/supplies them. I suppose you could adjust the preload between the two nuts with shims, but I haven't looked into it as my backlash is minimal. They are the exact same assembly as the single nuts, so larger balls are also an option. I actually ordered the single nuts and was planning on upgrading the balls down the road, but they sent the double buts with a note that said "free upgrade" which was nice, but I already had the saddle milled and ready for single, so had to mill more material to make room for the double nuts on my mill. I'm very pleased with the setup and with a very small amount of backlash compensation set in Mach 3 it's perfect/zero.

As a side note, I removed the balls from mine just to count and make sure they we're correctly filled. I had no real problems (using a little grease and tiny screwdriver) repacking all the balls. Took a little bit of patience, but not that difficult. If you have backlash issues and it's not the ballnuts/screws and everything including your screws bearing mounts/ preload is all right, I'd personally suspect the couplers. Triple check your set screws. I used to use a lovejoy type coupler initially and switching to a oldhams is the way to go. They are pretty easy to mill/make using steel or aluminum for the hubs and delrin for the "disc". Mine came out nice then snug, but you can always hand file the center discs sides to ensure an absolutely perfect fit with zero backlash. The lovejoys just don't seem to be as precise or solid in my experience, and for those using the low cost aluminum "flexible" couplers... throw them away... almost anything is better.
 
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