[toughtool]

Reinstalling the balls into a ball nut.
The story: A friend of mine (Larry) obtained a very nice ¾ inch diameter by 35 inch long lead screw and SKF ball nut assembly. So discarding the two ACME screws we were using for the X axis in our Gantry CNC design and redesigning the X drive in favor of a single lead screw and motor was a logical move. Then tragedy! Larry was moving the assembly around and laid the lead screw on the bench and turned his back. Yep, you guessed it. The lead screw unscrewed it self and all the balls ended up on the floor. A little search on the web and a company was spotted that reassembles ball nuts. Lead screw and nut was sent to get re-balled. Well after two months of waiting and a couple of phone calls , a quote of $1500 to re-assemble the balls into the nut. Wonder what planet they are from? Larry only paid $100 for this and a couple of smaller lead screw assemblies, and a bunch of other CNC stuff, from a guy getting out of the hobby. He now wanted to spend his time flying model helicopters and planes. Why couldn’t it be me to find a deal like that?
I took the lead screw home last night and after studying it, I re-assembled it about three different times. Took about 20 minutes each time. There is not much information available about how to re-install the balls but there are a couple of approaches. Scott’s 2006 assembly version ( http://www.cnczone.com/forums/linear...allscrews.html ) didn’t work for me because the clearance is to tight for a ball to pass between the lead screw arbor and the major diameter of the nut thread. There were also at least a full groove from the start of the thread and the first insert. Plus you have to install the balls after the first insert. Putting balls into the center groove looked like it would be a nightmare. Both ends of the ball screw has these arbors, although different lengths. Here is a description of this SKF ball nut unit: The nut has three paths of balls. Each path is one complete thread of the lead screw. Spaced apart, at 120 degrees, are plastic inserts, inserted into holes in the nut housing from the inside. These inserts have small protrusions that fill the lead screw’s thread and the nut’s grooves so that the insert does not rotate. There is also a ditch in the center of the insert to allow for balls to travel from the end of the thread back to the beginning of the thread, thus re-circulating the balls. This ditch allows the ball(s) to jump the peak of the thread and is about two and a half balls in length long. My first thought was to use a little IBM #23 grease, and fill the ball grooves of the nut housing. This is a medium, light colored grease, similar to wheel bearing grease, except it is not black, it’s odorless and thin enough to squeeze out between the cracks, and thick enough to hold the balls in place. The plan was to assemble the balls into the nut housing using hemostats. Then screw the ball screw into the nut. Later washout the grease with mineral spirits or lacquer thinner and re-lubricate with the recommend lubrication. After assembly, I found that wiping the grease from the lead screw after passing the nut along the ball screw worked just fine. The nut will slowly travel by it self all the way, with no binds, if the lead screw is held vertical. If the nut is held, the lead screw will screw it self even faster. Just like before the accident. I ended up installing 18 balls per segment but 19 balls appears to fit, with two balls in the ditch of the insert. So I am not absolutely sure if this nut requires 18 or 19 balls per circuit. If someone knows, let me know. To install all nineteen balls, a different method of assembly will be needed. I am thinking about trying to install all nineteen balls by making a sleeve with an OD (about .665”) that will just clear the ID of the balls (minor diameter of the lead screw), and with an ID (about .585”) to clear the lead screw arbor. Apparently ball nuts are sold with such a sleeve for installing onto ball screws. After placing the balls in the nut housing (with grease), inserting the sleeve should hold them securely in place. Then install the nut onto the ball screw according to this Youtube video. ( [ame=http://www.youtube.com/watch?v=CoTboHFy3eY]Ball Screws: Design and Installation - YouTube[/ame] ) You can not get the ball screw to thread onto the nut with just the grease with nineteen balls because the threading process rolls the balls and this pushes the ball(s) at the end of that thread out of the groove, causing a jamb. A sleeve should force the balls to start recalculating, by keeping them in their groove. Note. There will be an empty segment of nut groove between the first and second insert and between the second and third insert. These two areas about a half inch long and no balls should be there.
Finally, I would suggest that a wire-tie be installed onto each end of the ball screw, so the nut will not accidentally come off the end. These wire-ties can easily be removed once the ball screw assembly is installed into the machine. Joe

[BlairM]

Thanks Joe, I had been hunting for a long time last night for a solution to fix my little mistake. The grease works a treat and had it back on, albeit with some spare balls and a few lost. A bit more work to be done, but the method is good!