[Khalid]

I have 5/8'' -5TPI , 1300 mm (51.18 inches) long ball screw mounted on my X-axis. Currently i am getting two problems with it:
1) Whip
2) stalling of motor at two extreme locations.

I checked the ball screw and found a runout/sag in middle portion (about 4 to 5 mm).
My question:
a) Is that the ball screw run out cause its efficiency to loose and stalling the motor.?
b) How to remove the run out?


NOTE:The same size,length ball screw i installed at my Y-axis with no problem.
Sorry for bad english

[ger21]

Sounds like it might be bent and you may need to get it straightened.

[Khalid]

Okay i will check it..i will check it... but if its bent what may be the affect on ball screw and stepper motor?

[mll]

The travel may be to long for that size screw, I use a 20MM thick screw when traveling that distance. You are using comparable to a 16mm screw.

[LeeWay]

I have the same size ball screw about the same length. I get no whip @ 300 IPM. I only have one end captured by bearings though. The other ends rides in just a plain bearing, but not held firmly. I get no binding at the ends either. It sounds like you have both bearings pulling the shaft in on itself. This will make it want to deform if that is the case. With it binding at the ends, it also sounds like yopu have either the carrier bearings or the ball nut nut in the wrong location. I suggest loosening these things up some and then jogging. If it jogs better, lock one end of the shaft down and let the other just float. You may need to shim the ball nut holder or otherwise center it on the carrier bearing end.
Kinda hard to tell just what needs to be done without seeing the details or the machine in person.

[Khalid]

Thank you Leeway for your detail information. I will check those parameters and its gr8 to hear that the same length/size screw can attain 300IPM without trouble.
Best regards

[Khalid]

what if the ball nut in wrong location?.. means if i installed the ball nut toward the motor side then what will happen?

[LeeWay]

I don't think you can install a ball nut backwards. Either direction should be okay, depending on how you hardware is setup. You may gain extra travel if it's mounted one way or the other depending on the design of the mount.
It's the mount that I am referring to that may not be centered on the screw. It has to hold the nut perfectly centered throughout it's entire length of travel in order not to bind. There may be enough flex in the middle to prevent binding, so these need to be adjusted at the extremes of travel where the screw is held by the bearings. That's where you will locate the center or the sweet spot for the ball nut mount.

[Khalid]

Thanks Leeway for your response After close inspection of my lead screw attachments I found two problems. i will solve them but again i want to learn more so asking questions:
Please answer for:

1) If my ball screw has one end bearing 1mm down (i.e. 1mm slope).. then what will happen?
2) If my gantry (z-axis hold down) is tightly fitted with interference of about 1mm (pushing the ball screw back 1mm)...then what will happen?

[LeeWay]

I'm not sure what will happen. The closer you can set everything up to true, the better everything will work and it will last longer. Again hard to judge without seeing the machine and knowing what and how the bearings are installed. Rule of thumb. Shoot for perfect and when you get it as close to that as you can, that has to be good enough. If it's not, then you need to backtrack and maybe remake some parts until it is good enough.

[rexstep]

The OD of rolled ballscrews is always out of concentricity with the balltrack to some degree. This will also contribute to whipping. And other problems especially if you have a preloaded set-up.

Rick L.

[Khalid]

Rexstep... I dismantled the ballnut balls from screw.The balls appear 1/8" i.e. 3.175 mm. I found that no ball were missing, however it seems that the size of the balls are less. That was due to i am having some play in transverse direction in the ball nut with screw. I also found negligible backlash.
I want to install some bigger size balls in it say 3.25 or 3.3mm. Is that right?
As i have no ebay or foreign trade experience,if u please tell me from which ball bearing i can get the closer size of the balls. (I can manage to get such ball bearing here in my country to get the required size ball)
cheers

[Khalid]

I also plan to shift from lead screw to SPROCKET CHAIn arrangement. if u somebody help me locating the DIY thread for SPROCKET CHAIN...
Best Regards

[fatal-exception]

Good advise LeeWay! I back your previous comments 100%.

None of the rolled ball screws I've ever had felt tight on the screw. They are almost always loose. I would not replace the balls unless you were sure that they are worn. Precision ground screws fit perfectly with no slop in any direction and must be setup with a higher degree of precision. You can be out 'a bit' with rolled screws and still have a very low friction drive, this helps at the ends of the screw.

Something else not mentioned yet is that if the bearing journals at the ends of the screw are not concentric to the ball nut rotation, you will see binding action in a cyclic pattern near the pillow blocks. If your steppers cant overcome this, it could stall at varying locations. One of my mini mills is plagued with this problem at the top of the z axis. Thankfully I don't use that extent very often.

Paul

[Khalid]

Good advise LeeWay! I back your previous comments 100%.

None of the rolled ball screws I've ever had felt tight on the screw. They are almost always loose. I would not replace the balls unless you were sure that they are worn. Precision ground screws fit perfectly with no slop in any direction and must be setup with a higher degree of precision. You can be out 'a bit' with rolled screws and still have a very low friction drive, this helps at the ends of the screw.

Something else not mentioned yet is that if the bearing journals at the ends of the screw are not concentric to the ball nut rotation, you will see binding action in a cyclic pattern near the pillow blocks. If your steppers cant overcome this, it could stall at varying locations. One of my mini mills is plagued with this problem at the top of the z axis. Thankfully I don't use that extent very often.

Paul

fatal-exception, thanks for sharing your experience. Now a question arises can we straighten a ball screw with run out? what tools we needed? i have facility of machine shop but they dont work on ball screw.. I have available verticle lathe, Horizontal lathe, shaper, Mills,etc. can u al give me any idea or fixture, igs to work on it..

[rexstep]

Finding the right size balls for preloading is a trial and error event. Ideally, you would want four point contact but as someone pointed out, that tends to cause binding on rolled screws.

What we do to compensate for this is leave just the slightest amount of slop. The end result is usually about .0005" backlash. Also, you don't want to load it with just the oversized balls. Every other ball should be normal sized. This will make for better efficiency and longer life of the screw among other things.

Now, there's a certain amount of forgiveness regarding the eccentricity problem. When pressed, the balls will temporarily deform to a point and the screw will deflect some. Normally, these are not desired results they can provide the needed "give". If you have binding (or skidding), reduce the ball size by .0002" and try again.

www.precisionballs.com is the source we use.

Rick L.

[fatal-exception]

I got a bent c3 ground ball screw off ebay once, very dissapointing. The other 2 were fine. I tried straitening it on a hydraulic press since it only had a single hump in the middle. No luck. I can't remember the exact measurements, but I had it bent the other way somewhere in the range of 3/16" over a 12" length. It was like a spring. regardless of how much I bent it the other way, it would just spring right back to where it was. Finally, when I heard a crack, I knew that this wasn't the way tp try and straiten it. It's in my 'anchor' drawer now. The e-bay seller replaced it free of charge and shipping. That was nice of him!

If anyone has successfully straitened a ball screw, I'm sure there's ears that would like to know about your technique.

Also, how would that screw get bent anyways? There was no sign of heating, and the ball nut was still silky smooth with no backlash.

Paul

[rexstep]

I don't have a lot of experience with ground ballscrews but I've straightened dozens of rolled screws received out of spec from Thompson and Nook alike. Unlike ground screws, rolled screws have to undergo a straightening process at the factory but my main point is that ground screws are a totally different animal so what's been discussed here about bent rolled screws seems not to apply to your "anchor". Yes, I've heard a few snap sounds involving ballscrews; always bad.

Rick L.

[Khalid]

i have done some "non-technical" work on my screw and my machine is again alive Now my ballnut moves compensating the runout I manage to move my ballscrew at 100IPM at Full load condition.
I will soon post some pictures the way i fixed (I dont know the term what shud i say) it. Hope if u have the same kind of ball screw then dont break them in straightening try the other ways of compensating the run out
I hope some one will also post pictures of same runout problem and post some non-standard (what shud i say in place of non-standard) fixation pictures too.

[marchantdice]

Rolled ballscrews are manufactured using a method called "plastic deformation" the waste material (found on the spindle outer diameter) is not the cause of whipping. I think it is safe to assume your spindle is bent and therefore misaligned.

The OD of rolled ballscrews is always out of concentricity with the balltrack to some degree. This will also contribute to whipping. And other problems especially if you have a preloaded set-up.

Rick L.