Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: kolias on September 17, 2012, 10:01:03 AM
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First time I use ballscrews and have a hard time to understand how to set the antibacklash nut.
From the attached pics, this nut has a small raised face on one side and I was told this raised face goes against the bearing which has a recess to accept the raised face of the nut.
This nut also has two set screws which you tight them after the nut is set in the right position. So first I finger tight the nut against the bearing and then tight the two set screws which means that the nut will turn with the ballscrew.
I have two questions:
1. How tight is finger tight? It seems if I tight the nut with my finger too tight, the motor does not turn the ballscrew properly, it stalls momentarily.
2. Then when the nut is tight, it rubs against the bearing housing and therefore we have metal to metal friction. Should I put some grease in there to decrease the friction?
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You can contact the the bearing inner race but not the housing (rubbing)...sounds like you need a really skinny spacer in between to prevent interference from the nut and the outer housing. I dont think this is really anti-backlash (like between the nut and screw) rather anti linear play in the screw which would act like the former when changing direction of cut?
Robert
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Thanks Robert for the help
Perhaps I should have done this before but I removed one motor / coupling and then this small nut and looks like that the bearing is covered with some kind of rubber so this nut will rub against rubber (doesn’t sound good to me)
Where the shaft goes thru the bearing I cant see the bearing because its covered with this rubber
So I just screw the nut enough just to touch lightly this rubber and turn the set screws to lock it in place. Now looks like the ballscrew moves nicely by hand and tomorrow I will get the motors going and see how they turn.
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Hello Kolias,
Firstly, those bearing housings from China are, lets say....less than desirable. I gambled on a set myself, and lost. I had to do some modifications to them in order for them to even function halfway acceptable. Now on to your problem....
Your either missing a part, or you are just overlooking it. The nut should screw up against a short metal tube, that in turn presses against the inner race of the bearing. This tube should be located in the square part that has the rubber seal. The rubber seal should fit around the outside of the tube, and functions only to keep out contaminates. The rubber seals on my screws also got chewed up a bit. This was mainly due to the fact that the square jam nut was very poorly, and incorrectly fabricated. I had to fix those. You might have to shim between the outer races of the bearings also in order take out the play. I had to modify the bearing cap on the front too. They had the incorrect hole pattern, and the extrusion that goes against the outer race was not long enough to even touch the race. Maybe my bad set was the "one in a million", but I very highly doubt it.
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Thank you Sam for taking the time
I attach some pic to see mine. It looks like that I do miss the metal ring you mention in your pics.
In my pics, both rings you see are rubber but in any case the raised flange on the nut will touch the rubber seal anyway unless the metal ring I'm missing was extending in front of the rubber seals.
So where do I get this metal ring I'm missing? Am I out of luck?
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Where do you get one? You can make one I suppose if it comes down to it. As long as it hits the inner race of the bearing, and your able to screw the nut against it, you should be O.K. It does indeed extend out past the rubber seal slightly. Your local hardware store probably has something that will work, if your imaginative. You definitely must fix this. It is like having three wheels on your car. You'll get nowhere quick, and tear stuff up if you try to use it.
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ok Sam, I will get something to fix this problem. As you say better to fix it now than have major problems later.
Thanks for the help
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1. How tight is finger tight? It seems if I tight the nut with my finger too tight, the motor does not turn the ballscrew properly, it stalls momentarily.
Guess it depends on what one would feel. I would say in the range of 10 to 40 in-oz. What is important is that nut is tight enough to remove
most of the play in the bearing. So what you may want to try is tightening the nut until the bearing seats or causes a lot of drag in the bearing and then back off by a part of a turn. That will get you in the ball park of first adjustment for later testing.
How much you tighten everything is important. If loose there will be play in the bearing and you will have backlash and too tight you will be
using up available torque from the motor which is better used for machining. Example, it takes a total of 100 in-oz to move my mills table thus if the motor was rated at 200 in-oz that would only leave 100 in-oz for all the other things associated with that axis. If you leave 30% of available motor torque for headroom then there is not much torque to overcome other influences. Most folks concentrate on the ball nut backlash but the bearings can be a big influence on backlash. You will NEVER have zero backplash or play in the bearing, but you can
minimize what you end up with.
I am lucky such that i have an excellant torque gauge and can measure accurately the influence of each item in the axis chain. That allows me to tweak things and arrive at a good comprimise. Be meticulous and it will pay off in the end.
RICH
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Thanks RICH for the info, it will be very helpful after I resolve the problem described on this thread
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Had to go to the bottom of the problem and so I took two bearings apart to see what is inside
As you can see from the pic, I do have the ring which I thought was missing but the ring sits too far in and therefore when I screw the nut it touches the rubber seal
This may have been created by me because during the assembly I did not use proper methods to install / remove quite a few times the pillow blocks and maybe I have misplaced the proper bearings location. So now the back bearing is flush with the housing and the front bearing sits inside the housing by about 1/8" and thats why the ring does not extend to clear the rubber seal. The 2 bearings inside the housing are touching each other
My question: should I have a space between the 2 bearings inside the housing?
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the ring sits too far in and therefore when I screw the nut it touches the rubber seal
Same problem I had. The ring should be long enough to extend past the seal. This could have been accomplished by the bearings bore depth being shorter, or just using a longer tube. Preferably a shorter bore depth to solve the next problem.
the back bearing is flush with the housing and the front bearing sits inside the housing by about 1/8"
That is fine. The cap on the front has an extruded circular portion on it, that is supposed to mate against the outer race of the front bearing. It must extrude at least the 1/8", preferably a tad bit more. My set did NOT extrude the 1/8", hence a shorter bore depth would have also solved this problem as well. If the outer races of the bearings are not sandwiched firmly within the bearing housing, then the whole concept is useless.
should I have a space between the 2 bearings inside the housing?
As I stated earlier, the outer races must be held firmly inside the housing. There should be no movement of the outer races. When you tighten up the nut, it compresses the inner races together, taking out any play between the bearings. With those cheap bearings, however, you may have to place an outer race shim between the two bearings, in order for all the slack to be taken up. Otherwise the inner races mate together, and there is STILL play within the bearings.
Like Rich stated, you only need to tighten the nut until the slack is gone in the bearings. You don't need to tighten it very much. The screw should turn VERY easily, without much resistance at all. Keep in mind, the bearings should be oriented in a back-to-back configuration.
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Thinking which way will be the easiest to correct my problem
Finding a ring with the right diameter and wall thickness to extend the existing one looks almost impossible to me so I think it may be easier to make some shims and place them between the bearings.
Another alternative I’m thinking is to push both bearings towards the front. This will push the ring pass the rubber seal in front. Doing this looks like the best alternative unless I miss something. The threads of the ballscrew will sit against the rear bearing and when I tight the nut in the front will keep the bearings in place. This assumes that there is nothing in the pillow block casting to prevent the movement of bearings.
What do you think Sam?
Also how did you fix you problem; with a new longer ring? Where did you find the longer ring?
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I think the bearings should be seated in thier respective housing. If they were to touch each other on the OD it should not affect performance. You will feel any unwanted friction much like Rich described above. He's got this friction stuff down to a science, lol. Good thing that you have all the parts after all.
Robert
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I put the nut in the lathe and turned the shoulder on it longer, placed some shims between the bearings, and fixed the multiple problems with the cap on the front. I realize you do not have the options that I had, and you actually may not have all the problems that I had, so I certainly don't want to send you down the wrong path on anything.
The bearings will not go any further forward than the cap will allow. They may already be as forward as they will go. Mine had about .03" gap between the cap and the bearing, which allowed the whole assembly to float that much. Yours may be fine.
You only need to shim between the bearings (outer race) if tightening the nut will not take out all the slack between the bearings.
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Thanks Sam for the great help
I spent quite some time today to analyze what is going on with these bearings and finally I noticed that snagging the nut against the font rubber seal, the raised face of the nut does not rub against the seal.
The reason is that the seal is concave towards the center and this prevents the raised face of the nut to rub against it. To confirm this, I removed the ring and tuned the nut by hand and I definitely noticed the friction between the rubber and metal and putting back the ring I definitely noticed the nut tuning like a bearing – no friction at all.
So now I’m in the process to put back the two bearings I removed and all should be fine
Thank you all
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You only need to shim between the bearings (outer race) if tightening the nut will not take out all the slack between the bearings.
Just in case I need to shim down the line:
When you say to shim between the bearings I will assume you mean the two bearings inside the pillow block
But what do you mean by (outer race)?
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http://www.motorera.com/dictionary/pics/r/race.jpg (http://www.motorera.com/dictionary/pics/r/race.jpg)
Placing a shim between the outer races increases the distance between the inner races, and allows more slack to be taken up from tightening the nut.
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That is very good Sam, thank you