PDA

View Full Version : Ballnuts



Web Goblin
17-04-2011, 10:18 PM
I have came to the conclusion that ballnuts were made with the single purpose of making my Sunday afternoon a disaster.
Aftertrying to fit one to a ballscrew this afternoon it jammed part way on and wouldnt go any further. So I unscrewed it and reversed it to try from the other side. Same result, it jams part way on. So decided to take it off to have a look at it and found that some of the balls had gotten past the centre seals of the double ballnut and came out the centre. So I have about 20 balls lying on my bench at the moment and I`m not sure which race they came out of. I think the only way to be sure that it goes back together is to take all the balls out and start again. That should provide a few hours of fun.

Ian

Jonathan
17-04-2011, 10:35 PM
At least a few hours! I think you should machine a cylinder just under the minimum diameter of the screw (doesn't need to be accurate). You can put that inside the ballnut and then poke the balls in one at at time, making sure there's no gaps.

If it's a preloaded ballnut you might find it has two different size balls, OoOoOoOoO sort of thing. I'd measure them just to check.

Web Goblin
17-04-2011, 11:25 PM
Jonathan,
I still have the tubular sleeve that came with the nut so I can use that. It is a double nut so I am going to split it and try to do each half seperately. Hopefully this will be alot easier. I am going to get some light grease tomorrow as well so that I can use it to "glue" the balls in place. I can always wash it out again when the nut is assembled. I didnt know that some nuts had different size balls. I need to check that as well.

Ian

M250cnc
18-04-2011, 09:45 AM
Ian,

Use a scriber with a dab of grease on the end to pick up the balls and put them in place.

You need to turn the mandrel that came with the nuts to rotate to move the balls down the channel.

Please do not ask how i know how to do this. :rolleyes:

Using grease is OK to use as a lubricant as my nuts came pre greased, and unlike oil it tends to stay around.

Phil

Robin Hewitt
18-04-2011, 11:18 AM
Check the end of the screw for burrs where the helices start, give them a few swipes with a needle file regardless. Nowt like a bit of burring to misplace your balls.

Web Goblin
18-04-2011, 09:41 PM
Thanks for the info guys. I will have a go at it later this week.
I have been ding a bit of work on a lathe that a bought a short time ago. Just finished wiring up the inverter tonight and I`ve got it running!
Runs nice and smooth too. Need to fit the guards back on and make a new one for above the belt drive because I dont like the one that came with the lathe and then the next plan is to modify the frame covers to add some extra storage and maybe fit a suds pump.

Regards

Ian

Web Goblin
23-04-2011, 09:55 AM
I have finished off rebuilding the ballnuts. I ended up doing all three of them and it took me about 20 minutes per nut which wasnt too bad. Fitting them on the ballscrew is a different story. When they screw on they seperate. What I had to do was tighten them together with some large grips and fiddle about with the keyways to get them together. Its a right PITA. They tighten up quite a bit when together and take more effort to move them along the screw but I suppose thats the price to pay for zero backlash. The thing I`m really not impressed with is the end seals. They are made from hard material like nylon and held in place with two pointed grub screws set 180 degrees apart. Its really crappy. I think a softer better fitting rubber material would do a far better job of sealing the bearings and sweeping the screw and would stay in place better. I will try to post a pic later to let you see what I mean.

Ian

M250cnc
23-04-2011, 11:00 AM
Fitting them on the ballscrew is a different story. When they screw on they seperate. What I had to do was tighten them together with some large grips and fiddle about with the keyways to get them together. Its a right PITA. They tighten up quite a bit when together and take more effort to move them along the screw but I suppose thats the price to pay for zero backlash.Ian

In my case when i used double nuts they were too slack, i looked around on the net and found that they should be sprung in the middle. So i modified two big belleville washers and they seem to work well


The thing I`m really not impressed with is the end seals. They are made from hard material like nylon and held in place with two pointed grub screws set 180 degrees apart. Its really crappy. I think a softer better fitting rubber material would do a far better job of sealing the bearings and sweeping the screw and would stay in place better. I will try to post a pic later to let you see what I mean.

Ian

Those seals are really only good for keeping out crud while they are being built into the machine, what you really should be using is sprung ball screw covers, if you fail to do this then they are gonna wear/bind sooner rather than later.

Phil

Robin Hewitt
23-04-2011, 11:22 AM
Another vote for springs, I preload to 1/4 ton and they are really smooth with truly zero backlash. The spacer shim gives low backlash not zero, it can't compensate when they bed in. It shouldn't be hard to reassemble the nut though, is there any possibility you muddled the spacers up?

Jonathan
23-04-2011, 11:43 AM
Another vote for springs, I preload to 1/4 ton and they are really smooth with truly zero backlash.

How much less smooth are they than a single ballnut? I'm thinking of doing this with the spinning nut idea on the router.

Robin Hewitt
23-04-2011, 11:55 AM
If you spring the nuts it should be perfectly smooth, it can ride over imperfections without cogging. PITA to do but you get your reward when you put the DTI on it :tup:

Web Goblin
23-04-2011, 10:59 PM
Robin,
there is only one spacer in between the two parts of the nut. They look to be the same type as the ones in your photo apart from the spacer being in two halves.
I do like the idea of sprung washers though. This would allow the two halves of the nut to go together tightly enough to work correctly and allow for a little bit of adjustment to align them correctly. I think I will modify them before I go any further with my build.
I am still trying to think up a good idea for replacing the seals/screw cleaners on the nuts. If anyone has any ideas they would like to share :whistling:

Ian

Jonathan
23-04-2011, 11:04 PM
I do like the idea of sprung washers though. This would allow the two halves of the nut to go together tightly enough to work correctly and allow for a little bit of adjustment to align them correctly. I think I will modify them before I go any further with my build.

Springs is the standard way to do it. Just make sure that the force the spring applies is greater than the cutting force on your machine, otherwise the backlash will come back to get you...

Web Goblin
23-04-2011, 11:08 PM
Jonathan,
thanks for that. What angle of washers did you use? I was thinking of 60 degree ones to give a bit of adjustment space.

Ian

Jonathan
23-04-2011, 11:34 PM
thanks for that. What angle of washers did you use? I was thinking of 60 degree ones to give a bit of adjustment space.

I've not done it yet :lol: I'm not sure why you would need adjustment space.

http://en.wikipedia.org/wiki/Belleville_washer might be helpful...

Web Goblin
23-04-2011, 11:48 PM
The adjustment space would allow me to align the keyways on both havles of the nut. Say they were screwed together and the keyways did not line up you could rotate the nuts till they did line up and still have enough tension between them to keep the backlash down.

coredump
04-08-2011, 11:28 AM
Springs is the standard way to do it. Just make sure that the force the spring applies is greater than the cutting force on your machine, otherwise the backlash will come back to get you...

This is a common misconception. Having a spring force above cutting force is not enough to minimise backslash.
You should use a spring that have a spring constant high enough to give a displacement which is less than backslash.
displacement = Force / spring constant
Let say you have a fixed nut coupled with a spring loaded nut. The spring is preloaded 10mm to give 100N force (k = 10N/mm). If the cutting force is 10N, the spring will retract 1mm, above typical backslash, so in fact you end up like having fixed nuts with clearance between them. It's not backslash defined as having dead gap displacement when changing direction, but circle cuts will gives you ovals in these conditions as spring is compressed during direction change.

Belleville washer are the one to be used, and strong enough to minimise backslash. The best is to use the nut body itself to be the spring, this way you can achieve very high rigidity. Same thing with oversized balls.

Robin Hewitt
04-08-2011, 11:41 AM
Surely the preload has to be overcome before the spring can move any further? :whistling:

coredump
04-08-2011, 12:23 PM
Yes my deflection example is wrong when the total force applied to the nut is below preload threshold, my mistake.
But having preload rated against cutting force (ie force resulting in the cutter itself) is not enough! don't forget that you have also a motion!
There is force needed to achieve proper acceleration, and this add to the cutting forces.
Once you are above cutting forces, you need to have highest rigidity possible: shortest and strongest spring possible.
Having a very very high preload itself is not always the way to go: higher wear, need more driving power due to loss.

Jonathan
04-08-2011, 12:28 PM
Yes of course you need to include all the forces, cutting and F=ma. Taking my machine as an example, mass of gantry is about 40kg, a is 1m/s^2 at the moment (it will do 3m/s^2 but that doesn't help) ... so that means you have up to 40N extra from the acceleration.