Double ball nut worth it for long C7 grade rolled ball screw?

[arizonavideo]

I did the sliced nut trick once too. Back whenI used the Roton ball screws with single nuts, it was quite hard to get no backlash with the screws being all over the place in diameter.

I sliced a nut along one side with a tile saw, worked really well for hard stuff.

I then added over size balls. That was a fail because the balls would hit the grove and hang up. I then put the nut in a vice and compressed it so it was tight with the stock balls. This was a bit of a trick to not get it too small. Killed a few nuts. I still have the nut on a screw, never put it into service.

I still see a few places where this might be the best way to go. On a router table where you want low backlash with high speed and low drag. With a single nut you could set the preload with the "spring" system and even use less balls like two rows of the 3 rows of balls.

For the light loads of a router you would get low friction and vary high speeds with no backlash.

A few companies make nuts like that.

For the loads of a metal mill double nuts are still the easiest way to get low lash.

[handlewanker]

If you have a brand new screw then you could fit a double ball nut to have no backlash, but if the screw has seen use and has some wear in the centre part the double ball nut will not wok due to it not being resilient enough to take up the slack in the middle section.

A double ball nut has a hard precision spacer fitted between the two halves to give a precise preload but that only works properly on a new screw that has a consistent pitch and an unworn one at that.

If you have a used screw ….with some wear.....then fitting two single ball nuts flange to flange with a resilient hard rubber spacer, like an "O" ring, between them is the answer to your problems of backlash and it's completely DIY with off the shelf standard components.

The spacer will keep the preload on for the newer parts at the ends of the screw and the less new part at the centre...…...even a new screw with a single new nut can have some backlash.

Ball screw and nut manufacturers won't ever do that because it allows extended screw life for a worn screw long past it's use by date and that is lost revenue to them.....do the sums.

This effect is how an Acme threaded screw and a bronze nut with a spring loaded back up nut works, but ball screws can carry more loads without the friction.
Ian.

[pippin88]

Ian, you can replace the hard spacer in a double nut with a spring or rubber.

[handlewanker]

Hmmm......yes, I think that could be a practical thing to do.....with reservations.

As the two nut halves are keyed together to prevent them from rotating once preloaded there is nothing to hold the second nut from turning when the key is removed to fit the rubber spacer which is a vital thing to keep the two halves firmly in relation to one another......the key has to be removed because it will interfere with the new set up......the nuts have to be able to move axially to get the rubber to do it's job.

It is possible that the key is not secured and is just fitted in a slot, so in that case the key might be a good thing to keep in place if it allows axial movement with the rubber spacer

If a double nut solution is on the cards....especially with a somewhat used screw...... I think it would work better from the beginning to fit two standard nuts flange to flange with one in a housing and the other out in the air using the screw holes to lock them together and fit a hard rubber spacer between them.....it's vital that they do not move radially once they are locked together with the screws.

It really doesn't matter if the nuts are slack on the screw.....the rubber spacer will pull them up tight once secured with screws.
Ian.

[Bdrangdo]

I wrapped a thin piece of shim steel around the two halves and locked it into place using jubilee clips, that way you can loosen one side and effectively adjust the preload by turning one half against the other. Also the shim steel held the "O" ring in place and stopped it squeezing out. Dave.

[handlewanker]

LOL....what a clever idea...…..the shim steel effectively locks the two halves together both axially and radially and allows adjustment too.

I think that method could be applied to a double ball nut with the key and precision spacer removed and a rubber spacer substituted...…..having a straight body without the flange on the second half would enable the shim method to be applied.

The point is, when you have a well used screw and double nut combination the last thing you want is a rigid nut on the screw...…….it will be tight on the ends and loose in the middle where most of the work is done.

I can see a market for well used ball screws and nuts with this application...….normally they are a throw away item.

With this in mind, there would be no point in reloading ball nuts with oversized balls and hope to get a backlash free screw that would still have backlash due to the uneven wear on the screw......a double nut with a resilient spacer is the answer and a pair of slack ball nuts in that combo is better than tight ones.

The big difference is when the balls are damaged or the tracks in the nut are pitted, new balls won't solve that problem, then it's scrap box time....no sense in further damaging a screw......sometimes you have to bite the bullet.
Ian.

[damunk]

I am so stuck. I have spent over a week for alibaba members trying to sell me anything. JUst fed up now , researching and researching!

I have a new C7 chinese screw 400mm long, 20mm diameter and I got some scraping noise from the nut. the balls were viberating in the nut.
I stopped the viberation from the table and the noise went away.

I want to know it is true for me to buy the same ball nut and double it up and because it eliminate backlash, most like will help against noise?
I have 4cm of space to play with so I can get away with another 4cm ball nut.

You just listen them up and have either a nylon spacer? or just screws up against each other?

I dont get the science behind it.

[dazp1976]

I am so stuck. I have spent over a week for alibaba members trying to sell me anything. JUst fed up now , researching and researching!
I have a new C7 chinese screw 400mm long, 20mm diameter and I got some scraping noise from the nut. the balls were viberating in the nut.
I stopped the viberation from the table and the noise went away.
I want to know it is true for me to buy the same ball nut and double it up and because it eliminate backlash, most like will help against noise?
I have 4cm of space to play with so I can get away with another 4cm ball nut.
You just listen them up and have either a nylon spacer? or just screws up against each other?
I dont get the science behind it.

The science is simple.
You're basically spreading the 2 nuts (or double nut halves) away from each other.
This then makes them push against the screw grooves in both directions.
Now you have permanent contact of the balls and grooves so it moves as soon as the screw moves. Thus any backlash is eliminated.

Depending on how it's done, or how much pressure is applied. Your cutting forces could be stronger which would render it useless.
Wear may be higher but C7 screws are cheap enough to buy.

I prefer the foil packing method between the collar and slave nut on the double DFU's.

[dazp1976]

If you have a brand new screw then you could fit a double ball nut to have no backlash,
A double ball nut has a hard precision spacer fitted between the two halves to give a precise preload but that only works properly on a new screw that has a consistent pitch and an unworn one at that.
.

Not Necessarily. I've had brand new DFU screws/nuts with backlash as high as 0.08mm. My lowest new was 0.03mm.

[handlewanker]

I find that hard to believe unless you have a duff double ball nut......the whole purpose of a double nut is to create a rigid opposing force on the screw to eliminate backlash.....if you have backlash in a double ball nut then the spacer needs re-examining to determine if it's doing it's job or the the screw could be worn in which case you are lost and need to replace the screw and the nut.

You "can" create a double ball nut by adding a second single ball nut to the assembly with a hard rubber O ring between them to apply a resilient loading which will accomodate a slightly "used" screw......you have to understand the principle of a double nut.
Ian.

[damunk]

I have checked and there is no double ball nut for a 2020 screw. Would it be okay to use another single ball nut to double up with a spacer in between?

1) do you face the flanges of the ball nuts together?

2) which way do you screw on a ball nut onto a screw for the first time. front side or rear (flange side) so that the seal does not crease/fail.