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  1. #121
    Join Date
    Oct 2018
    Posts
    79

    Re: Ballscrew Basics

    No the 7202s are angular contact so I think the author meant 2 basic grade angular contact bearings with preload will still give backlash...

  2. #122
    Join Date
    Jun 2010
    Posts
    4252

    Re: Ballscrew Basics

    I think the author meant 2 basic grade angular contact bearings with preload will still give backlash...
    Really?
    With the correct preload?
    Forgive me if I think that is ... not quite correct.
    May I suggest you read some good engineering literature on the subject?

    Cheers
    Roger

  3. #123
    Join Date
    Oct 2018
    Posts
    79

    Re: Ballscrew Basics

    Well that was my question -

    However from the article

    Even a T7 rolled screw will benefit greatly from a god set of fixed-end bearings. This is one area which beginners often skimp upon, and they then wonder why their 0.002" ballscrew performs so badly, and has 0.008" of backlash, even with a "0 backlash" ballnut installed.

    I took to mean poor quality angular contact bearings - in fact he meant non-angular contact as the 'poor quality' reference - so then he is not saying quality of the set of angular contact bearings is critical - which is what I was expecting hence the earlier question...

  4. #124
    Join Date
    Jun 2011
    Posts
    692

    Re: Ballscrew Basics

    Getting proper preload on cheap AC bearings might be more difficult though. Matched AC bearings usually have the inner faces (the face pointing at the other bearing) of the races ground so that when the pair are installed tightly in contact with each other they will have proper preload.

  5. #125
    I concur! For DIY, for the double row bearing mentioned, use 5202 with seals. $13 ish on VXB.com. Same diameters as 7202, but a bit shorter, will need washers to get the length right if you run out of threads. Backlash is extremely low. Makes me want to go back in the lab to quantify that. We use hundreds of these.
    Mike Visit my projects blog at: http://mikeeverman.com/
    http://www.bell-evermannews.com/ http://www.bell-everman.com

  6. #126
    Join Date
    Oct 2018
    Posts
    79

    Re: Ballscrew Basics

    Skrubol you are correct, it also appears that the matched pair is to control preload - at least that is what I leared from

    https://www.ktaiballbearing.com/prod...tact-bearings/

    Matched Pair Angular Contact Bearings, also called duplex bearings, are machined as matched sets. The pairs are furnished with the race faces ground to provide preload when installed. To accomplish this, a gap is provided between the inner races or outer race of the pair.

    But the design used where you also fit a nut to the shaft - as long as you use a washer (on outer edge of bearing only I believe) then you can pre-load without a matched pair and saving vast amounts of money :-)

  7. #127
    Join Date
    Oct 2018
    Posts
    79

    Re: Ballscrew Basics

    Quote Originally Posted by Mike Everman View Post
    I concur! For DIY, for the double row bearing mentioned, use 5202 with seals. $13 ish on VXB.com. Same diameters as 7202, but a bit shorter, will need washers to get the length right if you run out of threads. Backlash is extremely low. Makes me want to go back in the lab to quantify that. We use hundreds of these.
    Thanks Mike I'll have a look at the dimensions/specs of the 5202 as an alternative to 7202's - either way I will be using double angular sealed bearings with preload via nut on the shaft.

  8. #128
    Join Date
    Oct 2018
    Posts
    79

    Re: Ballscrew Basics

    Quote Originally Posted by men8ifr View Post
    Thanks Mike I'll have a look at the dimensions/specs of the 5202 as an alternative to 7202's - either way I will be using double angular sealed bearings with preload via nut on the shaft.
    Ah 5202 is a double row angular bearing - I would prefer the 7202's so the preload can be set/adjusted.

  9. #129
    Join Date
    Jun 2010
    Posts
    4252

    Re: Ballscrew Basics

    I don't think the 5202 bears much resemblance to a matched factory pair of 7202 ACBs except in superficial appearance. Certainly not in performance imhe. A good bearing, but for a different application.
    Factory pairs of ACBs tend to be a shade more expensive than two stock ACBs. In a production environment the extra cost is not significant. For hobby use you can arrange the preload yourself by carefully doing up the retaining nut. Yes, it can be done by 'feel'. You will need one of those locking washers where you hammer a couple of tines over. Don't try to use a lock nut: that will wreck the preload.

    Cheers
    Roger

  10. #130
    Join Date
    Feb 2016
    Posts
    7

    Re: Thankyou

    Quote Originally Posted by Mat-C View Post
    Thanks for this writeup... the time and effort taken is really appreciated.

    Line on the rolled leadscrews: I would guess this is created first to give the displaced metal somewhere to go as it is forced out of the grooves? Or it might form naturally during the displacement process? (Wild guesses both).
    I don't know if anyone has given any confirmation on this or not, but I have seen that all of the rolled acme I have, has this line too, and the inner area of the poorly rolled ones, from zoro tools of all places, the groove is big enough and unprocessed enough to see that it is indeed from migration of cold rolled metal building up, though I have seen nearly non-existent line rod that was advertized as rolled from wholesale tool, and luckily I had a piece here I could examine under a strong magnifying glass, to see for sure, there is indeed a very tiny line wavering around through that plateau also called a crest in some places I have looked on threading details, (valley being the groove for the mating "crest" and walls to fit into for creating pressure when rotated) so my guess is they had a secondary rolling tool that was pressing down on these crests to flatted out the two rising bits of metal till they were smooth, which is probably good to avoid loss of lubricates out of that when they are turning.

    I have actually been looking at making some tooling that I may need to run coolant through, and thought of running an additional roller so I have two start that the middle additional thread is smaller and has two rollers that will finish pressing these together with enough force to seal it shut with out filling the void, so that a tube is spiraling through the rod below that crest, to press fluid or air through to cool the rod, though this is intended to be done with aluminum and use friction pressure to infiltrate it with micrograin carbide powder, to avoid wear, due to them being aluminum, so it does kinda make me wonder if rolled could be better, if like mine, a final grind is applied to force the accuracy by removing everything down to a few thousandths under where the lowest valley of pressure would be in any of the rod's threading.(also for tougher material, maybe using titanium in place of aluminum all under argon so no TiO2 or Al2O3 that forms passively is pressed into the rod. then impregnating with powdered tungsten carbide, or for more durability tantalum carbide. though the titanium is a poor thermal conductor thus why I have not thought of using it as my above mentioned custom threaded rod lol)

  11. #131
    Join Date
    Feb 2016
    Posts
    7

    Re: Ballscrew Basics

    oh also, on the "preloading" for removing backlash, in ballscrew/nut, I was under the impression that there was pressure on the entrance and exit points that the balls were pressing against continuously much like spring loading two nuts that can not turn independently to get some preload of nut threading against the screw threading, so movement is instant as long as the pressure is lower than your spring's pressure lol, anyway, the point I would figure is something you can envision, with soda cans where if you have the rolling through a space, standing on end, and the gap is tight, the cans are in a straight line, but if the gap is wider than the cans, they start forming a sort of zig zag pattern, so that the cans are packing in the gap as tightly as they can, with less than one can width from entrance to where the second can is at, since it is rolling against the other wall, and if the walls get widened over time (wear or pressure and compression off the wall) then the cans just go into an even wider zigzag pattern so that one side of the wall or the other has cans pressed firmly against it, thus pushing on one can instantly moves all cans and creates pressure on both walls instantly, so the balls would be doing the same thing but the zig zag is in a spiral too, to make things more complicated, so that as your walls of the nut and screw are worn the pressure pushes the balls firmly into the groove to form a wider zig zag pattern so that any rotation instantly generates pressure on walls of the groove so that no backlash is noticed. would also mean that packing them is probably a pain. this remains to be seen, as I plan at some point on making my own )

  12. #132
    Join Date
    Apr 2019
    Posts
    3

    Re: Ballscrew Basics

    Dear Swede,

    Thank You For sharing information.This was really a great article for those who don't have much knowledge of Ball Screw.

    I found a web page about ball screw in that they are explaining the information about ball screw and their type.

    Do visit to know more information
    THK Linear Guide Supplier | THK Ball Screw Distributor | Exporter in India - Seimitsu Factory Automation Pvt Ltd.

  13. #133
    Join Date
    Aug 2020
    Posts
    6

    Re: Ballscrew Basics

    Could someone explain the difference between DFU and OFU ball nuts? DFU means double nuts but OFU means off set double nut. What does the "off set" mean here?

  14. #134
    Join Date
    Apr 2009
    Posts
    144

    Re: Ballscrew Basics

    Quote Originally Posted by JKAVS View Post
    Could someone explain the difference between DFU and OFU ball nuts? DFU means double nuts but OFU means off set double nut. What does the "off set" mean here?
    The specific design can vary a bit by manufacturer, but basically it's just two ways of accomplishing the anti-backlash. The DFU uses a spring or spacer of some sort to apply preload. If your machine force overcomes this spring/spacer compressive strength, you will end up with backlash. The OFU is actually a continuous, one-piece nut, but has a small offset in the threads halfway through that applies the preload by providing two points of contact.

    The advantage of the OFU is that it's a solid piece that can withstand more force, so it's meant for heavy duty applications. The downside is that it requires a precision fit to the ballscrew (get them from the same manufacturer) and is not adjustable. Unless you have a specific use case for OFU, go with a DFU style nut.
    JGRO Complete - G540, 380oz Nema23s, 1/2-10 ACME, 30"x14", Craftsman router
    Joes 4x4 R&P in progress

  15. #135
    Join Date
    Jun 2010
    Posts
    4252

    Re: Ballscrew Basics

    The DFU uses a spring or spacer of some sort to apply preload.
    I don't like the spring versions as they will give backlash at some stage.
    The spacer versions can be hit and miss too, unless they are made with extreme precision.

    I prefer the double-nut version which is manually adjusted with a very fine screw-thread. You wind the second nut out (or in) until you feel some resistance, then you check how easily the ball-screw rotates in the assembly, and fine-tune until there is just a slight drag. That seems to give close to sub-micron backlash. Then you have to lock the adjustment of course: not with a lock nut but with a toothed washer.

    However, this only works with a good uniform ball-screw. If your ball-screw is worn significantly in (say) the middle, then what is tight at the ends will be loose in the middle. But then you need to replace the ball screw anyhow. You could keep it going for a while with a spring-loaded version, but with reduced accuracy. TANSTAAFL

    As for a zig-zag packing - forget it. Normally the balls in a ball-screw are not jam-packed together (and should not be so!), and the grooves should definitely NOT have that sort of slop anyhow. Does this mean those cheap poor-tolerance ball-screws should not be used in a CNC? Correct. They may be good for motorised window-openers, but not in a CNC.

    My 2c
    Cheers
    Roger

  16. #136
    Join Date
    Mar 2024
    Posts
    10

    Re: Ballscrew Basics

    hello friends,
    This is Steve from MSM Store.We're selling various of Linear Bearings,Shaft Holders,Couplings,Handles,Manual Screw Jacks,Press Plates,etc.Mainly all kinds of accessories of machines.If you have in need of such items,please feel free to contact me [email protected]
    You may also visit our site at https://www.msmstore.com/ for details.
    Thanks

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