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Why aren't ballscrews more common? The tech is old and no one likes backlash, right??

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  • Why aren't ballscrews more common? The tech is old and no one likes backlash, right??

    Title pretty much says it, the tech has been around for a long time especially in steering gears etc. I guess cost is the obvious factor...But I've read plenty about the lengths manufacturers have gone in the manufacture of precision lead screws from master screw generation, to complex temperature compensation and control, form grinding, error mapping, error compensation mechanisms, etc. Seems like after that kind of investment the cost of making a ballscrew actuator would be negligible.

  • #2
    Ball bearings don't handle shock loads at all well.

    Ball bearings don't handle contamination at all well.

    Ball screws will reverse drive under load.

    Lubrication is critical, not self lubricating.

    Wear compensation is difficult.

    Load carrying capacity is less than acme.

    Expensive.
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    • #3
      I imagine a ballscrew is just as difficult to make accurately (maybe more so) than a regular screw.
      ----------
      Try to make a living, not a killing. -- Utah Phillips
      Don't believe everything you know. -- Bumper sticker
      Everybody is ignorant, only on different subjects. -- Will Rogers
      There are lots of people who mistake their imagination for their memory. - Josh Billings
      Law of Logical Argument - Anything is possible if you don't know what you are talking about.
      Don't own anything you have to feed or paint. - Hood River Blackie

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      • #4
        Originally posted by SGW
        I imagine a ballscrew is just as difficult to make accurately (maybe more so) than a regular screw.
        Ground precision acme screws can be just as expensive. What Evan said.

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        • #5
          Evan points out some good reasons. The bottom line is that ballscrews move too easily on a manual machine that has no servo motor connected to give it some 'drag'. Thus making climb milling problematic without clamping down on the gibs. But then you don't want to do that as it accelerates way wear...thus ball screws are undesirable in a manual machine.

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          • #6
            Maybe my previous replly was ambiguous. I was trying to suggest that ballscrews aren't magically more accurate than a regular screw; they are subject to all the errors in manufacture as are regular screws. They don't have backlash, but that's not a source of error as long as one always takes up travel in the same direction. And they do have all the problems Evan listed.
            ----------
            Try to make a living, not a killing. -- Utah Phillips
            Don't believe everything you know. -- Bumper sticker
            Everybody is ignorant, only on different subjects. -- Will Rogers
            There are lots of people who mistake their imagination for their memory. - Josh Billings
            Law of Logical Argument - Anything is possible if you don't know what you are talking about.
            Don't own anything you have to feed or paint. - Hood River Blackie

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            • #7
              Actually, ballscrews are very common. So much so that they're readily available in the surplus market for a fraction of their original cost. From a design tradeoff and cost standpoint, I think they're used everywhere it makes sense. They're very common on CNC machines as well, but not so common on manual for all the reasons Evan mentions.

              I think a good understanding of backlash also helps explain this further. Backlash is just not as pernicious to the manual world as it is to the CNC world for a variety of reasons. Yes, it can contribute to chatter and other ills, but there is a lot going on to offset that. The much greater friction of the ACME screws versus ballscrews for example.

              On the flip side, CNC really has a problem with backlash and there aren't many factors that mitigate this. You can do things while profiling on CNC that just are not really possible to do manually and while doing them it is impossible to compensate completely for backlash. It's okay to take up the backlash in manual style, but what if the part you are machining requires arbitrary direction changes in mid cut with no opportunity to take up that backlash? Can you turn the handwheels exactly right to cut a smooth circle, ellipse, or other exotic shape directly? Probably not, but it's routine for CNC.

              Remember too that while many CNC machines are closed loop, meaning they can sense what the machine does when a screw is turned, it is a very limited closed loop compared to a manual operator's ability to tell what's happening. Heidenhein has a fascinating article on the value of using linear (DRO) scales with CNC machines to augment their basic closed loop encoder capability that helps illuminate some of these issues. Backlash is a source of unintended motion that lets the controller's notion of where the machine is subtlely "drift" off target. The linear scales bring it back to reality, but if you can just eliminate the backlash directly, it's a much cheaper solution than trying to bandaid around it.

              Best,

              BW
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              • #8
                Originally posted by BobWarfield
                Actually, ballscrews are very common. So much so that they're readily available in the surplus market for a fraction of their original cost.
                BW
                Very true, a guy local to me has got the job of refurbing a load of hospital beds.
                These have ballscrews in with double nuts on the raise lower and tilt.
                They have to come out and be replaced as part of the contract.
                Looks as if some have never been full travel in their lives, some still have the plastic transit wrapping on them but they will have to be swopped.

                [edit] just remembered there are some behind the seat of the truck, where's me camera ?

                I have even seen cheap black rolled ones on window opening mechanisms as they need fast travel with little effort.



                .
                .

                Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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                • #9
                  Another point to consider is there are all grades of ballscrews...El Cheapo ones with backlash for simple operations like automated close/opening machine guard doors, instrument grade precision ones, preloaded machine tool grade, etc.

                  It is common for the newbie machinist Bridgeport owner to dream of replacing his standard screws with ball screws, but those standard screws are there for good reason.

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                  • #10
                    Originally posted by John Stevenson
                    Very true, a guy local to me has got the job of refurbing a load of hospital beds.
                    These have ballscrews in with double nuts on the raise lower and tilt.
                    They have to come out and be replaced as part of the contract.
                    Looks as if some have never been full travel in their lives, some still have the plastic transit wrapping on them but they will have to be swopped.
                    My granmpa has one at his house that he, and my late grama, use. It moves a little wheelchair lift up and down on the side of the stairs outside. The lift is really cheap, well made, but ugly as heck. The ballscrew is completely exposed to the elements and is visibly rusting. Works just fine though and has for about 5 years now. Pretty sure they bought it used too, so tack on another 5-10 years of usage.

                    I'll bet ya though, it probably wasn't a low backlash model!

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                    • #11
                      The Dream

                      It sure would be nice to have Zero ,Perfect backlash. Just don't think it will ever happen in our lifetimes because of frictional resistance. It's always close but not perfect. Years ago I installed ballscrews in old Bridgeport NC'S and tolerances were greatly improved------but not what the company expected. Perfection is out there...just waiting on someone to defeat gravity and resistance. The Upsadasium didn't even work!

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                      • #12
                        Originally posted by D. Thomas
                        It is common for the newbie machinist Bridgeport owner to dream of replacing his standard screws with ball screws, but those standard screws are there for good reason.
                        For a long time for the reasons mentioned herein i'd held the belief that ballscrews, while having their place, were often falsely ascribed magical properties, they have their use but aren't the second coming.

                        The subject of ballscrews on manual machines came up at cnczone and to my surprise one of the very knowledgeable posters is a staunch advocate of ballscrews for manual machines. He says with abec7 with a high preload, the holding power is about the same as acme and with anti-backlash they are much nicer than acmes and the preferred way to go, i can see that for example climb milling or working with coordinates - as he says when you move the handwheel the table moves. Like most of the posts here, I'd not have thought them a first choice for manual machines, but this poster is not one who's expertise I'd quickly dismiss. i'm wondering if we should be so sure?
                        .

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                        • #13
                          How about double reverse Buttress threads?

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                          • #14
                            You can stop the backdrive problem with enough friction but ballscrews will inherently drive back through the screw. The reason is the high lead required to give enough room for the ball bearings. This gives a large helix angle. When the helix angle on a screw form is above about six degrees then it becomes possible to back drive the screw.

                            To overcome this problem on a manual machine will require some source of friction instead of the inherent self locking of a low helix angle acme thread. This is why they aren't suitable for manual machines.
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                            • #15
                              Originally posted by Mcgyver
                              The subject of ballscrews on manual machines came up at cnczone and to my surprise one of the very knowledgeable posters is a staunch advocate of ballscrews for manual machines. He says with abec7 with a high preload, the holding power is about the same as acme and with anti-backlash they are much nicer than acmes and the preferred way to go, i can see that for example climb milling or working with coordinates - as he says when you move the handwheel the table moves.
                              Yes, I know just what you are talking about. I have observed that particular prescription for the ABEC7 "made for ballscrew" angular contact bearings being made repeatedly. In fact, one will always read from the same poster about the magic of 20TAC47's (these are the ones that cost about $1000 unless you know the right people), the might be barely tolerable but still very expensive 7204CtYDUHP4's (if you don't know those right people), and the "I'd never use them, but they're still better than where you started," 7204BYG's. These bearings are always the cure for greater accuracy, and now I see they are the cure to apply ballscrews to a manual machine.

                              Call me a rebel, but when something gets to be so single mindedly and determinedly recommended, it just makes me want to look for other alternatives. These bearings may indeed have those properties, but the prescription is presented with such rigid consistency and scorn for any other possibility, that one almost has to wonder whether things can be so black and white or bearings so magical. I guess you can't really say unless you try them, or unless you have found an alternate solution that works equally as well. I will say that it is interesting to read the bearing catalogs, which the same prescriber often refers people to. There are differences in these magical bearings, but they do not look to me to be orders of magnitude differences at all when you look over the numbers. It is also interesting to read something like the Heidenhein linear scale paper where they estimate the inaccuracies just due to temperature change as a CNC machine heats up and compare those to the bearing differences documented in the catalogs.

                              Speaking of alternate solutions that work well, the Tormach mills, which are getting good reviews everywhere I have read, do not use ABEC7 bearings of any kind, let alone ABEC7 ball screw bearings. They don't even support the screws at the end, which I found particularly surprising. Now I have seen endless threads from any number of posters that argue there is a huge difference between ABEC7 and the other grades and that you just can't use those other grades on machine tools. You may as well just put the inferior bearings on a ("Gasp!") woodworking machine and not bother cutting metal.

                              Could be there are a lot of factors involved in getting an overall machine system to perform at its peak. Millman has tactfully pointed out that the reality does not always live up to the marketing hype. You may not need ABEC7's in your application, or the rest of your machine may not be up to what they are capable of anyway.

                              Best,

                              BW
                              ---------------------------------------------------

                              http://www.cnccookbook.com/index.htm
                              Try G-Wizard Machinist's Calculator for free:
                              http://www.cnccookbook.com/CCGWizard.html

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