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  1. #1
    Join Date
    Sep 2008
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    220

    Ballscrew support, fixed end.

    I've read through all the reads I could find and couldn't find a definitive answer. Is there a difference in the fixed end support? For instance there are a ton of C-7 quality fixed end supports on ebay fairly reasonable but is there any difference between them and a support for say a C-3 ballscrew. They both do the same job, preload the ballscrew so why make two different ones. Anyone know a definitive answer. I got a good deal on a new C-3 ballscrew and need a fixed end support, but don't want to spend a ton on one either.

  2. #2
    Join Date
    Apr 2009
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    5516
    The difference is the accuracy. A matched pair of C3 anglular contact bearings probably cost about 10 times what a C7 set cost. I'm sure you could use a C7 fixed end wih your C3 ballscrew, but then you effectively make your C3 ballscrew a C7 ballscrew,,,

    The bearings can also be arranged face-to face, or back to back. I believe the face to face method provides more preload ability and accuracy, but don't quote me on that.

    BTW, you're not preloading the ballscrew, you're preloading the bearings, which creates the "fixed" end.

  3. #3
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    Sep 2008
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    Quote Originally Posted by louieatienza View Post
    The difference is the accuracy. A matched pair of C3 anglular contact bearings probably cost about 10 times what a C7 set cost. I'm sure you could use a C7 fixed end wih your C3 ballscrew, but then you effectively make your C3 ballscrew a C7 ballscrew,,,

    The bearings can also be arranged face-to face, or back to back. I believe the face to face method provides more preload ability and accuracy, but don't quote me on that.

    BTW, you're not preloading the ballscrew, you're preloading the bearings, which creates the "fixed" end.
    That is what I meant to say, preloading the bearings. I have been doing more research and it seems that the fixed end is the important side, so I will probably just suck it up. I don't really need C3 but I got C3 for less than C7 prices in the size I needed, so...

  4. #4
    Join Date
    May 2007
    Posts
    767
    Just take a deep breath and think about the differences between C7 and C3 specifications for the bearings. The increased cost relates to the smaller tolerances permitted in the C3 specification. You do not say how big the lead screw is or how long or how fast it is to rotate. These are the factors that determine the design of the fixed bearing and the need / type of bearing support at the non driven end.

    For small lead screws it is often OK to use flanged deep row ball bearings in a very simple plain bore housing. Consideration of the thrust load on the fixed bearing will determine the type of bearing that is required along with any margin required to obtain satisfactory life. In practice there is not a lot to chose between angular contact and normal deep row ball races when used as a pre-loaded pair. Just do not use filled races unless precautions are taken to ensure the pre load is not going to force the balls into the filling slot rembering the filling slot can take different forms. That said races with filling slots are not common items. For moderate thrust load there are double row deep row ball races that are pre-loaded during manufacture and are suitable for most lead screw supports. From your description you have a better tolerance lead screw and that can only be good in terms of location precision along the axis. The use of C3 bearings in the fixed brearing block will do little if anything to improve the accuracy over that obtained from a C7 spec bearings used back to back and suitably pre-loaded as the rotational speed is low. If you want definative answers look at the data sheets on ball screws by NSK.

    Regards

    Pat

  5. #5
    Join Date
    May 2005
    Posts
    1810
    Somewhere on this site there was a retired bearing engineer that probably forgot more about high tolerance bearings than all of us combined currently know. I can't recall his screen name as it has been probably at least 3 years since I have been to this site but I will see if I can track down his posts on this very topic and report back. Or you can search for bearing preload or angular contact bearing - you might stumble across his posts. It would be worth your time.

    Scott
    Consistency is a good thing....unless you're consistently an idiot.

  6. #6
    Join Date
    Sep 2008
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    220
    FWIW, I looked at NSK's brochure for the ballscrew I have as well as their rolled ballscrew and they specify the same fixed end support for both. Fortunately I found one on ebay (NIB) relatively cheap and will use that.

  7. #7
    Join Date
    May 2007
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    767
    As the Japanese NSK site appears to be very slow I have attached the document which is part of their Motion Control series. This shows the use of a pair of single row flanged ball races for the fixed end support of a small ball screw. I have used this design in the compound slide of a small lathe.

    Regards

    Pat
    Attached Files Attached Files

  8. #8
    Join Date
    May 2007
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    767
    The need to support the far end from the drive for the ballscrew depends upon the diameter and unsuported length of the screw plus the maximum rotational speed. Here is a link to a paper that sets out the design criteria with some useful graphs to save doing the maths!

    http://www.jena-tec.co.uk/JT%20Serie...gn%20Guide.pdf

    Similar guides are available but this is the easiest to use.

    Hope this helps debunk some of the bearing mist -
    Regards Pat

  9. #9
    Join Date
    Mar 2006
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    2712
    I think the bearing engineer you refer to is NC Cams. Unfortunately, our good friend has passed away. MHRIP

    I've been reading your posts. I may have forgotten the part on C3, C5 & C7 on angular contact bearings but I think that refers to ball-screw accuracy. It could also refer to internal clearance of radial ball bearings like deep groove ball bearings.

    Angular contact ball bearing accuracy grades here in the USA are ABEC 1, 3, 5, 7, & 9.
    ABEC1 being the lowest, ABEC9 about as high as they get generally. The higher the number, the higher the accuracy.

    When mounted face to face, AC bearings are a little more forgiving as to alignment. Mounted back to back, they are more rigid.

    Ball screw support bearings are mostly axial (thrust) bearings, usually 60 degree contact angle. In lower force applications, a lesser angle is probably OK. Check the bearing specs to be sure.

    The ball screw support bearings are usually made with fairly high preload so they do not like sustained high speed.

    Lots of information about bearings out there. Read the tech sections of the bearing catalogs, good stuff.

    Too bad we don't have Dennis anymore.

    Dick Z
    DZASTR

  10. #10
    Join Date
    Sep 2008
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    220
    Quote Originally Posted by RICHARD ZASTROW View Post
    I think the bearing engineer you refer to is NC Cams. Unfortunately, our good friend has passed away. MHRIP

    I've been reading your posts. I may have forgotten the part on C3, C5 & C7 on angular contact bearings but I think that refers to ball-screw accuracy. It could also refer to internal clearance of radial ball bearings like deep groove ball bearings.

    Angular contact ball bearing accuracy grades here in the USA are ABEC 1, 3, 5, 7, & 9.
    ABEC1 being the lowest, ABEC9 about as high as they get generally. The higher the number, the higher the accuracy.

    When mounted face to face, AC bearings are a little more forgiving as to alignment. Mounted back to back, they are more rigid.

    Ball screw support bearings are mostly axial (thrust) bearings, usually 60 degree contact angle. In lower force applications, a lesser angle is probably OK. Check the bearing specs to be sure.

    The ball screw support bearings are usually made with fairly high preload so they do not like sustained high speed.

    Lots of information about bearings out there. Read the tech sections of the bearing catalogs, good stuff.

    Too bad we don't have Dennis anymore.

    Dick Z
    That is a shame about NC, I truly enjoyed reading his posts.

    For me, I think that it is fair to say that what I know about bearings pales compare to what I don't know about bearings. When I originally posed this question I made a basic assumption that the support bearings, being angular contact bearings which would need to be preloaded to remove axial free play would not necessarily need to be anything special. From what I have read on ABEC classifications, I am sure someone will correct me if I am wrong, they are simply dimensional tolerances for the bearing itself, but don't specify things like axial or radial run-out so in essence that is left up to the manufacturer. The end support I wound up buying was recommended in the catalog for the Class 3 ballscrew I purchased. Incidentally, the same support was also listed for the rolled ballscrews from NSK. The MSRP suggests it could be loaded with matched bearings since it was close to $500, fortunately I didn't pay anything near that or it wouldn't have happened. But the basic question is: If I am going to support a Class 3 ballscrew and am going to use a pair of angular contact bearings which I adjust the preload so that the axial play in the bearings is essentially zero anyway does it really matter (for most applications) if I use a bearing that someone is calling a class 3 bearing mount or a class 7 mount as long as I mount and adjust them properly?

  11. #11
    Join Date
    Jul 2007
    Posts
    1602
    The system of bearings, mounting, screw, nut, coupler all come together to define the accuracy of the system. The various components affect different aspects of the overall accuracy.

    The difference between a C3 and a C7 screw is lead accuracy i.e. all other things being equal if you turn a C3 screw a certain number of degrees you can expect that the distance the nut has moved is closer to what would be expected than if you turned a C7 screw of the same pitch the same amount.

    So by using a C3 screw, you are improving the lead accuracy portion of your system. That is something you can't tweak out of a system so you are ahead of the game. In addition, one could reasonably expect that the screw/nut portion of backlash will be less in a C3 screw than a C7 since the more accurate lead means that you don't need quite as much clearance to prevent binding.

    So I think you are ahead of the game. Use the components you can afford and do a really good job of assembling them and if need be massage them a bit to make them better and you are off to a great start.

    bob

  12. #12
    Join Date
    May 2010
    Posts
    290
    I don't think it matters.....but I'm one of those guys who has used deep groove bearings in place of angular contact bearings. I had no detectable play in the axial plane, and they were still as good as new after a year of medium duty use on a 26 x 52 router.

    The deep groove bearings aren't nearly as impressive as the AC's when documenting a build log......but they have performed well for me.

  13. #13
    Join Date
    May 2007
    Posts
    767
    The runout is specified in the bearing class (ABEC spec Numbering) along with the clearances. With a pair of bearings preloading one against the other takes up the clearancees leaving only the run out. In the pair used it is possible for the run out to be in oposite directions which will cause some initial wear and possibly shortened life. If the shaft and mounting are stiff enough possibly even a detectable rotational variation in turning torque might be felt as the preload is applied - however this will require some good close tolerance force fit type machining of boththe shaft and the bearing housings. However with modern bearings of know manufacture this is going to be rare.

    Recently manufactured ball races using cages (i.e. no filling slots) can support a thrust load that is more than enough for most applications at the speeds met with on a lead screw. For higher speed angular contact bearings are required but are over kill and if the thrust is a consideration then separate thrust races should be used as a pair to accomodate end thrust and a single ball bearing used to provide location as a slip fit on the shaft or if the thrust is bigger still then taper roler races are the way to go on a big machine.

    If you want a real cheap and quick bearing look at the 5000 series dual row ball bearings - these are pre loaded and are available in a range of stock sizes and low prices from all the reputable makers. The cost is less than half the cost of a pair of normal ball bearings so represent good value as well as being available with seals fitted to keep then clean.

    The linear location tolerance along the screw is what differentiates C7 from C3 with C5 grade also available. The C3 are better and would normally be used on higher class machines the accuracy of the end support bearings is by comparison not going to make any detectable contribution to the accuracy measured along the lead screw if using a pre loaded pair. The AC bearing might give a greater thrust capability but the limiting factor will be the number of balls in active contact with the screw. Look to the manufacturers tables for the bearings you have and you will see what I am driving at. Make sure the tables are for the actual make of bearing you are using as there are differences! If the bearings are unbranded then I would halve the table values but you should find there is plenty of thrust capacity and expected life. If you do the calculations then you will see that it is the constant reversing that will reduce the life more than the number of revolutions.

    Regards

    Pat

  14. #14
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    Mar 2006
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    2712
    There is end play in deep groove ball bearings. A C2 clearance is about as little as they make but it's still there.

    For a wood cutting router it may not matter. In the machines and instruments we play with, microns (.000040") matter.

    Pre-load is generally built into precision AC bearings and are sold in sets. This pre-load can be changed by grinding inner or outer race faces (by very qualified people). Or with the use of spacers. Tricky business.

    There is also a term "unloading force". Think about this. If the axial force is toward a pair of bearings, the bearing closest to the oncoming force is loaded further and the opposing bearing is unloaded by that amount. The pre-load has to be heavy enough to absorb those forces without unloading the second bearing of that pair.

    That probably is not of much concern to a wood router. The ball screw pushing a 4" dia. inserted drill through a block of steel is going to put a lot of force on the support bearings as well as spindle bearings.

    The work dictates the machine and it's design.

    Dick Z
    DZASTR

  15. #15
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    Mar 2006
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    2712
    Per NC Cams, 5000 series bearings aren't pre-loaded. I asked him about that with the same argument. I was properly "scalded" so I would remember that.

    There are double direction angular contact thrust bearings but they are not designed for radial forces. in fact, they're designed with slightly undersized outer rings. Made to be used with precision roller bearings in combination. Mostly used for machine spindles. I have a "used" set that I employ as paper weights on my desk.LOL

    Dick Z
    DZASTR

  16. #16
    Join Date
    Mar 2008
    Posts
    11
    The simplest and most economical solution is to use 52xxx series thrust bearing for the fixed end of the ballscrew. Mazak used them in their older M5 lathes. However, this requires careful tightening of the bearing because too much preload will quickly damage it. If the ballscrew is belt driven at the fixed end you will also need additional radial bearing to take over the belt tension. And, of course, such thrust bearings have lower speed limit than most other bearings.
    The accuracy of support bearings has minimal effect on the positioning accuracy of the system if the bearing(s) are preloaded - their runout is much smaller than the pitch error of the ballscrew

  17. #17
    Join Date
    May 2007
    Posts
    767
    Hi Richard

    You might have saved your self a bit of that roasting as the 5xxx series bearings are lightly pre loaded but not to the 200 lbs f plus of a matched pair of AC bearings. They are designed to support shafts where rigidity is a prime requirement. The applications are in the low cost mass production markets not the preserve of the high precision machine tools. The manufacturing method places a light pre load between the two AC race ways such that the angle of contact falls outside the bearing. That pre-load is probably way less than the 200 lb f minimum found in matched pairs of small AC precision bearings but is not specified neither is the axial movement with varying load. AC bearings on the other hand have nice graphs showing the deflection of the bearing under axial loading conditions.

    To those contemplating building machines on a low budget I offer the following from my experience in using 5xxx bearings the total end float under reversing loads of 100 lbs f is comparable to that with AC bearings pre loaded to 200 lbs f. I use these bearings for lead screw support in the 10 and 12 mm bore sizes for lead screw support single fixed bearing floating end bearing. As part of the setting up the end float under load in both axial directions has been immeasurable using a DTI reading tenths. The DTI arranged so that only the bearing deflection is measured. On the other hand I have had one bearing that did exhibit a couple of tenths (O.0002 inch) total + to - float and this was rejected at assembly / test run stage as being possibly faulty - this batch of bearings were unbranded.

    The design as with all things to do with machining start from the accuracy required both in terms of absolute positional precision and repeatability tolerance as well as budget. Then the fun or should I say compromising starts! Working back to apportion that total tolerance to tool wear as well as through all the bearings and slides with the expected tool loadings both deliberate and unplanned!

    Then you will have a design that has specified the grade of ball screw - types of slide - screw support bearings - type of positioning drive etc. To drive cost down the parts have to be lucky purchases or the components that are mass production stock items.

    Good luck to all those people building low cost machines with aspirations to achieve maximum potential accuracy with in modest budgets. Regards _ Pat

  18. #18
    Join Date
    May 2010
    Posts
    290
    Hey Pat,

    What does a mount look like when using just one bearing? I mean.....what keeps the bearing in place? Is there something to put pressure on the outer race at one side?

  19. #19
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    Mar 2006
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    2712
    Pat, (ref. #17) "SKF double row bearings are manufactured to normal (ABEC 1) tolerances as standard. The value for normal tolerances will be found in the tables on pages 60 and 61. [SKF General Catalog]

    "Internal clearances for double row angular contact bearings differ from deep groove ball bearings in that axial clearance rather than radial clearance is specified. The tables on the preceding page show the axial clearance of the SKF double row angular contact bearings in four standard clearance ranges; C2 (less than normal clearance), normal clearance, C3 (greater than normal clearance), and C4 (greater than C3 clearance). These are valid for bearings before mounting under zero measuring load."

    The tolerances are probably acceptable for the wood routers. I don't think so for precision machinery or heavily loaded axis ball screws.

    I view the ball screw support bearings as part of system where tolerances stack up. Axial clearances of the ball screw/nut, support bearings, flex in mounts, backlash in any gears or belts, stretch in belts, ball screw lead error etc. Each contributing to the total error.

    Kinda like the accumulation of the national debt.LOL

    Dick Z
    DZASTR

  20. #20
    Join Date
    May 2007
    Posts
    767
    Hi Richard

    Looks as if we are in agreement as it all depends where one stands in the circle. 5xxx brearings have no place in the high precision world end of story but they do have a place in lesser spec machines.

    Like your national debit comparison to the accumulation of errors which is why it is important to establish the precision aspired to and then look to the budget.

    Regards

    Pat

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