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  1. #11
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Current Activity: Viewing FOR SALE: Denford Novamill Has been a member for 8-9 years. Has a total post count of 1,394. Received thanks 137 times, giving thanks to others 5 times.
    Is the 50% figure for static or dynamic loading?

    You only really need to use taper bearings when dealing with high loads. For the majority of applications, ball bearings work fine, they have less resistance, and are more tolerant should dirt get into them.

  2. #12
    Quote Originally Posted by m_c View Post
    Is the 50% figure for static or dynamic loading?
    Yes that's static, forgot to mention that! I can't find a rating for dynamic, just interesting explanations about how the bearing responds to axial load.

    Quote Originally Posted by m_c View Post
    ... less resistance, and are more tolerant should dirt get into them.
    Fair enough, those are both clearly important for this application.

    According to the SKF site the axial internal clearence for a 40mm bore double row angular contact bearing is 11um. So that's like having 11um end float on the screw if I used the design with the single bearing. I'm not sure how bad that amount is - it doesn't sound like much. C2 clearance bearings only have 2um clearance.
    Last edited by Jonathan; 11-04-2011 at 12:42 AM.

  3. #13
    4th, and hopefully final, design:

    Click image for larger version. 

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    That's one double row angular contact bearing to take the thrust load, and one standard bearing. Stepper will bolt on to the 15mm plate. I can use 4" aluminium bar for the bits on the other side, and 2.5" for the rest purely because those are the sizes I've found cheap!

    Edit: Disregard that, not thinking... this one still does not preload the bearing properly. I'll add another angular contact bearing, but that's going to cost too much :sad::sad: Now I'm a bit stuck, perhaps a thrust bearing would do.
    Last edited by Jonathan; 11-04-2011 at 11:05 PM.

  4. #14
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Current Activity: Viewing FOR SALE: Denford Novamill Has been a member for 8-9 years. Has a total post count of 1,394. Received thanks 137 times, giving thanks to others 5 times.
    If you're using a double row angular contact bearing, then it doesn't need preloaded, as they come preloaded.
    Provided the orange and red bits tighten together onto the inner race, and the blue and green bits clamp the outer race, then that design will work fine.

    The only purpose the second bearing has, is to provide support to the shaft to stop it being pulled/deflected by the stepper/belt.

  5. #15
    Quote Originally Posted by m_c View Post
    If you're using a double row angular contact bearing, then it doesn't need preloaded, as they come preloaded.
    Are you sure about that, I was worried about what I posted here:

    Quote Originally Posted by Jonathan View Post
    According to the SKF site the axial internal clearence for a 40mm bore double row angular contact bearing is 11um. So that's like having 11um end float on the screw if I used the design with the single bearing. I'm not sure how bad that amount is - it doesn't sound like much. C2 clearance bearings only have 2um clearance.
    Here's the link:

    http://www.skf.com/skf/productcatalo...ogue=1&lang=en



    On a related topic, do you think 2 ballnuts like I drew in #13 is sensible for a router? It should enable backlash, and endfloat from the above, to be eliminated I think.

  6. #16
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Current Activity: Viewing FOR SALE: Denford Novamill Has been a member for 8-9 years. Has a total post count of 1,394. Received thanks 137 times, giving thanks to others 5 times.
    You'll have far bigger sources of play than 11 microns in a bearing!

    Whether you use two ball nuts or nut, will depend entirely on how much play you're willing to allow (or money you're wanting to spend!), and there will also be an increase in friction using two preloaded ballnuts.

  7. #17
    Quote Originally Posted by m_c View Post
    You'll have far bigger sources of play than 11 microns in a bearing!
    I was thinking it's best to eliminate as much play as I can. In that case it's either the design #13 or #7. The latter, with one bearing, is easier to machine and cheaper, but less stable and bigger pulley. I'm worried about getting the centre height in the design with two bearings accurate.

    Quote Originally Posted by m_c View Post
    Whether you use two ball nuts or nut, will depend entirely on how much play you're willing to allow (or money you're wanting to spend!), and there will also be an increase in friction using two preloaded ballnuts.
    I've worked out that linearmotionbearings only charges 18 per RM2510 ballnut (and about 31 per meter of the screw), so it's not much more to have a second nut. Friction is a good point since the whole idea of this is to make the machine run faster.

  8. #18
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Current Activity: Viewing FOR SALE: Denford Novamill Has been a member for 8-9 years. Has a total post count of 1,394. Received thanks 137 times, giving thanks to others 5 times.
    It's always best to eliminate play, but there are costs to that, as you're discovering.

    Most accurate way to machine bearing bores so they're in perfect alignment, is ideally do them in one go, but for that design wouldn't be too easy.
    However, as the double row bearing gets clamped in position, you could machine it's bore to allow the bearing to move around a bit, so when you finally assemble it, the bearings get lined up via the shaft, and it's just a case of clamping the doublerow in place.

  9. #19
    Hi Jonathan,
    If you would like some ideas or a rotating nut you can look at my mill build log. I have just uploaded some pictures showing the assembly of the rotating nut. I used 2 AC bearings and preloaded with a nut. I bought a double AC bearing from EBAY but was not happy with the amount of clearance between the balls and the race so I resorted to 2 seperate AC bearings. The nut is not a lock nut but the pulley below has a grub screw over the key. The nut stack is preloaded then clamped with the grub screw. If this causes problems the nut is thick enough to slit radially and add an M3 clamp screw. Hope this gives food for thought

  10. #20
    Hi Andrew

    Quote Originally Posted by Andrew Wilding View Post
    If you would like some ideas or a rotating nut you can look at my mill build log.
    That did get my attention - I was going to post but hadn't finished thinking!

    Quote Originally Posted by Andrew Wilding View Post
    I have just uploaded some pictures showing the assembly of the rotating nut. I used 2 AC bearings and preloaded with a nut. I bought a double AC bearing from EBAY but was not happy with the amount of clearance between the balls and the race so I resorted to 2 seperate AC bearings.
    It is interesting what you say about the bearings. I was indeed intending to get them from eBay, two of these to be precise:

    http://cgi.ebay.co.uk/5208-ZZ-DOUBLE...item53e20e2fe6

    I'm hoping if I press fit the bearing onto the shaft that should expand the inner ring, and tighten the bearing up nicely. Similarly with the outer ring, except if I follow m_c's excellent suggestion of boring it slightly oversize and clamping it then that's not going to happen. Did you get the single row angular contact bearings on eBay, and what size are they?

    The problem I've got is that I don't have the constant force applied to my bearings due to gravity, unlike on the various milling machines that have a rotating nut.

    I'm really not at all sure what to do now. I've said already two single row angular contact bearings is a lot of money...and yet it looks like that is certainly the best way to do it. Having said that there's someone on CNC zone who has done it with two standard deep groove bearings.

    If I could get away with a 16mm. 10mm pitch, ballscrew then I would save so much on the screw, that I could easily afford the cheaper smaller bearings. However I would prefer to have the 25mm screw just in case it fails completely and I have to spin the screw.

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