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  1. #1
    dsc's Avatar
    Lives in Lincoln, United Kingdom. Last Activity: 02-12-2016 Has been a member for 4-5 years. Has a total post count of 248. Received thanks 1 times, giving thanks to others 9 times.
    I've decided to ditch the double row AC + floating bearing approach and simply use two single row angular contact bearings mounted back to back:

    Click image for larger version. 

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    Am I correct to think that the bearings need to be a slide fit on the shaft and a press fit in the housings? preload will be via top cap, with the mounting bolt torqued to ##Nm, not sure on the value though. That or I'll use a locknut with a side grub locking screw.

    I'd like for this design to offer as much stiffness as possible, will it work, or is there something I'm missing?

    Regards,
    dsc.

  2. #2
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Current Activity: Viewing Has been a member for 8-9 years. Has a total post count of 1,453. Received thanks 146 times, giving thanks to others 5 times.
    The main thing is whatever bit gets adjusted floats.

    However that drawing won't work. Tightening the screw will simply pull the inners towards each other and preload nothing!
    You'll have to flip both bearings, which given your application, will probably be better in terms of where the main forces go...

  3. #3
    dsc's Avatar
    Lives in Lincoln, United Kingdom. Last Activity: 02-12-2016 Has been a member for 4-5 years. Has a total post count of 248. Received thanks 1 times, giving thanks to others 9 times.
    Indeed I've screwed up the lines showing the centers, so the above shows a face-to-face rather than back-to-back (I think). Re done here:

    Click image for larger version. 

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    Original I had tapper rollers in the design, the issue with those was no seals. ACs offer seals as well, but the idea stays the same.

    Thanks m_c.

    Regards,
    dsc.

  4. #4
    M_C beat me to it about wrong way around but I would also had that you'll want the locknut approach with locking grub screw rather than the just tightening a thread to a torque value.

  5. #5
    One of the snags with separated bearings in that configuration, is that the preload will change if there is differential expansion between spindle and housing

  6. #6
    dsc's Avatar
    Lives in Lincoln, United Kingdom. Last Activity: 02-12-2016 Has been a member for 4-5 years. Has a total post count of 248. Received thanks 1 times, giving thanks to others 9 times.
    Jazz I do have a fancy SKF locknut with a grub screw (pricey little bugger) which I could use, the only issue is accessing the grub screw as it's on the side. The cap / bolt is easier, as I can bolt it up from the top, but proper preload doesn't guarantee the bolt staying where it should without threadlock (I think).

    As for expansion John, I'm guessing you meant thermal expansion, which would force the housings further apart and increase the preload? this shouldn't be an issue as all the parts will stay close to room temperature all the time.

    Regards,
    dsc.

  7. #7
    Quote Originally Posted by dsc View Post
    As for expansion John, I'm guessing you meant thermal expansion, which would force the housings further apart and increase the preload? this shouldn't be an issue as all the parts will stay close to room temperature all the time.

    Regards,
    dsc.
    Would depend on the materials of the housing and spindle and what the source of the heat was. If the spindle is hotter it will decrease the preload.

    Is not usually a problem with slower running spindles like lathe headstocks or gearboxes which are oil filled as the oil evens the temperatures out. More of a problem with HS spindles where a lot of the heat is generated by the bearings and the rotor and the housing is cooled from the outside.

  8. #8
    Quote Originally Posted by dsc View Post
    I'd like for this design to offer as much stiffness as possible, will it work, or is there something I'm missing?
    Some car differentials and wheel bearing designs have a single use deformable spacer tube between the bearings. It has the advantages of gripping both sides of the bearings, the compressed tube and the tensioned shaft add to the stiffness of the assembly, and the high bolt up torque required to deform the spacer, make the nut more secure. Usually adjusted by measuring the increase in torque required to turn the spindle whilst tightening the nut.
    Last edited by johnsattuk; 05-01-2014 at 12:45 PM.

  9. #9
    Back to back they faced each other, ...

    Quote Originally Posted by dsc View Post
    I'd like for this design to offer as much stiffness as possible, will it work, or is there something I'm missing?
    If you want it to be as stiff as possible, then you could go back to using tapered roller bearings and add shaft seals, e.g:

    Bearing Station | Oil Seals
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  10. #10
    dsc's Avatar
    Lives in Lincoln, United Kingdom. Last Activity: 02-12-2016 Has been a member for 4-5 years. Has a total post count of 248. Received thanks 1 times, giving thanks to others 9 times.
    Pretty much everything is aluminium, apart from the shaft which is 316 SS. Heat sources will be far away, so the only way the housings can get warm is through consistent running, which is rather unlikely (operation will be more start-stop than on forever). In between the bearings there's a pulley, so the tube idea is not possible.

    Cap vs. locknut - still not sure, both have pros and cons, with no access from the side, the locknut is a) harder to torque to the right value (needs a locknut socket) b) harder to secure in place via grub screw (would need a longer thru hole on the mounting plate for allen key access). Cap / bolt has top access which is good, easy to torque to the right value, although would need Loctite or some threadlock to hold in place without undoing itself as I doubt the bearings can take huge preload needed to stop the bolt from undoing itself.

    With tapered roller bearings, I'm guessing I'd need to periodically re-apply grease? I can fit grease seals, but it complicate things a bit more from a design point of view. With sealed single row AC bearings, I can just whack them on the shaft and not worry much. With tapered rollers vs angular contacts - is there really that much more stiffness offered by the rollers?

    Just one addition check before I start doing proper drawings, both single row angular contact bearings should be an interference fit in the housings and a close slide fit on the shaft?

    Regards,
    dsc.

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