What bolts to use for an 8mm hole?

Discussion in 'Engineers' Talk' started by Smithy723, Dec 23, 2014.

  1. Smithy723

    Smithy723 New Member

    I have just purchased some bearings with an internal diameter of 8mm for a project I'm working on. I obviously know that an M8 bolt has a diameter of 8mm but am concerned about clearance. Will an M8 bolt fit in an 8mm steel bearing without being forced? If not, are their any recommendations for what I should go for? Are M7 nuts, bolts and washers easily available?
  2. tom.plum

    tom.plum Screwfix Select

    I don't think bolts of any size would work, they're not hard enough to act as bearing shafts,
  3. Phil the Paver

    Phil the Paver Screwfix Select

    You would need a hardened steel bolt with a shank shaft for the bearing to sit on a M8 bolt should be fine.

    Some thing like this will be fine in 10.8 minimum 12.9 would be better, these number are the hardness of the bolts.

    Smithy723 likes this.
  4. But, to answer your Q - an M8 bolt will fit in an 8mm hole. There is clearance.
    Smithy723 likes this.
  5. Phil the Paver

    Phil the Paver Screwfix Select

    A little tip for you if you find the bolt fit a little tight, which it should be.

    Put the bolts in the freezer for a few hours before you fit them, this will shrink the size a tiny bit allowing an easier fit, but wear gloves when fitting them, don't want you burning you hands on the frozen bolts.;)
    Smithy723 likes this.
  6. malkie129

    malkie129 Screwfix Select

    OMG, this is getting a bit complicated.:( If you are fitting a bearing on a bolt, that's really a bit of a no-no. I can only agree with Tom, no matter how hard the bolt is. You really need to have a precision machined spindle to suit the application. I don't often disagree with DA, but a 8mm bolt in a 8mm hole is size for size unless specified otherwise...ie, a force fit.
  7. Phil the Paver

    Phil the Paver Screwfix Select

    Sorry but I couldn't disagree with you more.
  8. Smithy723

    Smithy723 New Member

    Thank you all, this has answered my question. I am making a stabiliser rig for my GoPro so the forces that will are involved are tiny. I don't think I need to worry about the hardness or strength of the bolt, it's never going to fail under the loads I will be applying!

  9. I'm assuming that Smithy's bearing is a race-roller-cagey (or whatever they are called) type, so there is an inner and an outer sleeve with a ball race in t'middle. The bolt will fit through the inner sleeve, and I would expect that to be bolted tight and not to be spinning; the outer sleeve is usually an interference fit in whatever part is meant to be spinning.

    So 'all' the central bolt does it hold the bearing secure, prevent sideways movement and centralise the spinning bearing - so it needs to be a decent fit - and that's it. The central bolt shouldn't have any actual bearing 'wear' applied to it (unless it's a very low-load application).

    And, if that hole is called '8mm', then an 8mm bolt sill slip in chust fine and dandy. Just like if you want to put an 8mm bolt through a piece of metal you are drilling - what size drill bit do you use? Yes, an 8mm one.

    I know what you mean in theory malkie - an exact 8mm shaft will not fit in an exact 8mm hole. In practice I believe you will find that an 8mm bolt shaft will be fractionally less than 8mm. Phil's idea of freezing the bolt - and possibly warming the bearing - is a good one should Smithy find that it is a tight fit. Which I doubt.

    Smithy, will this 8mm bolt have any actual rotational wear on it - will it become one of the bearing surfaces?
  10. Smithy, having re-read your posts I am not at all sure what you are after here.

    You ask if M7 bolts and washers are available? Yes - you can get pretty much any size. But I certainly wouldn't use a 7mmbolt in an 8mm bearing hole as that wouldn't centralise the bearing properly - and if this device is going to be spinning fast, then you risk some astonishing vibrations.

    A 'stabiliser rig' for a GoPro? So this thing will actually only move in an arc (or oscillation) and not be 'spinning'? In which case things ain't so critical.

    But I'd still rather it be 'snug' rather than wobbly.
  11. Smithy723

    Smithy723 New Member

    Thanks DA,

    I agree, I want it to be as snug as possible but was concerned that I wouldn't be able to get the M8 in without forcing it. From what everyone is saying, it appears that it will go in just fine. If I do struggle, I'll try freezing the bolts first as suggested by Phil.

    If you're interested, the design I'm using is based on this:

    Thanks for all you assistance!
  12. Oooh, cool project.

    (What prevents the camera from just spinning? How do you aim it in the direction you want?)

    Anyways, that silly American was using imperial measurements - tsk, they are soooo last century :rolleyes:.

    If your bearings are definitely 8mm holes, then I am as sure as I can be that an 8mm bolt will be perfect for you. Since this thing ain't critical in your design, you could use a smaller dia bolt, but it just wouldn't feeeeeel so gooood. So I wouldn't.

    Obviously the bolt's head needs to be small enough dia so's it doesn't interfere with the bearing's race or outer sleeve - ie; it just acts on the inner bearing sleeve. I wonder if the inner sleeve is fractionally wider than the bearing race? Probably is.

    If, when you come to tighten that middle bolt you find the bearing doesn't spin easily, check to see if bolt's head (or nut) is touching the actual ball race or the rubber seal that covers it. If so, you may need a spacer washer which is the same outer diameter as the inner sleeve - that will keep the bolt head/nut away from the rest of the bearing surface.

    I wonder why he has that long flat plate at the bottom of the pendulum? It looks awkward and I can see it getting in the way. I wonder if it reduces unwanted rotational movement of the long shaft so the camera doesn't rotate too much by itself?

    If so, then fine. But if not, then you'd be better off with a much smaller pendulum weight at the bottom of that shaft - probably just a bunch of washers on that shaft held betwixt two lock nuts.

    Anyways, nice project - I love stuff like that :)

    Keep us posted. In fact, post your own topless YouTube vid.

    (Did I say that last bit out loud...? :oops: )
    Smithy723 likes this.
  13. malkie129

    malkie129 Screwfix Select

    No DA, for an 8mm bolt in metal, I would drill the specified clearance hole of 8.2mm ;)
  14. malkie129

    malkie129 Screwfix Select

    Sorry, replied to the wrong answer. :(
  15. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    You have 8.2mm drill bits?
  16. Nope, but he does have shaky hands :p.

    Malkie, you are quite right - and exact 8mm rod will not fit in an exact 8mm hole.

    But this is different. This is not top-end theoretical engineering. You know that if you drill a hole with an "8mm" drill bit, an "8mm" bolt will fit through it.

    Ah - you're yanking my chain... ;)
  17. malkie129

    malkie129 Screwfix Select

    Yeah, maybe. Happy Christmas Mate. :D

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