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Thread: Beam deflection

  1. #16
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    Thanks sossity, I do use Solidworks also, but have never tried those functions.

  2. #17
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    It's a great utility, dead simple to use. once you run the calc you can then change FOS and judge how much thicker you need the beam, or just rerun with thicker anyway and try it. To apply a force to a small area I just extrude 0.1mm in the area I want to constrain the force or restraint, otherwise it will apply the force over the whole face. I think you'd need to be very interested to work out how to use the full blown analysis tool though because you'd need to know a lot about mechanical engineering.

    I have to look it up each time but I think lifting gear usually has a FOS of 8 and anything structural is 4? I noticed that beam fails all over at FOS 4.

    I built this contraption so nowhere was less than FOS8 using that wizard. It doesn't sound a lot, 1000kg (I think the mill might actually be 1500kg which is the figure i used), but when its swinging there, its scary. I was very glad I put the extra time and steel into it. It would be scary as anything if it was bending and creaking, but when it is rock solid, gives you confidence. You need to check what its sitting on as well of course.
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  3. #18
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    Quote Originally Posted by Gestalt View Post
    Hi guys, got just of a bit of an academic question for the gurus here. I've got a metre length of 50 by 50 by 3 rhs (supported at each end and not restrained) that is to have a centre point load of 1000kgs. I need to know the deflection.
    I remember doing a bit of this 30 years ago, but I'm out of touch now. Something about second moment of area
    and bending moments I think. Any hellp welcome.
    Eric
    for me it's more like 40 years ago, but the acceptable deflection is something like (span length)/300, which in your case is just over 3 mm.

    5 mm deflection is probably OK if it's a once off carry and the beam will be junked afterward. If it's for a permanent installation then I'm less sanguine about 5 mm of sag.
    regards from Canmore

    ian

  4. #19
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    Quote Originally Posted by sossity View Post
    It's a great utility, dead simple to use. once you run the calc you can then change FOS and judge how much thicker you need the beam, or just rerun with thicker anyway and try it. To apply a force to a small area I just extrude 0.1mm in the area I want to constrain the force or restraint, otherwise it will apply the force over the whole face. I think you'd need to be very interested to work out how to use the full blown analysis tool though because you'd need to know a lot about mechanical engineering.

    I have to look it up each time but I think lifting gear usually has a FOS of 8 and anything structural is 4? I noticed that beam fails all over at FOS 4.

    I built this contraption so nowhere was less than FOS8 using that wizard. It doesn't sound a lot, 1000kg (I think the mill might actually be 1500kg which is the figure i used), but when its swinging there, its scary. I was very glad I put the extra time and steel into it. It would be scary as anything if it was bending and creaking, but when it is rock solid, gives you confidence. You need to check what its sitting on as well of course.
    Yes I'll certainly have a play around with that sossity.
    That project looks like something I'd do!! Tell us what you did when you lowered it onto the hoist frame. Ie. what did you use to get it off the frame?
    I know what you mean about scary. I lifted an old cast iron 2 foot thicknesser off the back of a ute with a cheap Chinese engine crane that I borrowed from my brother in law. It actually bent the main lifting beam, so I ended up rebuilding it with deeper section rhs. Then it lifted it no problems. Scary non the less.
    Eric

  5. #20
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    Quote Originally Posted by ian View Post
    for me it's more like 40 years ago, but the acceptable deflection is something like (span length)/300, which in your case is just over 3 mm.

    5 mm deflection is probably OK if it's a once off carry and the beam will be junked afterward. If it's for a permanent installation then I'm less sanguine about 5 mm of sag.
    Now you mention that Ian, I do remember the rule of thumb 300. It is a once off operation and it will end up less span anyway. The rafter spacing centres are a metre and they are 100mm wide and then I'm screwing the mounts for the rhs to the sides. These are 20mm wide and I'm also building a girder trolley for the rhs. So the actual span will be more like 850mm. I'm also having the rhs mounted at 45 degrees.
    Eric

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    Quote Originally Posted by simonl View Post
    Nice. Good to see it all went OK.

    What is it BTW? Looks like something from a refinery, like a cracking tower!

    Simon
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  7. #22
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    Quote Originally Posted by Gestalt View Post
    Yes I'll certainly have a play around with that sossity.
    That project looks like something I'd do!! Tell us what you did when you lowered it onto the hoist frame. Ie. what did you use to get it off the frame?
    I know what you mean about scary. I lifted an old cast iron 2 foot thicknesser off the back of a ute with a cheap Chinese engine crane that I borrowed from my brother in law. It actually bent the main lifting beam, so I ended up rebuilding it with deeper section rhs. Then it lifted it no problems. Scary non the less.
    Eric
    I used the best tool to get the mill positioned, - Machtool I think Phil must be one of the most experienced at handling huge machines so this was nothing for him. He used skates and 10mm round bar as rollers to get off the frame.

    There will be a lot of wise people shaking their heads at us for taking risks with so much potential energy. I think they are all too tired of going over it again and again so no-one has said anything. My take on it is that we are all entitled to risk our own lives based on our own assessment, thats basic Darwins theory. We are doing everyone a favour by removing defective genes from the pool. Personally, I think we have an obligation to massively over engineer something that could hurt others, now or in the future. As long as we are sure there's no possibility that kids could end up playing near something we've created, or someone with no way of knowing he was in any risk even, could end up getting hurt when it fails, then I think its up to us to to decide risk. If someone else could get hurt though, then I'd say you need to be sure its safe by way over strengthening it or just don't do it.

    Because I was putting Phil at risk I really went to town on the strength of that beam. I made it in three parts that bolt together with every joint in compression. That horizontal beam is literally as heavy as I can lift, and I don't mean a safe lift of 20kgs. Phil checked it all over to make sure he was happy and all joints were well braced. Even at that, he lifted it 1" off his trailer and left it 20 or 30 mins before going near it.

    As an aside, Phil rebuilt the spindle on that mill for me. I can get an unbelievable surface finish, like its polished. I know he fights with 'experts' all over the world on how to set up bearings, but its hard to argue with the results.

  8. #23
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    Quote Originally Posted by Gestalt View Post
    Now you mention that Ian, I do remember the rule of thumb 300. It is a once off operation and it will end up less span anyway. The rafter spacing centres are a metre and they are 100mm wide and then I'm screwing the mounts for the rhs to the sides. These are 20mm wide and I'm also building a girder trolley for the rhs. So the actual span will be more like 850mm. I'm also having the rhs mounted at 45 degrees.
    Eric
    now you are making me worry.

    It's a long time ago, but I vaguely remember that eccentric loads on any beam are not good.
    I can't remember how "not good" it was, but do remember that eccentric loading (such as mounting a square beam at 45 degrees) was really bad.
    regards from Canmore

    ian

  9. #24
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    If I was lifting a machine and Phil looked at it and told me he was happy with it then I think that would be good enough for me too!

    Simon
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  10. #25
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    Quote Originally Posted by ian View Post
    I can't remember how "not good" it was, but do remember that eccentric loading (such as mounting a square beam at 45 degrees) was really bad.
    I reckon Ian is right. This worked example indicates the beam will be square root of 2 (1.414 times) stronger if mounted with the sides running vertically than it would if mounted at 45 degrees. I don't know if the beam in the example is hollow or solid, but I reckon the result would be worse for hollow than solid, since there's nothing to tie the diagonally opposing sides together in a hollow beam.

    Tread carefully.
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    Quote Originally Posted by jack620 View Post
    I reckon Ian is right. This worked example indicates the beam will be square root of 2 (1.414 times) stronger if mounted with the sides running vertically than it would if mounted at 45 degrees. I don't know if the beam in the example is hollow or solid, but I reckon the result would be worse for hollow than solid, since there's nothing to tie the diagonally opposing sides together in a hollow beam.

    Tread carefully.
    again working from my vague memory

    hollow sections are much worse than solid.
    With a hollow section at 45 degrees, all that is resisting the load is the tiny bit of metal in the upper and lower corners of the section
    regards from Canmore

    ian

  12. #27
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    Quote Originally Posted by ian View Post
    now you are making me worry.

    It's a long time ago, but I vaguely remember that eccentric loads on any beam are not good.
    I can't remember how "not good" it was, but do remember that eccentric loading (such as mounting a square beam at 45 degrees) was really bad.
    That's interesting Ian, I would have guessed at it being stronger given the fact that the compression and tension sides are further apart. I'll set up another practical test with the load and report back.
    Eric

  13. #28
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    The scale is totally different, but this perpendicular load vs diagonal load discussion is reminiscent of the design flaw discovered in the Citigroup skyscraper in Manhattan after it was completed and occupied.

    https://en.wikipedia.org/wiki/Citigroup_Center

  14. #29
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    Well guys, hard to measure accurately, but it does seem a little less deflection. Not much in it though.
    Might be a good exercise for Solidworks.
    I'm feeling pretty safe.
    Eric
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  15. #30
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    I'm not sure you should be feeling safe tbh.
    From here https://www.creativemechanisms.com/b...ing-the-basics

    "A FoS of 1 means that a structure or component will fail exactly when it reaches the design load, and cannot support any additional load. Structures or components with FoS < 1 are not viable; basically, 1 is the minimum"

    Laying the rhs flat and the areas around the load on the beam had FOS < 1 in SW. i redid it at 45 degrees and it deflects 5.5mm, at 0.5mm less but there's defintely a lot of the beam with FOS less than one. i'd say that meant you were at risk of the beam crumpling at any moment?

    ScreenShot189.jpgScreenShot188.jpgScreenShot187.jpg

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