Does a magnet's pull equate to what it will lift?

  1. mart

    mart Member

    I have a magnet and the magnet pull is stated as 7.1kg
    Does this mean it will lift a steel item weighing 7kg?
     
  2. gordon stephenson

    gordon stephenson Forum Supporter

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    Try it,
     
  3. rikrobson

    rikrobson Member

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    I think its the maximum that it can attract, so in youe example you'll have ~1N of spare force. which means if you add another 100g then it will break the pull, thats assuming its a perfect contact. You'd need some saftey margin if you were trying to lift something, so it would probably lift 5Kgs easily but tugging on 7Kg would probably break.
     
  4. Nick67GT

    Nick67GT Member

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    Depends whether or not the pull is perpendicular.

    ie: try pulling the keeper plate off a relatively small magnet as opposed to sliding it off.....
     
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  5. Maker

    Maker Krombopulos Michael

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    It also depends on the thickness of the material, you won't get the full seven kg on 1mm steel.
     
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  6. Screwdriver

    Screwdriver Member

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    If that is a genuine measurement, the manufacturer will have placed the magnet on a perfectly machined thick block of steel (1 or 2 inches thick) and pulled on it until it comes away.

    The steel needs to be thick enough so that all of the magnetic flux can be contained before the material becomes saturated. If its "too thin" the magnetic flux will saturate the steel meaning it just can't transfer any more of that flux within the material and the pull force will be reduced.

    Interestingly you can test this by holding another piece of steel behind it. If it is attracted to the original piece then some of the magnetic flux has "escaped" and your test piece it "too thin" to give you the maximum pull force.
     
  7. Screwdriver

    Screwdriver Member

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    Say you had a bag of sugar and you wanted to use all of it for maximum sweetness. A regular cup of tea would take about 20 sugars before the tea becomes "saturated". It just can't dissolve any more sugar in that volume of tea. No matter how much more sugar you spoon into it, it doesn't get any sweeter, the sugar just piles up at the bottom.

    What you need is a bigger mug! Then at some size, you can dissolve the entire bag of sugar to give you the maximum amount of sweetness.

    Incidentally, I made up the numbers. Probably closer to 100 spoonfuls before your tea becomes saturated. I also happen to know that half a bag of sugar makes a lovely cuppa if you brew it up in a bucket!
     
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  8. Wedg1e

    Wedg1e They call me Mr. Bodge-angles

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    In mag particle inspection, one of the techniques involves applying a permanent magnet or electromagnet to the part under test and then spraying what is essentially iron filings in suspension over the part. As the filings follow the lines of magnetic flux, any break in the metal structure leads to a discontinuity in the lines and the defect can be clearly seen.

    In order to know whether enough magnetic field is being generated, a test piece of precise weight is lifted by the magnet.
    We manufacture these test weights; essentially blocks of carbon steel to suitable dimensions with a carrying handle. Of course the handles need screws so the resultant holes affect the weight and the screws don't weigh the same as the removed metal! The stupid thing is that the Standards specify a weight of 'not less than xxKg' but if we make the blocks even 100g over this, the customers will often send them back and ask for one closer to the minimum weight. You'd think they'd be happy if their kit could lift more than the required minimum but it seems they'd rather just scrape by :rolleyes:
     
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  9. Maker

    Maker Krombopulos Michael

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    Good analogy:clapping:.
     
  10. Gareth0123

    Gareth0123 Y'all hold my beer, while you watch me do this!

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    I've often backed up a thin work piece with an additional bit of plate when Mag drilling, so the Mag drill holds position, mainly on 127 X 76 X 13 universal beams and especially on vertical in situ jobs............ you've just got to remember that you have the backer in place when you switch the mag drill's magnet off.:whistle:
     
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  11. mart

    mart Member

    I did try it. Afterwards I took in to consideration the surface area of the magnet vs surface area of the weight, not just the weight factor alone.

    The magnet is only 2cm dia, and 7.1kg pull force seems a lot. I think what Nick67GT mentions about being perpendicular is a key point. I was testing the magnet's lifting capacity on a 5kg piece that was 20cm dia and 2.5cm thick but it could not be lifted.

    If I had a 7kg 2cm dia length of round bar as the test weight It may have worked lifting from the end of course.
     
  12. thinfourth pleb

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    Stonehaven
    You've made tea for a brickie then
     
  13. Screwdriver

    Screwdriver Member

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    Desert crew in Libya, always tried to join the Indian labour force for meal breaks. One of them would have a bushel of fresh tea, brew it up in a bucket (half a bag of sugar), served in a small glass. Best tea ever.

    I don't think surface area makes much difference to magnetic pull strength. It depends more on how much of the magnetic flux from the volume of the magnet flows through the object and the most critical aspect is distance. Thinner magnets can work better because the bulk of the body of the magnet is closer to the object. However if the object becomes saturated, that flux leaks out forum the sides of the object no matter what size or shape it is.

    The optimum shape for a magnet is when the diameter matches the length
     
  14. Maker

    Maker Krombopulos Michael

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    That sounds about right (Assuming it's neodymium), I've got one about 20mm dia and 6mm thick that will lift a 6Kg barbell. The thickness of the magnet matters too, as well as the grade of the magnet ("N52" is the strongest usually available).
     
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