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  • Force to bend steel bar

    Discussion in 'Calculations' started by oemodm, Apr 15, 2015.

    1. oemodm

      oemodm Active Member

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      Hello everyone,

      I have a 1 metre length of 9mm stainless steel 201 bar. I'm looking for help in approximating how much force/weight would be needed to bend it, 10cm from one end.

      I know this is appallingly basic. Any online calculators or equations I could get a general idea (5kg, 10kg, 40kg).

      thanks / simon
       
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    3. Dana

      Dana Well-Known Member

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      Not enough information... 9mm round bar, square, rectangular? Where is the bending load being applied, on the short end?

      In general the bending equation is S = My/I where S is stress (you will use the material's yield stress here), M is the bending moment (force applied times distance from the bend), y is the distance from the neutral axis (in your case this will be half of the bar's thickness) and I is the moment of inertia for the bar shape.
       
    4. oemodm

      oemodm Active Member

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      Thanks Dana


      [​IMG]
      Yield strength = approx 300mpa / (www.azom.com/article.aspx?ArticleID=6780)
      Distance from bend = 100mm
      Y = 4.5mm
      I = I've googled how to do this, its beyond my level.....help....

      Could someone help me calculate how much force/weight will bend this bar
       
    5. Dana

      Dana Well-Known Member

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      Moment of inertia is pi*r^4/4 or 1.025E-10 m^4

      stress = My/I

      so

      M = stress * I/y or 300,000,000 * 0.0000000001025/.0045 = 6.8 N-m

      Since the distance from the bend is 100mm, the force is 68.3N That's to just barely bend it so it stays bent, in reality you'll need more force to make a useful bend. And note that smaller forces will bend the bar, but it will spring back straight since the yield stress isn't exceeded.
       
    6. Lochnagar

      Lochnagar Well-Known Member

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      Hi Dana,

      Just one point:-

      You must have got a faulty calculator - because the first computation is arithmetically incorrect - and it follows that the second computation will be incorrect too. Sorry to be the bearer of bad news.:(


      Hope this helps.
       
    7. Dana

      Dana Well-Known Member

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      Oops! You're right, I missed multiplying it by pi.

      Corrected: I = 3.22E-10, M = 21.5, so force = 215N

      If I'd converted it to lbf, the error would have been obvious to this dumb Yank who never uses metric units... 48 pounds sounds right; 15 pounds doesn't.
       
    8. oemodm

      oemodm Active Member

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      Dana & Lochnagar - thank you both. 215N is roughly 20kg which is nice to understand. Thankyou both again.
       
    9. Innersteps

      Innersteps New Member

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      Hello everyone. I'd like to introduce myself short before asking a question.
      I'm Ramon from The Netherlands, and I'm planning on building a wire- bending machine as a hobby.
      One of the things to calculate is the force used to bend the wire. I could simply buy a stepper motor that has roughly 12 NM force and use gears to reach 600 NM or even 1200 NM but I'd like to know how to actually calculate the force needed, and simply understand the machine I'm building :)
      Also a gear would slow down the speed and taking 10 minutes for one bend to complete on wire that would need 50 bends is a bit too time consuming :)

      When calculating i (The Moment Of Inertia) for a 6 mm wire with 300 MPA of strength I use:

      Pi * D^4 / 64
      Or
      3.14 * 6^4 / 64 = 63.61725
      Where D is the diameter

      Stress (MPA) * Moment of Inertia / R * Distance from the bend = M
      Or
      300 * 63.61725 / 3 * 10 = 63,617.25

      63,7 NM to barely bend the wire, or am I doing something wrong?

      When lowering the distance to 6 mm I'd need less force to bend the wire, but that seems kinda odd to me.
      If I hold a bar in my hand, the closer I am to my hand holding the bar, the harder I'd need to push to create a bend. Or am I being mind-tricked!! :)

      Also I'd like to have a push in the right direction on how to calculate the "Elasticity" of the wire.
      If I'd like to make a 90 degree bend I'd need to bend for example 95 degrees to compensate for the elasticity? I'm not sure how it's named.

      Last question, for now.
      Something familiar as Ohm's law (In electricity)? For:
      stress = My/I
      M = stress * I/y

      Thanks in advance.
       
    10. Dana

      Dana Well-Known Member

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      You don't say if 300MPa is the yield stress (where it bends/stretches and stays bent/stretched) or the ultimate stress (where it actually breaks). You want to use the yield stress. But anyway, yes, stress = My/I where y is the radius of the wire, so M = stress * I/y, solve for M (moment).

      M divided by the distance from the bend point to the applied load is the required force to barely start to bend it. In reality, you'll need more force, say at least 2X to be safe.

      You will indeed have to overbend it slightly to allow for "springback" (that's the term you want), but that's harder to predict... try a few parts and adjust as required.
       

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