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Choosing Steel Thickness for a bracket

  1. Jun 22, 2016 #1
    Just need someone to double check my work.

    So the bracket is going to be cut out from a steel plate in a weird, rounded out triangular shape, and then a hole will be drilled through it. For simplicity though, lets just say the bracket is a ring with one half of it welded onto a wall. OD of the ring is 1.25", ID is 0.765", which results in a thickness of 0.2425".

    The equation for tensile stress is σ = Fn / A.

    Since the bracket is going to be pushed and pulled, the cross sectional area is:
    (0.2425" * H) * 2, where H is the height of the ring when it is placed on the table, which is the thickness of the steel plate I need to cut the brackets out from. Equation simplifies to 0.485H.

    The worst case scenario this bracket has to endure is 2 metric tons, which translates to 4410lbs

    The yield strength of the steel plate I intend to use is 36,000psi. As I understand, 0.6 represents the ratio of maximum bending stress to yield stress for the material, so I'm guessing it is the same for maximum tensile stress? Which makes the maximum allowable stress 21600psi.

    So if we solve for H,
    H = 4410 / (0.485 * 21600) = 0.421", so if I get a steel plate that is 0.5" thick to cut my brackets out from I should be fine.
  2. jcsd
  3. Jun 22, 2016 #2


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    A 1/2" bracket is a pretty hefty member.

    It's not clear what this bracket is supposed to support. Why don't you provide some extra details?
  4. Jun 22, 2016 #3
    The device is a large complex robotic arm controlled by multiple hydraulic cylinders, which in turn is controlled by a FPGA board. The arm is designed to regularly hold and move 2 metric tons of weight.

    These brackets will be welded directly onto the arm and the hydraulic cylinders will regularly push/pull the brackets to change the arm's orientation.

    I chose size 4" schedule 80 pipes for the arm's skeleton.
  5. Jun 23, 2016 #4


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    The 2 tonnes is at the arm end I presume.
    The hydraulic cylinder is attached to the arm somewhere between both ends of the arm.
    You need to find out what force the hydraulic cylinder will be acting with at its attachment points.
  6. Jun 23, 2016 #5
    I phrased my comment wrong. My bad.

    The bottleneck here is the cylinder. I got the strongest one available (3000 operating psi) for its size which equates to about 2.4/1.8 metric tons of push/pull. I don't know how much weight the arm can lift, but it's not too important right now. I'm trying to make the arm utilize the full power of what these cylinders can offer.
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