Rod with threads strength?


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Thread: Rod with threads strength?

  1. #1
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    Default Rod with threads strength?

    I'm designing something to break straps - a home DIY type deal.

    I have a hydraulic ram with a maximum of 30000 lbs of force.

    This will be acting on a pair of 1" 4140 HR (annealed) rods under tension. They are threaded to M24x1.5. (I plan to use a CV axle nut that is 20mm thick).

    I would like to figure out the yield strength of this arrangement. I took a stab at it below:
    I think it's 60,200 psi but I don't know if that's annealed, hardened or if it matters
    The minor diameter of the thread is 21.92mm. = 0.58" area * 60200 psi = 34916 lbs per side <-- no idea if this is correct

    At what point would the threads just rip out?
    I've never dealt with forces this large so I'm trying to figure out if I'm way off the mark. It would be nice if it doesn't catastrophically fail and hurt me with a flying piece of broken machine... In the movies, it would break with a fireball and I could dive behind a pine door and I'd be OK right???

    -Michael

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  2. #2
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    Default Re: Rod with threads strength?

    Quote Originally Posted by hackish View Post
    I'm designing something to break straps - a home DIY type deal.

    I have a hydraulic ram with a maximum of 30000 lbs of force.

    This will be acting on a pair of 1" 4140 HR (annealed) rods under tension. They are threaded to M24x1.5. (I plan to use a CV axle nut that is 20mm thick).

    I would like to figure out the yield strength of this arrangement. I took a stab at it below:
    I think it's 60,200 psi but I don't know if that's annealed, hardened or if it matters
    The minor diameter of the thread is 21.92mm. = 0.58" area * 60200 psi = 34916 lbs per side <-- no idea if this is correct

    At what point would the threads just rip out?
    You're on the right track, however, I would add considerable margin of safety to it.
    1. You don't know how the threads were put there. Were they cut? Rolled? Before tempering? These things kinda matter.
    2. Minor diameter? Sure. Maybe, if I had a certified source of known material. Otherwise, I'd take about 40% of minor diameter.
    3. If you gotta 30k ram, I'd be inclined to set up test module on a short chunk of rod with the INTENTION of causing failure. Use a pressure gauge and a few lightweight multiplication probs to get the final breaking strength. Do it once. Do it twice. See how repeatable it is. Then build the whole thing with solid knowledge. Testing Trumps Theory every time.

    Quote Originally Posted by hackish View Post
    I've never dealt with forces this large so I'm trying to figure out if I'm way off the mark. It would be nice if it doesn't catastrophically fail and hurt me with a flying piece of broken machine... In the movies, it would break with a fireball and I could dive behind a pine door and I'd be OK right???
    Yes, this is true.
    You must, however dive AWAY from the blast.
    Hand grenades, fireballs, even landmines are all mitigated by diving away from the blast.
    This has been well-demonstrated in hard-hitting documentaries like Rambo and pretty much every cop/action/army movie since.
    The body of data is simply indisputable.

    W.



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    Default Re: Rod with threads strength?

    Since the load is distributed on 2 of these rods, the maximum force should be 15000 lbs per rod. I could switch over to imperial and go with a 1"x12 nut for 0.663 tensile area. That's 39912 lbs per rod.

    I know machinists like to crap on engineers for what they do, but I wonder how much time would be spent on what I feel is a simple design to try and figure out what would happen if something breaks. I could add a piece of cable on each end of the arm to stop the i-beam from becoming a projectile, but this feels like overkill.

    -Michael



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Rod with threads strength?

Rod with threads strength?