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Column Buckling Load Calculation

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  • Column Buckling Load Calculation

    I have a 0.90" dia x 7.125" long CR 1020 round bar I use longitudinally as a ram extension or "push" bar for a job in my H-press. Both ends are 'free' in column load parlance.
    • Using a safety factor of 2, how much load (psi) can be applied before column buckling becomes a danger.
    • If the round bar is canted away from the line of force, how does the misalignment reduce the max non-buckling load that can be applied.
    • Are there any other questions I should be asking.


    I have already loaded the round bar to something in excess of 4T w/o accomplishing the objective. Before carrying on with the existing setup, I would like some feedback about the prospective buckle threshold.

    .

  • #2
    Originally posted by EddyCurr View Post
    I have a 0.90" dia x 7.125" long CR 1020 round bar I use longitudinally as a ram extension or "push" bar for a job in my H-press. Both ends are 'free' in column load parlance.
    • Using a safety factor of 2, how much load (psi) can be applied before column buckling becomes a danger.
    • If the round bar is canted away from the line of force, how does the misalignment reduce the max non-buckling load that can be applied.
    • Are there any other questions I should be asking.


    I have already loaded the round bar to something in excess of 4T w/o accomplishing the objective. Before carrying on with the existing setup, I would like some feedback about the prospective buckle threshold.

    .
    Hmm... we'll have to see what others get. I think I made a mistake but I come up with a critical load of 2900000 lbs or 1450 tons. I calculated this using Euler's Formula (see link below). Since the length is nearly 8 times the diameter, I assumed that Euler's Formula was appropriate.

    http://www.efunda.com/formulae/solid...ns/columns.cfm

    Comment


    • #3
      Thank you. Before posting, I started using online calculators for Column loads/Eulers but found myself getting bogged down by unfamiliar terms that relate to the calculations. On an iPad at the moment and kept losing progress when refreshes occured while switching tabs for defn's.

      If 1450T is correct, it seems I have some latitude.

      .

      Comment


      • #4
        I don't think it will take that kind of load myself, but that's instinct not calculated crippling.
        http://www.engineeringintro.com/mech...buckling-load/
        Mark

        Comment


        • #5
          Originally posted by EddyCurr View Post
          Thank you. Before posting, I started using online calculators for Column loads/Eulers but found myself getting bogged down by unfamiliar terms that relate to the calculations. On an iPad at the moment and kept losing progress when refreshes occured while switching tabs for defn's.

          If 1450T is correct, it seems I have some latitude.

          .
          Not sure what you are trying so maybe this advice is worth exactly what you're paying for it (nothing!) but when it comes to using a press, my concern is rarely with a "push bar" buckling because that mode of failure is fairly benign. What does worry me is something shooting out of the press at high velocity. This occurs when the ends are not well constrained and/or you are storing a lot of energy in elastic deformation (usually because the applied load is not normal to the cross section, which is what Euler's assumes). Contrast this with pure buckling when the stress exceeds the elastic limit so the result is a bent rod - bending the rod absorbs most of the energy and the ram is moving so slowly that you can stop it (or stop pumping) before you bugger anything up too badly.

          So, if you're concern is related mostly to safety, I would say don't worry about what the buckling strength of your rod is but instead focus on how you can properly constrain the ends so that when slight misalignments occur, you are less likely to shoot the rod out. Similarly, pay attention as you are pressing - if you see the rod deform, you are storing energy and need to be aware that it can be violently released.


          Edit:
          Mark's post suggests that we should be looking at "crushing" versus "buckling" load because the length is less than 15 times the diameter. Interesting... I was never taught a rule-of-thumb for when Euler's was appropriate or not.
          Last edited by Fasttrack; 08-23-2016, 02:44 PM.

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          • #6
            Eulers assumes straight and normal compression forces. Anything off axis or an uneven pressure across an end means the value is derated. Trouble is it's been so long I can't find my references!!!

            Pete
            1973 SB 10K .
            BenchMaster mill.

            Comment


            • #7
              Start off with the compressive strength of the steel. Matweb gives the yield of A36 as 22,000 psi
              So does this:

              http://amet-me.mnsu.edu/userfilessha...6HotRolled.pdf

              So then you would use 11,000 psi at 2:1.

              Your 0.9 diameter is 0.63 sq in, so a bit under 7000 lb is your limit that way. You won't do better any other way. And I suspect that buckling is not going to be your limit, at 8:1 ratio.
              1601

              Keep eye on ball.
              Hashim Khan

              Comment


              • #8
                Originally posted by J Tiers View Post
                Start off with the compressive strength of the steel. Matweb gives the yield of A36 as 22,000 psi
                So does this:

                http://amet-me.mnsu.edu/userfilessha...6HotRolled.pdf

                So then you would use 11,000 psi at 2:1.

                Your 0.9 diameter is 0.63 sq in, so a bit under 7000 lb is your limit that way. You won't do better any other way. And I suspect that buckling is not going to be your limit, at 8:1 ratio.
                That all reads as very right to my thinking. I think that these "assumptions" relating to the situation fit a lot better.

                The off axis issue is still a major one. As is the rigidity and possible spring energy. To make sure the bar is not loaded in an off axis manner I'd want to start with machined faces on the ends. Then I'd seat it on the part and line up the top end with the ram. Then I'd lift the bar up tight against the face of the ram and check that it is centered on where it was a moment before. If this looks good to the eyes then the pressure can be accepted as being within the column and the risk of off axis pressure causing the rod to fail by bending becomes very slight.

                If the bar can't pass such a test then something needs to be shimmed or faced.

                There may not be much you can do to ensure that the pusher bar can't skid off the work piece. But it seems that you should be able to take some measure on the ram to avoid a pusher from skidding off the ram's face.
                Chilliwack BC, Canada

                Comment


                • #9
                  The yield stress of A36 is about 36,000psi. 22,000psi is given as an allowable stress for certain specified loadings in some (older) codes, which might explain the confusion and mis-information.

                  Don't believe everything you read on the internet.
                  Last edited by cameron; 08-23-2016, 06:15 PM.

                  Comment


                  • #10
                    Originally posted by cameron View Post
                    The yield stress of A36 is about 36,000psi. 22,000psi is given as an allowable stress for certain specified loadings in some (older) codes, which might explain the confusion and mis-information.

                    Don't believe everything you read on the internet.
                    The 36000 is given tensile. 22,000 given compressive, and yes it is noted "allowed".

                    As for misinformation, and the internet, I found the same information elsewhere in hard copy. I would not term it"misinformation", but a number intended to be conservative for various reasons. In any case, it is about 1/6:1 difference, and I do not think the OP is trying to run at the bare minimum-minimum possible size, so it does no harm.

                    It would run the 7000 up to about 11,000. Important in certain cases, but presumably not so much here.
                    1601

                    Keep eye on ball.
                    Hashim Khan

                    Comment


                    • #11
                      Yes, safety is a concern.

                      I am replacing 8 swing arm bushings for four of the several 70's era small Hondas I own. Most slipped out with around 1T of push, one or two took closer to 2T.

                      I used a different shorter set up for the other bushings, pushing them inward - but that didn't work on this last bush. There is some wear/damage on the outboard circumference of the bushing and I concluded that this may have effectively peened over the lip. Hence the decision to switch directions. Still no joy and I am uneasy about pushing harder using this second set up without a pause to reflect before doing so.

                      In the current set up, the push bar passes through the housing for the opposing swing arm bush. The end of the push bar is relieved to fit over the inner sleeve of the bushing which protrudes beyond the outer sleeve. The bottom piece of the fixturing is a piece of DOM heavy wall tubing, a slight chamfer on the ID of this serves to locate the swing arm housing.

                      While there is some provision for restraining the push bar, I feel I have reason to wonder about it buckling.

                      Since initially posting, I have also had second thoughts about the DOM tubing underneath.





                      An option is to turn down the push bar dia and use it to push the inner sleeve and some of the rubber out, then saw or file through the wall of the outer sleeve.

                      .

                      Comment


                      • #12
                        Originally posted by J Tiers View Post
                        Start off with the compressive strength of the steel. Matweb gives the yield of A36 as 22,000 psi
                        So does this:

                        http://amet-me.mnsu.edu/userfilessha...6HotRolled.pdf

                        So then you would use 11,000 psi at 2:1.

                        Your 0.9 diameter is 0.63 sq in, so a bit under 7000 lb is your limit that way. You won't do better any other way. And I suspect that buckling is not going to be your limit, at 8:1 ratio.
                        If not buckling, is your thinking that crushing or 'riveting' is more likely - initiating at something like 3.5-5.5T (depending on whether 22,000 or 36,000 is used) as indicated on the H-Press gauge?

                        .

                        Comment


                        • #13
                          Fasttrack, 10KPete and BCRider.

                          My photo above of the set up will give a better idea of how the push bar is constrained. While better than nothing and adequate for the low pressures required for the other bushings, I feel something more robust to further constrain parts and guide load paths is in order for higher pressures.

                          Comment


                          • #14
                            Why "push"the bearings in or out using "columns in compression"?

                            Why not use a "Porta Power" unit away from the press and maker a "fixture/tool" to "pull" the bearing out (and/or "in" as well).

                            A fully "kitted up" Porta Power unit is very useful and very versatile as well.

                            The real and/or perceived "column risk (factor)" can be ignored or eliminated.

                            https://www.google.com.au/search?q=g...#q=porta+power

                            https://www.google.com.au/search?q=p...TvDs0Q_AUIBygC

                            Using a press removes the option of "taking the job to the work - or the work to the job.
                            Last edited by oldtiffie; 08-23-2016, 09:40 PM.

                            Comment


                            • #15
                              Have you applied any heat to the swing arm? With that much pressure it should have moved; could someone have Loctited the bushings in? Heating it up to the point that Loctite would release would also expand the swing arm slightly, quite possibly enough to allow it to break free. Also, more pressure than you've already used might deform the swing arm.
                              Location: Saskatoon, Saskatchewan, Canada

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