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  • OT (sort of): Need a little engineering help

    Hey guys,

    I want to build a device that will go in front of a hockey goal to block part of the opening of the goal. I know that the specs for a hockey goal require the front frame to be built from 2-3/8" schedule 40 tubing. The device would consist of a square steel tube frame that would have a net laced into it. I am trying to determine is what size square steel tube I need to use for this thing.

    I am not an engineer by any stretch of the imagination. My normal approach to this problem would be to build it out of 2X2X1/4" square steel tubing and hope for the best. However, I need to build it as light as possible and still not have it deform under the stress of a slap shot.

    What I know:

    1) A regulation hockey puck weighs 6 ounces, some practice pucks weigh up to 10 oz.

    2) The speed of the puck during a slap shot can be as high as 108 mph.

    3) Human life will not be endangered if the device fails.

    What I would like to know:

    1) What force (in psi) will the puck impose on the steel frame if struck head on?

    2) Are there other forces at play besides a direct strike from the puck that would contribute to deformation?

    3) What would be an appropriate safety factor to use in this instance?

    3) How do I apply this information to determine what size (structural) square steel tube to use?

    Thanks,

    Tim
    Last edited by tmc_31; 02-27-2015, 12:35 PM.

  • #2
    Will this part have an opening in the middle or be solid all the way across?
    Kansas City area

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    • #3
      Any concern about a player sustaining a more serious injury slamming into the corner of the square HSS, as compared to the 2 3/8 " round?

      Comment


      • #4
        I believe that the force will be way too low to damage 2x2x.25 steel tubing, but really it's impossible to calculate without knowing some very specific properties of the puck (due to it's material properties and shape). Short of some skilled FEA (where getting the material properties of the puck will be the most complicated part), there's no easy way to predict.

        But I estimated that if 95% of the energy from the strike is absorbed into the steel, you are only looking at an average force of 70lbs.... but again, due to the geometry of the puck, and the material properties, the impulse time and area seeing the actual strike could vary widely. 70lbs is a lot of force when concentrated on a small area.....

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        • #5
          Toolguy, it will have a net stretched across the front of it. The frame is only there to support the net.

          Good point cameron but that hazard exists whether or not the frame breaks or deforms. Since this is used as a training device where only one person is using it at a time I think the increased hazard is minimal. Still I will do well to grind off any sharp corners at the weld points. The edges of structural square tubing are pretty well rounded anyway.

          Tim

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          • #6
            Make it out of 1 1/4" x 1 1/4" x 0.100" wall mild steel square tubing.---Brian
            Brian Rupnow

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            • #7
              Whar are the dimensions of the square and how will it be supported?
              Location: The Black Forest in Germany

              How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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              • #8
                Thanks for the responses guys,

                Royldean, I think you are right, 2X2X.25" should certainly be sufficient. However I am trying to come up with the smallest/lightest square tubing that will withstand that force. How did you come up with the 70 lb figure? Am I correct in assuming that would be 70psi?

                Brian, thank you. Would you mind sharing how you came up with that? (keep in mind that this is for my and others education as much as for coming up with the right size tubing).

                BF, frame is an irregular shape that fills probably 65% of the opening of a hockey goal (72"X48"). The longest tube that is supported at each end is the 72" bar across the bottom.

                Tim

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                • #9
                  I don't think that average force is the way to go here. When the hockey puck strikes it, there will be a very high amount of force over a very small area when it first makes contact. Then this force will decrease as the puck or the steel distorts to allow the contact to spread over a larger area. But the damage will have already been done by the high initial force. If you want to prevent any damage to the frame, you are going to have to make it stronger than the average force calculation will indicate.

                  I would look at some kind of covering or coating that will be tough enough to hold up but that has enough "give" to take up that high initial force. That covering would also make it more user friendly when someone runs into it.

                  The easiest way to do this is probably experimentally. Set up a test section and hit it with high velocity hockey pucks.



                  Originally posted by Royldean View Post
                  I believe that the force will be way too low to damage 2x2x.25 steel tubing, but really it's impossible to calculate without knowing some very specific properties of the puck (due to it's material properties and shape). Short of some skilled FEA (where getting the material properties of the puck will be the most complicated part), there's no easy way to predict.

                  But I estimated that if 95% of the energy from the strike is absorbed into the steel, you are only looking at an average force of 70lbs.... but again, due to the geometry of the puck, and the material properties, the impulse time and area seeing the actual strike could vary widely. 70lbs is a lot of force when concentrated on a small area.....
                  Paul A.
                  SE Texas

                  Make it fit.
                  You can't win and there IS a penalty for trying!

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                  • #10
                    Ha, Paul, I know just the guy that can test this for me. My son is a hockey player and he asked me to build this for him. Why should I have all the fun, He has been a fan for many years and has just started playing in an adult league a year ago or so. His son (my 6 yro grandson) also has been playing for 2 years. It is a lot of fun to watch those little guys (and gals) go.

                    That piece of tubing is going to temporarily deform some during the strike. The puck will also deform some absorbing some of the energy. The trick is going be to find out how large the tubing member will have to be to not deform permanently under the forces involved.

                    I am thinking this is a problem that could be solved as a beam stress calculation with a 72" beam supported on each end and the force applied at the center of the beam. I am not sure how the beam would react with a strike (quick impact) as opposed to a slowly applied force.

                    For the sake of argument, I am assuming that the impact will be spread out over about 1/2" due to the deflection of the puck.

                    Tim

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                    • #11
                      That size I gave is not one arrived at by engineering calculation. It is based on 50 years design and fabrication experience. 1" square tubing will work, but it will bend---not from anything a puck will do, but from bodies running into it. 1 1/2" will work fine, but it is heavier than you really need. As people have pointed out, any hit from a puck on the network suspended from the square tubing will evenly distribute the shock load over a large area and have no effect on the square tubing. A hit directly on the frame by a slap-shot puck isn't going to damage it, because there is enough "give" in the structure that it will briefly deform and mitigate the "point load force" of the puck on the tubing.
                      Last edited by brian Rupnow; 02-27-2015, 08:34 PM.
                      Brian Rupnow

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                      • #12
                        Thanks Brian,

                        Given your background and barring any better information forthcoming, I will go with 1-1/4" .100 wall square tubing. Thanks for your input!

                        Tim

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                        • #13
                          Post a picture when you get it finished. Good luck!!!---Brian
                          Brian Rupnow

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                          • #14
                            Originally posted by tmc_31 View Post
                            Am I correct in assuming that would be 70psi?
                            No, 70lb would be the average sustained load during the impulse. The stress would be that load multiplied by some factor (such as the inverse of the cross sectional area if the loading were pure tension or compression, etc). But loading in this case would be quite unique and difficult to calculate by hand based on the cross section of the tubing and the form of the structure... but FEA makes that part really easy.

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                            • #15
                              Sounds complicated. I would have thought that a sheet of 3/4" plywood and half a dozen conduit clamps would be all it takes...

                              bob

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