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Thread: Beam integrity with reinforced holes??

  1. #1
    New Member
    Join Date
    May 2021

    Beam integrity with reinforced holes??

    Experience: 34 year CAD Technician in both Structural and Mechanical design. (AAS Mechanical) (CNC specialist)

    I'm retired. Bought a motor home and planning a long trip. Salt water fishing addiction. I need to bring my boat and a mode of transportation. I need a trailer that carry's both.

    Long story short, can't find a trailer manufacturer that will build it. Every one is too busy. So I had to design the trailer myself and found a local welder with high credentials & CNC capabilities to do the fabrication.

    Because the loading varies with 'boat on' / 'trike off' and vice verse, the axle needs to be a sliding system to accommodate a moderately constant tongue weight. Hence my question.

    The main frame member is a 5x2x.125 6061 T6 tube. The sliding axle clip requires drilling holes in the beam webs. I've reinforced the holes with a 2024 T3 .12 wall tube. The sliding system gives me a 2.25" increment move up to 24.75" total slide distance. I've attached a PDF with cuts from the axle area.

    The loads are being distributed fairly equally along the beam through the cross members.
    The total weight of boat and trike are 2k lbs.
    The trailer frame members weigh approx 850 lbs.
    Front deck adds 230 lbs the the frame weight.

    It's the loading at the axles between the 2 fixed cross members (72") that I'm concerned with. I'm assuming that the floating cross member will reduce the loading at the holes down to a 40"+/- span of the 72" span?

    Not looking for finite specifics. Just want to know if these reinforced holes pose a significant problem in the integrity of the beam web?? I am pretty assured the holes being used by the sliding clip will hold integrity, but I'm worried if the ones not being used will create a weak point?

    Thinking??? Put a bolt/nut in all the holes? But that's an enormous cost for all that stainless steel! That's 52 locations. And the MMA adhesive is stronger than a bolt. I'm hoping that the staggered pattern and the 2024 T3 inserts will create a truss bridge style support in the web?

    Thanks in advance for any help!

    PS: Thank you very much for the 'Trailer Weight & Balance Calculator' Engineers Edge!! It made locating the axles a no brainer! Can I suggest that you add a tandem capability to that calculator? And more load points?
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