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Thread: guidance needed on support plate for ambulance stretcher mounting

  1. #1
    Associate Engineer
    Join Date
    Jun 2020
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    1

    guidance needed on support plate for ambulance stretcher mounting

    hi, to all
    newbie to this site.
    i would like to ask for help regarding regarding the dimensions for mounting a stretcher support plate,in an ambulance.
    were mounting a ferno power load stretcher that has a interrior mounting install plate. we have to place a supporting bracket under the chassis to withstand the following spec taken from strykers install manual.
    "Each bolt and support structure must be able to withstand 2,750 lbf (12,230 N) in upward tension and 600 lbf(2,669 N) in shear in all horizontal directions."
    found in this install manual REMOVED
    so i guess we can use steel or aluminom as a plate,which we would tap, but what would those dimensions need to be?
    thanks so much
    david

  2. #2
    Principle Engineer
    Join Date
    May 2015
    Posts
    184
    Your description is vague and the best engineering design cannot be made without a complete review of the design elements in your installation. So this cannot be a recommendation.

    However, based on your requirements = 2750 lbf tension and 600 lbf shear lets talk about an initial approach to the problem:

    Let’s use grade 5 hardware, 85 ksi proof load, 92 ksi yield and 120 ksi tensile as an example.


    Sure there is stronger stuff but these are common fasteners.

    In tension, a 1/4 “ fine thread (28 tpi) bolt with a stress area of .0364” could theoretically support the tension load but fine threads aren’t recommended for aluminum. What about shear? Are you shearing across the threads or on the plain shank of the bolts?

    We don’t have your answer so dividing 600 lbs by the reduced stress area of the threads. 600/.0364 = 16.5 ksi. Now shear stress limits are usually just a bit over half the tensile strength but we have unusual loading and stress concentrations pushing sideways on the threads. Figure a stress concentration of 3.0 and your shear component could be 16.5 x 3 or 49.5 ksi. That too might just barely work but there is really no factor of safety in your overall design using the -28 grade 5. (So don’t)


    If you make the giant step to a 3/8” – 16, grade 5 bolt, the stress area of the threads more than doubles to .0775 sqin. The proof load is about 6500 lbf at roughly 380 in-lbs of torque and you have more than doubled your shear load. You really want to have the shear load acting through the body of the bolt and not the threads.

    This 3/16” bolt might be a starting point. The nature of the loading, both magnitude and number of cycles expected can influence the design of fasteners as can corrosive environments and temperature. So we cannot guarantee the fitness of this suggestion.
    It is offered as an example of preliminary bolt sizing efforts. You must either hire a professional engineer to do a complete study or take responsibility for the design yourself.

    Consider using nuts and washers instead of tapping the plate if you can to increase the grip length of the bolt in tension. Threads should point down so the bolts don’t fall out if the nuts come off.

  3. #3
    david:
    Are you also looking for assistance on the thickness of the under chassis plate?

  4. #4
    Technical Fellow jboggs's Avatar
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    Mar 2011
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    Myrtle Beach, SC
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    A question: When the plaintiff's attorney in the human injury lawsuit from the accidental death in which your ambulance was involved has you on the stand and asks you how you got your engineering design input, are you going to say "I got it for free from some guys on an engineering forum I never met?" Think about that.

    I'm not saying you shouldn't ask your question. I'm just saying in a field like this, so ripe for potential injury and lawsuit, actual fault has little to do with the outcome. It might be worth getting some paid professional and documented advice.

    How do you think I got this gray hair? Just sayin...

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