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Thread: Mini-crane (aka engine hoist) design.... strength questions

  1. #1
    Associate Engineer
    Join Date
    Mar 2013
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    2

    Mini-crane (aka engine hoist) design.... strength questions

    Hello all! This is my first post, so please delete/move if I made a mistake on placement. Thank you for any advice you can lend. Here's what I'm doing...
    I am pouring a concrete slab to place a car lift on. I am also trying to incorporate an engine lift, or at least ability to add one, into the slab. 1. I am going to bury a tube/pipe 4 feet deep into the ground, the top of it being flush with my concrete slab. The hole will be dug with a 10 inch diameter and filled with concrete around the pipe and then integrated into the slab surface. I will then have a 14 foot tall pipe that will set/sleeve into that pipe whenever needed for lifting stuff(engines, heavy equipment, anything). Meaning, 4 feet will be in the ground, 10 feet above ground. I am hoping to do this with a 4" diameter vertical tube/pipe but I need to see if it can hold the load safely. I want to design the lift to safely pick 1000lbs(most use will be far less than that though). From the amature research I've been doing, I am using bend moment to determine stress the pipe must endure. IE: Pipe is 10ft tall(sticking out of ground) with a 10ft long horizontal I beam. When lifting 1000lbs at the furthest point on the horizontal I beam(10ft out) I get a bend moment of 10,000lbs at the point where the vertical tube enters the sleeve in the ground or where the top I beam attaches to it. I am going to double that number for a safety factor, meaning I need the vertical tube to be spec'd at 20,000ft lbs of bend moment. Sound roughly correct? Now here's my main question........ How do I calculate the bend moment a pipe/tube can take? The tubes I'm looking at using are 4" outer diameter, with either a 1/4" or 3/8" wall. Both wall thicknesses have a tensile strength of 70,000psi and a yield of 60,000psi. Is there a way to calculate the bend moment strength based on tensile or yield strength? Thanks a ton!
    Last edited by Kelly Bramble; 03-09-2013 at 07:26 AM.

  2. #2
    Associate Engineer
    Join Date
    Mar 2013
    Posts
    2
    Picture was missing from previous post



    cranethree.jpg
    Last edited by Kelly Bramble; 03-09-2013 at 07:29 AM.

  3. #3
    Principle Engineer
    Join Date
    Mar 2011
    Posts
    175
    Dumbdumb,

    This is not a job for an amateur.

    Go get a professional to do this or you risk killing yourself or others.

  4. #4
    Technical Fellow
    Join Date
    Feb 2011
    Posts
    1,043
    Hi Welcome to the forum and before getting to your questions, have you considered the frictional component?

    A 500-lb engine hanging off a 10' high pipe and hanging out at 10' from that will have enormous load on the contact point with the vertical pipe and the one embedded in the concrete. Your calculations, which I have not bothered to check out yet, indicate a load of 10,000-lbs at that point.

    If you think a mere human, can make that slew then I have a surprise for you.

    Before getting to calculations you need to design the thing to work before going to the trouble of calculating loads and sizes.

    I can design and build pretty much anything I want but I still use a cherry picker type moveable shop crane. I suggest you save yourself a lot of extra work, and aggravation (after building) and just go with one of them.

    It is just overkill to design something that will hoist an engine while the lift is at full height. Run the lift up, undo everything that has to be undone from below, then drop the lift and hoist the engine. It is insanity to have a 500-lb engine hanging out at 10' and 10' high. For that you would need an overhead hoist on rails and we have had requests for those here too.

    My answer to these kinds of questions is always the same -- use a cherry picker shop hoist. It can go anywhere safely in the shop with the load on it.

    Just one old fart's opinion.

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