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  1. #1
    welder689 is offline Aluminum
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    I'm going to be welding a 5th wheel to a truck frame. Basically, it's similar to what you'd see on a tractor-trailer, just smaller.

    The 5th wheel pivots on a mount that's made of 3/8" plate. I'm going to weld it down to a larger piece of 3/8" plate that will be bolted to the frame rails of a diesel-dually cab/chassis set-up (new truck).

    Customer does not want any welding to the truck frame itself.

    Truck will be pulling a car hauler that holds three cars.

    How many bolts, how big, and what grade should be used to bolt the plate to the frame rails?

    I'm thinking either 7018 or 7024 to weld the 5th wheel to the plate. The 5th wheel bracket is not factory-beveled, but I'm thinking of beveling it for extra penetration.

    The brand new 5th wheel came with no instructions or recommended weld procedures.

  2. #2
    macona's Avatar
    macona is offline Diamond
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    Frames in general should not be welded to.

    I personally wouldnt touch this job due to liability.

    Have him go to where he bought it. They should be able to recommend where to have it installed.

  3. #3
    welder689 is offline Aluminum
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    macona,

    He ordered the 5th wheel from a trailer supply company.

    He has another truck just like it.

    Just curious why you think it's risky? The 5th wheel is designed to be welded to a plate, I'd say. There's no mounting holes on it.

    You think a well done 7018 weld could be a problem?

  4. #4
    macona's Avatar
    macona is offline Diamond
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    I might step up to 11018 for that one. Best guess 5/8 to 3/4" grade 8 with Locking nuts.

    I have seen a few of these 5th wheel adapter kits that attach to rails that bolt onto the bed. I think he should return this one and get one of those. Its a kit with explicit instructions how and where to attach it. Plus it is removable for when you want to haul something.

    Heres the thing. If anything, I mean anything, happens to that truck you may have lawyers knocking on your door. There is no way you could defend yourself since you have never done an install like this and frankly you dont know what you are doing. You have no calculations on determining plate thickness for the given width, no info on figuring shear for the bolts and their placement. Are you certified for this welding procedure?

    All these things that would be brought up in court.

    If you still insist on doing it do it as a cash sale, no paperwork, no receipt. You never did it.

  5. #5
    bob308 is offline Hot Rolled
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    icc rules no welding on truck frames between the wheels.

    now i think most of the times these people crying liability are full of it. but this time i would not even touch it. bolt it on make the brackets and bolt it. that is how i mounted them in the past. and that is how all of the ones i have ever seen were mounted

  6. #6
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    There should of been mounting instructions with the new hitch. Look online for the company and email them for instructions. 10 years ago i would of done it in a heartbeat, but not now for the reasons above...Bob

  7. #7
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    Maybe you can find it here if it isn't marked...Bob http://www.hitch-web.com/default.asp

  8. #8
    dfw5914's Avatar
    dfw5914 is offline Stainless
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    Typical RV 5th wheel hitches are attached to the truck frame with at least eight 1/2 inch grade 5 carriage bolts. The kits are almost always model specific in order to pick up suitable mounting points.

  9. #9
    welder689 is offline Aluminum
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    aametalmaster,

    Before I even posted here, I did a Google search on the manufacturer and called them. I spoke with a woman in product support. She said that the way my customer wanted the hitch mounted was acceptable. I asked her if she had any welding procedures or recommended consumables or instructions that would have came with the mount. She e-mailed me two pdf files...one was mainly just a parts breakdown on the hitch with operating instructions for the user, the other was a general welding safety bulletin. Neither is much help.

    I have a friend that is a mechanical engineer. I'll check with him to see what he thinks as far as plate thickness, correct hardware, etc. Thanks.

  10. #10
    JL Sargent is offline Titanium
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    Why even weld it?

    I have installed a couple of these type 5th wheels and typically the unit is pinned into rails which are bolted to the frame. Its much more common for gooseneck plates to be welded to the frame I suspect. The pinned frames are desireable because its removeable and you can use the truck for other stuff.

  11. #11
    welder689 is offline Aluminum
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    JL,

    I wasn't planning on welding it to the truck frame at all. The 5th wheel he gave me pivots on a bracket made out of plate steel. I was going to weld that bracket to a BIGGER piece of plate (the width of the frame rails) and then bolt the bigger plate to the frame rails.

    This would then be removeable if the truck were ever to be used for something else.

    All I'm trying to determine is the correct weld procedure for that and proper hardware.

  12. #12
    gbent's Avatar
    gbent is online now Diamond
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    I don't know what kind of a frame you are bolting to, but do not drill the flanges (top and bottom surface) of the frame. If you want to bolt something to the top of a frame, weld angles that come down the side of the frame web, and drill the web. This way the holes do not weaken the highly stressed flange.

  13. #13
    Timw is offline Stainless
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    I worked on semi trailers for 30 years, I've replaced dozens of King Pins and Fab-ed Upper couplers, replaced first 10' of flat bed trailers. I used either 6011,6013 or MIG. You don't want high strength welds, they tend to crack. With regular strength welds they can flex with the load and don't tend to crack.
    I would make it strong enough that the Fifth Wheel will fail before my work.

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