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Old 07-28-2009, 01:29 PM   #1 (permalink)
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analyze a failure & suggest a fix

I'm building a motorhome and trailer out of an old Duece and Half, and I'm building an enclosed trailer to tow behind it. Ran into a little problem with the tongue extension. Take a look and give me a some input about how I might want to go about fixing this.

This is what I did:







I extended the tongue about 30", using 3.5 x 3.5" x .25" - square tubing. Which allows me to do this:





And here's the problem:



When the trailer binds-up, it puts an incredible side-load on the tongue and now it's cracked. I know - the obvious solution is to not bind it up, and then I wouldn't have this problem. But I did this just testing out the limits as far as how maneuverable the trailer is with this tongue extension.

I was planning to add something like these gussets on each side of the trailer tongue, but now I'm wondering whether that's going to be strong enough.



They would extend far enough back to reinforce the tongue where it contacts the bumperettes (contact point shown in the picture below).

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Old 07-28-2009, 01:31 PM   #2 (permalink)
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Is it cantilevering off the bumpers when fully turned or is it just the twisting forces of a pintle?

The tongue is an awfully long pry bar if its from hitting the bumper. Maybe add a softer bumpstop of some kind so it has little give when it hits.
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Old 07-28-2009, 01:32 PM   #3 (permalink)
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i'd seem to think a butt weld is not strong enough in that application.

the fish plate you have drawn up should do the trick, but im no expert.

good luck finding an answer
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Old 07-28-2009, 01:34 PM   #4 (permalink)
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Cantilevering. I know that I'm going to do something to limit that, but I didn't think it would be this big of an issue. The problem is, I can't really tell exactly WHEN it's going to bind until it does. I'm thinking that at the very least, I should add some bumpstops to cushion things where the tongue contacts the bumperettes.
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Old 07-28-2009, 01:35 PM   #5 (permalink)
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Oh, and just for reference, the trailer weighs just under 4000lbs empty and will probably never exceed 7000lbs fully loaded.
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Old 07-28-2009, 01:36 PM   #6 (permalink)
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Whatever the solution is, I need to make this happen quick-like. I'm using this trailer to move and have to be out of here by the end of this week!
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Old 07-28-2009, 01:42 PM   #7 (permalink)
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Maybe it is just the way I see it but that picture makes it look like the weld that failed is focused more on the hitch adapter and not on the square tube. If you follow it around from the top it look like there is very little weld on the square tube. I like the idea of the gussets you made as long as they are welded to the adapter and not just bolted through.
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Old 07-28-2009, 01:43 PM   #8 (permalink)
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I'd be building that fish plate, but for a little extra, what about building a 1-2" spacer for the pintle on the truck side?
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Old 07-28-2009, 01:44 PM   #9 (permalink)
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Whatever the solution is, I need to make this happen quick-like. I'm using this trailer to move and have to be out of here by the end of this week!
the gussets you designed are perfect. cut em out and burn em in. that will handle the weak link in that area.

after you move you can figure out the best possible solution for the cantilever effect.

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Old 07-28-2009, 01:45 PM   #10 (permalink)
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Can you move the pintle back about an inch with a spacer or something? That would give you more clearance, but won't help with the break. Were you going to bolt the plate on or fill weld it?
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Old 07-28-2009, 01:50 PM   #11 (permalink)
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I'd be building that fish plate, but for a little extra, what about building a 1-2" spacer for the pintle on the truck side?
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Originally Posted by roczuk View Post
the gussets you designed are perfect. cut em out and burn em in. that will handle the weak link in that area.

after you move you can figure out the best possible solution for the cantilever effect.

-Tristan
Sounds like two good ideas.
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Old 07-28-2009, 01:53 PM   #12 (permalink)
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OK. Thanks guys. I'll go ahead and add these gussets. Spacing the pintle back would work great, except that it would interfere with the loading ramp.
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Old 07-28-2009, 01:54 PM   #13 (permalink)
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Oh, and material thickness? I'm thinking gnar-kill. Maybe .375-ish? Hell, I could go 1/2".
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Old 07-28-2009, 01:55 PM   #14 (permalink)
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Not knowing what your using for a welder I'll make this comment.

Whille looking at your weld bead I noticed you welded vertical down.

If you reference the D1.1 welding code (for structural steel) It does not allow vertical down welds. This is do to the lack of fusion, gravity is just pushing the weld pool down the joint and not letting it dig in.

If you grind that out, and weld vertical up, you'll fight gravity and allow the weld to literally dig in. It may not look as nice, but the penetration will be significatly deeper.

One of my welding professors years aso left me with this comment.

"A pretty weld never held shit", that's not always the case, but in this instance it may apply.
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Old 07-28-2009, 02:04 PM   #15 (permalink)
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The gusset will help but will only lead to more massive failure. Something has to give in that situation and you can't build a trailer tongue big enough to handle those forces.

When I built my trailer I gusseted the hitch on the top only, but mine doesn't stick down like yours. I also beveled the end of the tube all the way around to burn in a good full penetration weld, then I threw a fillet over the top.

If you did run a vertical down you could have poor adhesion and that could lead to a quick failure as noted above. You certainly want that weld done in position.

In any case, I'd grind the weld out, reweld it in position if possible then gusset it.
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Old 07-28-2009, 02:08 PM   #16 (permalink)
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Tig the new joint, fish plate it similar to how you have figured out already and then add proximity sensors that will trigger lights in the cab so you dont go too far anymore.
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Old 07-28-2009, 02:17 PM   #17 (permalink)
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Yeah, it was welded vertical-down. I know that's not 'right', but I've never had a problem with it before. I'm going to grind out that weld and re-weld it this afternoon, then I'll add the gusset plates as soon as I can pick them up from the shop that's cutting them for me.

I'm going to add some bump-stops to the tongue where it hits the bumperettes. Got a set of bumpstops off the frontend of one of my Tacomas that have threaded studs on them. Should work pretty well. At least they'll soften the blow a little bit. It's not like I'm jack-knifing this thing at speed. I just don't know when it's actually bound-up until I hear that 'clunk'.

I think it would probably be a good idea to add formed sheetmetal gussets that would run from the lunette-eye mount back to the base of the A-frame. Don't have time to do that right now, but it's something that I'll make time to do sooner than later.
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Old 07-28-2009, 02:19 PM   #18 (permalink)
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Tig the new joint, fish plate it similar to how you have figured out already and then add proximity sensors that will trigger lights in the cab so you dont go too far anymore.
That's a good idea. I could just use something like the door-buzzer sensors or some other type of trigger switch to tell me when I'm getting close. But honestly, I don't think this is going to be as much of an issue as I get a little more familiar with the truck and trailer.
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Old 07-28-2009, 02:26 PM   #19 (permalink)
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Unless you need the bumperettes, I think I'd take them off or make them an inch or two shorter, if you can't move the pintle rearward.

The gussets you've drawn up look like a good solution for that area, but I'd rather see a long formed gusset that extended from the lunette eye back to the original tongue if at all possible. Probably grossly overkill, but this is PBB; you always need more triangulation.
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Old 07-28-2009, 02:29 PM   #20 (permalink)
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The quick analysis.

Judging from the first picture under “here’s the problem”: the fillet weld on the top appears to have excess convexity and overlap on the far end. The side welds cracked because you have essentially a small fillet in tension on a cyclically loaded tubular member. It’s a t-joint design. Not a butt joint. Depending on how wide the adapter is to the tube it could be a flanged but joint. The weakest point in the weld is the area with the smallest cross sectional width. In this case is looks like the leg of the weld of is much larger on the hitch adapter. This left a lack of filler material (smaller leg) at the toe of the weld on the tube section. Leading to a small effective throat and weaker weld.

Solution: Use a grinder or burr to completely remove the crack. Go a little further than the visible ends of the crack. This will help ensure it is completely removed. If it is left in there the crack will propagate back through the new weldment. Re-weld the side fillets. Ideally you should use the grinder to bevel the ends of the square tube. This will turn the weld into a single bevel groove weld then go back over the top of that with the fillet.
After all that put the gussets in. That will help disperse the tensional forces directed at the side welds.

Cool hauler!

Last edited by Hunt; 07-28-2009 at 02:32 PM.
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Old 07-28-2009, 02:30 PM   #21 (permalink)
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Unless you need the bumperettes, I think I'd take them off or make them an inch or two shorter, if you can't move the pintle rearward.

The gussets you've drawn up look like a good solution for that area, but I'd rather see a long formed gusset that extended from the lunette eye back to the original tongue if at all possible. Probably grossly overkill, but this is PBB; you always need more triangulation.
If I take the bumperettes off (or shorten them), I'll just be able to go a little bit further before I'm in the same situation. Hah. I'm liking the formed gusset idea though.
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Old 07-28-2009, 02:39 PM   #22 (permalink)
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Judging from the first picture under “here’s the problem”: the fillet weld on the top appears to have excess convexity and overlap on the far end. The side welds cracked because you have essentially a small fillet in tension on a cyclically loaded tubular member. It’s a t-joint design. Not a butt joint. Depending on how wide the adapter is to the tube it could be a flanged but joint. The weakest point in the weld is the area with the smallest cross sectional width. In this case is looks like the leg of the weld of is much larger on the hitch adapter. This left a lack of filler material (smaller leg) at the toe of the weld on the tube section. Leading to a small effective throat and weaker weld.

Solution: Use a grinder or burr to completely remove the crack. Go a little further than the visible ends of the crack. This will help ensure it is completely removed. If it is left in there the crack will propagate back through the new weldment. Re-weld the side fillets. Ideally you should use the grinder to bevel the ends of the square tube. This will turn the weld into a single bevel groove weld then go back over the top of that with the fillet.
After all that put the gussets in. That will help disperse the tensional forces directed at the side welds.

Cool hauler!
Gotcha. I did put a big bevel all the way around the tube, but I'll be sure to make a nice groove when I grind this out. And I was using a Millermatic 251 MIG welded, running vertical-DOWN (oh no). I'll try welding vertical-up this time, but I always have trouble with the weld dripping and turning to shit when I run vertical-up. The lunette-eye mounting bracket is 3.5" wide and the tube is 3.5x3.5" so the gusset plates will be real easy to install and weld to the lunette-eye mount and the tubing.
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Old 07-28-2009, 02:55 PM   #23 (permalink)
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If you reference the D1.1 welding code (for structural steel) It does not allow vertical down welds. This is do to the lack of fusion, gravity is just pushing the weld pool down the joint and not letting it dig in.
You might want to re-read a few portions of your D1.1
3.7.1 (2) “When tubular products are welded, the progression of vertical welding may be upwards or downwards, but only in the direction(s) for which the welder is qualified.”

NORMALLY things are welded upwards. You have to really know what you are doing to run down hill but it doesn’t say you can’t.
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Old 07-28-2009, 02:59 PM   #24 (permalink)
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Well I'm obviously not 'qualified' because my vertical-down weld failed. If it was a good weld, it would have at least cracked where the weld met the tube, right? This failed right in the middle of the weld. I'm not an allstar welder, but I really didn't think I'd have any problems with this. I'm just glad I found out about it now....
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Old 07-28-2009, 03:01 PM   #25 (permalink)
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Cool rig, Duane was telling me about it last weelend. Gotta build thread?
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