Plug wielding---- when is it needed?

[ray66v]

Plug welds are commonly used in body shops, during the replacement of sheet metal components, as a substitute for factory spot welds.

 

[Shield Arc]

I just did a project like this not to long a go for a friend's galvanized boat trailer.
I drilled a series of 3/4-inch holes in both sides, and top of the box tube. inserted solid square stock in the box tube. Gaped the box tube just a touch, made a full pen weld all the way around the box tube, then made all the plug welds.

 

[Shield Arc]

I forgot to add, in this case I would also use oval, or diamond shaped fish plates on both sides of the box tube, right at the break in the box tube.

 

[Gary Fowler]

I don't think you would gain any strength on that narrow of a plate by putting in plug welds after it is welded down both sides. Plug welds are useful when putting a thicker plate to a thinner one to give more attachment points and equalize the stress rather than put all on the edges. When I put chain hooks on my FEL, I used a 1/4" plate 4x6" for a repad. It already had holes in it as it was made as a trailer leaf spring holder. I plug welded up the 4 ea 1/2" holes then put my hook over the top of them. This stiffened up the plate, added additional weld area to the 1/8" thick bucket material. Did I need to do that, probably not but the holes were there and I didn't want to have them collect water/dirt anyway.

 

[Shield Arc]

Please excuse the really crude MS Paint drawing!
Here is what I was trying to say how I would make this repair.

 

[dragoneggs]

View attachment 366634

Not as hard posting a picture as I thought.....just have to be able to read!

Looks to me that also.....reason for not just laying a piece under the box metal is that----- the downward pressure would only have to shear 3/8, whereas by putting plate on side of box, the downward shear would have to effectively shear 8inches of 3/8 metal......am I thinking correctly?

I am not a welder but it looks to me from the little picture that it was a bending failure not shear. If that is the case, it will be important to reinforce the top/bottom and make sure there is at least as much material thickness. Putting a tube inside the original tube will not have as much bending strength. The bending stress is highest at the outer most edges. That is why I-beams are the most efficient in carrying bending loads. My :2cents:

 

[Arc weld]

I think Shield Arc has a good method for repair since the hitch will have to be cut off and reattached anyway. It might depend on the dimensions of the existing tube and if a smaller size would fit nicely inside it. Another method to reinforce it would be just like most tandem axle trailers. Another length of tubing going on the bottom of the A frame hitch. That method might depend on how much ground clearance you needed.

 

[Shield Arc]

Another option on the same lines of thinking as Arc weld suggested. You could buy some structural tee. Skip weld it on the top and bottom. And still add a fish plate over the weld repair on the sides.
When I skip weld something like this, before painting I like to run a bead of caulking between the welds so water doesn't get under the metal and rust it out.

 

[CNC Dan]

Caulking the seams is a good tip.
You can also get weld through primer.

As for the T iron for reinforcement, i would go for some small channel iron. Creating additional box sections. That would put more metal farther away from the nutral axis.

 

[dragoneggs]

Another option on the same lines of thinking as Arc weld suggested. You could buy some structural tee. Skip weld it on the top and bottom. And still add a fish plate over the weld repair on the sides.
When I skip weld something like this, before painting I like to run a bead of caulking between the welds so water doesn't get under the metal and rust it out.

Hey I know nothing about welding but why not just square up the break, weld it back together all the way around and then weld a flat plate to top and bottom sides? While a tee would work also, in theory one should not design a bending member without a stabilized edge or a return flange if subjected to compression stresses due to bending. In this case, the material thickness will likely prevent a crippling failure but in thin material this is an issue. Sorry for my two cents here, I come from aerospace structure background.