I have posted about building a trailer. Would like to weld and bolt
the A frame underneath the actual trailer. (For more peace of mind I
I think a grade 8 bolt has a tensile stregth of 150,000 psi. Would
puting a bolt throught the top of the tongue tube into the bottom of
the frame tube through threading or something give enought strength to
hold the thing together in a worst case senario? (like a half inch
bolt)? Or would it be necessary to go through both tubes with a long
bolt. (This tubing is 3 inches and the thickness is 1/4 inch. Would
one bolt on each side cover it?
Trailer would probably have one 3500 lb axle. Probably be 10 feet
Since 150,000 psi is the bolt, does that mean it is stronger than a
weld with 7018 rod which is 70,0000 psi?
Just trying to learn.
You need to do a bit of googling. When you discuss metal strengths you
almost always talk about tensile strength, i.e., the strength of the
item being stretched. But when you actually use the item it is
frequently used in a manner where the load is in shear. i.e., trying
to slice the bolt in half.
Tensile strength and shear strength are not the same thing, however
shear strength can be calculated approximately from tensile strength.
Another point that you might want to ponder is that a properly made
weld is as strong, or stronger, then the parent metal -- so why the
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I guess as a "fail safe" if a weld was not good. Also, actually it
would help hold the unit in place and centered before welding.
By the way, this 3 inch tubing I got is 3/16 I found out last night,
I'm not trying to be a smart ass but really, if you don't feel
confident enough in your welding to build a trailer, then don't build
it. Practice a bit.
Assuming that you are even marginally competent it shouldn't take more
then a day or of practice to become able to make good 100% penetration
welds in 3/16" or 1/4" mild steel.
One of the secrets is to position as many of the parts as possible in
such a manner that you have easy access to them and they are as near
as possible to being flat welds. Position welding is harder to learn
so try not to do any, or at least as little as possible.
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I am going to be practicing before i ever build this thing, I am just
looking for ideas. I have welded several projects already, none of
them really have broken. Most were with Mig though and I was going to
use stick for the trailer. (My Mig is only 135 amps, 110v.)
The key term here is psi--pounds per square inch. The total joint
strength depends not only on the pure strength of the bolt metal, but
the area it is applied over.
For a 1/2-13 bolt, A=pi*r^2=3.14*0.25*0.25=0.196 => except that you
need the area inside the threads, not the total bolt diameter. Inside
the threads the area is closer to 0.138. F0,000*0.138 ,700 lbs.
Looks good, right? Maybe. If you apply that much force to both sides
of a metal tube, one of two things will happen: you will crush the
tube out of square and it will bend at the bolt, drasticly and
dramatically, like a pretzel. Or the bolt will apply so much stress
at the bolthead or nut that it will simply tear right through the side
of the tube. Washers will help some, but not as much as you might
think. There is often as much or more art to a properly designed
bolted joint as there is to a weld.
Others have said this before: you need to support the tube to keep it
from crushing from the force of the bolt. One good way to do this is
to drill about a 3/4" hole, use a piece of 1/2" (nominal) pipe inside
the hole to span from one side of the tube to the other, and weld it
in. Then bolt through that. Otherwise, your nice 3x3 tube will crush
flat and become simply two pieces of 3/16 sheet metal as far as forces
Most mild steel can have a strength of 30,000 psi or less. The art of
bolting something like this is rarely about the strength of the bolt.
It's mostly about effectively spreading the forces of the bolt over
the area you need it to support. The big advantage of a welded joint
is that it allows you to transfer the stress forces over a larger
area. For example, if you have two pieces of your 3x3 tube meeting at
a right angle, one on top of the other, and weld all four sides (at 3"
of weld at each side), with about a 1/8" wide weld seam, you now have
a total area of about 1.5 square inches--more than 10 times what the
joint area is with one 1/2" bolt. And besides, the forces are now
being applied where the metal is strongest--at the corners, where it
is supported by the wall section, and not in the middle of the faces
where it will tend to make the tube crush. If you want to make this
joint effectively with bolts, you're better off using four 6" pieces
of angle iron, and using them as brackets, which are then bolted into
the four corners of the "+" formed by the crossing tubes, with each
bolt holding two pieces of angle iron on opposite sides of each tube.
Now the stresses on the bolts is mostly from the forces trying to cut
them like you would with a pair of scissors, aka "shear force", not
trying to simply pull the bolt straight through the tube. Also, the
angle iron is supporting the thin wall of the tube, helping prevent it
from crushing, and spreading the bolt force over a larger area ther
full width of each piece of tube. You also have four bolts holding
the joint together instead of one, and the angle iron design keeps the
tubes square to each other, not allowing the "+" turn on the bolt into
an "=" configuration. Using a piece of pipe inside the tube in this
situation is probaly still advisable, but it is now much less
critical. Once again, though, if you welded these angles on, or even
just two of them, your total joint area would go up even more than the
My thought for you is to avoid bolts all together for the main frame,
except to attach other pieces like lights and fenders which don't need
a lot of strength. If you're worried about a particular welded joint
being strong enough, add another piece of metal to spread the forces
out over a larger area, preferably in such a way that it will apply
forces to both pieces and not just add more weld area. Think bolted
joint but weld it instead, if you will.
If this is hard to picture from what I wrote, let me know and I'll
throw together a few sketches for you.
I kind ofunderstand what you are saying, some scketches would help
Would using the angle iron bracket with the spring hangers still a
godd idea? (Not bolting the angle but welding it? Maybe it would
distribute forces better?
Might take me a little bit, depending on what my workload is like
For the spring hanger angle bracket, I think I'd still bolt it, just
because it would be easier to adjust or modify later if needed. While
you can cut and grind out welds, it gets old fast.
OK, using just the examples we were discussing earlier, 3x3x3/16 tube
Simple through bolt:
I think you can see here that all the stress is on the bolt, and is
limitted to the area inside the threads of the bolt. A fairly poor
design, stresswise. Tension on the bolt will want to flatten the
tube, even if it isn't overtightenned, as the load is applied.
Basic joint welded on 4 sides:
I think you can begin to see how much more area there is to distribute
forces this way.
Angle iron braces bolted on:
One possible way to make the bolted joint stronger. In a case like
this, you may be even better off using more smaller bolts, like (2)
3/8" bolts instead of one 1/2 bolt at each side of each joint. I
haven't calculated it out, but I think the simple welded joint might
still be stronger than this.
Angle iron braces welded on:
The extra angles give you more weld surfaces, and gives you more metal
to transfer stress from one tube to the other. There are other ways
to gusset and brace joints like this that may distribute forces
better, possibly even using less material, but thinking of ways like
this to reinforce your joints will give you less total stress on any
individual weld. Just keep in mind that this is also at the cost of
more metal and more weight.
There are other things to consider when welding tube. Welds that
cross the face of the tube instead of welding along the length cause
local stress points and retemper the metal in that location that can
cause the tube to bend easier, so even the examples I've given you can
have problems. Knowing when you can get away with some of these
things and when you can't is where experience comes in. Without the
benefit of experience (yours or someone else's), you are basically
stuck with trying it and finding out. Engineers now do this with
computer models, but even they will sometimes build samples of joints
to break apart, just to verify that the computer is correctly
modelling everything that happens. For you, it might be simpler to
make a version of any joint you are considering, clamp it in a vise,
and beat the heck out of it with pipe wrenches and hammers to see
where it breaks. If you make it in a smaller metal (1x1, 3/4x3/4, or
whatever) you can at least see which design of several you may be
considering holds up the best. For joints in areas without a lot of
stress, I wouldn't bother.
I'm sure there are others here who could give you better ideas, but
this is also something you can look for when looking over other
trailers and structures to get ideas. What did the manufacturer
reinforce, and how? If they didn't reinforce it with another piece,
how did they arrange the metal to get the best weld area and the best
transfer of forces from one piece to the other?
Thanks for that explanation, it really helps and I am learnign alot.
One more question, If I wanted to take my 10 foot sections and
extended them 2 feet to make a 12 fott trailer, would you do it? I
mean, would Just butt welding the tubing fully make it a reliable
joint? Or shoudl I play it safe and just make it a 10 foot trailer?
10' is a very good length for a trailer. If you can live with that, I
think it'd be best.
With that said, you can probably get away with extending to 12', but I
would put the joint in the portion of the trailer behind the axle:
leave the section between the hitch and the axle one piece.
Personally, I'd probably not bother joining two pieces of tube for the
back 2', but add angle, maybe 3' long so it overlaps a foot, then the
strength of the angle helps you rather than relying on the strength of
If you decide for looks or load that you want the tube to run all the
way to the back, then I'd do one of the following:
Gind the edges of the tube at an angle almost the full thickness of
tube wall, so just a little flat is left to get the pieces to line up
where they touch. Fill this "vee" with weld. It's simply, quick and
easy if you have a 4-1/2" grinder. If you don't, get one--they really
are that useful.
If that sounds like too much work, then I might tack the sections of
tube together with a couple of rod stubs or bit of scrap here and
there to leave a gap all the way around, then fill the gap with weld.
The big danger here is getting distortion from different shrinking
rates as the metal cooled, since there is nothing supporting the gap.
So if you do this, consider doing an inch at a time on each side of
the tube and then keep going on to the next side, so that you're
putting less total heat into each joint at once. Clamping the whole
mess to an angle to keep it straight as it cools can help too.
Another way to do it might be to use pieces of flat stock or cut-off
scraps of tube, and cut them down so that they just fit inside the
tube. Leave maybe 3/16 to 1/4" between the two tube ends with your
added pieces showing through the gap, and weld these backing pieces in
until you've completely filled the gap.
The advantage of all of these over a plain butt weld is that besides
being stronger in general, if you then grind them flat, you still have
most of your weld strength. Grinding flat would allow you to apply
another piece over the joint, maybe just a 2-1/2x8" piece of 1/8" or
so, without leaving gaps under it where it has to go over a big heavy
weld where a lot of water can collect and rust can start. Ideally,
this added piece would either have the ends tapered to points, or only
be welded on the long sides (along the corners of the tube) to reduce
the stress and metal changes at vertical welds, possibly defeating the
purpose of adding the piece. If the ends are taperd, these added
patches are often called "fishplates". You could also add a short
piece or two of angle along the joint to achieve much the same effect.
That is excellent information! I am learnign alot by talking (or
posting) to you.
I think I may leave it alone at 10 feet or possibly just buy two new
side pieces to the proper length. (but I am trying ot do this as
cheaply as possible).
When I make the frame where the front and rear pieces meet the side
pieces, is it a good idea to grind down the edges where they "T"
before I weld. Or just weld where they butt togther?
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