Tensile stregth of bolt. (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 bolt?

Bruce-in-Bangkok (correct email address for reply)

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, not 1/4.

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. F=150,000*0.138=20,700 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 are concerned.

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 bolted version.

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.

--Glenn Lyford

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. Bruce-in-Bangkok (correct email address for reply)

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.)

I kind ofunderstand what you are saying, some scketches would help though.

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 today.

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. --Glenn Lyford

OK, using just the examples we were discussing earlier, 3x3x3/16 tube and 1/2" bolts.

Simple through bolt:

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:

think you can begin to see how much more area there is to distribute forces this way.

Angle iron braces bolted on:

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:

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?

--Glenn Lyford

Would you bolt through both sides of the tubing.

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?

Yep. --Glenn Lyford

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 the joint.

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.

--Glenn Lyford

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?

For a corner joint, there's usually not as much need to grin down. If your material was thicker, I might worry about it, but for 3/16, I'd just put a fillet in the corner. --Glenn Lyford