Strength of steel tubeing

Looking to make a receiver hitch I can put on the back of my camper to pull my boat.
Boat trailer is rated at 3500 lbs but weighs less than that so we can use 3500 lbs safely.
That puts us in the class 2 hitch range.

Frame rails of camper are about 6 feet apart.
I am going to bolt about a 2 foot length of 3 inch channel iron (already have) to the trailer frame rail.
This will allow me to make the hitch bolt on rather that welding it to the frame rails.
I already have a 2.5 inch od 2 inch id receiver tube that is 1/4 inch wall. Its for a class 3 hitch but it fits my ball mounts and I already have it so I will use this.
I also have some 1/4 inch plate that I was going to cut into triangles with the 2 short sides being 1 foot long.
I will use these for gussets on the channel iron sides and also on top and bottom of the receiver tube.

You can look at me crude drawing to get a idea of what I am trying to explain.

My question is.......
Between the 2 pieces of channel iron or frame rails I was thinking about using a 2.5 inch od x 2 inch id x 1/4 inch thick tube.
The length of this tube will be just under 6 feet long.
Is that strong enough?
Is it over kill and I could get away with 2 inch od x 1.5 inch id x 1/4 inch tube?

Thanks

You will need the height of the 2 1/2" tube at least. But for that length span I would go with 3" tube and drop to 3/16" wall to get the same weight. For most fabbing like this you are better off to go larger but thinner than use 1/4" wall. The strength/weight ratio is better. That is how I would do it anyway.

I assume you are using the 2 1/2" tubing because it fits between the flange of the 3" channel iron and will allow for a weld all the way around, good plan in my opinion. I never under build, I always over build, I got my engineering degree from the University of Nigeria and learned early that bigger and heavier is better.

At the Nigerian School of Engineering they teach to use the SWAG system for calculating the yield points of structural components. SWAG silly wild a-- guess. This is in Professor Bubba's textbook

I don't see adequate RBM in 2.5 tubing in that span to support much tongue weight.You have to plan for a short but heavier load for a situation like crossing a set of RR tracks where the back of the motor home and front of the trailer go up and then down, putting greater load on the hitch. you could also put a second crossmember two feet ahead of the first and run a diagonal brace up to it from the front of the tube.

I planed on using the 2.5 inch tubing because......
That is what the receiver hitch on my pickup is made of and is rated for more weight.
It is what a professional welding shop used on a friends camper to build a hitch and he pulls a bigger boat with it.
It allows for easy gusset to the receiver because both are the same size.
It has a good deflection rating for this weight per a online calculator.
It allows the use and easy welding to the 3 inch channel because it fits in the channel.

After reading what Jon said I think I will change a few things because I really trust his advise over University of Nigeria advise.
I am going to change the tube to 3 inch by 3/16.
Same weight but less deflection over the span per online calculator.
I am going to change the 3 inch channel to 1/4 inch flat. I will have to measure but I think I can get a wider piece than
3 inches inside the camper I beam and will allow for better fit to the 3 inch tube.

I am trying to keep the weight down as much as I can but I will look into adding some kind of
brace like showcrop says even though I do not know what adequate RBM means.

While I do not figure it into the rating of this hitch because of the thin walls this hitch will
also have a 4 inch steel tube (bumper grade) welded to the top of it.
I use this to store my sewer hose.

Been thinking about this since I posted last night and I was going to add another post recomending almost exactly what you said here. You can run the flats long like your channels to spread the stress on the trailer frame and run the gussetts just the same. I would run 4 gussetts, 2 each end, top and bottom of the tube. Since your tube will be 3/16" and you trailer frame is likely less there is no need for thicker gussetts or plates than 3/16", but if you have 1/4 that works too. Is the trailer frame tube or channel?

The camper frames are 5 inch I beam or H beam what ever you call it.
Looks to be 1/8 or 3/16 thick. My eyes are not that good to tell the difference laying on the ground.

Doing like showcrop said my be a problem as the sewer tank is right there.

That's not a bad idea but I don't think it's necessary for you. You will be very limited on what you can pull and I think using the larger tube will get what you need. I'll bet the web of the I-beam will be less than 3/16 thick which is why I think a longer end plate will help. I would use 3-1/2" grade 8 bolts with steel lock nuts on each side, one behind the tube, one in front and another 8" or so forward of that.

I am concerned with the overall length of the tubing. The frame-frame tube length on a truck mount would be around 3 feet, you are planning on 6' with no supports.

Yeah, well I guess I can't help myself after all these years, in the oilfield we use 1" plate and gussets where you could get by with 1/4" angle iron because there is absolutely no doubt that someone is going to overload it, get rough with it and use it in 100 ways that it was not designed for so abuse and stupidity has to be factored in.

the section modulus of 3x3x3/16 tube is 1.73 inches cube
the effective span length of the tube is 6 ft minus 2x1ft gusset plates = 4 ft
The maximum bending moment for a 3000 lb load = 3000 x 4 divided by 4 x 12=36000 inch lbs
the stress would be 36000 divided by 1.73 inches cube which equals 20,809 psi which is below 24000 psi allowable

(quoted from post at 13:24:57 10/24/15) the section modulus of 3x3x3/16 tube is 1.73 inches cube
the effective span length of the tube is 6 ft minus 2x1ft gusset plates = 4 ft
The maximum bending moment for a 3000 lb load = 3000 x 4 divided by 4 x 12=36000 inch lbs
the stress would be 36000 divided by 1.73 inches cube which equals 20,809 psi which is below 24000 psi allowable

Click to expand...

How about when you increase the load by 30%? as when you drive through a dip that you didn't notice.