Where can I place the trailer axle? Under or upper

[alsayyed]

i have been working on my trailer for almost a month or so doing some maintenance and removing the axles which are replaced by 3 inch square tube instead a round one. Know when I have  tried to place the square tube under the spring leaf a friend suggested whey not place the tube over the spring leaf, I said to my self this cannot be right, because if I have done that then all the load will be concentrated on the tires, so that mean there are no significant of keeping  the springs there if the springs will not carry the load.
So I need so help in solving this problem.

[POC]

I can't see what you have there, but you could technically put the axle over the springs, and use some kind of bracket to attach the axle to the springs.
The springs would be "pulled" on by the axle, instead of "pushed".
The only negative I could see would be loss of springs travel, as the axle would probably contact the frame or something sooner this way. 
This pic might help...

[Jeff]

The other more severe problem with the axle above would be that you are putting the u-bolts under constant load. The whole weight of the trailer and its contents would be loading those bolts and not be shared by the springs and shackles and tires.

[Mark K]

My 5 ton Hudson trailer has the axle above the springs, factory made. Replaced all the U-bolts when I rebuilt the trailer. My axles are round, u-bolts go around the axle on each side of the leaf spring. there is a flat plate on the bottom of the spring stack that is held in line with a pin just like in the picture. I've had 5 ton of fertilizer on it with no problem.

[SPIKER]

placing the axle between the frame and springs is a common way to attach them, as described above with the plate and u bolts (or square shaped U bolts) in some cases.   this is actually more common as this reduces ride height of the trailer deck reduces spring twist as bumps are hit as well as less twisting shearing stress on the U bolts due to the angles of the axle about the arc of the springs.   It does place the U bolts in 100% tension suspending the weight of trailer & load below the axle, it is fine as the tension strength of the U bolts is probably higher than the shear bending moment strength as often they are cut threaded rod rather than rolled threads which will weaken the bolts in shear or bending more than tensile strength...

Mark

[Magicman]

If the springs are arched enough to allow enough travel so that the axle will not contact the trailer, then the desired height of the trailer would be the final determining factor.

I own two factory built trailers with the axles below, and one with the axle above the springs.

[LorenB]

The other more severe problem with the axle above would be that you are putting the u-bolts under constant load. The whole weight of the trailer and its contents would be loading those bolts and not be shared by the springs and shackles and tires.

An excellent point.  Here's some analysis, for what it's worth:

If your u-bolts are 1/2" in diameter, the cross sectional area of each bolt is: pi x R(squared) or about 0.20 sq. in.

If the bolts are made of relatively mild steel, the strength of that steel should be about 35,000 psi. 

That means each bolt should carry about: 35,000 psi x 0.20 sq. in. or about 6800 pounds. 

Assuming a safety factor of 3, a safe load would be about 2200 pounds per bolt. 

Each side of the axle should have two u-bolts, for a total of four load-carrying shafts.  All eight u-bolts should be able to support a safe load of 17,000 pounds or more. 

I suspect that the u-bolts are made of higher strength steel, so these numbers are probably conservative.  On the other hand, they may not be 1/2" bolts either. 

In any case, I suspect that the u-bolts could carry the load even after many years of rusting, as long as you inspect them occasionally to make sure they are still mostly there.