the weight of a 31 inch tire ranges between 40 and 50 pounds depending on width, so let's say it's 45, and the weight of the steel of the carrier will probably be what, another 30 or so? not sure what size stock you're using. so that means that you're carrying about 80 pounds on that single pivot. that tire has about a 15 inch radius and it's probably what, another foot to the pivot from the pivot side edge of the tire? so say about 2 and a half feet from the center of gravity to the pivot point? 2.5 x 80 = 200 ft-lbs of torque on that 4 inch weld. what's the sidewall thickness on the outer tubing of the hinge?
looking at the close up of the hinge part you posted, it seems that you're mounting the outer tube to the side of the frame channel? am i looking at that right? if that's the case, you may want to think about moving it to rear of the channel so that the tube faces front and back along the jeep, instead of side to side. the reason i say this, is because it will change the nature of the stresses in welds. normally you open the carrier so it points towards the rear of the jeep for the most part, maybe a little toward the passenger side. with the pivot mounted on the side of the frame rail, this position of the carrier will put the welds under a shearing stress that will want to twist the welds apart. generally, this is the type of stress any kind of fastener is weakest against. think of a screw or bolt, generally you twist the head off or snap the shaft twisting etc. you don't see one popping the head off because it was being pulled under tension, and you don't see it buckling because it was being pushed under compressive loads. moving the tubing to the outside will change the load to tension on the top and compression on the bottom of the welds with no stress in the middle. ideally anyway. in reality it will probably want to pivot at the base of the weld so the entire weld will likely be under tension. either way, you'll probably be able to resist much more stress under these conditions then with the tubing to the side. the general rule of thumb is that material is twice as strong under compressive and tensile loads then it is in shearing loads. if you need another example, take a pencil or pen and some sticky tack (or chewing gum) and stick the pencil to a flat surface. now try to remove the pencil, which represents the tube, by twisting it. the tack, which represents the weld, will give fairly easily this way since it's the only thing resisting the force applied. now try again by levering the pencil up from one end. if it's good tack, it should be a bit harder this way. the tack will want to stretch before it comes loose and the table or wall will act to help hold up the pivoting end of the pencil. anyway, I hope that made at least a little sense and that i didn't bore everyone to death. good luck!
RIP: '88 YJ 2.5L Ax-5 NP231
Posi-Loked. Herculined. Optima yellow top. 1" Shackle, 2" BDS. Cragar 397's Aussie front.
92 YJ 4.0L Ax-15 231
5" springs, 1" shackle 31's or 35's depending on my mood