Just a quick update here. I prepared the trailer to weld the rear section deck plate on by building and installing 4 temporary angle iron pull mounts. While I was workign on them, I took a little time to start working on welding the diamond plate runners to the ramps. The ramp is constructed of 2" square tubing with a wall thickness of .083". The ramp measures 12" wide by 60" long. I clamped the ramp to the tongue of the trailer to give a strong/flat surface to hold the ramp during welding. I laid down 2 to 3" long beads and only welded about 4 beads at a time, letting them cool completely before doing any more. I also kept the beads being welded far away from each other to keep heat input to a minimum. The beads are placed no closer than 4" from the start of one to the finish of the next closest. I also set the machine up 1 setting colder than I had it set when I welded the deck on the main trailer frame.
Guess what, the ramps have 1/8" to 3/16" bow in them as well. The best I can tell I did everything right on the ramps and apparently the top of the square tube still shrunk. The only thing I could have done different is to put a shim under the center of the ramp long sides to create a small bow before welding.
I'm kinda concerned now that the rear section may warp as well even using the skip weld technique. I think creating a preload bow condition will help though. Is there any chance the preload bow (in an up direction) could make the trailer bow up more when welded from possibly having too much tension put into the preload? (instead of the frame pulling down in a sag from the top of the tube shrinking like it did on the 1st two sheets)
I guess the solution is to use my best judgement with the preload tension and stop every-so-often and remove the tension to check to see if and how much the trailer main rail is pulling/shrinking the opposite direction and adjust the tension accordingly?
BTW, I'll use the ramps as a test to see how applying heat from an actylene torch will shrink the bottom of the box tube to straighten them out...good practice:).
If you apply the same welding techniques to welding the trailer that you did to welding the ramp, all other things being equal, the ramps will warp more than the trailer. That is because the trailer members are bigger, and thus resist warping better. I think you are on the right track with the couter bowing, because no matter how carefull you are, putting a weld on one side of a member will warp it. The only difference technique will make is how much it warps. Fixturing, such as your couter-bowing technique, is one good way to couteract the warpage that will occur.
Hi again Goose.
It's sounds like you are having too much fun.
As you have seen, clamping down your work piece and sequence welding are good ways to minimize warpage, but they don't eliminate it. Even putting a pre-bow in your frame will not eliminate warpage (unless you get amazingly lucky and put exactly the amount of tension required in your pre-bow which I assume is impossible to pre-determine). A pre-bow will however help to minimize your warpage even more.
Let's say, theoretically, that you put too much pre-bow in the frame and wind up creating a bow in the opposite direction of that which you were trying to prevent. As long as you don't kink or wrinkle the tubing in the process, it's no big deal. Just put some heat on the welded side to pull out the bow in the same manner as we discussed earlier. If you mess up and put too much heat in and create a bow in the opposite direction, than just heat the other side until it straightens.
Naturally you want to avoid putting excessive amounts of heat into the tubing as it may wind up becoming brittle or warp in the wrong direction. Just remember to not let the metal become bright orange or yellow while heat shrinking. Also, you can't keep shrinking the same spot over and over. That won't work.
You'll see how it works as you go.
You mentioned stopping every so often and removing the tension to see if the trailer is shrinking. I don't recommend removing the tension until you're finished. Releasing the tension will just allow the metal the freedom to shink in the way you don't want it to go and will make your job take a lot longer to complete.
Oh, by the way, you'll find that not only does the metal shink to form a bow, it will also shrink lengthwise, especially if you use water to force it straight as we discussed before. You trailer may wind up being a half inch shorter than it started out. A while back I was making large pipe racks similar to the design of your trailer, but they were 60' long and the longitudinal frames were 3 side by side 36" x 300# I beams. By the time we finished welding and staightened out the 6" deep bow, the whole thing shrank in length 1 1/4"!
I believe the only way to prevent the warpage from happening all together is to apply vibratory stress relief while welding. This is just hearsay though as I've never seen it done.
Have any of you knowledgeble folks reading along used vibratory stress relief? Does it work like the salesmen claim?
Anyway, good luck and don't sweat it too much. It's not as difficult as it seems.
Tim
From what I could see on your pics, the distortion is not too severe. I assume you will be placing some kind of sides on your trailer? You may try using these to "pull" the deck streighter.
You will in any case tend to get a natural bowing in the opposite direction when you have the trailer loaded up, because the wheels will support it in the middle with some of the load to the rear of the trailer. I am not suggesting that it will take out all of the current bowing, but it may just be that you have a much smaller problem than it presently appears to be.
Regards
Niekie Jooste
niekie has a point there....the axles will be in the middle of the trailer, and since it's a car trailer, the load will be closer to the front and the back. what that means is, your trailer will be pre-bent in the correct direction for hauling a car. if you hauled anything where the pressure point is distributed all along the length of the trailer that would be ok too. but if you heavily loaded the trailer in the middle where the bow is the deepest, that wouldn't be the best thing to do. so, maybe this whole dilemma isn't so bad after all...it could be to your benefit.
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