Although that much torque was able to bend the spring plate, the u-bolts still saw that much torque in them.
The u-bolts are made out of the weakest steel, so use the torque value as shown in that chart…
Old, old post but since this came up in a search I just did to confirm a spec I thought “posterity” could benefit a little clarification.
Ubolts aren’t ‘made out of the weakest steel’ as much as they are made with steel of the appropriate alloy for the job. They are designed to provide consistent clamping force of the spring(s) to the spring perch. Torqued properly the ubolts ‘stretch’ but not to ‘yield’ - yield meaning they have stretched too far to return to manufactured condition when the nuts are loosened.
Nevertheless, the supplier at a trailer dealer I worked for and others from automotive applications described ubolts as single-use items- install properly, once, replace them with new if ever removed. Of course in practice “we” know that most often doesn’t happen.
The ubolts OP provided the picture of do not appear to me to be correct, for one thing. Secondly, 80ftlbs is way too much torque which ruined the plate. Third, those ‘appear’ to be 1/2” ubolts from the pic which are unnecessarily large for a 1000# spring.
EDIT:
Turns out the U-bolts were 9/16", not 1/2"
Which again are huge…in this application
60-65ftlbs is an acceptable range for 1/2”ubolts on a trailer axle but 30lbs is not. Although I suppose one might speculate that’s ‘enough’ for these oversized 9/16 ubolts on these light springs, I’d rather use a smaller ubolt torqued properly- guessing 3/8 would be appropriate? Bigger is not always better.
Note that regular bolt torque specs do not specifically apply to ubolts- a standard grade 5 bolt torque spec
may be close, but ubolts traditionally are a specific alloy to be ‘tough’ yet still be bendable without weakening as well as allow the threads to be rolled into the rod stock.
Every ubolt can be overtightened which causes either the ubolt to yield, the nut to yield, or both. Even proper installation introduces a bit of yield. This is often why you can begin to loosen the nuts on otherwise visually unharmed ubolts and then a few turns or less later the nut stops turning or becomes very difficult to turn further: the nut stretched along with the ubolt threads and once you move it ‘down’ it binds on the unstretched original threads.
