Check your U-bolts!!

[Mastiff]

This happened to a buddy in his company supplied work truck driving down the freeway at 70mph . Didn't crash thank god. Truck was just two months old. Loose from the factory.

Warranty is a good thing in that case...

 

[rdn2blazer]

He ended up with a different also new work truck all together after that. I beleive it was repaired but he never saw that truck again.

 

[sreidmx]

Now I am not endorsing this but I have reused my front u bolts many times, they are fine thread 5/8 and I bring them down evenly and crank on them with the big impact, then I grab the 30 inch breaker like many other guys and tighten them further basically until you feel the nut stop willingly tighten up,

 

[4xcrazy]

I've had mine on and off a few times so far, tighten them up real good and retighten them a short time later.

I'm sure I could go out there and get them to move, as you say, but if they are already past their recommended torque setting, I'd quit tightening them before you start damaging the strength of the U-bolt or the nut/threads.

 

[Mastiff]

I've had mine on and off a few times so far, tighten them up real good and retighten them a short time later.

I'm sure I could go out there and get them to move, as you say, but if they are already past their recommended torque setting, I'd quit tightening them before you start damaging the strength of the U-bolt or the nut/threads.

The wrench is always on 150 ft-lbs, so I'm not torquing them _more_ in that sense.

 

[kennyw]

The wrench is always on 150 ft-lbs, so I'm not torquing them _more_ in that sense.

The first time you install the ubolts my thought is that they are not uniformly loaded/stressed due to friction resisting movement between parts and the uneven nature of not being able to torque 4 nuts simultaneously. During the first few drives, vibration and loading/unloading cycles help things move into a uniformly loaded/stressed condition. This results in a relaxation of torque.

Re-torquing back to spec should result in more even loading/stress state than the initial install. It still won't be perfect, but it should still be in the linear elastic region on the stress/strain curve and not add any additional stress than expected.

After the initial install and re-torquing, you start to introduce fatigue (cyclic loading/unloading over time) and creep (constant tension load over time). These residual stresses build up with time and skew/shift the stress/strain curve. You may see a reduction in torque over time due to these factors (it should take DECADES if the parts are designed and manufactured well). Re-torquing at this point increases the stress in the material above the initial design intent by a small percentage and moves you closer to failure but there should have been a factor of safety in the initial design that allows for this to a point. Corrosion should be an issue long before fatigue or creep even in a desert climate.

For u-bolts that can't remain at a relative nominal torque setting over a period of a couple months after being properly installed, I expect to be either made with inferior materials or contain manufacturing defects.

 

[Mastiff]

That all sounds reasonable, but then it seems pretty common for guys to go around and tighten stuff up, especially U-bolts, before or after every major outing. Stuff gets loose when you rattle it. Has anyone ever heard of a 5/8" U-bolt failing, like breaking? I really don't know, but those things seem pretty tough.

 

[kennyw]

Looking at pics of that beautiful yellow paint job would leave me to assume the most extreme duty your u-bolts see is the curbs at the mall.

The theory behind any bolt is that constant tension force will prevent the nut from rotating off when loaded statically. When something is sitting still or loaded in one direction that never changes it works well. However, subject it to vibrations and the nut will back drive on its own under the right conditions. Split ring lock washers have actually been proven to speed up the nut loosening with vibration over not using a lock washer at all.

Extreme wheelers that thrash their trucks off-road or drive fast on rough roads could easily see things loosen up as a result. Road only vehicles far less likely. For those guys I would recommend distorted thread lock nuts or other vibration resistant locking methods. If they still see u-bolts loosen after that I would suspect fatigue.

And there is a big difference between re-torquing because the nut loosened up from vibration vs. a case where tension was lost due to fatigue. That is the reason it is not recommended to re-use old u-bolts. You never know how much fatigue stress has accumulated until it fails.

You don't have aluminum wedges to correct your driveline angles do you? I have seen cases of cheap cast aluminum wedges compressing or splitting over time to cause u-bolts to loosen up.