Ubolts under torqued

Thanks Landphil.
That is exactly the post that prompted that part of my question.

>So, I'm interpreting that to mean you would need less "torque" with a lubricant to achieve the same amount of "fastener tension" . Right?

>OK, how much less?

Is it proportioal to the numbers that Nat posted as an example?:
Torqued dry: 107 ft/lbs
Torqued lubed: 76 ft/lbs

(are these maximum loads for Grade 8 14mm bolts??)

 

Metric fasteners are not on the same class/grade system as SAE.

Grade 5 = Class 8.8 (metric)

Grade 8 = Class 10.9 (metric)

I posted the values for a Grade 5/Class 8.8 14mm fastener.

The grade 8 torque values are quite higher, without knowing for certain what the u-bolts were made with, I would stay with the lower torque.

 

I have seen and repaired many loose u-bolts ( probably well into into the hundreds of cases) and resulting damage in my trade as a truck tech, but can honestly say I've never seen a broken u-bolt, other than in a major collision where anything and everything breaks. Not saying to overtorque, but I do torque to the top end of spec if there is a range given, and in the real world, it proves reliable.

 

I need to check mine. I loaned out my torque wrench.

Nice debate. Not do sure a u bolt can be rated like a regular bolt. I seem to remember the length being a factor. I'm a civil eng. when we bolt together bridges we spec put tension. We have to test each fastener in a skid more to determine the torque. The first five is cool. Then it gets boring in a hurry.

 

37 is definitely insufficient clamping force. I pretty much never dry thread anything critical. Depending on the application, everything either gets the appropriate loctite or anti-seize, or in some cases a locking nut.

Over torquing can actually cause a reduction in clamping force, and produce permanent damage to the nut/bolt. I would stick to 75ft/lbs.

 

I took my 07 TRD Sport in to have the repaired leafs retorqued after 2 weeks of running around. I asked the Tech what he was going to torque the bolts to, he told me he was going to 90 then retorque to 120, this is a business that does leaf springs for their bread and butter I told theTechnician that the trucks are coming of the line at 37 ft.lbs he just laughed and shook his head.
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Note : had a extra leaf added each side

 

Now wait a sec here guys, I saw this link to a comparison 2012 test among all the top mid-size trucks:

http://special-reports.pickuptrucks.com/2012/01/2012-midsize-shootout-track-day.html

The Taco comes out on top, and there's no "re-torquing of the u-bolts" caveat. So what the hell? This is a tempest in a teacup.

 

Im glad im not the only one that was... WTF'ed about the odd torque specs mine are same as wheel studs 89

 

This is true for critical industrial applications, but then you'd be using thread lubricant to reduce rundown torque to a minimum. The torque specified in the service manual (when written correctly) are meant to be used "dry" and already account for rundown torque.

Breakaway torque occurs mostly during loosening only, due to the wedge profile of screw threads. That's why the loosening torque will typically be higher than what the bolt was tightened to.

For our "imprecise" purposes, it is quite valid to just check the torque by trying to tighten the bolt with a torque wrench set at the desired value.

This is true only if the parts remain elastic. U-bolts are apparently torqued to the yield point. That's why it isn't recommended to re-use U-bolts. It's also the reason for checking the torque later, as the U-bolt may have stretched plastically over time.

 

My u-bolts came loose after I had the TSB leafs installed a while ago. After this discovery I torqued them proper with a torque wrench and I always visually inspect them when getting fuel.

 

You definition of "Breakaway Torque" is incorrect. It is applicable for loosening and tightening any bolt or nut.

In physics and mechanics, torque is rotational force, or the force necessary to cause an object to rotate around an axis. In most cases, more torque is needed to start the rotational motion than is needed to keep it going once it has begun. This initial force is called breakaway torque.

Torque is also measured on clean dry threads. How clean and dry are your threads after a few hundred or thousand miles?

 

What you are referring to is the difference between static- and dynamic friction. This is important for designing bearings and leadscrews. However, for a threaded fastener, the force required to run the nut down to the base of the bolt is trivial. Torquing a threaded fastener can be considered quasi-static - the torquing action doesn't start until the last 1/4 turn or so, after the nut has already contacted the clamping surface.

Keep in mind we are not building race engines here. For leaf spring U-bolts, torque values between 73-100 ft-lbs all work. It just needs enough to clamp the leafs to the axle, so the leaf centering pin doesn't take all the shear load (which would otherwise cause the leafs to crack at the pin hole). Hell, the new U-bolts that came with my All-Pro flip kit were larger diameter than the OEM U-bolts, so in theory they should be torqued higher, but I didn't bother.

When I removed my U-bolts after two winters, the portion of the threads in contact with the nut was like-new. Same observation with the wheel studs.

 

No. That isn't what he said.

" Breakaway torque occurs mostly during loosening only, due to the wedge profile of screw threads. That's why the loosening torque will typically be higher than what the bolt was tightened to. "

I was explaining that breakaway torque is equally applicable in both tightening and loosening fasteners.

I am not loosening anything unless I want to ensure the torque is correct. Loosen, Clean, torque to spec.

 

Torque is the force required to cause something to rotate on an axis, period. Torque applies even if you are just running the nut onto the bolt by hand. The only difference is when you reach the final torque stage you stop when the amount of force required to rotate the nut is equal to the desired specification, be it in lbs or ft lbs. There is no such thing as " Static or Quasi-Static" Torque. Static = not moving there for it cannot be defined as torque. I don't know WTF Quasi-Static means. I don't think it is even a real term.

I understand that the threads inside are clean, but the threads immediately on either side are not clean and will usually effect the breakaway torque needed to either loosen or tighten.

I'm not trying to argue the facts. I only wanted to clear up the many misconceptions in this thread about how to ensure the U Bolts are properly torqued. I do agree that it should be well over the 37 ft lbs that was quoted earlier. However I'm sure some will keep doing it wrong, they always have. Myself if I am going to go through the trouble of checking I would just remove the nuts clean the threads and torque them at 30%, 70% and 100% of the rated value for the bolt. Then I Know for a Fact and I'm not just guessing.

 

OK, today i finally got around to digging out my torque wrench and checking them on my 5 month old 2012.
I set it to 40#, and I could turn all 8 nuts without much effort!
Them set it to 50# and the same, then torqued to 70#.
Called it good.

So, are the factory guys all doing it wrong? Or are each of them actually reading the technical service manual with same typo of 37# vs 73#?

 

If I hit the ubolts with PB Blaster does that get them into the "Lubed" catagory?

 

I did mine dry so as not to confound the issue.
Then again, these were relatively new and have not yet had time to rust.

 

Having been tipped off by this most informative thread (thanks OP, 4WD) I torqued my U-bolts to 70 lb/ft. I don't know how loose they were but they turned real easy and I had to turn them a lot before they clicked. I would not be surprised if they were less than 40. Did they come that way from the factory or did they loosen due to break-in/stretching? I got a feeling they did loosen over time. I have over 100k on this '07 2nd gen.

My question is should I torque it even higher? Should I go to 90? Who here that has a 2nd gen has torqued higher than 70 lb/ft? I don't want to take a chance on busting something.