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Check your U-bolts!!

i had aluminum on an older truck
but now i have steal and they are bolted
maybe just bad center pins?
i hope you get it figured out
 
I have NEVER seen a stock 9/16" u-bolt on a 1973-1987('91). Only cheesy aftermarket 9/16" u-bolts.

I never torque u-bolts. Not on my own stuff, not on customer's stuff. I have never had one come loose. I also reuse u-bolts all the time.

Martin
 
82355 said:
I have NEVER seen a stock 9/16" u-bolt on a 1973-1987('91). Only cheesy aftermarket 9/16" u-bolts.

I never torque u-bolts. Not on my own stuff, not on customer's stuff. I have never had one come loose. I also reuse u-bolts all the time.

Martin
Click to expand...

By "never torque", you mean you just crank the hell out of them, or impact them?
 
For me -

Good compressor
Quality air gun
Grade 8 u-bolts

Tighten them until the gun is red in the face!

Knock on wood, I also have not had issues with u-bolts loosening up.
 
So if you don't exceed the elastic limit of the fasteners, (and without external factors like heat) you shouldn't have issues, right?

5/8-18 are the u-bolts? GM spec is 140ft lbs? http://www.americanboltcorp.com/tech/techtorque.pdf shows a suggested value of 240 ft lbs dry for grade 8. At first glance that would seem to indicate that one is nowhere near any limit of the fastener, however it would seem to make an unopinionated decision, you'd need to know what kind of steel the u-bolts are to determine if re-use is safe or not. No?

As a comparison, the 7/16" wheel studs are very, very close to the "maximum" normally used for that fastener diameter, per GM lug nut torque spec.

I have a vested interest in not having to replace u-bolts, but just because people (including myself) have gotten away with certain things for long periods, without issue, doesn't mean it's good practice.

I have never bought new u-bolts, and I've hit my re-used ones with the same torque wrench over their life. Thus far I've never found one loose, but that doesn't indicate they are or are not near failure, either.
 
nvrenuf said:
For me -

Good compressor
Quality air gun
Grade 8 u-bolts

Tighten them until the gun is red in the face!

Knock on wood, I also have not had issues with u-bolts loosening up.
Click to expand...



x2:thumb:


Then I get out the breaker bar to make sure.
 
Hammer them with the impact till they won't turn anymore is my u-bolt spec. Along with every other mechanic I have ever known.

Martin
 
Ouch, I use a torque wrench and set them to appropriate torque in a cross pattern. Then I re-torque them after ever run until they stop moving (usually takes 3 or 4 runs until they stop stretching).
 
dyeager535 said:
So if you don't exceed the elastic limit of the fasteners, (and without external factors like heat) you shouldn't have issues, right?

5/8-18 are the u-bolts? GM spec is 140ft lbs? http://www.americanboltcorp.com/tech/techtorque.pdf shows a suggested value of 240 ft lbs dry for grade 8. At first glance that would seem to indicate that one is nowhere near any limit of the fastener, however it would seem to make an unopinionated decision, you'd need to know what kind of steel the u-bolts are to determine if re-use is safe or not. No?

As a comparison, the 7/16" wheel studs are very, very close to the "maximum" normally used for that fastener diameter, per GM lug nut torque spec.

I have a vested interest in not having to replace u-bolts, but just because people (including myself) have gotten away with certain things for long periods, without issue, doesn't mean it's good practice.

I have never bought new u-bolts, and I've hit my re-used ones with the same torque wrench over their life. Thus far I've never found one loose, but that doesn't indicate they are or are not near failure, either.
Click to expand...
That video I posted fell short in the full explanation. :doah:

To safely re-use them you need to know the stress applied (related to the torque applied). Divide the force applied by the cross sectional area.

If that stress is below the Yield limit for the material, you can load/unload the material multiple times without issue until you reach a fatigue limit (typically in the thousands if not millions of cycles).

If you exceed the Yield limit, you can safely reuse the material a certain number of times, but it may not be predictable without precise testing. Each time the material is reused, the stress/strain curve shifts over and you accumulate a residual stress that is added to the stress applied when the bolt is re-torqued (the failure point doesn't shift over however). You can safely reuse the material until the residual stress and stress from re-torquing hit a certain point in the stress strain curve and the part begins to fail.

Generally if you stay in the "Yield" area you are OK. Most manufacturers don't want to be responsible for failures and recommend replacement every time to avoid any risk of failure. Add to that the normal imperfections in all engineering materials and you are taking a risk of failure every time you reuse a torque to yield fastener.
 
Seen a few guys with axles coming loose like this or whatever, but has anyone ever seen the actual u-bolt snapped? I know I haven't yet.

I've reused u-bolts a LOT, never put a torque wrench on them, I tighten them down till they are crazy tight and yes, I do go back a second time a few days after driving to check them again, but have never had any type of failure with them.

Lifted and lowered my old '77 Datsun pick up several times, I'd get tired of the lowered ride, raise it back up, then want it lowered again a few months later... (back in the younger years) had the ones on the Suburban off and on several times for mods to the suspension as well,, still,, never had any failure or breakage of the u-bolt.

Just interesting to see all the arguments about this that comes up from time to time.
 
From my other thread:

IMAG1596.jpg


I didnt build the truck, but I'll check everything now!
 
and what out of the ordinary, extreme force did you put on them to cause them to move, hard to tell in that pic, but I didn't see any actual broken u-bolt material. Looked stretched and maybe pulled threads.
 
4xcrazy said:
and what out of the ordinary, extreme force did you put on them to cause them to move, hard to tell in that pic, but I didn't see any actual broken u-bolt material. Looked stretched and maybe pulled threads.
Click to expand...

I was just slowly driving around on some dirt piles. The nuts came loose, probably because they were not properly tightened by the kid that installed the lift, and the truck was never actually used off road...
 
I used to have problems with u-bolts coming loose all the time, back when I used the hammer them on with an impact wrench method. I haven't had any issues since I started using a torque wrench. I tried everything before I figured out the real problem (u-bolt stretch). I used nylock nuts, jam nuts and I couldn't figure out why they kept coming loose until I did some Google searching. Now I just use a regular nut and washer.

I don't think normal driving is going to make them stretch, it's flexing the suspension while rock crawling that puts the real stress on the u-bolts.
 
4xcrazy said:
Just interesting to see all the arguments about this that comes up from time to time.
Click to expand...

I have not calculated if 140 ft-lb comes anywhere close to yield on a grade 8 ubolt. But, I am willing to bet it doesn't. 240 ft-lb would be closer to that.
 
Martin - It's certainly possible that both my trucks had been tampered with by previous owners. But it does seem odd that both trucks would end up matching... :dunno:

Kenny - Nice job explaining stress, strain, and torque-to-yield applications. :thumb:
Do we have any sources indicating that U-bolts are indeed a TTY application? It's an assumption that I haven't seen verified yet (and I'm not sure either). :dunno:

Edit (after seeing the above post): If we're betting that the loading doesn't exceed yielding stress, why would the bolts run into yielding problems?
 
Actually, I have over torqued a set of TTY head bolts once. The instructions specify torque to a specified setting. Then, turn a set number of turns. I went above the predetermined setting and could not reach the value I set my torque wrench to.

What happens when you reach the yield point is the torque value becomes unpredictable due to the stretching material. If u-bolts were torque to yield, running them down with an impact wrench would just twist them off because the material would continue to yield, then strain harden, neck down, and fail.
 
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