U bolt nut torque

[richfaa]

I do not see anything personal towards me or anyone else in you post. My quest for information and the willingness to share was apparently a bad idea in this case. It is something MOC members have done for the 8 years we have been on the forum. I will refrain or be more careful in the future. I guess it is a case of things change and I have not.

[Tom S.]

OK, here is my take on it from a shade tree mechanic perspective.

Dexter factory torques the u-bolts to 45 lbs with no weight on the axle or stress (other than that from torquing) on the u-bolts. Then Montana adds 12,000+ pounds to the u-bolts, and they stretch. So when we crawl under the trailers, especially if we have driven them for a while, the nuts are looser than the specifications call for.

Chances are, if Dexter had torqued them to 70 lbs to begin with, they might still have stretched, but the additional torque would have already pre-stretched them, perhaps enough to leave them within the 45 lb figure. Additionally, they get away with this because the springs are on top of the axle. I'm betting if they weren't, a lot more people would have experienced the same problem Rich had.

[bigred715]

Rich, your comments are ALWAYS welcome as far as I am concerned. You always seem to do a lot of research on some of these problems and most of us are not engineers or designers. Getting to the bottom of the problem is your goal and I for one appreciate your time and diligence in these matters.

[grayghost03]

Yep, Rich, any information you provide, that serves to get 5 pages of conversation and discussion is good information. Again thanks. And keep this kind of research going, it benefits all who read it, just by stimulating our thoughts.

[richfaa]

In my previous work life , aviation, data collection, verification of data and asking questions was a vital part of our job. usually asking one question provokes another and another question. The answer to this particular question is not yet answered as I learn more from folks posting here I have more question to ask of Dexter.

In may cases the answer is something we do not want to hear. At work I had a sign on my desk that said" Don't tell me anything I do not want to hear" That applies in all to many cases.. However the quest for the truth and facts is never ending.

[bncinwv]

quote:Originally posted by Tom S.

OK, here is my take on it from a shade tree mechanic perspective.

Dexter factory torques the u-bolts to 45 lbs with no weight on the axle or stress (other than that from torquing) on the u-bolts. Then Montana adds 12,000+ pounds to the u-bolts, and they stretch. So when we crawl under the trailers, especially if we have driven them for a while, the nuts are looser than the specifications call for.

Chances are, if Dexter had torqued them to 70 lbs to begin with, they might still have stretched, but the additional torque would have already pre-stretched them, perhaps enough to leave them within the 45 lb figure. Additionally, they get away with this because the springs are on top of the axle. I'm betting if they weren't, a lot more people would have experienced the same problem Rich had.

The biggest problem I am seeing here rests solely on Dexter and their ultimate responsibility. If what Tom says is even remotely correct, then as an engineer, I can only reach one conclusion. The design is not adequate. Torquing a nut and bolt, should not have to rely on any nuances related to bolt stretching, loaded vs, unloaded, the list goes on. If the bolt is subjected to stretching by merely adding the load that it is intended and hopefully designed to support, then my hillbilly injuneering brain tells me that the bolt design is inadequate! If a 1/2" bolt stretches then it should be replaced by a properly sized and analyzed bolt that would not be subject to stretching under the load that it is EXPECTED to carry. What the heck am I missing here??

Bingo

[richfaa]

Bingo you are a engineer and you are missing nothing.

[grayghost03]

Bingo, your missing the difference between torque and tension. Tension is the stretching of the bolt, this is good, this is what clamps pieces of material together, you will have elongation on a torqued bolt.

[bncinwv]

quote:Originally posted by grayghost03

Bingo, your missing the difference between torque and tension. Tension is the stretching of the bolt, this is good, this is what clamps pieces of material together, you will have elongation on a torqued bolt.

I understand the difference, what I am saying is that if the tension (or stretching) affects the torque and the initial elongation when the torque is applied, then there is a design flaw somewhere. The elongation is expected when the bolt is torqued, the elongation should not be exacerbated by the application of the normal load (the weight of the rig). Therefore, in my mind, either the pre-load torque is wrong, or the bolt design is wrong. This is not based on engineering, just my warped view of common sense which is usually biased by engineering.

Bingo

[bncinwv]

As a follow-up, I looked up the torque specs for the u-bolts on the rear suspension of our Chevy dually. Specifications call for 150 ft. lbs. Now, that struck me as odd since obviously the rear suspension of our truck should not carry the load that the rig suspension does? Something is not adding up here and all I will do is refer back to my original post about design.
Bingo

[Irlpguy]

I do not recall any suggestion that there have been actual failures of the U bolts themselves. Insufficient torque values seem to have been the problem identified, therefore the suggestion that we check the torque on our U bolts was made in the past and is valid.

Early on in this thread the statement was made that in order for the torque value to be correct you must lift the wheels off the ground before torquing them, also if you checked them when the unit was sitting on it's wheels the value would differ. Up to this point there has been "no" qualification of that statement from Dexter and had I or someone else not questioned that idea, then my feeling is that this idea would have been deemed the proper way to torque your U bolts, because 3 people at Dexter said so.

I am no engineer, but I have put a wrench to everything from chain saws to road building machinery and common sense tells me this was a statement made that was not substantiated in any way.

Axles with the springs mounted on top do "not" put the load on the U bolts, they are subjected to considerable lateral forces I am sure, but vertical load bearing is not one of those forces. The greatest amount of load bearing would be when the frame was lifted off the ground and the U bolts would then be carrying the weight of the axles and wheels and nothing else.

My conclusion on this topic is that the Dexter Manual may have been read incorrectly, prompting the question in the first place, and the "people in the know" perpetuated that misconception by also misreading a part of the Manual. They offered no substantive explanation nor could they put a value to the difference one would see in doing it one way as opposed to the other.

Here is my test performed this morning:

1) loosened the nuts on two U bolts on the same axle side with my level up supporting the weight of the trailer on that side.
2) I torqued those 4 nuts to a value of 60# and lowered the unit back onto it's wheels.
3) I rechecked the torque starting at 50# and worked my way up using the smallest increments on my torque wrench.
4) Since you cannot get the nut to actually move using only a pound or two increase it was not until 63 - 65# that I was able to move the nut.

Does that mean there is a difference in value by torquing one way as opposed to the other, perhaps, but it is so insignificant 99% of those checking the torque would not even notice it. Or could it simply be as I believe that it takes several additional pounds increase to actually tighten the nut.

I will perform this same test when my new axles are installed in the next couple of weeks, I expect to see the same result. I still have not heard back from Dexter.

[jlb27537]

Hello Group,

One thing we need to realize is the axle is UNDER the spring. The U-bolts do not carry any weight. They only keep the axle attached to the spring.

I also checked mine yesterday on my 2014 3150RL. The nuts on the U-bolts were a mixture of tightness. Some as low as 45ft lbs and some close to 70ft lbs.

I tightened all to 65ft lbs

[bncinwv]

65 pounds is correct for a 1/2" up to Case 3 hardness bolt. Guess I am going have to do like Irlpguy and jlb and get under the rig and check things out. Looks like it will turn into an annual maintenance list addition based on what all have been saying. Still curious as to what Dexter finally says regarding the procedure!
Bingo

[Irlpguy]

From Google Earth Street View today I could clearly see a phenomenon that I have not witnessed before.

On the grounds of Dexter Axle Corporate Headquarters there was a definite circular formation, the wagons were tightly arranged with many, many people inside the protection of the circle.

CSIS (Canadian Spy Agency) reported hearing over and over again, “don’t answer any of their questions, don't let them come near, those barbarians from the North have pointy sticks and will poke you in the eye if they get close enough”, also heard was “If you leave the protection of the circle and are caught off guard, you may be tied to a post and your feet will be held to a raging fire, if you talk we will all be up the creek”.

CSIS also reported hearing several young men state that they had all said the same thing and why oh why do these people want an explanation, surely our word is good enough, because we say it is.

“I have found a microphone, those bloody crackers from Montana and North have been listening to us, quiet everyone, we will have to use smoke n mirrors from now on”. That was the last heard by CSIS. Since then nothing but SILENCE…..

Some might say “oh that is all just crap” this guy watches too many Westerns. I say, HONESTLY folks I have heard nothing since they found the mic, only some smoke and flashes of light now coming from the mideast.

Hard not to love a good Mystery isn’t it.

[richfaa]

I could not stand it. As I mentioned one question spawns another. back to Dexter but Glenn was not in today and I want to talk to him since he is the Original source of the information. This post and other similar post got my attention.

"One thing we need to realize is the axle is UNDER the spring. The U-bolts do not carry any weight. They only keep the axle attached to the spring."

Again I have no education at all on the subject. I recalled that when weighing the rig at the rallies they weighed each wheel on the ground and there was weight , a lot of it, on each wheel. I have the weight document. So I wondered how does that weight relate if at all to the U bolt torque??? That weight is below the spring. I also wanted to make clear that the axle is under the spring. Were we on the same page on that. Does it make a difference if the spring is under the axle??

There is something not right here and my guess is due to my lack of education on the subject I asked the wrong question or was not specific enough. I will take a run over to the International shop where we had our School bus work done later and pick their brains on the subject.

[Irlpguy]

When I wrote my post last evening it was sort of tongue in cheek, let me explain.

I called my contact at Dexter asking only one question, just to keep things simple. I suggested he could respond by email which he did. At the moment I won't publish the exact question nor the response.

I followed this up with an email asking two more questions, which were then referred to someone in the engineering department to respond to. It appears they are prepared to make a response but must clear that response with someone superior and he/she was not available. I may or may not get that response on Monday. I am hoping someone will be brave enough to come out of the circle.

In the Dexter manual we all received with our Montana's starting on Page 65 and continuing to page 67 there are 4 "Caution" boxes, not one of those mentions U bolts.

Below the shaded/unshaded section "Suspension Fastener Torgue Values, a new topic begins. This is specific to and says "Worn spring eye bushings, sagging springs, or broken springs should be replaced using the following method.
1) Support the trailer with the wheels just off the ground.

This my friends is what Glen, his associates and my contact have all looked at, quoted and suggest is the reference to the torquing of the U bolts. Read the whole section and see if you can find anything that mentions U bolts other than: 3. Disassemble the U-bolts, nuts and tie plates.

My DW always says to me "why do you always have to question something that you think does not make sense". To that I reply "I do not accept everything that is said or printed as "gospel" and if someone makes a statement, then they must be able to provide me with a logical explanation for that statement or conclusion". I don't think that is asking too much.

[richfaa]

Glenn never referred to that section of the handbook. It was another who did. You are correct in reading the handbook which is why I asked the question of dexter in the first place because in reading that section I was not sure of the U bolt torque.Perhaps Dexter engineering can give the answer..BTW. Glenn is NOT a engineer.I asked.

My guess is that the difference will be in a over slung or under slung axle. Ours are over slung.

[grampachet]

If there are torque specs of 45-70 i am not going to worry about the recommendation of weight on or off the ground. Checking the torque is important but with the variable numbers recommended it leaves a lot of room for discussion. How could weight on or weight off vary as much as the reommendated specs? Just my .02.

[Tom S.]

quote:Originally posted by bncinwv

The biggest problem I am seeing here rests solely on Dexter and their ultimate responsibility. If what Tom says is even remotely correct, then as an engineer, I can only reach one conclusion. The design is not adequate. Torquing a nut and bolt, should not have to rely on any nuances related to bolt stretching, loaded vs, unloaded, the list goes on. If the bolt is subjected to stretching by merely adding the load that it is intended and hopefully designed to support, then my hillbilly injuneering brain tells me that the bolt design is inadequate! If a 1/2" bolt stretches then it should be replaced by a properly sized and analyzed bolt that would not be subject to stretching under the load that it is EXPECTED to carry. What the heck am I missing here??

Bingo

Bingo - All bolts stretch when enough torque is put on them. If they didn't stretch, they would break. It's one of the reasons why certain bolts aren't reused - because they have already been stretched. This (not reusing) especially applies to ubolts. A good examble of bolts stretching are head bolts. If you measured a new head bolt, and then pulled one out of your truck, you would find the used one longer than the new one.

[grayghost03]

Tom S. is right on the money. Also if you notice, most are fine thread, when you apply 70 ft/lbs of turn torque, you are putting up to 20 to 30,000 lbs of tension on a grade 5, 1/2" x 20. This deforms the threads, although not usually noticed by the eye, by backing off and reusing some bolts, you are actually slightly cross threading and the torque will no hold. Some bolts are different, when you get to plastic and elastic (not the material) things change. One example of reusable bolts that are torqued and reused are head bolts in our engines, although it is recommended, just like our u bolts to recheck torque after the first 500 to 1000 miles of use. Many of the newer engines have a head bolt that is called TTY(torque to yield) they stretch and then become solid, these should not be reused. A lot of information just to insure we use the proper fastener, but really important. Hence the interest in this thread.