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The U bolt dilemma

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Old 03-04-2013, 01:24 AM   #1
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The U bolt dilemma

Found this and Imwanted to share it here.
The U-bolt Dilemna
Posted on The wagon way website
By Tony K


To re-use leaf spring u-bolts or not? That is the question.

Seems like the most common answer is to never re-use but I want to know why. Seems to be a lot of opinion in this area but opinions are of little help, facts are what I am interested in.

I have never broken a u-bolt during the tightening process or found one of the nuts loose when taking them apart. And there are plenty of bolts on our rigs that are tightened much more frequently than the u-bolts and we don’t throw them away. Just how many times have you torqued the lugnuts on your rig? Probably a couple hundred times now including numerous times by the tire shop with a carelessly used impact gun all with no ill effects. So why should I throw out my u-bolts after tightening them once?

The always replace them crowd seems to be led by the aftermarket manufacturers. No surprise there as they make money when you buy parts. But many of their reasons for replacement seems more sales pitch than technical know how.

-How U-bolts Work-

In a nutshell u-bolts are used to hold the axle to the springs on our wagons. It is designed to prevent the spring from moving in anyway where the axle contacts the spring. When tightened correctly the tension on the bolt does just this. The u-bolt is always in tension and there is no cyclic loading on the fastener which would lead to failure. To help the u-bolt maintain this tension, longer nuts are used which are sometimes referred to as Hi nuts or deep nuts. Hi nuts are used primarily because more threads provides a better holding force on the bolt with less likelyhood of nut thread failure. Longer nuts mean better torque retention and torque is what determines how much clamping force the u-bolt provides.

U-bolts inherently are not designed to stretch nor is it part of the tightening sequence. While there are bolts designed to do this, aka TTY or Torque to Yield fasteners, u-bolts are not one of them. They are held by the amount of torque used on the nut not the amount of stretch you get during the initial install. And you should never use an impact gun when installing u-bolts.

For economic and other reasons the threads on u-bolts are rolled. Which, it can be argued, may actually be stronger than a cut thread due to metalurgical reasons that are beyond the point of this write up. Hi nuts on the other hand have cut threads because all nuts are made this way. Not because they are designed to cut into the u-bolt threads. Like everything I am sure there are exceptions.

For way more info about u-bolts go to this link, its a great explanation of u-bolts in general http://www.clampsinc.com/guidelines%20for%20ubolts.htm

And for a great layman’s explanation of bolt strength this is a good read. It also explains tensile and yield strength really well. http://www.mechanicsupport.com/artic...eStrength.html

-When Do You Have to Replace-

There are times when replacement is the only option. If there is apparent damage, heavy rust, nuts that required excessive force or heat to remove, are all reasons to replace the u-bolts. I would also recommend replacement if you are installing new springs as there is not much to gain by keeping your 20 year old u-bolts. And under no circumstance would I ever re-thread a u-bolt to try and save it, throw it away.

But if you have no trouble removing them for a repair that is not related to the u-bolt, why can’t you just re-use them? They are very large bolts and are not torqued all that much considering their size.

A quick look on the Internet didn’t come up with anything decisive. Forums are full of info, but short on facts. Even the Toyota Factory Service Manual doesn’t mention any need to replace the u-bolts during a repair or spring replacement. I need to call my local dealer and see what they say. And I have not found a reference from a single vehicle manufacturer that recommends to retorque the u-bolts on a new vehicle once it leaves the lot.

And I found it odd to read that most u-bolt makers recommend re-torquing but not re-using, whats the difference? And using the u-bolt makers argument about the nuts cutting into the u-bolt, re-torquing shouldn’t be an option as the threads have already been “damaged” (ie cutting into the u-bolt) by the initial tightening process. Tightening up damaged threads will only do more damage. This is the site that seems to be quoted without credit on numerous other sites http://www.lhrods.com/donotreusse.htm

One other example I found for replacement was the argument comparing what happens when you torque a new u-bolt with a lubricated nut and when you do the same with an old dry u-bolt with damaged threads. And of course a damaged thread won’t compare, that’s why you need to replace it…. Damaged dry threads will give you a faulty torque reading. I would like to see a comparison between a new and an undamaged used u-bolt. This link is the site that compares damaged to new. It’s a pdf file from Dayton Parts. http://www.plazafleetparts.com/uploa...100/ubolts.pdf

One other thing the manufacturers are saying is that if you had to remove a u-bolt for some reason one week after installing it you shouldn’t re-use it. To my thinking this would be absurd. But this is exactly what they are saying.

Does removing a nut from a u-bolt re-cut the threads, no. But it may feel like it is if the threads are truly damaged or jammed up with rust.

What seems to be “stretch” when you remove u-bolts may just be the tension from removing one side at a time. While most of us are careful to tighten both sides equally, during the removal this is often overlooked. Remove one nut and the other nut will seem to already be loose when we go to remove it. This has more to do with the u-bolt shifting than any so called stretch.

-In Conclusion (for Now)-

By no means do I think re-use is the only answer. Some special applications may actually require a u-bolt change but I have not found anything to support the idea that u-bolts are a use once then throw away item. Use some common sense and when in doubt, err on the side of caution and just replace them. And the always replace it crowd just may be a response to the lawsuit happy world we seem to have created. But if there is no apparent reason to, damage etc, I am going to do what I have always done in the past, re-use…
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Old 03-04-2013, 02:06 AM   #2
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I agree that rolled threads are stronger than cut threads. It's like being forged. The molecules get smashed together and the metal gets harder.
At work people always come by my shop and ask me to machine something out of a bolt. (Cause i hide all my round stock) The threaded part always seems case hardened. like with a hard shell and soft center, but if there is a shoulder or unthreaded body, it's way softer than the threads.
Not saying that its right or wrong to reuse u bolts (i reused mine ) but I know that bolts stretch. They can stretch enough to be measured with a steel rule in some cases. i've seen 3/8 inconel bolts go from 16 threads to 12 threads from re-using when the specs called out for replacement.
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Old 03-04-2013, 05:38 AM   #3
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There is no dilemma for me.... Always change them.

You've already spent money to FIX or upgrade whatever suspension components you've installed - then go the extra route and apply NEW hardware to hold it all together.

It's cheap...... and you don't have to buy them from online stores or big name shops. A local suspension shop will custom build ujoints for you in a matter of minutes and they're less expensive than anything you buy online.
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