U-Bolts... To re-use or not to re-use?

[HunterA]

U-bolts for auto and commercial vehicles, that hold and align the leaf springs. The general consensus is not to re-use u-bolts. There are a few common reasons, but I fail to see how any are valid.

One of the most common is the torque to yield idea that the u-bolt stretches but deforms and cannot be re-stretched, and is therefore garbage when undone. My thought is that wheel studs get really abused and re-stretched all the time, and no one bats an eye when they re-install tires on the old studs etc.

The other is the thread deformation concern that says the deep nuts used on a u-bolt deform the threads when torqued to spec. Upon undoing the nut, the u-bolt is again garbage. I find that theory a little hazy, as wheel studs again have deep threads. A commercial dually tire system called Budd wheel studs, have an inner nut that is also the stud for the outer wheel. These inner nuts have very deep thread engagement. Torqued to a good 550 lbs.

So for a 1.25" U-Bolt, I find it difficult to not re-use it, provided the threads are clean and not corroded away.

Thoughts?

 

[tygerdawg]

NOT an expert, but...what does the vehicle's service manual state is proper procedure? THAT's the authority in this question.

Wheel studs are probably a different material than u-bolts. Specially heat-treated for higher loads and multiple loadings on a safety-critical application. It could really ruin your day if your wheel studs broke. Case in point is an Isuzu Trooper II that I owned years ago. A very problematic vehicle. Starting with 3rd tire rotation, we began snapping off all the studs. The dealer blamed the tire store's heavy handed monkeys and their impact wrenches. After my engineering investigation, it turned out that the Isuzu stud supplier screwed up and produced a bad lot of heat treated parts. The Isuzu Regional Guy started replacing everything for free because he smelled "class action lawsuit".

If the u-bolts are "cheap enough", then why take the chance? What would happen if they DID fail?

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[CoryPad]

The recommendation to replace u-bolts is based on conservative engineering principles. The bent shape of the bolts, the flexible joint, means there is more possibility for damage and preload variation.

Regarding steel and heat treating, in general u-bolts and wheel studs are similar.

 

[gerhardl]

Agree with both above. Main difference is that forces on the U-bolts are different and could be more complex and more likely to be unforeseen (torque, bending, stretching, corrosion (winter, salt) etc) and above constructional limits.

Seen apart from rules and safety regulations is the great question: what could be the consequences if a re-used unit failed? Perhaps less than for an airplane or U-boat, but anyway .......