U Bolt Thread Failure

What diameter, pitch, and class of fit are they?

Dia of the U bolt is 20.2 and the thread size is M22*1.5

Sounds like you need to do a better failure analysis of the threads of your U bolt first.

I also wonder if you might have counterfeit material. How do you know that this material is Boron steel?

The material is tested and found ok.

I posted three distinct sentences. One of these sentences is a question that starts with the word "How". You have not answered this question. Did you at all review the link I provided that shows some of the types of thread failures? Who am I kidding. If you need to have the word "how" explained then I'm wasting my time.

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The material used is SAE15B41H and is chemically tested and the results matches with the supplier TC.

Length of thread ?

Material of nut ? Mechanical properties ?

Units of the torque ? Nm ? ftlb ?

Threaded fastener failure analysis

According to some computations I made there could be 4 reasons:

- the steel has not the right hardness

- the number of engaged threads is too small

- the manufacturing tolerances are too big.

- the torque was computed for a friction coefficient > than the one you obtain at assembly due to used lubrication (grease). I was confronted with such a case several years ago. The introduction of a new grease with MoS2 lead to a series of failures at assembly for exactly same part as you mention. However the failures were not the thread shear.

A good thread with your specification should accept the mentioned torque (assumed to be in Nm since you did not answer the questions) if the friction equivalent coefficient is over 0.14 and at least 8 threads are engaged. This is valid for correct cut threads.

It would be good to have a picture of the part and of the nut.