Technical question - U-BOLT microscopic cracks on the tooth of the rolled thread

[Coury Ferguson]

<snip>The suggested methods of detection of surface cracks on threads has some practical difficulties:
1.How can we probe the narrow surface of the threads for ultrasonic and eddycurrent crack detection?Should we make special probes to match the thread profile?

Thanks,
V.J.Brahmaiah

The next method you might try would be Magnetic Particle. That would show the indication (s) on the thread, as long as the material is magnetic, in my opinion.

 

[brahmaiah]

The functional requirement of the u-bolt is rather critical.There is fluctuating load on the bolt continuously.A failure can be serious.
I think you have to prevent the defect by correcting the process inputs.

V.J.Brahmaiah

 

[Wes Bucey]

The functional requirement of the u-bolt is rather critical.There is fluctuating load on the bolt continuously.A failure can be serious.
I think you have to prevent the defect by correcting the process inputs.

V.J.Brahmaiah

Yes! Prevention is more important than detection on continuing basis. It may be acceptable to get some outside experts to help diagnose what part of process causes this condition. I would not put my faith in trying to inspect quality into the part.

 

[Umang Vidyarthi]

Hello Fridenson,

Any business dealing is a taboo on the cove. Yet you may send PM for any help in this regard.

Umang

 

[Umang Vidyarthi]

The suggested methods of detection of surface cracks on threads has some practical difficulties:
1.How can we probe the narrow surface of the threads for ultrasonic and eddycurrent crack detection?Should we make special probes to match the thread profile?

2.When hairline cracks are on the threads,How can we x-rax the threads?

3)But the last method suggested is special.

Could you please tell us more on above points?

Thanks,
V.J.Brahmaiah

1. The requisit probes are readily available. Just google to find suppliers in your locality.
2. X-Rays are capable of detecting hairline cracks.
3. Here is something on NDI with Eddy current.

Hope this helps

Following is answers to mr. V.J.Brahmaiah:

Material is SAE 5140 H
No heat treatment carried before rolling
Hardness value 16-18 RC
See exact location of crack on the thread in picture attached
Its U-BOLT for connecting Rear Axle of a vehicle to its leaf springs.2 U-BOLT holding each leaf spring total 4 U-BOLT per Axle.

Regards, Shlomo - Mechanical Engineer

First & foremost, can you tell us what method of thread rolling are you using?

The defects I have noticed from your post here comes under the category of 'folds', distinctly different from 'cavities'. Another pecularity is that all the threads have identical 'folds' of approx. equal dimension/shape and location. Very intresting and indicative that the defect is due to misalignment of thread tracks on the thread surface.

Now, one way of rolling external threads is to use two tools, id est, a fixed roll and a moving roll; or a pair of dies. A repeated copying of the profiles of the tools, forms the rolled thread. Surface thus extruded contacts one tool's surface and contacts other tool's surface after half a revolution. When metal is displaced from blank, thickening occurs on both sides, this deformation leads to defect at the crest of the thread.

Another point of importance is that the identical defects leads me to think about 'pitch error' and/or error of helix angle. Rolling of threads with these errors leads to forming of folds, since crest of one roll is offset vis-a-vis the other.

Hope this helps.

Umang