Stud Material & Manufacturing Questions

[Auspex]

Hi,
I am designing a bolted joint and I'd like to bounce a couple material selection and manufacturing questions off the knowledgable folks here.

Imagine I have a 50" stud with 3" of 1.75"-8 thread on each end. Due to geometry and space constraints the clamped member is not as stiff as I would like it to be. To reduce the magnitude of the cyclical loading on the threaded member I would like the center 42" of the stud necked down to a smaller diameter like 1.625" or smaller. I plan on preloading the stud to 185 kip and want to design for 1e6 cycle life. I am thinking the diameter I specify will depend on the strength and yield of the material as well as manufacturing spec that may be needed to aid in fatigue resistance. Depending on how narrow I dare go with the center section, I am estimating the stud will see a +15% load cycle (with a 1.625 center). The cool part is that the load cycle drops off exponentially as I reduce the center diameter.

I am assuming the stud is a custom component that I need to have manufactured. I am guessing a heat treated 4340 bar with rolled threads may be the ticket here, but that's about as far as my knowledge goes. Should I consider a different material? How hard should the material go, but still be able to roll these threads? Are there any other tricks I should employ on the center like heat treating, nitriding, etc? Keep in mind fatigue is the name of the game here....this is a high cost downtime application.

Thanks in advance!

 

[GregLocock]

Why not forge the stud?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Auspex]

Thanks Greg,
I was assuming I would start with a forged bar and machine or roll from there, but I am not that familiar with stud forging. Do you mean forging the bar complete and then rolling the threads? Is it a cold or hot forging process and what material properties could I expect that would aid in fatigue?
Thanks for the help!

 

[GregLocock]

Yes, forge the 'waisting' into the rod before you start forming the thread, don't cut metal in the loaded areas at all. I'm not a fatigue dude, but a forged stud with rolled threads would be a very good place to start for fatigue life.

"The cool part is that the load cycle drops off exponentially as I reduce the center diameter."

So your concern is that the misalignment or motion between the two ends will cause bending stresses in the threaded section?




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Auspex]

Thanks Greg. I had thought of rolling the radius leading down to the necked section or perhaps peening it, but forging the whole center sounds like a good option. Wonder if forging without machining or grinding would leave too rough of a surface finish...negatively affecting fatigue? Any recommendation for a forge shop with the capability to forge something like this?

As for the load cycling comment. I hope the L/D ratio and skinny center should circumvent bending stresses. My comment relates more to the ratio of stiffnesses between the tiebars and clamped member....the clamped section needs to be appreciably stiffer than the tensile section. The column stiffness of the material is proportional to the cross sectional area. As the diameter of the stud decreases, so does the resulting tensile cyclic loading in the stud, but at a nonlinear rate. In essence, you can get a lot of mileage out of reducing the stiffness of the tensile members, moving further below the infinite fatigue life curve.

 

[hydtools]

We made long studs with the center section turned to 90% of the minor thread diameter. The transition from the thread to waist was a compound arc like an elipse shape. By turning, the studs were straight and the threads concentric.

Alternatively, you could hot forge to upset the ends before turning and threading. Straightness may be a problem. The transition may have to be turned to clean it.

Sorry I don't remember the hardness range for good thread rolling. The thread roll manufacturer could help you.

Ted

 

[CoryPad]

I think your idea of 4340 bar is a good one.

Your part could be made with upset ends - this is common for automobile axle shafts that are near the size you are contemplating. You also could use radial forging. You might try FormTech in Royal Oak, MI - they are the largest independent forger in the USA:

http://www.formtech2.com

Regards,

Cory

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