| bnam |
Sat Jul 11, 2020 8:43 pm |
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I understand how bolts and nuts have max tightening torques based on their grade. A m8 gr 8.8 has approx 18ft-lbs. But, doesn't that apply only when the nut or the threaded hole is at least as strong as the bolt.
So, if an m8 gr8.8 bolt was being used on an aluminum or magnesium threaded hole, would the max recommended torque applied have to be reduced depending on the strength of the aluminum/magnesium?
Thanks! |
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| modok |
Sat Jul 11, 2020 9:53 pm |
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The stud/bolt tends to be the weaker part.
Because on the stud.... the root diameter of the thread is only so large, while the nut or threaded hole....the material around the threads can be as large as needed. Tho infinite would not be optimum, as this would reduce it's ability to flex. Actually the ideal would be similar to a slightly oversize nut.
a stud threaded into a softer material you will need a lot more thread engagement than if it were threaded into a steel nut.
Some of this can be accomplished by more threads, just threaded in deeper, but this has limits.
The stud stretches as load is applied so the first thread ends up handling the largest share of the load(35%?) , and the second thread less (20%?), and so on......maybe the sixth thread is basically doing almost nothing.
AT FIRST, but that may change.
The first thread needs to deform some amount in order to let the deeper threads have more share of the load. This can happen by wear, or it can happen by the internal threads deforming now, or perhaps deforming later.
IMO, this does happen. So using 2-3 nuts worth of engagement does help in practice, tho if you read technical papers on it you would think it would not help in simple theory.
If that first thread deforms too much it will sheer, and can have a cascade failure and peel all the threads out one after the other.
That all would be easy enough to calculate and figure out in a laboratory condition.....
What makes it worse tho, is aluminum and magnesium can have unpredictable fatigue life because these materials can have the properties to both age harden AND creep. Add in vibration, heat, cycling, and oxidation and it becomes more complex, and....no longer a question of will it hold or not, more a matter of how long under the real conditions.
And then what will happen?
It may relax so the "torque" is actually less, or it may pull out, or some of both.
So a 8x1.25 stud in magnesium CAN hold 18 ft-lb, right now, but, it's not going to stay that way forever. Not in an engine.
And threading it in deeper will not solve it, but is helpful.
When Gm designed the LS engine, they created 10x2 main studs. Extra coarse threads, and they are threaded in WAY deep. Very annoying that they invented a new thread, but I think it was a good design. Why they used the same bolts in an iron block versions I will never know :roll: |
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| Cusser |
Sun Jul 12, 2020 7:45 am |
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bnam wrote: I understand how bolts and nuts have max tightening torques based on their grade. A m8 gr 8.8 has approx 18ft-lbs. But, doesn't that apply only when the nut or the threaded hole is at least as strong as the bolt.
So, if an m8 gr8.8 bolt was being used on an aluminum or magnesium threaded hole, would the max recommended torque applied have to be reduced depending on the strength of the aluminum/magnesium?
Thanks!
On the VW, most 8mm studs, bolts into aluminum/magnesium get torqued to 14 ft lbs, and into steel get torqued to 18 ft lbs, like the six clutch pressure plate bolts. |
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| bnam |
Sun Jul 12, 2020 9:54 am |
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| What about m6 into aluminum (not fresh) what would be the recommended torque? |
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| modok |
Sun Jul 12, 2020 2:41 pm |
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| probably around 75-85 in-lb |
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