Stripping threads on class 10 nut

[321GO]

[tt]I'm in a pickle...help!

I'm using a bolted connection which experiences thread stripping on the nut when reaching close to max torque value.
The bolt is a class 10.8 and the nut is class 10, to my knowledge both classes should match, right? 10 vs 10?

I've determined the torque from a generic torque sheet, of course for the corresponding pitch and friction value's present, 600Nm to be precise.

From my understanding bolts and nuts threads are designed to not shear, i.e. the bolt shank should fail before stripping, right?

How can this still be happening? [/tt]

 

[Screwman1]

I think that your bolt is likely a class 10.9....
The ISO fastener standards are designed that the system will allow the bolt to be tightened to it's proof load if both classes match, without any damage to either the bolt or nut. From your results, I am guessing that you have added lubricant to the threads prior to torquing up the parts. Most torque tables are designed for dry bolts and lubing them can easily double the clamp load.
The ISO nut heights are designed so that you will get bolt breakage at least 10% of the time, but other than that the failure modes can be a mixed bag, so what you are seeing may be a normal failure mode.

 

[321GO]

[tt]Thanks for the swift reply!

I don't fully understand the 10% remark. I mean shouldn't bolt breakage be normal? The heights are designed for this right?

Does this basically count for all nut designs? Because the one used is a nyloc nut which seems rather low in height.

p.s. no lubrication was used for sure![/tt]

 

[dgallup]

Is your nut from a reputable supplier? There are a lot of cheap fasteners out there that have class markings but made out of junk.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[321GO]

[tt]I also have doubts regarding that. I will check the hardness to be sure.

Also these are the nut specs, it also clearly states 'low type'. And now i'm wondering if this 'low type' still has enough thread length as to make sure bolt failure occurs first?

DIN 985
In compliance with ISO 10511
Low type
With polyamide lock[/tt]

 

[TVP]

DIN 985 is for stainless steel nuts, not carbon steel nuts that are quenched and tempered to Property Class 10.

 

[MintJulep]

Is it a half height "sheer nut"?

For some reason nylock type nuts are sometimes supplied waxed, drastically lowering friction.

 

[MintJulep]

See notes at th bottom of the table.

http://www.crestindustrial.com/prevailing-torque-lock-nut-values---metric.html

 

[Screwman1]

I was referencing back to ISO 898-6 (Mechanical Properties of nuts with specified proof load values). This spec requires that the nut height be a minimum of 0.5D.
The old nut specs had a higher height that would assure bolt failure, but they have been reduced to save materil and cost so that in 898-6 para 3.1 it describes the 10% bolt breakage goal.
If your nut is stainless it can't be a class 10. that requires carbon or alloy steel. If it is low height, it will not fail the bolt, that requires a standard height nut.

 

[MintJulep]

If you picked your torque from a "generic torque chart" that may or may not state what K factor was used....

The calculations in your chart might use K = .2 or even .3 since those are "handbook values" for at least some handbooks and some nuts.

But if you have waxed nuts K may be as low a .1 so your preloads could be two or three times higher than what your think.

 

[321GO]

[tt]Feedback time!

"Is your nut from a reputable supplier? There are a lot of cheap fasteners out there that have class markings but made out of junk"

I had my doubts also so it's hardness was tested and it was within the specified range, albeit on the low side, but still

"DIN 985 is for stainless steel nuts, not carbon steel nuts that are quenched and tempered to Property Class 10"

Maybe but it is definately a class 10 that is zinc coated

"Is it a half height "sheer nut"?

For some reason nylock type nuts are sometimes supplied waxed, drastically lowering friction"

I don't know what you mean by shear nut, but it is a 'normal' nyloc nut (blue insert).

I'm also fairly certain it isn't waxed. Why? Because it's a 'low budget' nut and also from what i've read is that the waxing is mostly done on stainless steel nuts, but i could be wrong here.

----------

DIN 985 cals it a low profile nut for some reason but min. thread height is still 14mm well above 0.5D as i see it. So bolt shank should fail eventhough it's a 'low profile', right?

"The old nut specs had a higher height that would assure bolt failure, but they have been reduced to save materil and cost so that in 898-6 para 3.1 it describes the 10% bolt breakage goal"

How should this be intepreted? To my knowledge torque table value's create 90% yield stress(combined tensile & torque). Up till this point nuts are safe even when over torqing till shank breakage, right?

Under the 'new' guidlines nuts are only safe up to this 90% point but not beyond?


Thanks a bucket
[/tt]

 

[LittleInch]

Am I the only one to question where 600 Nm came from? what size bolts are you using? I've looked up a couple of referecnes for standard bolt sizes just to see where that could come from and 600Nm doesn't come up. If you're exceeding the bolt strenght or getting that close to yoield then any failure could happen, especially if your bolts are actually 10.9, but sold as 10.8

However there does seem to be some sort of incompatability between your info, the comments and what you actually have in your hand. Low type is only just 0.5 D of thread...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[321GO]

[tt]LittleInch, 600Nm is for M20x1.5 (fine thread) class 10.9 (10.8 was my wrong)

I think that's correct though.

Thanks![/tt]

 

[LittleInch]

OK - I've seen everything from 550 to 620 Nm for the same bolt grade and size though.... Even a little bit of grease can make 10% difference though so maybe you're just tightening them too much??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[1gibson]

Are your surfaces parallel and flat? Have you tightened the same fasteners with a flat plate in between them and had the same failure?

 

[321GO]

"OK - I've seen everything from 550 to 620 Nm for the same bolt grade and size though.... Even a little bit of grease can make 10% difference though so maybe you're just tightening them too much??"

I think this has to do why this is happening indeed

"Are your surfaces parallel and flat? Have you tightened the same fasteners with a flat plate in between them and had the same failure?"

I know this is important, but they are parallel so in this case it's not the case

-----------
After further investigation i can report the following...

1) actually the bolt was a stronger class than 10.9 as assumed. The nut was as such basicaly 'underclassed' for the bolt material

2) the designation on the nut was between two lines like this |10| and after reading into the matter seems to be done so that one can clearly indicate it as an old 'DIN' design with a lower nut height. Newer 'ISO' nut's are a little higher and as such have a slightly higer proof load (1040 MPA instead of 1000 MPA if i'm not mistaken). Above seems to only apply to coarse threads though and since mine is fine threaded this does not apply. I'm still not 100% clear why the ISO nuts have an increased proof load though, for an class 8 nut this increase is pretty substantial (940MPA instead of 800MPA or something like that).
I mean the old DIN nut heights to my knowledge were actually designed to prevent thread stripping, so why the increase in heights? This I really don't get at the moment. Was it because the old DIN heights were simply too low???

3) since the nut was a class |10| acording to it's hardness and still stripped i'm asuming the bolt endured 1000MPA stress

So basically the problem was the 'underclassing' of the nut for the bolt. The only thing that still strange is that the nut stripped at a torque which 'should' be correct for a class |10| according to different torque tables (for zinc plated nuts that is).

The only conclusion that i can come up with is that indeed the nut as being a nyloc nut (not stainless though) would of been 'waxed' - as pointed out by others - and as such would have a much lower friction coefficient than the initially assumed 0.125 for a zinc plated nut

We now have chosen to torque it as a (zinc coated) class 8 nut, because class 8 pre-load is actually sufficient for the application. In reality the pre-load will probably be higher that that for a class 8 due to the presumably lower friction coefficient value but since this torque value is much lower that the initial 600Nm there is no real risk of thread stripping, which seems a good compromise in this case.

Thanks

 

[Tmoose]

Just to muddy the water, are you taking the prevailing torque of the nylon insert into account with the "new" torque and preload?

 

[321GO]

Kind of.. torque value's are about 20Nm above table value.

It seemed really abc at first but this thing backfired on me and I did not expect it to be this 'difficult'

Good leason learned though.

 

[Screwman1]

A very quick and dirty way to get close to 75% of yield (of the weakest component) is to simply torque several samples to failure and then use 65% of the average failure torque as your seating torque. this will put you quite close to 75% of yield of the weakest component in the joint.

 

[321GO]

Nice tip, thanks.
Actually that's similar to what we did. We tested till failure and then used 80% of that.

I guess with your suggestion that would be 75% of yield for the total combined stresses in the component right, i mean tensile & torque combined?

Thanks again, great help