I want to tap an M12 Bumax fastener (fu = 145ksi, stainless) into a 4" thick piece of machined 6061-T6, and want to be able to develop the full strength of the bolt. The thread stripping calculation requires approx 0.75" of thread length, which is 1.5 diameters, that is to say a lot more than a full strength nut. I am curious about the threads not taking the load evenly, though the aluminum is ductile and in principle should be fine.
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High strength stainless bolt tapped into 6061 Aluminum 1
- Thread starter glass99
- Start date
TugboatEng
Marine/Ocean
Definitely use an insert. The aluminum can give you the pull out strength without an insert but the risk of galling will be high during tightening.
The load is taken up mostly by the threads near the surface. It's a diminishing returns thing as you go deeper with the female thread. I'm not sure what all is involved but the obvious reason would be that the deflection of the bolt material and the thread material would favor the stiffest load path which is nearer the surface. I'm sure someone has run a test: What goes first - the bolt or the threaded aluminum?
EdStainless
Materials
The problem with high strength bolts into Al without an insert is that if you avoid galling when you load the bolt you get a little deformation of hte Al. This can result in a small 'hump' around the hole making it impossible to get good flat fit up in the future.
What alloy is your bolt? I would typically look for using a 17-4PH insert for SS bolts.
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P.E. Metallurgy, consulting work welcomed
What alloy is your bolt? I would typically look for using a 17-4PH insert for SS bolts.
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
- Thread starter
- #6
We could use a helicoil which would of course help with the galling side of things, but I am curious about the structural performance. An elastic analysis of bolt threads shows that its only the first couple of threads doing anything because of the bolt stretch. The normal wisdom is that a tiny bit of yielding in the first threads give you an even distribution, but I'm not sure how far you can go with that assumption. I assume this is similar to the bolts holding down the head to an aluminum engine block. We could probably do a pullout test, but I feel like I can't be the first person in the world to ask this question.
The alloy is 316 stainless (Bumax bolts)
The alloy is 316 stainless (Bumax bolts)
The helicoil increases the area the bolt works on; it effectively converts the fastener into a larger fastener without requiring a larger fastener.
That said - I don't recall ever wanting to put a high strength steel fastener directly into aluminum, if for no other reason than avoiding galvanic corrosion, so I suspect that others thinking the same way never considered not using an insert of some kind.
The studies I recall showed the first full thread is 40%, the second about 30%, the next about 20%, and the 4th close enough to 10% as to be inconsequential. Increasing the area with an insert directly increases the load carrying capacity.
That said - I don't recall ever wanting to put a high strength steel fastener directly into aluminum, if for no other reason than avoiding galvanic corrosion, so I suspect that others thinking the same way never considered not using an insert of some kind.
The studies I recall showed the first full thread is 40%, the second about 30%, the next about 20%, and the 4th close enough to 10% as to be inconsequential. Increasing the area with an insert directly increases the load carrying capacity.
TugboatEng
Marine/Ocean
Dave, your concerns about galvanic corrosion have been addressed.
McMaster-Carr Page:
But if the assembly stays relatively dry, the galvanic corrosion should never be an issue to begin with. And certainly not more so than would be seen with the bolt itself.
McMaster-Carr Page:
But if the assembly stays relatively dry, the galvanic corrosion should never be an issue to begin with. And certainly not more so than would be seen with the bolt itself.
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- #10
TugboatEng
Marine/Ocean
I totally misinterpreted your comment, I think my brain read high strength steel as helicoil. I was really surprised that an aerospace engineer would be opposed.
- Thread starter
- #11
@3dDAVE:
Interesting that a helicoil actually improves the strength of the female thread. Is there any data on this? I had assumed they were only good for galling.
Re: bimetallic corrosion. In my normal world of facades its pretty routine to use stainless fasteners in curtain wall. The assumption is that the stainless is the cathode and the aluminum is the anode, and that the stainless has much smaller quantity of material.
Also, is there an ASTM or MIL spec or something which defines the strength of long female threads? There are plenty of formulae out there for thread strength, including in standards, but none of them limit the length of threads. I can't believe how hard it is to find definitive data on this.
Interesting that a helicoil actually improves the strength of the female thread. Is there any data on this? I had assumed they were only good for galling.
Re: bimetallic corrosion. In my normal world of facades its pretty routine to use stainless fasteners in curtain wall. The assumption is that the stainless is the cathode and the aluminum is the anode, and that the stainless has much smaller quantity of material.
Also, is there an ASTM or MIL spec or something which defines the strength of long female threads? There are plenty of formulae out there for thread strength, including in standards, but none of them limit the length of threads. I can't believe how hard it is to find definitive data on this.
See page 11 of
Really long thread engagement has problems that become noticeable at 5X to 10X diameter engagements due to thread lead errors.
Most results are from practical testing rather than theoretical calculations; not much going to be definitive.
Really long thread engagement has problems that become noticeable at 5X to 10X diameter engagements due to thread lead errors.
Most results are from practical testing rather than theoretical calculations; not much going to be definitive.
Hi glass99
I agree with using an insert into the aluminium as others have said it reduces galling and it improves the female thread shear strength in the aluminium but it only does that due to the increased shear area gained by the helicoil insert. There are other issues which you haven’t touched on, like whether there is cyclic loading or temperature changes involved with your application, these are things that should also be considered in order to establish the bolt preload.Formulas which give the length of thread engagement required to develop full strength of the bolt are only really a guide because theoretically you can develop any strength you want by increasing thread engagement. However in practice once you have reached an thread engagement of about 1.5 times bolt diameter there is little to be gained after that, this fact is mainly due to the fact that thread loads are not evenly spread over the engaged threads which you have already mentioned.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
I agree with using an insert into the aluminium as others have said it reduces galling and it improves the female thread shear strength in the aluminium but it only does that due to the increased shear area gained by the helicoil insert. There are other issues which you haven’t touched on, like whether there is cyclic loading or temperature changes involved with your application, these are things that should also be considered in order to establish the bolt preload.Formulas which give the length of thread engagement required to develop full strength of the bolt are only really a guide because theoretically you can develop any strength you want by increasing thread engagement. However in practice once you have reached an thread engagement of about 1.5 times bolt diameter there is little to be gained after that, this fact is mainly due to the fact that thread loads are not evenly spread over the engaged threads which you have already mentioned.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
- Thread starter
- #14
@3dDave: thanks for the Stanley doc
@desertfox: Is that 1.5x bolt diameter an "official" thing? Or maybe you are saying we should do a test? I just want to show that the bolt is stronger than the thread, so no load cells etc required.
The application is an architectural column supporting a rich man's rooftop canopy milled from 4"x4" aluminum billet, and we are anchoring into the carbon steel substructure with these bolts. The temperatures are just normal outside temperatures for NYC. The preload in the bolts will be nominal/not controlled.
@desertfox: Is that 1.5x bolt diameter an "official" thing? Or maybe you are saying we should do a test? I just want to show that the bolt is stronger than the thread, so no load cells etc required.
The application is an architectural column supporting a rich man's rooftop canopy milled from 4"x4" aluminum billet, and we are anchoring into the carbon steel substructure with these bolts. The temperatures are just normal outside temperatures for NYC. The preload in the bolts will be nominal/not controlled.
Hi glass99
See the cautionary note on page 7 of this link
Sorry I am in Europe so not familiar with things in the States
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
See the cautionary note on page 7 of this link
Sorry I am in Europe so not familiar with things in the States
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
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