stainless screws into aluminum (pt II) 5

[StevePAA]

a few yrs back (!) I started this thread on trying to find a K value for SS screws into aluminum.

http://www.eng-tips.com/viewthread.cfm?qid=229169

The above thread is closed so I figure I'd start another.

I tried to get the company to do a load cell study to measure clamping force vs torque to calculate a K, but that didn't happen. So, I ended up taking a less scientific approach myself and broke off a bunch of SS SHCS into a piece of 6061-T6 (using a SS flat washer under the screw head) and recording the torque where they failed. I also broke off SS screws into SS hex nuts to compare. What I found was that the failure torques were usually within 10% of each other- the alum didn't make a big difference. This makes sense to me because the friction between the washer and screw head should be the largest friction source.

In addition I did minimum thread engagement verifications and found 6061-T6 to be good stuff (no surprise) and that you do not need a lot of engagement with an 18-8 SS screw to have the screw fail before the threads strip in the alum- generally ~1.2X screw diameter is about the minimum you would want. I first calculated min engagements based on shear strength of the alum thread area vs fastener tensile strength and then verified and the calcs were accurate: yes, math works.

For my "test" I used a SS flat washer under the screw heads, 18-8 SS SHCS and tightened until tensile failure. From failure torque, SHCS size, and matl strength I back-calculated a K value. I realize this isn't the right way to do this but I wanted to get some data. What I found was that K was approximately .35 for #10 thru 1/4 sizes. This is slightly less than the ~.40 values if you back calculate from the NASA and ITT tables on SS fasteners.

For comparison I did a few tests into the alum bar with a grade 2 zinc plated 1/4-20 HHCS and FW and found K at failure was .23; it failed at a significantly lower torque than the SS 1/4-20 screws.

So, stainless on stainless= lots of friction as already known.

Again, these K values are from failure torque and this might be giving lower K values than actual because the screws are well into yield at this point. Opinions on this?

I think the summary is that if you are using clean, dry SS screws (no lubricant), with a SS flat washer, then you can basically use the NASA table for torque and will be ok

 

[CoryPad]

You are correct regarding low k values. Take a look at the attached graph.

 

[StevePAA]

thanks CoryPad, that is what I figured; since failure is outside the ~linear range, K will be higher than what's calculated from failure torque.

since 18-8 has such a large spread between yield and ultimate I'm surprised that my failure K results are not further off than they seem to be at 10-15%. Again, this is comparing my numbers to tables that I really know nothing about- other than someone with a lot more resources than me compiled them. Oh well, it's the best I can do with no load cell.

Oh, and how do you go back and edit a post once it's posted? I did not see an edit button.

 

[CoryPad]

You are not allowed to edit directly. You can Red Flag your post with recommendations for editing and the site staff will consider doing it for you.

 

[StevePAA]

ok, I will edit here.

Edit: What I found was that K was approximately .35 for #10 thru 5/16 sizes. This is slightly less than the ~.40 values if you back calculate from the NASA and ITT tables on SS fasteners.

CoryPad: Did you prepare that graph for this post? if so, very nice. if you prepared it, would it be ok if I use it? it's an easy way to illustrate the issue along with just a brief explanation.

 

[CoryPad]

Yes I prepared that graph for this thread (using data I had from my own testing). Yes you may use it.

 

[StevePAA]

Again, great graph and thanks for letting me use it- will save me some time.

 

[Tmoose]

"you can basically use the NASA table for torque and will be ok"

So far that has been the case for me, too.

There have been a few projects with Fastener or structural or sealing joint failures, and to date the root causes have originated in much more basic stuff than establishing fastener "tightness" with greater precision than torquing provides.

Dan T

 

[StevePAA]

for some background, this issue came about when checking dwgs for another company. An assy dwg has a SS 10-24 screw into mild steel with (for example) a 30 in-lb torque spec called out on the dwg. The same screw, same dwg, into alum had (for ex) 15 in-lb torque spec. I asked "why are you using about half the torque for the same screw..." Answer: "because it's threaded into aluminum; you HAVE to use a lower torque...it's weaker than steel." The values they're using are supposedly from a torque table found online somewhere- I haven't seen it nor know anything about it. My concern is they are not tightening the screws nearly enough since their torque values are about half of what's listed in the ITT and NASA SS fastener tables. I tried to explain that they're missing the entire point of tensioning the fastener- forget the parent material; the variables are friction, screw size, screw material strength- that's it. The end goal is to tension the fastener to 70-80% of it's tensile yield. However they do not seem to understand this most basic concept of bolted joints.

Math didn't convince them so maybe broken screws will. Probably not.

 

[CoryPad]

Steven,

The mating thread material strength is important when considering the required thread engagement. Sometimes you have to de-rate joints that have lower strength materials when compared to traditional steel alloys.

 

[boo1]

What NASA reference are you using?

 

[StevePAA]

The NASA reference is one I found on the web- I don't have a link handy but you should be able to find it by searching for NASA stainless screw torque or similar. There is also another table on SST fasteners that is from ITT Harper and appears to be pirated by Fastenal. They are in close agreement but NASA starts at 1/4".

On the mating thread material strength, yes I understand that and I've verified that they have plenty of engagement to break screws before threads strip.

 

[tbuelna]

StevePAA,

A threaded fastener body will fail during installation due to the combined stresses it is subject to. The combined stresses are tension due to the screw thread mechanism, torsion due to the wrenching force applied, and bending due to any under-head misalignments at the clamped interface.

The wrenching forces at installation can vary widely due to friction and interferences. This is especially true with small diameters and coarse loose tolerance threads.

If you haven't done so already, I'd recommend measuring the running torque on a sample of your fasteners.

As for tensioning your fasteners, you only really need to tighten them enough to prevent them from loosening, or to prevent the joint from separating or sliding under load. Tensioning fasteners to 70% of yield is only a general rule of thumb for optimizing the number and size of a fastener pattern. With aluminum parts and ferrous fasteners, you must also consider the effects of thermal mismatch between the materials when calculating preload.

Good luck,
Terry