K value for SS fasteners into Alum
K value for SS fasteners into Alum
(OP)
I have found almost no info on this. I'm looking for a K value for a 18-8 SST bolt threaded into bare aluminum. Anyone have info on this? With all the aluminum used in automotive today someone must have done a lot of research on this.
please don't say "determine it experimentally" :) I know about variations that can lead to errors on this but there are published/accepted k values for many typical fastener finishes/conditions that are used every day
please don't say "determine it experimentally" :) I know about variations that can lead to errors on this but there are published/accepted k values for many typical fastener finishes/conditions that are used every day





RE: K value for SS fasteners into Alum
Regards,
Cory
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RE: K value for SS fasteners into Alum
KENAT,
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RE: K value for SS fasteners into Alum
So, I was wondering how much SS into AL would change the situation. The below link is a nice study but seems contradictory? The text mentions K=.25 for SS into AL, but the chart shows K=.42 for untreated threads.
http://facstaffwebs.umes.edu/eyilmaz/torase2k.pdf
For my application, I believe they are not lubricating the bolts but I will check. They might be using vacuum grease but will not be able to use a better lubricant other than that. This will be an assemble and then never take apart situation once we reach a production stage. Whatever they are doing I haven't heard of any galling issues so far.
RE: K value for SS fasteners into Alum
We use stainless steel fasteners in aluminium structures on system that are mounted in aircraft that fly at low altitude over the sea. We have not had problems with corrosion.
Most of our machined parts are Aluminium 6061-T6. Most of our sheet metal parts are Aluminium 5052-H32. All of our parts either are anodized, or chemical filmed. Most of our threaded connections use thread inserts of some kind, mostly helicoil, or PEM. Both fasteners are stainless steel. Usually, we secure the screws with Loctite 242.
Several points here...
- Loctite 242 is a lubricant. If it is used, I use k=0.15.
- On unlubricated connections, I use k=0.2, as per my machine design textbook (V.M Faires). I am not aware of problems. I cannot prove that I optimally tensioned the screws.
- An anodize finish on aluminium massively reduces its friction coeffient. You can do threaded assemblies of anodized aluminium parts. If it is bare aluminium or chemical filmed aluminium, look out!
- I cannot recall galling problems with stainless steel fasteners in the stainless steel thread inserts.
- I called up the self locking helicoil inserts once, a long time ago, and everyone hated me for it. Keep your torque calculations simple, and don't make people hate you.
![[smile] smile](https://www.tipmaster.com/images/smile.gif)
JHGRE: K value for SS fasteners into Alum
I'm not sure about SS on bare AL but I know that with SS on SS regular lubricants don't always work well, you some times need specific anti-galling compound or equivalent.
drawoh - interesting what you say on stainless in AL and helicoils. We did the exact opposite on aircraft stores back in the UK, virtually no SS into AL and helicoils all over the place.
KENAT,
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RE: K value for SS fasteners into Alum
I cannot recall ever having problems with 4-40UNC stainless steel screws into aluminium.
If I tap a hole into aluminium, it would be 6061-T6. 7075-T6 would be a high strength design with inserts. 5052-H32 would be sheet metal, and not thick enough for tapping, and I would use PEM nuts. We have used some Alumium 356-T6 castings, but I, personally, did not handle them.
Our machine shop complained about the 356-T6. Apparently, it is soft, and more difficult to machine.
Could you have been tapping into soft material?
I am doing a Google search here on thread galling. It appears that a lot of stainless steel fasteners are manufactured with lubricant.
JHG
RE: K value for SS fasteners into Alum
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at http://eng-tips.com/market.cfm?
RE: K value for SS fasteners into Alum
recommended drills, that in small drill sizes, the holes
would shrink or end up undersize after drilling. Parts
greater than .250 dia. were not a problem. Not certain
about 6061T6 however.
RE: K value for SS fasteners into Alum
Thanks for the tips
Drawoh: does your machine dsn book give a k=0.2 for SS into AL or are you just using a "generic" k?
RE: K value for SS fasteners into Alum
RE: K value for SS fasteners into Alum
RE: K value for SS fasteners into Alum
I am using a generic k.
If you are using torque to control screw tension, you need to control the friction. Anodizing the aluminium and using a thread lubricant makes for a more consistent connection.
Search this website. This is a question I asked on here at some point.
JHG