K value for SS fasteners into Alum

[StevePAA]

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

 

[CoryPad]

Is the stainless fastener uncoated? Galling is likely to occur with this combination. If you are using T = KDF, then you could use 0.25 or 0.30. The automotive industry rarely uses austenitic stainless steel combined with Al. The only applications I can think of are HeliCoil thread inserts (which generate zero preload and hence are not concerned with friction coefficient) and exhaust manifold studs, which usually are not in a soft, uncoated condition. Good luck.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[KENAT]

Not directly on topic but to add to CoryPad caution, stainless into Aluminum can be problematic from a corrosion point of view, we normally avoided this combination in aerospace/defence. However, where I am we do it a lot as our equipment is used in a fairly benign environment.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[StevePAA]

I have found that K is ~.35-.40 for 18-8 SST bolts back-calculating from SS fastener charts from ITT, fastenal(copied from ITT?) and unbrako; NASA data is about the same as ITT.

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.

 

[drawoh]

StevePAA,

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...

[ol]
[li]Loctite[ ]242 is a lubricant. If it is used, I use k=0.15.[/li]
[li]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.[/li]
[li]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![/li]
[li]I cannot recall galling problems with stainless steel fasteners in the stainless steel thread inserts.[/li]
[li]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.[ ][/li]
[/ol]

JHG

 

[KENAT]

I'm just remembering that I had trouble with a smaller (4-40) thread in bare aluminium where the SS fastener would bind. I thought it was because they hadn't cleaned the thread out properly after anodize, machine shop said as much, but it may have been galling. The binding would happen long before end of travel while I was just inserting it 'finger tight'.

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,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[drawoh]

KENAT,

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

 

[KENAT]

Maybe getting a bit off topic but the drawing specified 6061-T6 as I recall.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[dinjin]

I remember that when drilling into aluminum with the normal
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.

 

[StevePAA]

The parts will be 6061-T6 with no finish.

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?

 

[prost]

For those of who are ignorant about the "K" value (like your truly), what is "K" and what is it used for?

 

[TVP]

k is the nut factor in the traditional rule of thumb equation T = kDF where T is torque, k is nut factor (essentially friction), D is thread major diameter, and F is pretension force.

 

[drawoh]

StevePAA,

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