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SS fastener galling

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John Ackerman

Joined: 19 Jun 2006
Posts: 130
Location: Prescott, AZ

Posted: Fri Feb 16, 2007 11:30 am Post subject: SS fastener galling

I want to install a magnetometer shelf behind the baggage  

compartment , bolting it to the longerons with AN3 hardware.



I ordered the appropriate stainless steel bolts, nuts, and washers;  

placed the first set in the first hole, and began tightening. The nut  

tightened up early, long before reaching the end of the threads, and  

could not be backed off. I finally had to twist hard enough to shear  

the bolt in order to remove it.



Experimenting with a SS bolt held in the shop vise gave the same  

result - the nut "galled" on the bolt and froze solid. I could not  

remove the nut except by shearing the bolt. I didn't tighten  

particularly fast, either, so although there was some friction  

heating, it was not abnormal. Substituting a regular steel AN365 nut  

solved the problem - it behaved perfectly normally. Likewise,  

substituting a steel AN3 bolt for the SS bolt and using a stainless  

nut gave no problem. A generous coat of Boelube solid wax lubricant  

allowed the stainless nut and bolt to work together.



Now "everybody" in my previous life (high temperature chemical and  

electrochemical technology) knew better than to use stainless-on- 

stainless threaded fasteners because of this very same galling  

phenomenon, but it just did not occur to me that it would be a  

problem, especially at room temperature. Talk about forgetting  

lessons learned!



Solution: use a steel AN365 nut and accept the miniscule magnetic  

field it causes.  BTW, the nut wighs about 2 g and the bolt about 4  

g. I'm thinking that there's not any significant dissimilar metal  

corrosion problem - the difference is much smaller than steel vs  

aluminum.

Boelube would work  for the first few SS-on-SS assembly/disassembly  

cycles, but it could conceivably dry out or be removed  by repeated  

assembly and disassembly and lose its protective film. Somebody else  

down the line could also use new SS hardware and not know...



FWIW

John Ackerman 40458

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rvbuilder(at)sausen.net
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Posted: Fri Feb 16, 2007 11:59 am Post subject: SS fastener galling

Any chance you got a nut with a different thread count per inch than the

bolt?  I personally have never run across this before and it sounds very

odd.  I have used the Lowes Depot variety of SS nuts and bolts together

tons of times with no problems.  Maybe they just have a bit more "slop"

than what you are using.



Michael Sausen

-10 #352 Limbo

Do not archive



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John Ackerman

Joined: 19 Jun 2006
Posts: 130
Location: Prescott, AZ

Posted: Fri Feb 16, 2007 6:06 pm Post subject: SS fastener galling

Any chance you got a nut with a different thread count per inch than the

bolt?

Very little. Several bolts and nuts were used, all were 1/4" -28tpi  (AN3, 10-32) purpose bought from ACS. Besides, thread mismatch shows up earlier in the tightening process, and allows one to back the nut off. Here, both nut and bolt worked fine with their carbon steel equivalents. Finally, lubricant stopped the problem, at least temporarily. 



 	  Quote: 	
		  I have used the Lowes Depot variety of SS nuts and bolts together

tons of times with no problems.  Maybe they just have a bit more "slop"

than what you are using.

	


 I think you're right about the "slop" in non-aviation consumer hardware.



The galling problem occurs with other metals where the two threaded parts are made of the same stuff, but SS is far worse than non-stainless steel. I think it's related to the poor "machineability" of most stainless steels - they tend to become plastic, or "gummy" under severe localized stresses. We would almost always purposely make the two threaded parts of different materials, or would avoid threads entirely in favor of other connecting means where galling was a problem. Our experience with  SS galling involved equipment run at high temperatures (350°-650°C, or about 575° to 1200°F). in that case, we could frequently assemble OK, but be unable to disassemble after a heating cycle. It didn't take long for it to become SOP to avoid stainless-on-stainless threads. 



Today's experience points out that galling can be a problem at room temperature, too.



What happens, I believe,  is that drag causes heating on a microscopic scale. The metal becomes very plastic locally, and the two parts conform to each other and stick, causing even worse heating on a micro scale. The whole thing gets stuck  because a lot of the gross thread area has very good contact  - and excellent adhesion because the crystal structures of both parts are essentially identical. Backing off does not help once this positive feedback process gets started. 



A really sloppy fit leaves lots of non-adhering surface (gaps), so the problem is not so bad. Also, appropriate lubricant  or anti-seize compound keeps the two surfaces from intimate contact.



 	  Quote: 	
		  	
John Ackerman 

40458 fuselage

On Feb 16, 2007, at 12:56 PM, RV Builder (Michael Sausen) wrote:

[quote]--> RV10-List message posted by: "RV Builder (Michael Sausen)" <rvbuilder(at)sausen.net (rvbuilder(at)sausen.net)>

Any chance you got a nut with a different thread count per inch than the

bolt?  I personally have never run across this before and it sounds very

odd.  I have used the Lowes Depot variety of SS nuts and bolts together

tons of times with no problems.  Maybe they just have a bit more "slop"

than what you are using.

Michael Sausen

-10 #352 Limbo

Do not archive

--

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Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

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