18-8ss screw binding

[BraunP]

Hello great knowledge of this form,

I have run into a interesting problem, and would like your input. For a project I am using 1/4"-20 18-8ss bolts that are connected to 18-8ss nylon insert lock nuts. The issue that has be seen is that sporadically the nut will lock up while I am trying to tighten up the screw. and it locks up about 1/4 down the length of the screw. after that, it will not move forward or backwards. Because of this, I have resulted into cutting the bolt and putting in a new one.

My guess in to what is happening, is that as the nut is moving down the bolt. some inconstancy are rubbing onto the thread of the nut or bolt causing some metal to shear off and causing it all to bind up. pretty much the screw material is too soft and being "machined" off inside the screw thread. that is my best guess.

so any ideas what could be happening.

 

[CoryPad]

Galling, common problem, perform a search on this site for more information.

 

[BraunP]

thank you, 1) I did not know the term 2) I knew there was a great knowledge on this form =)

 

[sreid]

Use Antiseize.

http://www.mcmaster.com/#antiseize-lubricants/=wrih4e

 

[dgallup]

Using dissimilar metals will greatly reduce galling. Stainless steel in general is bad about galling, there are special SS alloys that are gall resistant. Nitronic 60 is one.

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[tbuelna]

1/4-20 UNC threads are usually quite sloppy, and should not seize up.

With a 1/4-20 UNC nylock nut there is probably only about 2.5 to 3 metal-to-metal thread pitches engaged before the nylock insert comes into contact. Sometimes it can take a fair amount of torque to force the bolt threads past the nylock insert.

As a quick test, try running a plain 1/4-20 UNC CRES nut onto the bolt to see if the same problem occurs.

 

[dgallup]

18-8 on 18-8 is one of the worst combinations possible:

http://www.nickelinstitute.org/~/Me...lingCharacteristicsofStainlessSteel_9006_.pdf

See table XI on page 24.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Compositepro]

It could be galling or it could also be cross-threading when installing the nut. The nylon patch can contribute to either problem. A socket wrench that holds the nut square to the screw might help.

 

[BraunP]

They were attached with a allen key on one side and a socket on the other. thank you Dgallup for the link. that is very interesting.

 

[BraunP]

ok. now I have a question, if I used 18-8 black oxide screws, with 18-8 black oxide nuts, would that help reduce the problem? before they where uncoated screws.

 

[CoryPad]

Yes, carbon steel fasteners with black oxide surface treatment should be resistant to galling.

 

[KENAT]

No Cory, the OP is talking about still using 18-8 stainless but with black oxide coating, I'm guessing like this

http://www.mcmaster.com/#socket-head-cap-screws/=wu5ozs.

Posting guidelines faq731-376

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

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[BraunP]

yup. that is what I was talking about. my thought is that the oxide coating will act as a protective layer/ lubricant for the threads. That is just my thought.

 

[CoryPad]

Black oxide on austenitic stainless will be better than bare with respect to galling. A lubricant would help, verify torque-clamp force behavior if this is a critical joint.

 

[racookpe1978]

The black oxide will help screwing on. Lube will help.

But later, when unscrewing, the black oxide will have scratched through, and the galling will re-appear.