'Galling' of S/S nuts/bolts

[Poignard]

Just spent an un-jolly afternoon struggling to unscrew the A4 nuts from some new A4 s/s bolts. The threads had seized solid ('galled') when I tightened up. Eventually I had to use a nut splitter.

Any tips to prevent it happening again, apart from using bronze fastenings (which are too expensive and probably not the best metal to use with a steel fitting)?

 

[sarabande]

AG90 spray

http://www.actioncan.com/contact


Other sprays and compounds are also available


Be religious about cleaning the threads before you assemble the items.

 

[Poignard]

Thanks Sarabande

The threads were clean - the nuts and bolts were brand new. I was just doing a trial assembly.

I've never heard of AG90 before but it looks like the answer. Is it generally available?

 

[HunterWanderer]

Any oil is normally good enough to stop galling. Its a problem that occurs with machined threads rather than rolled. Its caused by lumps of chromium in the alloy being so hard they dont machine smoothly. Rolled threads are becoming more widely available. try big distributors such as http://www.ukstainless.co.uk/ I have no link just happy customer.

 

[cliff]

Any oil is normally good enough to stop galling. Its a problem that occurs with machined threads rather than rolled. Its caused by lumps of chromium in the alloy being so hard they dont machine smoothly. Rolled threads are becoming more widely available. try big distributors such as http://www.ukstainless.co.uk/ I have no link just happy customer.

Just to set the record straight, galling has SFA to do with "lumps of chromium in the alloy being so hard they dont machine smoothly". Thread galling seems to be the most prevalent with fasteners made of stainless steel, aluminium, titanium, and other alloys which self-generate an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the contacting and sliding thread surfaces, protective oxides are broken, possibly wiped off, and interface metal high points shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme, galling leads to seizing - the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out.
Another factor affecting thread galling in stainless steel fastener applications is thread roughness. The rougher the thread flanks, the greater the likelihood galling will occur. In an application where the bolt is galling with the internal thread, the bolt is usually presumed to be at fault, because it is the breaking component. Generally, it is the internal thread that is causing the problem instead of the bolt. This is because most bolt threads are smoother than most nut threads. Bolt threads are generally rolled, therefore, their thread flanks are relatively smooth. Internal threads are always cut, producing rougher thread flanks than those of the bolts they are mating with. The reason galling problems are inconsistent is probably due largely to the inconsistencies in the tapping operation. Rougher than normal internal threads may be the result of the use of dull taps or the tapping may have been done at an inappropriately high RPM.

 

[lenten]

thats told us thoroughly cliff----now whats the best way to stop it-----regards lenten

 

[cliff]

thats told us thoroughly cliff----now whats the best way to stop it-----regards lenten

1) Buy your nuts and bolts from a reputable source - Not fleabay or one-hung-low's bolt emporium.
2) Run the nuts on by hand, if they are tight remove them and re-tap using a sharp tap and lots of tapping fluid (extreme pressure lubricant)
3) Check bolts - if rough clean / polish the threads on a stainless steel wire wheel.
4) Assemble using Molyslip (Molybdenum Disulphide) or similar. There are several proprietary lubricants available, at a price (Molyslip is comparatively cheap)
5) Tighten s l o w l y

 

[Bosun Higgs]

when we cold formed stainless steel in the steelworks in sheffield we either used molydisuphide or copper coating to prevent cold welding ie galling. It shouldnt really be an issue on well formed fasteners - you undoubtedly have some with poorly finished threads. But if you use moly or copperslip you should be OK.

 

[vyv_cox]

Just to add to Cliff's post, the process of galling is known as 'adhesive wear' and occurs by exactly the same process as seizure of unlubricated steel surfaces under load and motion. It occurs by localised welding of adjacent high points, properly called 'asperities'. These tear out, leading to increased problems as the particles of debris themselves form further welds.

Shell, and most other oil companies, researched this topic quite intensely around 20 years ago. Their findings then were that molybdenum disulphide was the best palliative, although other thread lubricants such as copper and nickel compounds were nearly as good. There are reasons why sulphur compounds are not preferred in other applications, such as hardened carbon steel, hence copper is perhaps more widely used.

 

[Halo]

thats told us thoroughly cliff----now whats the best way to stop it-----regards lenten

The best way to stop it to titanium nitride coat one of the parts. This is what we do at work on top of the range stainless steel surgical instruments.

In the practical boaty world others have given the answer to active prevention but to be honest it does not seem to occur that often unless you get anal about cleaning stainless before assembly.

 

[forelle541]

At work we use Never-Seez a bit like silver copperslip. Works really well.

 

[vyv_cox]

but to be honest it does not seem to occur that often unless you get anal about cleaning stainless before assembly.

Certainly wasn't true in my case. My mast was stepped after transiting the Canal du Midi. I greased all the bottlescrews beforehand. We sailed to Port Leucate, where I checked rig tension but was unable to turn the babystay bottlescrew. It was seized totally solid, could not shift it in a vice with my big adjustable spanner.

 

[macd]

It was seized totally solid, could not shift it in a vice with my big adjustable spanner.

...and you knew, even as you were doing it, that the whole shebang was totally snafu. But can you explain why, through some ephemeral process at the molecular level, the steel knows to do this at precisely the most inconvenient time and place? Been there, done that, but where do you get the T-shirt?