CoryPad, Deformed top lock silver plated 5/16-24 UNJF running over stainless bolt (alloy unknown not plated) Customer currently using 380 RPM right angle nut runner. Run down distance is 1.5".
Without a lubricant, this will be close to impossible to prevent galling. 1.5 inches divided by 0.04 inches per revolution equates to > 30 revolutions. With a deformed thread nut? Not good.
Galling would be more likely at 10 rpm or 1000 rpm. I don't think a tweak on your current speed will do much.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
In my limited experience, the silver plating is effective for a rundown of approximately twice the nut's thickness... then it's gone. It's a complete waste of money if you have any other option.
Fastener expert Joe Greenslade claims:
" Slowing down the installation RPM speed will frequently reduce, or sometimes solve completely, the problem. As the installation RPM increases, the heat generated during tightening increases. As the heat increases, so does the tendency for the occurrence of thread galling."
I recently took some new unlubricated 300-series nuts and bolts, put them together SLOWLY WITH MY FINGERS, and they gauled and seized so tightly that I could not take them apart with a wrench. On the other hand, I've noticed that a hard heat-treatable stainless isn't as bad and I believe can be fully tightened dry without galling. There's more to it than speed.
Stainless steel, Inconel and titanium alloy fasteners are all prone to cold welding/galling. An anti-seize, such as silver plate or moly-disulfide, is usually applied to cres nuts. Cetyl alchohol or moly disulfide is used with titanium fasteners. I've also seen teflon/electroless nickel used on cres nuts that require lots of installation and removal cycles.
A common problem with deformed thread, self locking nuts is that the prevailing torque (ie. thread interference) tends to vary widely with the smaller thread sizes. The deformed thread locking feature is suppose to be within certain limits for running torque, but they never are. So I would check the running torque on a representative sample of the nuts you are using.
300 series austenitic cres alloys(with the exception of 303, which should never be used for a fastener) are the most prone to galling, likely due to their work hardening characteristics. Most high quality aerospace cres fasteners are heat-treatable alloys like A286 or 13-8ph, which are less prone to galling. One thing that also really seems to help prevent galling is to have a difference in hardness between the two mating parts.
