Over the years we have looked at the nuisance of galling in several separate blogs.
That is because, every year galling makes it to the leader’s group of “Heavy Maintenance Royal Pains”, alongside, magnetism, barber pole-ing, and cupping or oil-canning.
So, let’s begin with this for anyone absent those days; Galling is the seizing of mating parts. A sort of Cold-Welding as it were.
When it is time to disassemble for inspection or repair, often the parts (hubs, casings, flanges), won’t come apart. That leads to hours of lost time and materials, often to find internal parts were savable, had you not destroyed the case. One industry I occasioned to visit finally gave up and inspections switched to mandatory replacements. Ouch!
Galling is a special thrill with the stainless assemblies often found in industries that prohibit the use of lubricants (food service and production).
Parts tend to gall as a response to friction and low-yield strength deformation. Eliminate the friction, eliminate the deformation, minimize galling. Not so fast. Can’t use most lubes in food service due to contamination.
If you’re using stainless shafts, you are most likely accustomed to a certain amount of gumminess in machining; a product of reduced strength that also contributes to deformation.
When mating parts encounter friction (resistance) they bind. Subsequently, they may deform and “cold-weld” together (gall). You can try minimizing binding during installation by slowing the installation speed; perhaps use a hand feed to detect potential galling areas; then back off pressure and speed. You can make sure the parts fit nicely (snug) to eliminate pulling the part together using the threads like a turnbuckle.
SMOOTHER & STRONGER THREADS would help as they would minimize friction and resist deformation. Using dissimilar steel parts with dissimilar hardness would also help deter galling. Try using “Rolled Threads”. Rolled Threads are smoother than cut threads so they have less surface defects which means less propensity for fatigue failure. But let’s keep the focus on galling. Rolled Threads are stronger because they are compressed or displaced into shape. That work-hardens (strain-hardens, cold-works) the thread; especially if it is a stainless steel.
“Roll-Threaded Rod Minimizes Galling”, may not be headline-making news. But it is a byproduct of that thread production method. Life is short. Be easy on yourself. Take advantage of available benefits before reengineering the whole job.
-Howard Thomas, November 8th 2021
