Why Prevent Galling?
If you have worked long in mechanical system assembly, operation, or maintenance, you may have seen cases, where simple measures taken to prevent galling of a threaded fastener could have prevented major disruption to an operation. The losses of such disruption are often a combination of extremely high repair costs and lost opportunities.
What can happen if you don’t Prevent Galling?
It might be the valve that can’t be operated in a critical system. Replacement in some systems may require a whole plant shutdown to cold iron or freeze seals to replace or repair the faulty valve. Lost operation time and an expensive repair are the results.
The galling could cause a stainless-steel bolt to be sheared off trying to remove it from a large part like a ship thruster propeller blade. The ship might have to be drydocked because the fastener can’t be repaired by divers.
There are many cases of stainless fasteners high-performance race engines that can’t be removed from the engine block during prep for an important race. Repair isn’t possible in the time allotted, so the engine must be swapped out or the race scrubbed. All the time and money of preparation is at risk, as is the reputation of the racing team.
What is the cost when you don’t Prevent Galling?
These are some good examples of the high cost of galling in threaded fasteners. Like accidents, virtually all cases of damage caused by galling in threaded fasteners can be prevented with strict adherence to a set of best practices and just being extra careful with susceptible fastener materials like stainless steel and aluminum.
What is Galling?
In its simplest form, galling is wear on a sliding threaded surface, to the point where the material is removed from one part and deposited on the other. The result is freezing in place or even worse sometimes, failing fatigue or over-torquing. Junior mechanics are more prone to causing galling if they have not been trained when to recognize the early signs of galling and the steps to take once it starts.
How to Prevent Galling?
If a lubricant like Neolube #1 or #2 can be used on the threads, that’s one way to greatly reduce the occurrence of galling. At other times when a lubricant can’t be used, the materials in the design must be built-in. For example, materials like BUMAX 88 have a higher molybdenum content that adds a little extra galling prevention.
In design, courser threads tend to be less galling prone than more fine threads. In manufacturing, rolled threads tend to gall less than cut threads, but it is not common except in the food industry where most lubricants are prohibited.
Using higher-quality fasteners that have a perfect fit can also reduce the likelihood of galling. That can be part of the original design, but the right fasteners must be used in repair work if the Neolube type of dry lubricant isn’t used. Never using a fastener that has even the slightest damage is also important.
What can cause Galling?
A bolt that’s dropped on a hard surface with a little nick in a thread can be a sure source of galling. The same applies to any dirt or debris on the threads. A good wire brush cleaning before reassembly can go a long way. If you are using lock nuts, beware of the extra torque they can apply throughout the assembly process. Using lock washers may help reduce fastener galling in some instances.
Just being careful in assembly to “feel” if a fastener is starting to gall is important. Plowing on with an impact wrench or applying increasing pressure to a fastener that’s starting to gall is the kiss of death. Stopping at the first sign of galling, the slight resistance, and working the fastener back and forth as it’s removed and re-applied is excellent prevention. Go slow!
Use High-Quality Lubricants to Prevent Galling
The details of design and galling prevention are a lot more in-depth than presented here. However, one thing is sure that if you are using a high-quality dry lubricant like Neolube #1 or #2, your chances of galling go way down. It’s just good mechanical practice to do all the things discussed above AND lubricate your threads with a graphite-based lubricant.