FASTENERS 101™
Thread Galling

What Is Galling

Example of Thread Galling — Nut seized on a bolt that has broken
from excessive torque during installation

Galling is a form of abrasive wear (chafing) that has the potential to occur when any two metals rub together with poor lubrication. When two metals rub against each other they begin to wear down. As heat (friction) and pressure increase, the rubbing surfaces begin to stick and break each other down (tear) on the atomic level. This results in micro amounts of transfer between the two surfaces. As these factors continue or increase, the amount of transfer increases resulting in a non-smooth surface. At this point, the transfer will present itself as a rough marred surface. In extreme cases this abrasive wear can cause the two materials to fuse together, a process also known as cold welding or friction welding. Once galling begins the material will begin to lump or ball up which can cause issues with performance.

What Materials Are Prone to Galling?

There are several materials that are considered more likely to gall during installation. Stainless steel, aluminum, titanium and other similar materials are all very corrosion resistant. They acquire this level of resistance by creating an invisible film layer (protective oxide film) which protects the metal from corrosive elements. As friction is introduced to this film, it causes excess heat and wears off leaving the reactive metals exposed. Once this occurs, the chance of galling increases dramatically. Note: Galling can occur in any metal that is rubbing against another metal but is not nearly as prominent in harder materials.

What Is Thread Galling?

Thread galling is the process of galling forming in the threads of nuts, bolts, threaded inserts and other fasteners. As a nut is threaded onto a bolt, the high points begin to rub causing the oxide layer to dissipate. Once gone, the threads continue to generate heat (and more pressure as tightened) which will result in the metals sticking to each other. This is the beginning of the thread galling process.

Galling can cause damage to the threads of the fasteners, but they can still be removed using extracting tools or lubricant. In severe cases, the two materials completely weld together, preventing loosening of the parts and adding the difficult step of forced removal (extraction) and new parts to the process. The frustrating aspect of this is that both the nut and the bolt can cause clear inspection individually but fail to function effectively as a mechanical unit.

Causes of Thread Galling

Galling in fasteners can occur naturally but there are many factors that only increase the risk of galling.

Dirty Threads
Damaged Threads
Installing Under Load
Installing Too Quickly
Installing in Poor Environmental Conditions
Not Applying Lubricants
Using Locking Fasteners (Specifically Nylon Insert Lock Nuts)

How To Prevent Thread Galling

While galling is a natural occurring process that cannot be completely mitigated, there are a few ways to minimize the chance of thread galling.

Add fastener lubrication	The most efficient way to minimize risk of galling is to add an anti-seize solution. Some environments have special requirements like kitchens. Make sure the correct anti-seize is used based on the environment.
Reduce installation speed	As installation increases friction can cause a fastener to get hotter and hotter, reduce the speed will not stop the chance of galling completely but it will allow the heat to dissipate.
Take load off assembly before tightening	When tightening the nut onto the bolt, ensure there is no or minimal load on the assembly. Increased load will force the threading to push against itself while spinning, increasing the friction.
Use a torque wrench to avoid over-torque	Over-torqueing a nut can cause damage to the threading.
Use two different grades of the material	Some success has been found by using one softer grade and one harder grade material. Although in some situations this is not possible such as near the ocean where only grade 316 stainless steel should be used.
Ensure fastener threads are not damaged prior to installation	Damaged threading will increase the friction during installation.
Ensure there is no debris in fastener threading prior to installation	Debris sitting inside of threading will also greatly increase friction during installation. If bolts are covered in dirt use compressed air to remove it before use.
Use coarse threading where possible	Fine threaded fasteners are great tools but require many more turns per inch during that coarse threading. This, in turn, will generate more friction per inch leading to an increased chance of galling.
Keep fasteners in a controlled environment	Try to keep fasteners in a cool environment. If left in the sun, the fasteners will heat up resulting in an increased chance of galling.

If seizing begins, immediately stop and remove the nut and bolt, use new fasteners and apply lubricant.

What Is Anti-Seize Lubricant?

Anti-seize lubricant is formulated to reduce the friction created during the installation of fasteners. Formulas vary and may use any combination of grease, graphite, copper, aluminum, nickel, calcium, and other ingredients. We recommend using anti-seize lubricant before installing nuts and bolts, especially to prevent stainless steel galling. Shop our lubricant of choice here: MRO Anti-Seize Solutions, Anti-size extends the fastener life, adds an additional layer of corrosion resistance, and allows for easier disassembly of parts. Each bottle comes with a built-in brush which is used to apply a small amount of anti-seize to the male and female threads before installing. Anti-seize lubricant is not the same as WD-40 and other oils, it is specially formulated for fasteners and makes installation easy.

Nylon Lock Nuts and Galling

nylon lock nuts increase the risk of gall

Nylon lock nuts are designed to cross thread, preventing the nut from backing off the threads and holding the nut in place. When the nylon inserts cross threads, the threads may gall or seize. This insert is designed to add extra tension to the assembly which during the installation can also cause added friction significantly increasing the change of thread galling. Nylon lock nuts are not designed to be removed after installation; an alternative to using a nylon lock nut would be using a lock washer and flat washer or solid two-way lock nut. If you plan on using a nylon lock nut ALWAYS apply lubrication before installation.

Application of Hot Dipped Galvanized Steel

Hot dipped galvanized (HDG) steel is also prone to galling and when used, should be matched with HDG exclusively. For example, an HDG carriage bolt will only properly thread onto an HDG hex finish nut. Because there is a thick zinc coating that is applied to these bolts by dipping the steel into molten hot zinc, the nuts are tapped larger than stainless steel, zinc plating, plain alloy and other nuts to accommodate for the thickness of the galvanized zinc coating.