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Thread Galling

Cold Welding and Thread Seizing

What Is Thread Galling?

Thread Galling is most common in stainless steel, titanium, aluminum, and other corrosive resistant metals whose chemical composition contains protective oxides. Galling, commonly referred to as seizing, occurs during fastener tightening and installation. Pressure builds between the contacting surfaces of the male and female threads and the protective oxides or coatings begin to shear, causing high points and imperfections in the raw material to become locked together. In serious cases, this problem is even referred to as "cold welding" because the galling surfaces lock together which illustrates the severity of the issue. Metal on metal contact and the resulting friction are the main causes thread galling.

Galling can cause damage to the threads of the fasteners being joined, however the fasteners may be removed using extractor drill bits or lubricant. In severe cases of thread galling, the two materials being joined 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.

Cut Threads vs. Rolled Threads
Threads on a screw are rolled onto a "blank" with a cold formed head. Threads on a nut are cut into the fastener. The combination of a cut thread and a rolled thread can create galling.


Nut seized on a bolt that has broken from excessive torque during installation
What Materials Are Prone To Galling?

Thread galling is a common occurrence with stainless steel, aluminum, titanium, and hot dip galvanized fasteners. Stainless steel, among other alloys, generates a protective oxide surface film which can sometimes lead to difficulty with installation. The beneficial nature of using stainless steel, corrosion resistance, also causes the inconvenience of thread galling and seizing. Customers sometimes report nuts becoming "stuck" on bolts. Thread burrs may contribute to thread galling; stainless steel is a softer metal that is susceptible to burring and galling.



What Should I Do If My Fasteners Start To Gall?

When a customer reports that they are "snapping bolts" or their nut and bolt are stuck together, it is usually an instance of applying too much torque and the result is excessive friction and seizing. The nuts and bolts are doing their job, threading together and forming a tight bond. If you are installing nuts and bolts that begin to gall, stop immediately, apply anti-seize lubricant, and replace the fastener with a new one if you are unable to advance with reduced torque and speed.

Though this problem may be frustrating, it is avoidable.
The main issue to address is friction between the adjoining parts. A slower installation process (reduced RPMs) is key to reducing galling. The idea behind this method is to reduce the friction (and thus heat) between the parts preventing the "welding" process. Our recommendation, due to its ease of implemention, is to always use lubrication in the form of anti-seize lubricant.



STOP
What To Do If Fasteners Gall
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, such as MRO Marine Grade Anti-Seize, before installing nuts and bolts. 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 femate 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.



Why Do Nylon Lock Nuts Gall and Seize?

Nylon lock nuts are designed to cross thread, preventing the nut from backing off of the threads and holding the nut in place. When the nylon insert cross threads, the threads may gall or seize. We recommend using anti-seize lubricant, such as MRO Food Grade Anti-Seize, before installing lock nuts and bolts. Anti-seize lubricant adds an additional layer of corrosion resistance. A very small amount of lubricant needs to be used on the male and female threads before installing, resulting in easy threading. 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.



Application Of Hot Dipped Galvanized Steel

Hot dipped galvanized (HDG) steel is 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 nuts. 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, clear zinc, plain alloy and other nuts to accomodate for the thickness of the galvanized zinc coating.