Thread galling is wear that happens when two threaded parts rub against each other. Over time, the build-up of friction causes the threads to stick together. Vibration, changes in temperature, and pressure causes the threads to rub against each other. This build-up of friction can cause excessive wear and tear on the threads, leading to seized bolts, and stripped threads. Galling makes it impossible to disassemble the threaded parts without breaking them.
Thread galling is commonly found in bolted joints, and it’s often caused by insufficient lubrication or incorrect assembly.
One of the main reasons thread galling happens is because of how the threaded parts are manufactured. On a bolt, the threading is cold rolled. This means, the bolt blank was rolled between thread dies to form the threads. The threads on a nut are formed with a cutting tool.
If you looked at a matching nut and bolt under a microscope, you’d see that neither set of threads is perfect. This difference in manufacturing methods can cause some resistance when the nut and bolt are threaded together.
Thread galling is also caused by heavy pressure on the threaded assembly. If a threaded part is holding weight, or being pressed up against something, the threads are being pushed into each other which can cause galling over time.
Vibrations or movement in an assembly also causes galling. Because the threads aren’t perfectly matched together, if the parts are moving there will be friction and heat causing galling.
Temperature changes also cause thread galling. The metals expand and contract and the threads rub against each other.
When most people experience thread galling, it’s happening with stainless steel fasteners. Any type of metal can be affected by thread galling. However, harder metal alloys are less susceptible.
Metals that are more prone to thread galling are stainless steel, aluminum, brass, and titanium. Galling can happen to other materials depending on the design of the threaded assembly, and the surrounding environment.
Thread galling can happen right away when the threaded parts are being assembled. If you tighten a nut onto a bolt and experience friction, the threads will heat up and stick together. That galling can happen in any environment, but it can be avoided with proper assembly and anti-seize lubricant.
Galling can also happen over time. It’s more likely to happen when threaded parts are exposed to high temperatures, high pressures, and high levels of vibration. When there’s more movement or pressure, the threads are rubbing against each other more. And in high temperature applications, the metals are weaker and more likely to deform.
You can’t guarantee that thread galling will never happen, but there are steps you can take to reduce the chances of it happening.
Thread galling can be prevented by using proper lubrication before assembly. Adding a small amount of anti-seize lubricant onto your bolt creates a barrier between the metal threads, reducing the chances of galling. You’ll also want to clean off all the threads to avoid any debris between the threads.
A lot of thread galling happens during assembly. If you’re screwing a nut onto a bolt and it gets stuck, you might try to keep threading it on. This will cause thread galling and can snap the bolt, or ruin the threading on both parts.
Making sure the parts aren’t cross threaded, and assembling the parts slowly helps prevent galling.
Using two different metal alloys (like a softer metal and harder metal) can also reduce thread galling. Two stainless steel parts can stick when they’re being assembled, but a stainless steel bolt and brass nut are less likely to have threading issues. Of course, there are many situations that require certain alloys to be used, like in the marine industry.
Thread galling can be repaired in some cases by adding new threading to the parts. However, it’s usually best to aim to avoid it or replace the damaged parts.
Finer threads are sometimes easier to cross thread, but thread galling can occur in both large and small threads. The size of the thread doesn’t necessarily make it more or less prone to galling.
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