How to Prevent and Solve Galling in Bolts and Nuts: Expert Solutions

Understanding Galled Threads: What They Are and How to Deal With Them

Galled threads. It sounds like something you might find in a sci-fi movie or maybe a bizarre mechanical term. But in reality, it’s something that anyone who works with fasteners or deals with metals regularly should know about. Galled threads are one of those pesky issues that can lead to a lot of frustration, wasted time, and even damage to your materials or machinery. In this feature, we’re going to break down exactly what galled threads are, how they happen, and most importantly, how to avoid them (or at least fix them when they occur).

What is a Galled Thread?

To put it simply, galled threads happen when the threads on a fastener or nut become damaged due to excessive friction. It’s like two pieces of metal getting stuck together in a battle of attrition, gradually eroding each other under pressure. This results in a build-up of metal debris in the threads, causing a rough and often jagged surface. When a fastener experiences galling, it can seize up, making it difficult or impossible to unscrew or adjust without causing further damage.

The damage caused by galling typically happens when materials undergo repeated contact and friction—whether it’s when you’re tightening a bolt or simply moving a nut. This friction generates heat, and that heat can lead to the transfer of material from one part of the fastener to another, leaving behind those unsightly and problematic grooves. The process can be described as “cold welding,” where the materials, though they aren’t molten, stick together due to heat and pressure.

What Causes Galling?

Galling is usually the result of two factors: friction and the materials involved. When you apply pressure to a fastener, like a screw or bolt, friction is generated between the threads as the parts come together. But in some cases, the friction gets so intense that the metal starts to stick to itself, causing small bits of the metal to tear away. It’s almost like the threads are “grabbing” onto each other in a vice-like grip.

The main culprit behind galling is the material itself. Some metals are just more prone to galling than others, due to their hardness, malleability, and how they react under pressure. Galling is most common when soft metals like aluminum, copper, or stainless steel come into contact with each other or harder materials.

What Metals Are Prone to Galling?

Certain metals are particularly susceptible to galling, often because of their softness and tendency to deform under pressure. These include:

• Stainless Steel: Although stainless steel is tough and resistant to corrosion, it is also one of the most common materials prone to galling. The issue becomes even more apparent when you’re dealing with high-strength stainless steels or fine-thread fasteners.
• Aluminum: Aluminum alloys are light and corrosion-resistant, but they are relatively soft, which makes them more likely to experience galling under friction.
• Copper and Brass: Like aluminum, copper and its alloys are soft and prone to galls, especially when they’re used in tight-fitting threaded components.
• Titanium: Although titanium is strong and resistant to corrosion, it is also susceptible to galling when paired with certain materials, especially when there isn’t adequate lubrication.

So, while stainless steel is often the go-to metal for its strength and corrosion resistance, it’s also the metal that seems to gall most frequently, especially when it’s used in threaded connections.

Does Titanium Gall with Stainless Steel?

Ah, titanium and stainless steel. Two powerhouses in the world of materials, but can they peacefully coexist without galling? Well, it depends. While titanium is a strong, lightweight metal with great corrosion resistance, it can still experience galling when paired with stainless steel—particularly in high-stress environments or when lubrication isn’t applied. However, titanium is less likely to gall than softer metals, so it can still hold its own in many cases. To avoid issues, it’s usually a good idea to use coatings or lubrication when connecting titanium and stainless steel components.

How to Identify Galling

So, how do you spot galling when it happens? It’s usually pretty obvious once you know what to look for. Here are the main signs of galled threads:

1. Visible Scarring: Galling typically leaves behind visible marks on the threads, making them look scratched, rough, or even torn. The metal may have a discolored appearance due to the heat generated by friction.
2. Difficulty Tightening or Loosening: One of the most frustrating signs of galling is when the fastener becomes hard to turn. You might feel a rough or sticky sensation as you tighten the screw or bolt, and when you try to remove it, the threads can feel “stuck” or stripped.
3. Metal Debris: In severe cases, you might notice small metal shavings or debris around the fastener. This is often a sign that the metal has started to transfer between the threads, leaving behind residues of the material.
4. Seizing: Galling can eventually lead to a full-on seizing of the fastener, where the threads lock together and prevent any movement. This can sometimes result in the entire fastener breaking or snapping.

Why Galling Happens (And Why It Most Often Happens)

To understand why galling happens, think about the materials that are involved. As mentioned earlier, softer metals tend to be more prone to galling, and high friction makes it worse. When metals are pressed together tightly, the microscopic peaks and valleys in the metal surfaces create friction, and over time, that friction can build up. Eventually, the pressure causes the metal to “cold-weld,” bonding together in a way that makes it difficult to unscrew or loosen.

This is why galling often happens with stainless steel, aluminum, and other soft metals. When these metals are under high stress or in environments where they experience repeated movement (like on fasteners), the chances of galling increase.

Does Galling Lead to Seizing?

Yes, in extreme cases, galling can lead to seizing. Once the threads become damaged, the fastener can become stuck, and no amount of tightening or loosening will work. If this happens, you could find yourself in a situation where the only solution is to cut the fastener off or replace it entirely. In some cases, continued tightening of a galled fastener can lead to the breakage of the fastener itself, or worse, the threads may be torn off entirely, leading to irreversible damage.

Can Lubricants or Coatings Prevent Galling?

Here’s the good news: Yes, lubricants and coatings can help prevent galling, and there are a variety of options depending on the situation. Using the right lubricant or coating can reduce friction, disperse heat, and help prevent the transfer of material between threads.

• Anti-seize Lubricants: These are specifically designed to prevent galling by reducing friction. They contain ingredients like graphite, copper, or zinc, which help to lubricate the threads and minimize the risk of cold-welding.
• PTFE Coatings: Coatings like Teflon (PTFE) can be applied to threads to reduce friction and make it harder for the threads to seize up.
• Dry Lubricants: These are particularly useful when you don’t want to deal with the mess of wet lubricants. Dry lubricants, like graphite or molybdenum disulfide, create a low-friction layer that can keep galling at bay.
• Coated Fasteners: Some fasteners are made with special coatings that help reduce friction and prevent galling. These coatings can be made of various materials, including nickel, zinc, and even ceramic coatings.

Options to Fix Galled Threads

If you’re dealing with galled threads, don’t panic—there are ways to fix the problem. Here are some common methods:

• Use a Thread Repair Kit: If the galling is minor, you can use a thread repair kit (like a helicoil or timesert) to re-cut or replace the damaged threads.
• Use a Thread Chaser or Die: A thread chaser or die can be used to clean up damaged threads, especially if the fastener is still usable.
• Re-tapping: In cases where the threads are completely shot, you may need to re-tap the hole to create new threads.

Watch this easy-to-follow tutorial as Joe from Eastwood as he demonstrates how to use a tap-dye set.

How to Avoid Galling

Prevention is always better than cure, and when it comes to galled threads, there are a few things you can do to avoid it altogether:

• Use the Right Fastener: Make sure the materials you’re using for your fasteners are compatible and less prone to galling.
• Lubricate: As mentioned earlier, lubrication is your best friend. Use anti-seize or dry lubricants when dealing with materials that are prone to galling.
• Proper Torque: Tighten the fastener to the right torque specification. Over-tightening can increase friction, while under-tightening can make it easier for galling to occur.
• Choose the Right Materials: Avoid using soft metals like aluminum or brass with stainless steel or other metals that are prone to galling.

In conclusion, galled threads are a common yet frustrating problem in the world of fasteners and metals. However, by understanding what causes them, recognizing the signs early, and taking proactive measures like lubrication, you can avoid most of the headaches that come with them. And if you do run into a galled fastener? Don’t worry—there are ways to fix it and keep things running smoothly.