Everything You Should Know About Thread Galling
January 31, 2023
Everything You Should Know About Thread Galling
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.
What causes thread galling?
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.
What metals are most likely to be affected by galling?
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.
Which environments does it happen in?
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.
Can you prevent galling?
You can’t guarantee that thread galling will never happen, but there are steps you can take to reduce the chances of it happening.
Proper lubrication
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.
Proper assembly techniques
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 metals
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.
Does thread galling happen more with smaller threads?
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.
Spex offers a large selection of custom precision machined parts for whatever your project needs. We are an ISO 9001:2015 certified company, and our team specializes in precision machining and supply chain efficiency.
Our machined components are available in an array of different materials and finishes and are manufactured to meet all the highest quantity needs.
Reach out to our team to get a quote for custom precision fasteners.
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Countersink vs. Counterbore Holes in Machining
March 7, 2023
Countersink vs. Counterbore Holes in Machining
Machined parts can have holes of all different shapes and sizes. Each type of hole has a name that describes the shape of it. Two popular types of machined holes are countersunk holes and counterbored holes.
Countersink vs. Counterbore Holes
The difference between countersink and counterbore holes is the shape of the top of the hole. Countersink holes have a cone-shape, while counterbore holes have a cylindrical shape.
Both types of holes are designed to hide the head of a fastener, making it flush with the part when it’s assembled. The different type of hole is designed to be used with different fasteners.
Fasteners that have an angled head require a countersunk hole to be flush. A fastener with a flat head needs a counterbored hole.
Countersunk holes
A countersunk hole is a cone-shaped hole that is drilled into a material. A cone-shaped cutting tool is used to machine the chamfer at the desired angle, and then the rest of the hole is drilled with a regular drill bit.
Countersink holes are often used in woodworking or when working with softer materials. The angle can be adjusted, but the 3 most common are 60°, 82°, and 90°. ISO standard screws often have a 90° angle. In the aerospace industry, countersunk fasteners usually have an angle of 100°
Advantages and disadvantages
Countersink holes are used to hide the screw head by making it flush with the metal. This protects the screws and increases their lifetime. This also improves the visual appearance, and can make the part or assembly more aerodynamic.
Drilling countersink holes does add to the machining cost compared to drilling a regular thru hole.
Counterbore holes
A counterbore hole is a type of machined hole created to seat a fastener flush with, or below, the surface of a part. Counterbore holes are typically shallower than a countersink hole and have straight sides rather than angled sides.
Washers and lock washers can also be used in counterbored holes to secure the fastener.
Advantages and disadvantages
A counterbore usually has more holding strength compared to a countersink hole because the force applied by the socket cap screw head is parallel to the axis. The force applied by the screw or bolt is distributed evenly over a larger surface area. This is not the case with a countersunk hole, which has tapered sides.
Countersink vs. Counterbore: Applications
Countersinks are typically used for wood and metal screws, while counterbore holes are used for larger fasteners such as lag bolts, and socket head fasteners.
Countersink and counterbore holes are often used in applications with moving parts, and for assemblies with limited space. Because they provide more space for lock washers, counterbores are used in heavy-duty applications including machinery, construction, and automotive applications.
Callout symbols
The callout symbol on a machining blueprint for a countersink hole is “⌵” and the callout symbol for a counterbore hole is “⌴”. The symbols are designed to match the top shape of each hole.
Countersink and counterbore sizes are expressed in terms of their diameter, depth, and angle of countersink. The diameter is the width of the countersink or counterbore, while the depth is the distance from the surface of the material being drilled down to the top of the pilot hole.
Spex offers a large selection of custom precision machined parts for whatever your project needs. We are an ISO 9001:2015 certified company, and our team specializes in precision machining and supply chain efficiency.
Our machined components are available in an array of different materials and finishes and are manufactured to meet all the highest quantity needs.
Reach out to our team to get a quote for custom precision fasteners.
Related content
Types of Precision Fasteners
Using Custom High-Strength Bolts
Contact us
Talk with one of our team members to get a quote for precision screws.
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