Written by Hiroki Goto | Engineering Manager
NBK America LLC
Have you ever had problems with fastener threads getting stuck? If so, and you attempted to remove the seized fastener by force, you likely ended up with little success to show for it other than a broken screw.
Galling is a form of wear that can lead to seizing, a phenomenon that occurs when a screw is unable to rotate and is locked into place. Unfortunately, these events are common and frustrating, occurring regularly during fastener installation when screws are tightened.
Friction and adhesion are typically blamed for this. However, outside factors can also play a role, such as foreign matter or burrs. Let’s discuss these culprits.
Friction and adhesion
Sometimes referred to as “cold welding,” galling occurs when a fastener expands because of the pressure or friction that results during the tightening of a screw upon installation. This causes adhesion in the fastener’s mating threads that can, in more severe cases, lead to seizing. This means the fastener threads are stuck and removal without damage is tough.
Galling and seizing are more likely to occur in high-temperature environments, which makes sense during installation as friction causes heat. Interestingly, the material of the fastener can also make a difference.
For example, stainless steel screws are more likely than other types of screws to seize because they have a:
• Higher coefficient of friction – twice that of general steel, which means that heat is easily generated
• Lower rate of thermal conductivity – one-third of that of general steel, which means any heat generated is not easily released
• Larger expansion coefficient – about twice that of general steel, which means the male and female threads will adhere easily to one another
Whenever fastener threads are prone to adhesion, there’s a greater chance they’ll fuse together. Sometimes with minor galling, it’s still possible to remove the screw without damage. But if the surfaces are completely fused, this is next to impossible. Also, if enough pressure builds between the contact threads, it can break any oxide coating that’s on the fasteners, further leading to adhesion and seizing.
Although galling is typically caused during fastener installation, the environment can also play a factor. For instance, if left outside in the hot sun or in warm conditions for a prolonged period of time, fasteners are likely to heat up. As mentioned, high-temperature conditions can lead to a greater chance of galling, whether that’s caused by friction during installation or the external environment.
Any debris sitting inside of the threading can also pose a risk of increased friction during installation. So, be sure to first check and only use clean parts. If necessary, use compressed air to ensure a clean surface before fastener use.
Similarly, foreign matter such as burrs can cause galling. A burr is a raised edge or small piece of material that’s stuck on the thread of a screw, typically resulting from the thread-rolling process.
Any damaged threading will increase the friction during fastener installation.
It’s important to note that galling and seizing can be prevented by checking the condition of the fastener threads before tightening and using the appropriate torque. Over-torquing a nut can cause damage to the threading, so it can help to use a torque wrench.
Pneumatic torque wrenches are designed to provide fast, safe, and simple fastening for applications that require controlled bolting solutions.
The load also matters. An increased load will force a fastener’s threading to push against itself while spinning during installation, adding to the friction. So, when possible, decrease the load of the assembly beforehand. Although changing the thermal conductivity or thermal expansion coefficient is difficult, the friction coefficient can be decreased during fastener installation. This is possible through the use of lubrication and/or surface treatments, such as anti-galling and anti-seize coatings.
Teflon coating is one example of surface treatment for seizing prevention that provides the added advantage of chemical resistance. Fluorine is another type of coating that has proven highly effective. As there are several options, depending on the application, be sure to discuss the ideal surface treatment with your fastener supplier to make the right choice.
As an alternative to coated screws and lubricant, surface-hardened screws reliably prevent galling and seizing. Over time, any coating added to fasteners will peel off, becoming a source of debris and contamination. This is particularly troublesome in certain industries, including in semiconductor manufacturing and food-processing machines.
Equipment that’s regularly exposed to high-temperature environments also faces a similar issue. Surface-hardened screws solve this problem, offering a simple fastening solution that helps prevent unnecessary galling challenges.