Galling is a common complication in industrial operations that involve fasteners. Often it leads to damage or seizing of threaded components that can impact the quality and performance of expensive equipment.
Understanding what causes galling can help prevent it. Here are the main factors that contribute to its formation: Friction and Heat, Surface Condition, Debris, Thread Smoothness and Cleanliness.
Friction
Although galling can occur between a wide range of metals, it is particularly pronounced with stainless steel hardware. Stainless steel is soft compared to other common alloys, and it tends to adhere to itself when rubbed against other harder materials. This adhesion, in turn, causes excess friction and heat.
This excessive friction and heat can cause a number of issues, including corrosion. In addition, it can lead to premature wear of tooling and equipment. Luckily, there are steps that can be taken to reduce the likelihood of galling.
The first step is to use a lubricant. This can be pre-applied or added during assembly. A good lubricant will reduce the friction between surfaces and help prevent galling. However, even with a good lubricant, some metals are more prone to galling than others. If a galling problem is encountered, the installer should slow down the tightening speed to allow the heat generated by the friction to dissipate.
Heat
Although friction causes heat, the two surfaces do not actually reach a melting point in galling. Instead, the metals become fused together via adhesion and form miniature welds on their respective surfaces.
Ductile materials like aluminum and austenitic stainless steel are prone to galling in unlubricated environments. Precipitation hardened metals, however, are more resistant to galling than soft alloys.
Engineers, designers and manufacturers who handle stainless steel fasteners should understand what causes galling. This condition can damage critical parts, impact productivity and increase maintenance costs.
To reduce the chance of galling, it is recommended to use a anti-seize lubricant before installation. This lubricant is specifically formulated to prevent galling by creating a barrier between the bolt and nut. Additionally, it is recommended to reduce the torque speed during installation to minimize friction. This may help avoid the formation of the gilded oxide layer, which is known as galling.
Debris
Debris is a collection of fragmented pieces or parts, often from a broken down or destroyed item. It can also refer to the scattered remains after a natural disaster or accident. For example, the city after a hurricane was strewn with debris.
In science and engineering, debris often refers to any substance that has become separated from its original form. For instance, a piece of glass that has been shattered is considered debris. It is also common for surgical instruments to be contaminated with debris, which can lead to cross-infections or nosocomial infections if not cleaned and disinfected.
In metalworking, galling is a problem that can be avoided by using the right stainless steel materials and taking the necessary precautions. It can be very expensive and frustrating if galling occurs on a project, so it is important to take the right steps to avoid this issue. This includes the use of proper lubrication, choosing the right tools and being aware of what causes galling to reduce the chances of it occurring in your projects.
Surface Finish
Surface finish is a physical property that describes the texture and roughness of a part’s surface. It affects both aesthetics and functional characteristics like lubricity, galling resistance, and material strength. Different manufacturing processes produce varying surface finishes, so careful forethought is needed when selecting a process for any given product.
The more polished or rough a surface is, the more likely it is to gall. For this reason, it is important to use a high-quality lubricant with your threaded connections and to keep the wrench speed low. Electropolishing can help you achieve a smoother surface that will resist galling.
When selecting a surface finish, it is also helpful to reference a roughness chart. This will help you determine how smooth or rough a surface is by describing parameters such as Ra, Rq, and cla (center line average). Look for a chart that includes both American and metric units to avoid confusion when reading the results.what is galling
