Galling is a persistent issue that many engineers and fabricators encounter when working with fasteners. It happens when pressure and friction cause bolt threads to seize against the threads of a nut or tapped hole. The fasteners don’t just become overly tight; they effectively "cold weld" together. Even if you inspect all your fasteners before use, they might still fail after installation. So, how do we avoid this?
The main culprits here are typically metal fasteners. During the installation process, the protective oxide layer on these metals can wear off due to increased pressure and heat between the contacting thread surfaces. Without this oxide layer acting as a barrier, the raw metal threads come into direct contact, causing the nut and bolt to fuse together.
To prevent galling, there are several strategies you can adopt:
First, install slowly and correctly. Slower installation speeds reduce friction and heat generation, which are major contributors to galling. Over-tightening can exacerbate the problem, so maintaining control during installation is crucial. Additionally, applying lubrication and coatings can significantly reduce friction. When choosing a lubricant, always consider the final application—some lubricants may not be suitable for environments requiring food or laboratory-grade purity. It's also important to account for how lubricants alter the torque-tension relationship, as over-tightening remains a risk even when using lubricants. Some effective lubricants include molybdenum disulfide, film lubricants containing metals like silver or copper, and tribological coatings like PTFE, PVDF, PFA, and FEP. Extreme pressure waxes can also be effective in certain scenarios.
Choosing the right type of fastener threads is another way to minimize galling. Rolled threads tend to have smoother surfaces compared to cut threads, reducing friction and the potential for galling. Similarly, opting for coarse threads instead of fine ones can help since coarse threads have a larger thread allowance, reducing the number of friction-generating surfaces.
Another approach is to use dissimilar metals of varying hardness for the nut and bolt. This ensures that only one material is likely to wear away its protective oxide layer, thus minimizing galling. However, be cautious about creating a galvanic couple between the dissimilar materials, as this could accelerate corrosion, especially if the fasteners are exposed to liquid environments.
While stainless steel, aluminum, and titanium are particularly susceptible to galling during installation, it's not just an issue tied to tightening bolts. Thread friction can build over time, leading to delayed galling. Therefore, addressing this problem requires vigilance throughout the lifecycle of the fasteners, not just during initial installation.
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