Nylon insert locknut issues
July 21, 2025
By Bob Lund | Engineering Manager
By Bob Lund | Engineering Manager
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Each year in the fall and winter, we see a dramatic increase in customer complaints about nylon insert lock nuts. I’m sharing this now to help you understand what’s happening, should this issue arise in your plants in the coming months.
Over time, and in certain environments, nylon and other plastics become brittle. This occurs naturally, and there’s not much we can do to prevent it entirely. There are some steps you can take to help, but you can’t stop it altogether. When the nylon insert becomes brittle, the interaction between the bolt and the nut—specifically the nylon ring—can result in different types of failures:
The ring being pushed completely through is the most commonly reported issue, likely because it’s the easiest to see. However, cracked or deformed nylon is probably more common. Nylon 66 is typically used for the nylon ring in lock nuts. Like any plastic, it becomes brittle over time. We see a few issues like this in the summer months, but not many. In the winter, these problems become very common, and we field calls regularly. The drier air in winter draws moisture out of the nylon more quickly—this isn’t a very technical explanation, but you get the idea. Nylon is also affected by UV rays, so keeping the parts out of direct sunlight will help them last longer. There are several manufacturers of nylon insert nuts. They all use white as a generic base color but include a colored ring specific to their brand. This serves as a sort of head marking in our industry—the red ring folks are a different manufacturer than the blue ring folks, etc. We receive about equal numbers of complaints across all colors, so switching manufacturers isn’t necessarily the solution. Even though it might work in the short term, it has no greater chance of success than getting a different lot from the same manufacturer you’re currently using. Other variables can worsen the issue. If you’re using a lubricant and a hand wrench, you’ll see fewer problems than if you’re assembling nuts and bolts without lubricant and using an impact wrench. Impact wrenches and other high-speed installation tools are generally not friendly to fasteners, and when a nylon ring starts to become brittle, problems will show up first during impact wrench installation. How do you fix this? First, everyone should understand that this is a normal issue when using this type of product. It’s similar to galling when installing stainless steel parts—you can go months or years without problems, then suddenly experience multiple issues, even across different lots. Using a lubricant and slower installation speed are both effective ways to reduce nylon insert troubles and stainless steel galling issues. You can also rehydrate the nylon. A simple method that may allow continued use of an impact wrench—while lowering the percentage of issues—is to soak the nuts in water overnight. Deionized water works best, as it shouldn’t affect corrosion down the road, but even tap water will work fine to rehydrate the nylon and will have minimal impact on plating in terms of advancing corrosion. Give this a try and see if your problems go away. You should at least see a significant drop in the number of issues. Fastenal Company disclaims all warranties, both express and implied, relating to the information, reports, opinions and analysis disclosed to the Customer by Fastenal Company. Fastenal Company shall not be liable for any errors or omissions or any losses, injuries or damages arising from the use of such information, reports, opinions and analysis by the Customer.
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