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They're DIN 985 self-locking nuts

DIN 985 self-locking nut

that have a plastic ring inside that is meant to cause extra friction and prevent self-loosening of the nut.

How effective are they against self-loosening?

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I've had good experience with them, but nothing too demanding, so I'm not putting that as an answer. They are annoying to close on long bolts - you can't turn them with your fingers... –  Eli Iser Feb 4 '13 at 8:32
    
They work great, especially when coupled with a lock or star washer. Give them a try, I think you will like the performance. –  shirlock homes Feb 4 '13 at 11:43

2 Answers 2

In the General Aviation aircraft industry, AN365 / MS20365 full-profile Nylon insert self-locking nuts are an important staple. Use them with the appropriate washer and in appropriate locations.

Nylon insert self-locking nuts major weakness is heat, they are recommended to only be used in areas that never exceed 250°F (~121°C). The heat softens the Nylon, reducing its gripping power.

For high temperature use, metal construction self-locking nuts are used. Nylon is a little more gentle on the threads, the full metal nuts either have one end ovaled or have the end castellated and compressed to grip the threads and can be like running a thread cutting die on the bolt if disassembled multiple times.

Straked Top Lock, Nylon Insert, Torque Nut Self-locking Nuts

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Their effectiveness will vary by usage and manufacturer. They will obviously "hold" differently if the usage is a paint shaker versus say a more static object. As well, a poorly produced nut will be less effective than a higher quality product.

As a commenter mentioned, you can further improve the performance by coupling with a lock washer.

If you really want to ensure it doesn't budge, you can also apply a thread-lock compound to the bolt threads.

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