High strength nuts cannot be used randomly. They must be matched with bolts of equivalent grade. Many engineering accidents happen because contractors pair high strength bolts with ordinary low-grade nuts.
For example, a 10.9 bolt must use 10 grade nut. The nut’s yield strength needs to meet or exceed the bolt, so the bolt breaks before the nut thread shears, which is the designed safety failure mode.
Surface treatment consistency is also important. If you use hot dip galvanized bolts, HDG nuts are recommended. The galvanizing layer will increase thread thickness, and tapping may be required to guarantee smooth assembly. When purchasing high strength nuts, ask for material test certificates to verify mechanical properties.
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