The mechanical properties of Finished Hex Nuts are covered in the Society of Automotive Engineers (SAE) SAE J995, “Mechanical and Material Requirements for Steel Nuts”. This specification covers the materials, manufacturing methods, mechanical properties and testing requirements for these nuts. See Tables 2 & 3 for the mechanical properties.
The dimensions for Finished Hex Nuts are covered in the American National Standards Institute (ANSI) B18.2.2, “Square and Hex Nuts-Inch Series” (See Table 1). The size of the hex is the same as a cap screw head to permit using the same standard wrench size. Thread dimensions are per ANSI B1.1 and are manufactured to either a unified national coarse (UNC) or unified national fine (UNF) Class 2B thread tolerance.
In general, the proof load of the nut should be greater than the minimum tensile strength of the bolt to ensure that if overloaded, the bolt breaks rather than the nut threads shearing or stripping. Grade 5 Hex Nuts are designed to be used with Grade 5 Cap Screws and Grade 8 Nuts with Grade 8 Cap Screws. A stronger nut can nearly always be used with a weaker bolt (i.e., a Grade 8 Nut is acceptable for use on a Grade 5 Cap Screw).
Finished Hex Nuts are typically purchased plain or zinc electroplated. Grade 5 Nuts typically have a clear trivalent chromate finish, while Grade 8 Nuts typically have a yellow chromate finish.
The marking requirements are covered in SAE J995 and were last modified in July 1999 to improve the marking requirements. The following table and figure show the marking requirements: