Hex head bolts have a head with six flat sides. The shoulder is an additional shank length that flares out before the start of the bolt thread. Different standards apply to heavy hex shoulder bolts based on their diameter.
The "Electromechanical Design Handbook" by Ronald Walsh states that these bolts can be made from chrome, nickel, molybdenum or vanadium alloys. Hex shoulder bolts have the unified coarse thread (UNC) class. Hex shoulder bolts must have a Rockwell hardness of C32 to C43.
The American Society of Mechanical Engineers (ASME) ASME Standard B18.2.1 is the square and hex bolts and screws, inch series standard. ASME Standard B18.2.8 is the clearance holes for bolts screws and studs standard, including clearance holes for hex shoulder bolts.
American Society for Testing and Materials (ASTM) Standard A193 B5 is the standard for stud bolts, hex head bolts, heavy hex nuts and screws. ASTM Standard A325M -- 09 gives the minimum tensile strength standards for metric hex shoulder bolts. ASTM A490M -- 10 ASTM A490M - 10 is the standard specification for high-strength steel bolts, Classes 10.9 and 10.9.3. This standard applies to heavy hex structural bolts with diameters M12 through M36.
According to the "Machinists' and Metalworkers' Pocket Reference" by Ronald Walsh, the Fastener Quality Act (1990) applies to any nut, bolt, screw or stud 0.25 inches or 5 millimetres in diameter or larger which is through-hardened, is required to have a grade designation on the fastener. This includes hex screws and hex stripper bolts.
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