Reduced-Height Inch-Series Hex Nuts for Vibration-Resistant Double-Nut Locking in Industrial and Structural Bolted Assemblies.
ASME B18.2.2 Lock Nuts, designated hex jam nuts in ASME B18.2.2 Table 4, are inch-series internally threaded fasteners with a reduced body height compared to a standard ASME B18.2.2 regular hex nut, specifically engineered to function as a locking element in double-nut configurations where the jam nut is tightened firmly against the face of a primary full-height nut to create a mechanically locked bolted joint that resists self-loosening under vibration, dynamic loading, and thermal cycling without the need for thread-locking adhesives, nylon inserts, or spring washers. The ASME B18.2.2 standard defines precise dimensional requirements for hex jam nuts including width across flats, width across corners, reduced jam nut thickness H1, chamfer, and Class 2B thread specifications for nominal sizes from 1/4 inch through 4 inches, maintaining the same across-flats dimension as the standard hex nut in each size but with a significantly reduced thickness that identifies them unambiguously as the jam nut in a double-nut assembly. When manufactured to ASTM A1082, the specification for high-strength stainless steel bolting material, ASME B18.2.2 Lock Nuts provide excellent corrosion resistance and reliable mechanical integrity in aggressive service environments including marine equipment, offshore structural connections, chemical processing plant machinery, food-grade production equipment, and coastal infrastructure where the combination of vibration loading and corrosive atmosphere would cause standard carbon steel jam nuts to corrode rapidly and lose their effectiveness as a locking element over time. The correct installation procedure for a double-nut locking system using ASME B18.2.2 lock nuts requires the jam nut to be applied first and set to a specified low torque, followed by the full hex nut tightened to full specified torque against the jam nut , this sequence correctly loads the thread flanks in opposing directions and locks the assembly. ASTM A1082 ASME B18.2.2 lock nuts are available with EN 10204 Type 3.1 material certification for all specification-controlled industrial procurement requirements.
| Nominal Size | Basic Major Diameter of Thread | Width Across Flats, F | Width Across Corners, G | Thickness, H1 | Runout, FIM | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| Basic | Min. | Max. | Min. | Max. | Basic | Min. | Max. | |||
| 1 1/8 | 1.1250 | 1 11/16 | 1.631 | 1.688 | 1.859 | 1.949 | 5/8 | 0.595 | 0.655 | 0.029 |
| 1 1/4 | 1.2500 | 1 7/8 | 1.812 | 1.875 | 2.066 | 2.165 | 3/4 | 0.718 | 0.782 | 0.032 |
| 1 3/8 | 1.3750 | 2 1/16 | 1.994 | 2.062 | 2.273 | 2.382 | 13/16 | 0.778 | 0.846 | 0.035 |
| 1 1/2 | 1.5000 | 2 1/4 | 2.175 | 2.250 | 2.480 | 2.598 | 7/8 | 0.839 | 0.911 | 0.039 |
ASTM A1082/A1082M (2016) Table 1 lists 14 duplex (ferritic-austenitic) grades, S31100, S31260, S31803, S32101, S32202, S32205 (2205), S32304 (2304), S32506, S32550 (255), S32750 (2507), S32760, S32906, S32950 and S39277, and 5 precipitation-hardening grades, S15700 (Type 632), S17400 (Type 630), S17600 (Type 635), S17700 (Type 631) and S35500 (Type 634). Grade selection depends on design, service conditions and the mechanical properties required.
Dimensions follow ASME B18.2.2, tabulated on this page. ASTM A1082/A1082M sets chemistry (Table 1), heat treatment and mechanical properties (Tables 2 to 4) and marking (Table 5). ASTM A962/A962M general requirements also apply, and A1082 prevails where the two conflict (clause 4.1). Inch-pound units apply unless the order specifies A1082M (clause 1.4).
ISO 15156-3 Table A.24 accepts wrought duplex stainless steel for any equipment when it is solution-annealed and liquid-quenched or rapidly cooled, has 35 % to 65 % ferrite and has had no ageing heat treatment. With 30 ≤ PREN ≤ 40 and Mo ≥ 1.5 % the limit is 232 °C (450 °F) and 10 kPa (1.5 psi) H2S partial pressure; with 40 < PREN ≤ 45 it is 232 °C (450 °F) and 20 kPa (3 psi). Table A.27 accepts wrought 17-4 PH (S17400) at 33 HRC max after a double age at 620 °C (1150 °F) or a modified double age, for named wellhead, valve and subsurface components, up to 3.4 kPa (0.5 psi) H2S and pH 4.5 or above.
Two supplementary requirements apply to nuts when the purchaser specifies them. S5 requires proof load testing of nuts, with the number of tests and the proof loads agreed between purchaser and manufacturer. S6 requires cross-sectional hardness testing of nuts per ASTM A370 Annex A3, with the number of tests agreed. Certification reports chemical analysis, mechanical tests and the heat treatment method (clause 10.1).
Table 4: solution annealed at 1880 to 2060 °F [1025 to 1125 °C], water quenched. Up to and including 2 in.: tensile 116 ksi [800 MPa] and yield 80 ksi [550 MPa] min. Over 2 in.: 110 ksi [760 MPa] and 75 ksi [515 MPa] min. Elongation 15 % min and hardness 33 HRC or 310 HB max for both. The marking grade symbol is 32750 (Table 5).
