Prevailing-torque lock nuts for vibration-prone marine, offshore, and rotating-equipment service
Lock nuts hold preload where transverse vibration would loosen a plain hex nut. Marine deck equipment, subsea pump foundations, downhole tool string assemblies, rotating compressor skids, and any bolted joint subject to fatigue cycling benefit from a prevailing-torque locking feature. Ferralium 255 (UNS S32550, EN 1.4507) lock nuts combine that vibration resistance with super duplex chloride-pitting and sour-service compliance per NACE MR0175 / ISO 15156-3 Table A.10, hardness ≤ 32 HRC, ferrite content 35 to 55%. RFQ: info@torqbolt.com / WhatsApp +91-22-66157017.
| Type | Standard | Locking mechanism | Service envelope |
|---|---|---|---|
| Nyloc (nylon insert) | DIN 985, ISO 10511 | Polyamide ring deforms over thread | Up to 120 °C, dry service |
| All-metal prevailing torque | DIN 980, IFI-100 | Distorted thread crown engages bolt | Up to 232 °C, qualifies for sour service |
| Stover (one-way) | ASME B18.16.6 | Top-thread distortion only | Up to 232 °C, single-direction install |
| Castle / slotted | ASME B18.2.2 | Cotter-pin retained | Service-temperature unlimited |
| Jam nut (thin) | ASME B18.2.2 | Wedge-locked against primary nut | General service, low cost |
Polyamide-insert nyloc nuts are restricted by polymer service temperature (typically 120 °C max), and the polymer ring is destroyed on installation, making the joint single-use. For high-temperature sour service or subsea wellhead applications where re-tightening cycles are expected, specify the all-metal prevailing-torque type per DIN 980. Castle nuts with cotter-pin retention remain the default for safety-critical lifting and crane hooks. Jam nuts are acceptable as secondary retention but never as the primary preload-bearing element.
Per-heat EN 10204 type 3.2 MTC, hardness traverse, ferrite-content per ASTM E562, NACE MR0175 conformance with Table A.10 reference. Marking on the nut face: manufacturer ID, type code (NM, AM, ST), heat number. NORSOK M-650 QTR available on request.
