Spring lock, Belleville, internal-tooth, external-tooth and serrated patterns for vibration resistance and preload retention
A lock washer between the nut face and the flange surface holds preload when transverse vibration would otherwise back the nut off. Spring (helical) lock washers per ASME B18.21.1 are the most common: a slit, axially-deflected coil that bites into both bearing surfaces on torque-up. Belleville disc-spring washers per DIN 6796 distribute preload through elastic deflection and are preferred for soft-flange or thermally-cycled joints. Internal- and external-tooth lock washers per ASME B18.21.1 are used in low-preload electrical bonding service. Ferralium 255 (UNS S32550) lock washers carry the same NACE MR0175 / ISO 15156-3 Table A.10 sour-service stamp as the matched stud and nut. RFQ: info@torqbolt.com / WhatsApp +91-22-66157017.
| Pattern | Standard | Mechanism | Use case |
|---|---|---|---|
| Spring (helical) lock | ASME B18.21.1, DIN 127 | Slit coil deflects axially, bites bearing | General service, low to medium preload |
| Belleville disc-spring | DIN 6796, ISO 6770 | Conical disc deflects elastically | Thermally cycled joints, soft flanges |
| Internal-tooth | ASME B18.21.1 | Inward-facing teeth bite under-head | Electrical bonding, low preload |
| External-tooth | ASME B18.21.1 | Outward teeth bite OD bearing | Visible-tooth service, low preload |
| Serrated flange (Schnorr-type) | DIN 6798 | Radial serrations across full face | High-vibration, no separate lock-nut needed |
Spring lock washers have come under criticism in recent ISO and NORSOK bolting guidance because the helical deflection is largely consumed before full preload is reached, leaving the locking action minimal. Where the joint is safety-critical or subject to fatigue cycling, prefer either (a) Belleville disc-spring washers, (b) prevailing-torque lock nuts, or (c) a paired flat washer plus serrated washer combination. For electrical earthing or grounding straps the internal- or external-tooth pattern is acceptable since preload is not the primary functional requirement.
Per-heat EN 10204 type 3.2 MTC: chemistry per ASTM A479, hardness, ferrite-content per ASTM E562, NACE MR0175 conformance with Table A.10 reference. Spring rate (for Belleville) and minimum-deflection-load (for spring lock) characterised per the controlling DIN / ASME standard.
