Lock Nuts

Lock Nut Types, Vibration-Resistance Mechanisms
& Engineering Selection

Part 01 — Types, Vibration-Resistance Mechanisms & Engineering Selection

Lock Nut Classifications · DIN 985 · DIN 982 · DIN 6923 · DIN 935
Nyloc · All-Metal · Prevailing Torque · Serrated Flange · Castellated

DIN 985 · DIN 982 · DIN 935 · DIN 6923 · ISO 7040 · ISO 7042 · Nyloc · All-Metal Prevailing Torque · Stover Lock Nut · Serrated Flange Nut · Castellated Nut · Disc Spring Lock Nut · Ramp Lock Nut · Junker Test DIN 65151 · 

Self-Loosening Mechanics and Lock Nut Engineering Principles

Bolt self-loosening under vibration is one of the most well-documented failure modes in mechanical engineering, responsible for a disproportionate share of industrial accidents, equipment failures and unplanned maintenance. The self-loosening mechanism, characterised by Gerhard Junker in the 1960s and formalised in DIN 65151, occurs when cyclic transverse (lateral) loading between the clamped joint members overcomes the thread friction that holds the nut stationary — producing incremental rotational back-off of the nut with each load cycle until the joint is loose. Critically, self-loosening does not require axial vibration: transverse loads perpendicular to the bolt axis are the primary driver.

Lock nuts interrupt the self-loosening mechanism through one of three engineering approaches. Prevailing torque (friction-based): the nut thread is permanently deformed or obstructed (polymer insert or metal deformation) so that an additional resisting torque must be overcome in both the tightening and loosening directions, regardless of joint preload. Direct tension indicators and locking geometries (interface-based): serrations, raised ridges or deformed faces bite into the bearing surface and resist rotation through mechanical interlock rather than thread friction. Positive mechanical retention (pin-and-slot): a cotter pin, safety wire or clip physically prevents rotation, providing a fail-safe locking mechanism independent of friction or preload.

Lock Nut Types — Complete Engineering Descriptions

Locking Mechanism Comparison — Selection Matrix

Dimensional Data, Governing Standards
& Prevailing Torque Reference

Part 02 — Dimensional Data, Standards & Prevailing Torque Reference

DIN 985 / DIN 982 / DIN 6923 / DIN 935 · M3 – M36
Width Across Flats · Nut Height · Flange Diameter · Prevailing Torque

M3 · M4 · M5 · M6 · M8 · M10 · M12 · M16 · M20 · M24 · M30 · M36 · Width Across Flats · Nut Height · Flange Diameter · Prevailing Torque Min · Prevailing Torque Max · DIN 985 · DIN 982 · DIN 6923 · DIN 935 · 

Nyloc Lock Nut — DIN 985 / ISO 7040 Dimensional Data

Governing Standards

DIN 985 and ISO 7040 cover full-height nylon insert lock nuts (Nyloc), specifying dimensions, prevailing torque limits and proof load per ISO 2320 for metric sizes M3–M36. DIN 982 and ISO 7042 cover all-metal prevailing torque lock nuts with deformed metal section, M3–M36. DIN 6923 and ISO 4161 cover serrated flange nuts, M5–M20. DIN 935 and ISO 7035 cover castellated (slotted) hex nuts, M6–M64. ISO 2320 defines the prevailing torque test method and acceptance limits applied across all prevailing torque lock nut standards. DIN 65151 and ISO 16130 define the Junker vibration test procedure for lock nut qualification.

Material Grades, Mechanical Properties
& Surface Treatments

Part 03 — Materials, Mechanical Properties & Surface Treatments

Carbon Steel · SS 304/316 · Duplex 2205 · Alloy Steel
Nyloc Temp Limit · NACE · HDG · Zinc · Dacromet

Carbon Steel Class 8 · Class 10 · A194 2H · A194 2HM NACE · SS 304 A2 · SS 316 A4 · Duplex 2205 · Super Duplex · Nyloc PA66 · Zinc B633 · HDG A153 · Dacromet ISO 10683 · Black Oxide · Passivation A380 · 

Critical Material Interactions with Locking Mechanisms

The PA66 (polyamide 66) nylon insert in a DIN 985 Nyloc nut has a maximum continuous service temperature of +120°C. Exposure above this temperature causes the nylon to soften and lose its interference against the bolt thread, permanently destroying the prevailing torque function. For applications above +120°C or where the fastener is exposed to oils, fuels, strong acids or bases that degrade polyamide, the all-metal DIN 982 prevailing torque nut is the mandatory alternative. PA66 nylon also absorbs moisture, causing slight dimensional swelling that can marginally increase prevailing torque values in high-humidity environments — verify prevailing torque range compliance under installed service conditions for precision applications.

Lock nuts in H≶S-containing service zones must comply with NACE MR0175 / ISO 15156 hardness limits. For all-metal prevailing torque lock nuts in carbon steel, the deformation process used to create the prevailing torque section can locally work-harden the nut to hardness levels exceeding the NACE maximum of 22 HRC (237 HB). This makes standard DIN 982 all-metal lock nuts non-compliant for sour service. The NACE-compliant options are: (a) ASTM A194 Grade 2HM prevailing torque heavy hex nut (hardness verified ≤235 HB through full cross-section); or (b) duplex 2205 or super duplex lock nuts per NACE MR0175 Part 3.

Inspection, QC Protocols, Applications
& Export Documentation

Part 04 — QC Protocols, Applications & Export Documentation

EN 10204 3.1/3.2 · Prevailing Torque Test ISO 2320 · Thread Gauging
Machinery · Piping · Offshore · Automotive · Wind Turbines

EN 10204 3.1 · 3.2 MTC · ISO 9001:2015 · Prevailing Torque ISO 2320 · Junker Test DIN 65151 · Thread Go/No-Go · AQL ISO 2859 · PMI XRF · ISPM-15 · VCI · Vibration · Machinery · Piping · Offshore · Wind Turbine · Automotive · 

Inspection and Quality Control

The primary QC test for all prevailing torque lock nuts (DIN 985, DIN 982, Stover, serrated flange) is the prevailing torque test per ISO 2320. A calibrated torque wrench engages and tightens the nut onto a reference bolt to the seating surface, then disengages. The nut is removed with a calibrated torque wrench, and the measured prevailing torque during removal (off-torque) is compared against the ISO 2320 limits (minimum and maximum values for each thread size and property class). Both min and max limits must be satisfied — a prevailing torque above maximum is as non-conforming as one below minimum. For production batches, prevailing torque testing is performed at AQL 1.0 sampling frequency per ISO 2859 for standard supply, and 100% for safety-critical applications.

Prevailing torque lock nuts — particularly DIN 985 (Nyloc) — lose prevailing torque effectiveness with each assembly/disassembly cycle. The prevailing torque on the first assembly is the rated value per DIN 985; by the fifth re-use, it can drop below the ISO 2320 minimum. For safety-critical applications (pressure vessels, structural, ATEX), single-use is mandatory: the lock nut must be discarded after first disassembly and replaced with a new unit. Re-use of prevailing torque lock nuts beyond the manufacturer's specified maximum is a maintenance procedure non-conformance and must be documented in the bolting management plan.

Width across flats, nut height, and (for DIN 6923) flange diameter and serration geometry are verified per the applicable standard. Thread gauging (Go/No-Go per ISO 1502 or ASME B1.2) is performed on each batch. For DIN 985 Nyloc nuts, the nylon insert inner diameter is gauged — if the insert bore is too large (worn tooling), the prevailing torque will be below minimum; if too small, the Go gauge will not pass. For all-metal DIN 982 nuts, the deformed section geometry is visually inspected for consistent deformation pattern and measured for the reduction in thread bore diameter at the deformed section.

Type 3.1: Minimum for all EPC, OEM and structural project supply. Must state material standard, grade, heat/lot number, chemical analysis and mechanical test results. For lock nuts, the certificate must explicitly state the prevailing torque test results (min/max per ISO 2320) as measured characteristics — a generic MTC without prevailing torque data does not confirm the lock nut's functional performance. Type 3.2: Co-validated by purchaser-nominated TPI body. Mandatory for offshore, NACE, nuclear-adjacent and structural safety-critical applications. For NACE 2HM lock nuts, full cross-section hardness mapping is required as part of the 3.2 certificate.

Applications by Industry

Lock nuts are the mandatory fastening solution in rotating machinery subject to dynamic loading: compressors, pumps, turbines, gearboxes, electric motors and fans. Nyloc (DIN 985) lock nuts are used throughout machine enclosures and access panel bolting in moderate vibration environments. All-metal prevailing torque (DIN 982) lock nuts are specified for elevated-temperature applications such as exhaust manifold studs, turbine casing bolting and motor end-bell covers. Ramp-locking nut designs (Nord-Lock, Hardlock) are specified in the most severe vibration applications where DIN 985/982 prevailing torque is insufficient to pass the Junker test at the design preload level.

Heavy hex prevailing torque nuts per ASTM A194 Grade 2H (or 2HM for sour service) are specified in ASME B31.3 process piping and ASME VIII pressure vessel bolted flange joints subject to vibration (pulsating flow, compressor discharge, pump suction). The prevailing torque provides supplementary vibration resistance beyond the flange preload. Offshore and petrochemical EPC projects specify EN 10204 3.2 MTC, PMI, full cross-section hardness survey (for 2HM) and TPI countersignature on all lock nuts in safety-critical service.

Ramp-locking nuts and wedge-locking washer systems are standard on wind turbine tower flange bolting, where the dynamic cyclic loading from wind-induced tower bending produces the exact Junker-type transverse loading that causes conventional lock nuts to fail progressively. VDI 2230 (Systematic Calculation of Highly Stressed Bolted Joints) is the design methodology applied to wind turbine tower bolts, requiring lock nuts to pass the Junker test with ≥90% preload retention after 100 cycles at the design preload.

Castellated nuts with split cotter pins (DIN 935) remain mandatory in safety-critical automotive wheel bearing retainers and steering knuckle assemblies where positive mechanical retention is required as a fail-safe against preload loss. In railway track fastening, spring clip and wedge-locking systems are preferred. In aerospace, as-specified self-locking nut designs (MS and NAS standards) and safety wired nut assemblies are mandatory in primary structure and flight control bolted joints per AC 43.13-1B (FAA) or equivalent authority advisory.

Stainless steel 316L and duplex 2205 lock nuts are specified throughout offshore topside equipment, cable gland plates, structural framing connections and instrument panel bolting in chloride-laden environments. NACE MR0175-compliant A194 2HM prevailing torque nuts cover all sour service H≶S-zone fastening where vibration resistance is required in addition to corrosion resistance. All stainless steel lock nuts for offshore applications require EN 10204 3.2 MTC with PMI and TPI countersignature.

Export Packaging and Preservation

• Lock nuts packed in heat-sealed polypropylene bags, batch-labelled with PO number, type (DIN 985 / DIN 982 / DIN 6923 / DIN 935), grade, thread size, heat/lot number and quantity
• VCI (Volatile Corrosion Inhibitor) poly liner for plain and black-oxide carbon steel lock nuts destined for sea freight or extended site storage
• Nyloc (DIN 985) lock nuts must be stored at ambient temperature (below +40°C) away from UV light and oils — heat and UV degrade the PA66 insert and reduce prevailing torque effectiveness
• Bags packed in double-wall corrugated cartons with foam or kraft paper void fill; prevailing torque mechanism (nylon insert, deformed section) must not be damaged by inter-nut abrasion during transport
• Cartons secured on ISPM-15 heat-treated timber pallets with stretch wrap and minimum two steel strapping bands per pallet layer
• MTC, prevailing torque test report (ISO 2320), thread gauge certificate and all project documents in waterproof sealed envelope attached to crate exterior
• Outer crate labels: PO number, item tag, material grade, type, standard, heat number, quantity, gross weight, country of origin, handling/storage instructions