LOCK NUT

1. Industry Context

Lock nuts are critical mechanical locking elements used in safety-critical bolted assemblies where vibration, dynamic loading, thermal cycling, or rotational forces may cause conventional threaded joints to loosen.

Across modern industrial infrastructure, loss of preload remains one of the most common causes of joint failure. Studies within heavy engineering sectors show that self-loosening under transverse vibration accounts for a significant percentage of mechanical failures in rotating machinery, structural assemblies, pressure equipment, and transport systems.

Therefore, lock nuts are not simply fastening accessories — they are engineered reliability components designed to maintain clamping force throughout the service life of an assembly.

1.1 Global Industrial Dependence

Lock nuts are standard requirements across EPC specifications and OEM equipment designs in:

• Structural steel connections
• Rotating machinery
• Pipeline systems
• Offshore platforms
• Heavy transport equipment
• High-temperature process plants
• Railway and infrastructure assemblies
• Power generation turbines

Engineering procurement specifications frequently mandate locking systems complying with:

• ISO fastening systems
• ASTM material standards
• DIN mechanical locking designs
• BS structural applications

SM Fasteners manufactures lock nuts within an ISO 9001 certified quality management system, supporting global supply chains requiring full traceability and inspection compliance.

1.2 Why Conventional Nuts Fail

A standard nut relies purely on friction generated by preload.

Under operational conditions:

• Vibration reduces friction
• Thermal expansion alters preload
• Cyclic loads induce micro-movement
• Joint embedment reduces clamp force
• Surface relaxation occurs

This leads to self-loosening, described by the Junker vibration mechanism.

Lock nuts introduce secondary locking mechanisms that resist rotation independent of friction alone.

1.3 Role of Lock Nuts in Modern Engineering Design

Lock nuts provide:

✔ Preload retention
✔ Vibration resistance
✔ Rotational locking
✔ Enhanced fatigue resistance
✔ Safety redundancy

They are essential where joint failure could result in:

• Equipment shutdown
• Structural instability
• Leakage of hazardous media
• Personnel safety risk

2. Technical Definition

A Lock Nut is a threaded fastener incorporating a mechanical, prevailing torque, deformation, or auxiliary locking feature designed to prevent loosening after tightening.

Unlike standard hex nuts, lock nuts maintain resistance against rotation even when preload decreases.

2.1 Functional Definition

Engineering Definition

A lock nut is a threaded fastening device that maintains clamp load and resists rotation through controlled interference, elastic deformation, or integrated locking mechanisms.

2.2 Fundamental Locking Methods

Lock nuts operate using one or more of the following principles:

1. Prevailing Torque Locking

Resistance created by thread interference.

Examples:

• Nylon insert lock nut
• All-metal distorted thread nut

2. Mechanical Locking

Physical restraint prevents rotation.

Examples:

• Castellated nut with cotter pin
• Tab locking systems

3. Friction Enhancement

Increased frictional resistance.

Examples:

• Serrated flange lock nut

4. Chemical Locking (System Level)

Used with thread lockers rather than internal nut design.

2.3 Distinction from Standard Nuts

Feature	Standard Nut	Lock Nut
Rotation resistance	Friction only	Mechanical + friction
Vibration performance	Moderate	High
Reusability	High	Type dependent
Safety critical use	Limited	Preferred
Preload retention	Variable	Controlled

3. Load Mechanics & Force Behavior

Understanding lock nut performance requires analysis of bolted joint mechanics.

3.1 The Bolted Joint System

A bolted joint consists of:

• Nut (lock nut)
• Clamped members
• Washer/interface surfaces

The nut converts applied torque into:

• Bolt tension
• Joint compression
• Frictional resistance

3.2 Preload Generation

When tightening torque is applied:$$T = K \times F \times D$$T=K×F×D

Where:

• T = Applied torque
• K = Nut factor (friction coefficient)
• F = Preload force
• D = Nominal diameter

Typical nut factor values:

Condition	Nut Factor (K)
Dry	0.20–0.25
Zinc plated	0.18
Lubricated	0.12–0.15
PTFE coated	0.10–0.13

Lock nuts must maintain this preload despite external disturbances.

3.3 Clamp Force Mechanics

The bolt behaves like a spring.

• Bolt elongates elastically.
• Joint compresses.
• Stored elastic energy maintains clamping force.

Loss of preload occurs when:

• External load exceeds joint stiffness
• Surfaces embed
• Thermal expansion mismatch occurs

Lock nuts reduce loosening even after partial preload loss.

3.4 Self-Loosening Under Vibration

Transverse vibration causes microscopic sliding between joint faces.

Sequence:

1. Shear displacement occurs.
2. Friction moment drops.
3. Nut rotation begins.
4. Preload collapses rapidly.

Lock nuts introduce prevailing torque or positive locking to stop rotation during this stage.

3.5 Load Types Acting on Lock Nuts

Static Tensile Loads

Structural steel assemblies.

Dynamic Cyclic Loads

Rotating machinery, engines, rail systems.

Shear Loads

Flange joints and frame connections.

Impact Loads

Mining and heavy equipment.

Thermal Loads

Power plants and petrochemical reactors.

Lock nut selection must consider the dominant load regime.

4. Joint Design Principles

Proper lock nut performance depends more on joint design than on fastener strength alone.

4.1 Fundamental Design Rule

A lock nut prevents loosening — it does NOT compensate for poor joint design.

4.2 Bolt Stretch vs Joint Stiffness

Optimal joints satisfy:$$\text{Bolt Elasticity} > \text{Joint Compliance}$$

Design implications:

• Use longer grip lengths when possible
• Avoid excessive joint rigidity
• Maintain sufficient bolt elongation

4.3 Required Thread Engagement

Recommended minimum engagement:

Material Combination	Engagement Length
Steel–Steel	1 × diameter
Steel–Aluminum	1.5 × diameter
Stainless–Stainless	1.25 × diameter
High temperature alloys	≥1.5 × diameter

Insufficient engagement causes:

• Thread stripping
• Load concentration
• Fatigue initiation

4.4 Preload Target

Engineering best practice:$$Preload = 70\% – 85\% \text{ of proof load}$$

Benefits:

• Maximized fatigue life
• Minimal joint separation
• Improved vibration resistance

4.5 Lock Nut Selection Criteria

Engineers evaluate:

Mechanical Requirements

• Required preload
• Dynamic loading
• Fatigue resistance

Environmental Conditions

• Corrosion exposure
• Temperature range
• Chemical compatibility
• Sour service (H₂S)

Maintenance Strategy

• Reusable vs single use
• Inspection accessibility

Regulatory Compliance

• ISO / ASTM / DIN / BS standards
• NACE MR0175 / ISO 15156 where applicable

SM Fasteners supports engineering-driven selection through material traceability, certified production, and customized locking configurations.

4.6 Interaction with Washers

Lock nuts may be paired with:

• Hardened washers
• Belleville washers
• Insulating washers
• PEEK washers for electrical isolation

Proper washer selection prevents embedding losses and maintains preload stability.

4.7 Failure Mechanisms Prevented by Lock Nuts

Failure Mode	Cause	Lock Nut Contribution
Self-loosening	Vibration	Rotation resistance
Fatigue failure	Preload loss	Clamp force retention
Fretting	Micro movement	Surface stabilization
Leakage	Joint separation	Compression retention
Structural instability	Dynamic loads	Mechanical locking

4.8 Engineering Responsibility

Correct specification must define:

• Nut type
• Material grade
• Property class
• Surface finish
• Inspection level
• Certification requirements

SM Fasteners integrates these parameters within ISO 9001-controlled manufacturing and documentation systems aligned with global EPC procurement expectations.

5. Product Types and Variants

Lock nuts exist in multiple engineered configurations, each designed to resist loosening through a specific mechanical principle. Correct selection depends on vibration severity, load type, operating temperature, maintenance philosophy, and regulatory standards.

SM Fasteners manufactures lock nuts across international dimensional systems and advanced material grades, enabling compatibility with global EPC and OEM specifications.

5.1 Classification by Locking Mechanism

Locking Principle	Lock Nut Type	Primary Function	Reusability
Prevailing Torque	Nylon Insert Lock Nut	Elastic interference	Limited
Prevailing Torque	All-Metal Lock Nut	Thread distortion	High
Mechanical Lock	Castellated Nut	Pin locking	Reusable
Mechanical Lock	Slotted Nut	Positive restraint	Reusable
Friction Lock	Serrated Flange Nut	Surface bite	Moderate
Jam Locking	Thin Jam Nut	Secondary locking	High
Chemical Assisted	Standard + Threadlocker	Adhesive retention	Limited
Polymer Lock	PEEK Insert Lock Nut	High temperature insulation	High

5.2 Major Industrial Lock Nut Types

5.2.1 Nylon Insert Lock Nut (Nyloc Type)

Design Concept
A nylon collar positioned at the top of the nut produces radial pressure on threads.

Engineering Characteristics

• Controlled prevailing torque
• Excellent vibration resistance
• Low installation torque variation
• Reduced galling for stainless assemblies

Limitations

• Temperature limited (~120°C typical)
• Not suitable for hydrocarbon fire zones
• Limited reuse cycles

Typical Standards:

• DIN 985
• ISO 10511
• ASTM F594/F594M

5.2.2 All-Metal Prevailing Torque Lock Nut

Locking achieved via:

• Elliptical deformation
• Top thread crimping
• Slotted crown deformation

Advantages

✔ High temperature capability
✔ Chemical resistance
✔ Aerospace and oil & gas suitability
✔ No polymer degradation risk

Common Standards:

• DIN 980V
• ISO 7042
• ASTM A194 Grade 2H (modified)

5.2.3 Castellated Lock Nut

Designed for positive mechanical locking using cotter pins.

Applications:

• Rotating shafts
• Axle assemblies
• Structural pivot joints

Standards:

• DIN 935
• ISO 7035
• BS 1768

5.2.4 Slotted Lock Nut

Similar to castellated designs but with deeper slots for load-bearing shafts.

Used where:

• Absolute rotational security required
• Safety certification mandates visible locking

5.2.5 Serrated Flange Lock Nut

Integrated washer face with serrations.

Function:

• Increased friction coefficient
• Load distribution
• Reduced washer requirement

Limitations:

• Not recommended for hardened surfaces
• May damage coating layers

Standards:

• DIN 6923 (locking variants)

5.2.6 jam lock nut (Thin Pattern)

Used in double-nut locking systems.

Procedure:

1. Main nut tightened to preload.
2. Jam nut tightened against main nut.
3. Thread compression prevents rotation.

Typical in:

• Pipe supports
• Machinery adjustment assemblies
• Precision alignment systems

5.2.7 High-Temperature & Polymer Lock Nuts (PEEK Insert)

SM Fasteners provides PEEK-based locking systems for extreme environments.

Advantages:

• Continuous service temperature up to ~250°C
• Electrical insulation
• Chemical resistance
• Non-magnetic properties

Applications:

• LNG instrumentation
• Offshore electronics
• Semiconductor equipment
• Chemical reactors

6. Dimensional Logic and Geometry

Lock nut geometry directly influences:

• Load distribution
• Prevailing torque
• Thread engagement
• Stress concentration
• Installation accessibility

6.1 Standard Geometry Elements

Parameter	Symbol	Function
Nominal Diameter	d	Thread size
Pitch	P	Load transfer efficiency
Width Across Flats	s	Tool engagement
Nut Height	m	Thread engagement
Bearing Face Diameter	dw	Load distribution
Locking Zone	—	Prevailing torque region

6.2 Height Variants

Type	Height Ratio	Application
Standard	0.8d	General use
High	1.0d	High strength joints
Thin (Jam)	0.5d	Locking secondary nut
Heavy Pattern	1.2d	Structural steel

Higher nut height increases:

• Thread shear area
• Fatigue resistance
• Load distribution

6.3 Thread Geometry Considerations

Engineering performance depends on:

Thread Form

• Metric ISO
• Unified (UNC/UNF)
• Whitworth (BSW/BSF)

Pitch Selection

Pitch Type	Behavior
Coarse	Faster installation, dirt tolerant
Fine	Higher preload accuracy
Extra Fine	Vibration resistance

Fine threads improve resistance to loosening due to smaller helix angle.

6.4 Dimensional Specification Table

(Metric ISO Lock Nuts — Reference Data)

Size	Pitch (mm)	Width Across Flats (mm)	Height (mm)	Approx Weight (kg/100 pcs)*
M6	1.0	10	6	0.9
M8	1.25	13	8	1.8
M10	1.5	17	10	3.6
M12	1.75	19	12	6.0
M16	2.0	24	16	13.5
M20	2.5	30	20	26
M24	3.0	36	24	46
M30	3.5	46	30	95
M36	4.0	55	36	170

*Weights aligned with SM Fasteners production references.

6.5 Thread Engagement Mechanics

Minimum threads engaged:$$N = \frac{L_e}{P}$$

Where:

• $L_e$​ = engagement length
• $P$ = pitch

Recommended engagement:

6–10 full threads for structural joints.

7. International Standards & Compliance

Lock nuts must comply with global interchangeability requirements.

SM Fasteners manufactures according to ISO, ASTM, DIN, and BS systems to ensure worldwide compatibility.

7.1 ISO Standards

Standard	Scope
ISO 7040	Prevailing torque hex nuts
ISO 7042	All-metal lock nuts
ISO 10511	Thin nylon insert nuts
ISO 2320	Prevailing torque testing
ISO 898-2	Mechanical properties of nuts
ISO 4032	Hex nut dimensions

7.2 DIN Standards

DIN Standard	Description
DIN 985	Nylon insert lock nuts
DIN 980	All-metal lock nuts
DIN 6923	Flange lock nuts
DIN 935	Castellated nuts

7.3 ASTM Standards

ASTM Standard	Application
ASTM A194	High-pressure nuts
ASTM A563	Structural nuts
ASTM F594	Stainless steel nuts
ASTM F836	Washer compatibility
ASTM B633	Zinc coating

7.4 British Standards (BS)

BS Standard	Application
BS 3692	ISO metric fasteners
BS 1768	Castellated nuts
BS 1083	Structural assemblies

7.5 Property Class System (ISO)

Mechanical strength of nuts corresponds to bolt property class.

Nut Property Class	Compatible Bolt Class
5	5.6
8	8.8
10	10.9
12	12.9

Rule:

Nut proof load must exceed bolt tensile capacity.

8. Thread Standards & Tolerances Table

Thread System	Standard	Angle	Typical Tolerance
Metric ISO	ISO 965	60°	6H
UNC	ASME B1.1	60°	2B
UNF	ASME B1.1	60°	2B
BSW	BS 84	55°	Medium
BSF	BS 84	55°	Medium

SM Fasteners supports mixed-system projects requiring interchangeability across global supply chains.

9. Interchangeability & Engineering Compatibility

Critical procurement considerations:

• Thread form compatibility
• Property class matching
• Coating thickness tolerance
• Temperature rating
• Galvanic compatibility

Incorrect substitution may cause:

• Thread stripping
• Reduced preload
• Hydrogen embrittlement risk
• Premature joint failure

9.1 EPC Procurement Verification Checklist

✔ Standard reference defined
✔ Property class specified
✔ Material grade confirmed
✔ Coating requirement identified
✔ Certification level required (EN 10204 3.1 / 3.2)
✔ Inspection testing defined

SM Fasteners integrates these parameters into manufacturing documentation packages aligned with international EPC workflows.

9.2 Dimensional Control Philosophy at SM Fasteners

Production control includes:

• CNC gauging
• Go/No-Go thread inspection
• Optical profile measurement
• Statistical process control (SPC)
• ISO 9001 documented calibration systems

Ensuring dimensional interchangeability across multinational projects.

10. Material Grades and Selection Criteria

Material selection for lock nuts is one of the most critical engineering decisions affecting:

• Mechanical strength
• Preload retention
• Corrosion resistance
• Temperature capability
• Galling behavior
• Compliance with industry regulations

Unlike standard fastening applications, lock nuts frequently operate in dynamic and aggressive environments, making metallurgy a primary reliability factor.

SM Fasteners manufactures lock nuts using a full industrial alloy portfolio supported by ISO 9001 certified manufacturing, controlled raw material traceability, and certified inspection systems.

10.1 Industrial Material Categories

Material Group	Typical Grades	Key Characteristics
Carbon Steel	C35, C45	Structural applications
Alloy Steel	4140, 4340, B7	High strength & fatigue resistance
Stainless Steel	A2-70, A4-80	Corrosion resistance
Duplex Stainless	UNS S31803	Strength + corrosion resistance
Super Duplex	UNS S32750	Offshore & chloride environments
Nickel Alloys	Inconel, Monel	Extreme temperature
SMO 254	6Mo Stainless	Seawater resistance
Hastelloy	C276	Chemical processing
PEEK Polymer	Engineering thermoplastic	Electrical isolation

10.2 Mechanical Property Classes (ISO 898-2)

Lock nut strength must always equal or exceed bolt strength.

Property Class	Proof Load (MPa)	Typical Application
5	500	Light structures
8	800	Structural steel
10	1000	Heavy machinery
12	1200	Critical dynamic joints

10.3 ASTM Material Grades

ASTM Grade	Material	Industry Use
ASTM A563 DH	Carbon Steel	Structural
ASTM A194 2H	Alloy Steel	Pressure vessels
ASTM A194 7	High temperature	Power plants
ASTM F594	Stainless Steel	Marine & chemical
ASTM A453 Gr 660	High-temp alloy	Turbine systems

10.4 Material Selection Matrix

Environment	Recommended Material
Indoor structural	Carbon steel
Outdoor construction	Zinc-coated alloy steel
Marine atmosphere	A4 / Duplex
Offshore splash zone	Super Duplex
Sour service (H₂S)	NACE compliant alloys
Chemical plants	Hastelloy / SMO 254
Cryogenic LNG	Austenitic stainless
Electrical isolation	PEEK lock nut

SM Fasteners provides engineering support for material selection aligned with EPC project specifications.

11. Mechanical Properties Comparison Table

Material	UTS (MPa)	Yield (MPa)	Temp Limit °C	Corrosion Resistance	Relative Cost	Typical Industry
Carbon Steel	600	360	300	Low	Low	Construction
Alloy Steel	1000+	850	450	Moderate	Medium	Oil & Gas
SS 304	700	450	400	Good	Medium	Food/Process
SS 316	800	500	450	Excellent	Medium	Marine
Duplex	900	650	300	Very High	High	Offshore
Super Duplex	1000	750	300	Extreme	High	Subsea
Inconel 625	1030	700	980	Exceptional	Very High	Aerospace
Hastelloy C276	790	355	1000	Acid Resistant	Very High	Chemical
SMO 254	650	300	400	Seawater	High	Desalination
PEEK	—	—	250	Chemical inert	High	Electronics

12. Corrosion Resistance vs Environment

Environment	Carbon Steel	SS316	Duplex	Super Duplex	Nickel Alloy	PEEK
Atmosphere	Fair	Excellent	Excellent	Excellent	Excellent	Excellent
Seawater	Poor	Good	Excellent	Outstanding	Outstanding	Excellent
Chlorides	Poor	Good	Excellent	Outstanding	Outstanding	Excellent
Acidic Media	Poor	Moderate	Good	Very Good	Excellent	Excellent
H₂S Sour Service	Limited	Controlled	Suitable	Excellent	Excellent	Excellent
High Humidity	Moderate	Excellent	Excellent	Excellent	Excellent	Excellent

Materials selected by SM Fasteners follow corrosion mapping based on project exposure conditions.

13. Heat Treatment Processes

Heat treatment directly controls strength, toughness, and fatigue performance.

13.1 Typical Heat Treatment Routes

Quenching & Tempering

Used for:

• Alloy steel lock nuts
• ASTM A194 Grade 2H

Process:

1. Austenitizing
2. Rapid quenching
3. Tempering

Benefits:

• High proof load
• Improved toughness
• Fatigue resistance

Solution Annealing

Applied to stainless steels.

13.2 Hardness Limits (Typical)

Material	Hardness Range
Class 8	22–30 HRC
Class 10	32–39 HRC
Class 12	39–44 HRC
NACE Sour Service	≤22 HRC

Compliance with NACE MR0175 / ISO 15156 is mandatory for H₂S environments to prevent sulfide stress cracking.

14. End-to-End Manufacturing Workflow

SM Fasteners follows a controlled manufacturing process integrating metallurgical verification, precision forming, and inspection traceability.

14.1 Raw Material Verification

Incoming material inspection includes:

• Mill Test Certificate verification
• Heat number traceability
• Chemical composition validation
• Positive Material Identification (PMI)

14.2 Manufacturing Process Flow

Step 1 — Raw Material Preparation

• Certified bars or wire rod
• Ultrasonic inspection where required

Step 2 — Cold Forging / Hot Forging

Cold forging advantages:

• Grain flow alignment
• Higher fatigue strength
• Minimal material waste

Hot forging used for:

• Large diameter lock nuts
• High alloy materials

Step 3 — Machining Operations

• Facing
• Chamfering
• Slotting (castellated types)
• Flange forming

CNC machining ensures dimensional accuracy aligned with ISO tolerance classes.

Step 4 — Thread Formation

Thread Rolling (Preferred)

• Improves fatigue resistance
• Work hardening increases strength
• Superior surface finish

Thread Cutting

Used for:

• Large diameters
• Exotic alloys

Step 5 — Locking Feature Creation

Depending on type:

• Nylon insert installation
• Elliptical deformation
• Crown slot machining
• Serration forming
• PEEK insert integration

Step 6 — Heat Treatment
Performed under controlled furnaces with calibrated temperature systems.

Step 7 — Surface Preparation
Cleaning, shot blasting, or pickling prior to coating.

Step 8 — Coating / Surface Engineering
(covered below)

Step 9 — Inspection & Traceability Marking

• Batch coding
• Property class marking
• Manufacturer identification

15. Surface Finishing and Coatings

Surface engineering protects lock nuts against corrosion, friction variation, and galling.

SM Fasteners offers coating systems compatible with global project specifications.

15.1 Surface Finish Comparison Table

Finish	Corrosion Protection	Temp Limit	Friction Control	Typical Use
Black Oxide	Low	300°C	Stable	Indoor machinery
Zinc Plating	Moderate	120°C	Good	Construction
Hot Dip Galvanized	High	200°C	Variable	Structural steel
Mechanical Galvanized	High	150°C	Controlled	Bridges
Zinc Nickel	Very High	300°C	Stable	Automotive
PTFE Coated	Excellent	260°C	Low friction	Offshore
Dacromet/Geomet	High	300°C	Excellent	Wind turbines
Passivated Stainless	Excellent	400°C	Stable	Chemical plants
Nickel Plating	High	600°C	Moderate	Power plants

15.2 Coating Selection Considerations

Engineers must evaluate:

• Hydrogen embrittlement risk
• Coating thickness tolerance
• Thread fit after coating
• Temperature exposure
• Chemical compatibility

SM Fasteners performs baking procedures post electroplating where required to reduce hydrogen embrittlement risk.

15.3 Galvanic Compatibility

When dissimilar metals contact:

• Electrochemical corrosion may occur.
• Insulating washers or PEEK lock nuts are recommended.

15.4 Friction Control & Torque Stability

Coatings influence the nut factor (K).

Typical values:

Surface Condition	Nut Factor
Dry Steel	0.22
Zinc Plated	0.18
Lubricated	0.14
PTFE	0.11

Correct torque calculation must reflect coating condition.

15.5 Surface Engineering Philosophy at SM Fasteners

• Controlled coating thickness
• Salt spray performance validation
• Batch traceability
• ISO 9001 controlled subcontract processing
• Compatibility with offshore and petrochemical standards

16. Inspection & Quality Control

Lock nuts used in industrial environments must comply with strict inspection protocols to ensure mechanical reliability, interchangeability, and traceability throughout project life cycles.

SM Fasteners integrates inspection systems within an ISO 9001 certified quality management framework, supported by calibrated instruments, documented procedures, and full material traceability.

16.1 Incoming Material Inspection

Verification conducted before production:

• Mill Test Certificate (MTC) validation
• Heat number verification
• Chemical composition confirmation
• PMI (Positive Material Identification)
• Visual surface inspection
• Ultrasonic testing (when required)

16.2 In-Process Inspection

During manufacturing:

• Forging dimensional verification
• Thread profile inspection
• Prevailing torque measurement
• Heat treatment temperature monitoring
• Coating thickness measurement
• Statistical Process Control (SPC)

16.3 Final Inspection Requirements

Inspection Type	Method	Purpose
Dimensional Inspection	Vernier / CMM	Geometry conformity
Thread Check	GO/NO-GO Gauges	Tolerance compliance
Hardness Testing	Rockwell / Vickers	Mechanical verification
Proof Load Test	ISO 898-2	Load capability
Prevailing Torque Test	ISO 2320	Locking performance
Coating Thickness	XRF / Magnetic	Corrosion protection
Visual Inspection	ISO 3269	Surface defects

16.4 Non-Destructive Testing (NDT)

Applied where project specifications require:

• Magnetic Particle Testing (MT)
• Dye Penetrant Testing (PT)
• Ultrasonic Testing (UT)
• Eddy Current inspection

16.5 Certification & Documentation

SM Fasteners supplies:

• EN 10204 3.1 Material Test Certificates
• 3.2 certification (third-party witness when required)
• Heat treatment reports
• Mechanical test reports
• Coating compliance certificates
• Certificate of Conformity (CoC)

17. Mechanical Properties Table (Grade-Wise)

Nut Property Class	Proof Stress (MPa)	Compatible Bolt	Typical Use
Class 5	500	5.6	Light equipment
Class 8	800	8.8	Structural
Class 10	1000	10.9	Heavy machinery
Class 12	1200	12.9	Critical assemblies

18. Proof Load & Tensile Capacity Table

(Metric Lock Nuts)

Size	Stress Area (mm²)	Proof Load Class 8 (kN)	Proof Load Class 10 (kN)
M6	20.1	16	20
M8	36.6	29	36
M10	58	46	58
M12	84.3	67	84
M16	157	126	157
M20	245	196	245
M24	353	282	353
M30	561	449	561

19. Tightening Torque Chart

(Typical values — lubricated condition)

Size	Grade 8 Torque (Nm)	Grade 10 Torque (Nm)
M6	10	13
M8	25	32
M10	50	65
M12	85	110
M16	210	270
M20	410	520
M24	710	900
M30	1420	1800

Torque must always consider coating friction factor and lubrication condition.

20. Preload Calculation

Lock nut effectiveness depends on achieving correct preload.

20.1 Engineering Formula

$$F = \frac{T}{K \times D}$$

Where:

• F = Preload Force (N)
• T = Torque (Nm)
• K = Nut Factor
• D = Nominal Diameter (m)

20.2 Worked Example

M16 Lock Nut — Grade 8

Given:

• Torque = 210 Nm
• Nut Factor = 0.14 (lubricated)
• Diameter = 0.016 m

$$F = \frac{210}{0.14 \times 0.016}$$

$$F = 93,750\ N$$

Result:

Preload ≈ 94 kN

Recommended target:

70–80% of proof load.

21. Weight Chart — SM Fasteners Reference

Size	Weight / Piece (kg)	Weight / 100 pcs (kg)
M6	0.009	0.9
M8	0.018	1.8
M10	0.036	3.6
M12	0.060	6.0
M16	0.135	13.5
M20	0.260	26
M24	0.460	46
M30	0.950	95
M36	1.70	170

Aligned with SM Fasteners manufacturing data for logistics and project estimation.

22. Surface Finish Performance Comparison

Coating	Salt Spray Resistance	Hydrogen Risk	Offshore Suitability
Black Oxide	Low	None	No
Zinc Plated	Medium	Moderate	Limited
Hot Dip Galvanized	High	Low	Yes
Zinc Nickel	Very High	Controlled	Excellent
PTFE	Excellent	None	Outstanding
Dacromet/Geomet	Very High	None	Excellent
Passivated Stainless	Excellent	None	Chemical plants

23. Industry Applications

23.1 Construction & Structural Steel

• Steel frame connections
• Bridge assemblies
• Wind towers
• Seismic joints

Lock nuts prevent preload loss caused by structural vibration and environmental exposure.

23.2 Oil & Gas Sector

Upstream

• Drilling rigs
• Wellhead equipment
• Subsea systems

Midstream

• Pipeline supports
• Compressor stations

Downstream

• Refineries
• Process vessels
• Flange systems

NACE compliant materials supplied where sour service exists.

23.3 Power Generation

• Steam turbines
• Boiler structures
• Nuclear auxiliary systems
• Renewable energy installations

High-temperature all-metal lock nuts preferred.

23.4 Petrochemical & Chemical Processing

• Reactors
• Heat exchangers
• Pump systems
• Corrosive chemical handling

SM Fasteners supplies nickel alloys and SMO 254 lock nuts for aggressive environments.

23.5 LNG & Offshore

• Cryogenic piping
• Offshore platforms
• FPSO modules
• Desalination plants

Super Duplex and PTFE coated lock nuts widely applied.

23.6 Automotive & Heavy Equipment

• Suspension assemblies
• Powertrain systems
• Mining equipment
• Earthmoving machinery

Prevailing torque lock nuts resist dynamic vibration.

23.7 Railways & Infrastructure

• Track fastening systems
• Signal equipment
• Bridge expansion joints

Positive locking designs ensure operational safety.

23.8 Shipbuilding & Marine

• Deck equipment
• Propulsion assemblies
• Corrosion-prone structures

Duplex and stainless solutions recommended.

23.9 PEEK Lock Nut Applications

SM Fasteners’ advanced polymer fasteners enable:

• Electrical isolation systems
• Semiconductor manufacturing
• Medical equipment
• Hydrogen energy systems
• High-frequency instrumentation

Benefits:

• Non-conductive
• Non-magnetic
• Chemical inertness
• Lightweight performance

24. Failure Mechanisms & Prevention

Failure Mode	Cause	Prevention Using Lock Nuts
Fatigue cracking	Preload loss	Prevailing torque
Shear failure	Joint slip	Maintained clamp force
Hydrogen embrittlement	Electroplating	Controlled baking
Stress corrosion cracking	Material mismatch	Proper alloy selection
Galling	Stainless friction	Lubrication/coatings

25. Export Capability & Global Supply

SM Fasteners supports international EPC procurement through structured export processes.

25.1 Industrial Packaging

• VCI corrosion protection
• Oil wrapping
• Thread protectors
• Batch identification labels
• Moisture barrier packaging

25.2 Export Crating

• ISPM-15 compliant wooden crates
• Heavy-duty palletization
• Container load optimization
• Shock protection for machined parts

25.3 Documentation Package

Typical export dossier includes:

• EN 10204 3.1 / 3.2 MTC
• Inspection reports
• Dimensional reports
• Heat treatment certificates
• Coating compliance certificates
• Certificate of Conformity
• Packing list & traceability data

26. Engineering Selection Guide

Condition	Recommended Lock Nut
High vibration	All-metal prevailing torque
Structural steel	HDG lock nut
Offshore	Duplex / PTFE coated
High temperature	All-metal lock nut
Chemical exposure	Hastelloy / SMO 254
Electrical isolation	PEEK lock nut
Adjustable assemblies	Jam nut system

27. SM FASTENERS — Engineering & Manufacturing Capability

SM Fasteners integrates:

✔ ISO 9001 certified quality systems
✔ MSME registered manufacturing facility
✔ UKAF certification framework
✔ Precision cold forging & CNC machining
✔ Advanced alloy manufacturing capability
✔ Custom fastener engineering
✔ Global EPC project supply readiness

Material capability includes:

• Stainless Steel
• Carbon & Alloy Steel
• Duplex & Super Duplex
• Hastelloy
• Inconel / Incoloy
• Monel
• Nickel Alloys
• SMO 254
• High-performance PEEK fasteners

28. Procurement Readiness Summary

SM Fasteners lock nuts deliver:

• International standards compliance
• Full mechanical traceability
• Engineered locking performance
• Corrosion-resistant material solutions
• Certified inspection and testing
• Export-ready documentation
• Custom manufacturing flexibility