Prevailing Torque Nut

1. Industrial Context

Prevailing Torque Nuts are critical self-locking threaded fasteners developed to maintain clamp load integrity under dynamic operating conditions where conventional nuts lose preload due to vibration, thermal cycling, or fluctuating loads.

Across modern engineered systems, bolted joints increasingly operate under:

• cyclic stress
• impact loading
• rotational vibration
• pressure pulsation
• thermal expansion mismatch
• fatigue-sensitive assemblies

Traditional locking approaches such as double-nutting, spring washers, or adhesives introduce variability, installation complexity, or inspection limitations. Prevailing torque nuts provide a mechanically integrated locking mechanism that delivers predictable resistance to loosening independent of external locking devices.

These fasteners are widely specified in:

Industry Sector	Functional Requirement
Oil & Gas	Anti-vibration joint retention
Offshore Structures	Dynamic wave loading resistance
Power Generation	Thermal expansion stability
Petrochemical Plants	Reliability under cyclic pressure
Structural Steel	Wind and seismic resistance
Railways	Shock and oscillation durability
Heavy Equipment	Rotational vibration control
Automotive OEM	Controlled clamp force retention
LNG Facilities	Cryogenic contraction tolerance

SM Fasteners manufactures prevailing torque nuts aligned with global EPC procurement specifications, ensuring compatibility with ISO, ASTM, DIN, and BS fastening systems.

2. Technical Definition

2.1 Definition

A Prevailing Torque Nut is a nut incorporating a locking feature that creates controlled frictional resistance independent of clamp load, preventing spontaneous rotation after installation.

The locking torque exists even when:

• axial load = 0
• joint relaxation occurs
• vibration attempts rotation

This differentiates it from friction-dependent locking devices.

2.2 Engineering Definition

A prevailing torque nut develops a specified torque required to rotate the nut on mating threads due to intentional deformation or insert interference.

The locking mechanism produces:

$$T_{prevailing} > 0 \quad \text{before clamping force develops}$$

2.3 Fundamental Functional Characteristics

Characteristic	Engineering Meaning
Prevailing Torque	Torque required to overcome locking feature
Clamp Load	Axial preload generated during tightening
Reusability	Depends on locking mechanism type
Self-Locking	No auxiliary washer or adhesive required
Controlled Friction	Predictable installation behavior
Anti-Loosening	Maintains joint integrity

3. Functional Role in Bolted Assemblies

A bolted joint must satisfy three simultaneous engineering objectives:

1. Generate sufficient preload
2. Maintain preload during service
3. Prevent rotational loosening

Prevailing torque nuts primarily address Objective 3 while supporting Objectives 1 and 2.

3.1 Why Bolted Joints Fail

Field failure investigations show loosening occurs due to:

• transverse vibration
• differential expansion
• embedment relaxation
• gasket creep
• cyclic shear forces

Loss of preload leads to:

• fatigue cracking
• gasket leakage
• structural instability
• flange separation

Prevailing torque nuts act as a mechanical energy barrier against rotation.

4. Load Mechanics & Force Behavior

4.1 Bolt–Nut System Mechanics

A bolted joint behaves like a spring system:

• Bolt = elastic tension spring
• Clamped parts = compression spring

Preload ensures joint compression exceeds external load.$$F_{external} < F_{preload}$$

When vibration introduces micro-slip, rotation begins unless resisted.

Prevailing torque provides resistance torque:$$T_{resist} = T_{friction} + T_{locking}$$

4.2 Torque–Tension Relationship

Installation torque consists of three components:

Torque Component	Typical Share
Thread friction	40–50%
Bearing surface friction	35–45%
Useful preload generation	10–15%

Prevailing torque nuts introduce an additional controlled friction component.

General Torque Equation

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

Where:

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

4.3 Prevailing Torque Influence

Total tightening torque:$$T_{total} = T_{preload} + T_{prevailing}$$

Therefore, installation procedures must consider:

• Minimum prevailing torque
• Maximum prevailing torque
• Desired preload

Failure to account for prevailing torque results in:

• under-tightening
• over-stressing
• inaccurate clamp load

SM Fasteners supplies prevailing torque data during project submittals to support accurate tightening procedures.

5. Joint Design Principles

5.1 Self-Locking Mechanism Physics

Self-locking occurs when rotational energy required exceeds vibration energy input.

Design objective:$$T_{prevailing} > T_{vibration}$$Tprevailing​>Tvibration​

Locking mechanisms typically operate through:

• elastic interference
• plastic deformation
• polymer compression
• distorted thread geometry

5.2 Frictional Energy Dissipation

Prevailing torque nuts convert vibrational motion into:

• frictional heat
• micro-plastic deformation
• energy damping

This prevents reverse rotation.

5.3 Clamp Load Stability

Prevailing torque nuts:

✔ Maintain preload after relaxation
✔ Reduce self-loosening probability
✔ Improve fatigue life of bolts
✔ Stabilize gasket compression

5.4 Design Considerations for Engineers

Design Parameter	Engineering Guidance
Joint Type	Dynamic or cyclic joints preferred
Access	Limited access favors integrated locking
Maintenance Interval	Long-term installations
Temperature Range	Verify insert compatibility
Reuse Requirement	Select metal locking types
Inspection Requirement	Visible mechanical locking preferred

5.5 Prevailing Torque vs Other Locking Methods

Method	Reliability	Reusable	Inspection Ease
Double Nut	Medium	Yes	Moderate
Spring Washer	Low	Yes	Easy
Adhesive	High	No	Difficult
Safety Wire	High	Yes	Labor intensive
Prevailing Torque Nut	Very High	Depends on type	Excellent

6. Preload Calculation — Worked Engineering Example

Given

• Bolt Size: M16
• Torque Applied: 210 Nm
• Nut Factor (K): 0.18
• Diameter (D): 16 mm = 0.016 m

$$F_p = \frac{210}{0.18 \times 0.016}$$

$$F_p = 72,916\,N$$

Approximate Preload = 73 kN

Engineers must subtract prevailing torque contribution when determining installation torque.

7. Failure Mechanisms Prevented by Prevailing Torque Nuts

7.1 Self-Loosening (Junker Effect)

Primary failure mode under transverse vibration.

Prevailing torque resists rotational slip initiation.

7.2 Fatigue Failure

Loss of preload causes fluctuating stress amplitude.

Maintained clamp load reduces fatigue crack propagation.

7.3 Shear Joint Failure

Proper preload ensures shear loads transfer via friction instead of bolt shear.

7.4 Hydrogen Embrittlement Risk

High-strength prevailing torque nuts must control:

• hardness limits
• coating processes
• baking procedures

SM Fasteners manufacturing integrates controlled heat treatment and post-plating hydrogen relief processes.

7.5 Stress Corrosion Cracking

Critical in:

• offshore environments
• sour service (H₂S)
• chloride exposure

Material selection must comply with:

• NACE MR0175
• ISO 15156

8. Engineering Advantages in Critical Systems

Prevailing torque nuts enable:

• predictable torque retention
• reduced maintenance shutdowns
• improved structural reliability
• standardized installation procedures
• compatibility with automated assembly lines

They are therefore preferred in EPC project specifications requiring lifecycle reliability.

9. Functional Selection Criteria

Engineers select prevailing torque nuts based on:

Selection Factor	Consideration
Vibration Severity	High vibration favors metal locking types
Temperature	Nylon inserts limited by temperature
Chemical Exposure	Stainless or nickel alloys required
Reusability	All-metal preferred
Load Level	Match property class to bolt grade
Inspection Requirements	Mechanical locking preferred

10. Role Within SM Fasteners Engineering Capability

Prevailing Torque Nuts supplied by SM Fasteners are manufactured within certified quality systems:

• ISO 9001 Quality Management
• UKAF accredited processes
• MSME certified manufacturing
• Full traceability from raw material to dispatch

Engineering integration includes:

• controlled prevailing torque validation
• compatibility with international bolt standards
• advanced alloy and corrosion-resistant material capability
• availability of PEEK-based high-performance locking solutions for electrical isolation and chemical resistance applications

11. Product Types and Variants

Prevailing torque nuts are classified according to the locking mechanism used to generate resistance torque. Selection must consider temperature capability, reuse requirement, vibration severity, and inspection accessibility.

11.1 Primary Classification

Type	Locking Principle	Typical Standards	Reusable	Temperature Capability
Nylon Insert (Nyloc) Nut	Polymer interference	ISO 7040 / DIN 985	Limited	≤120°C
All metal lock nut	Thread deformation	DIN 980V / ISO 7042	Yes	High
Stover Nut	Elliptical distortion	ASTM A563	Yes	High
Flex Lock Nut	Slotted crown distortion	IFI Standards	Yes	High
Two-Way Lock Nut	Dual deformation zones	OEM Specs	Yes	High
Center Lock Nut	Mid-body deformation	DIN 980M	Yes	High
Serrated Flange Prevailing Nut	Combined friction + locking	DIN 6927	Limited	Medium
PEEK Insert Lock Nut	High-performance polymer insert	Custom / Aerospace	Limited	Up to 260°C

SM Fasteners manufactures these configurations in metric and imperial systems aligned with EPC procurement requirements.

11.2 Nylon Insert Prevailing Torque Nuts

Functional Principle

A nylon ring produces radial compression against mating bolt threads.

Locking Behavior

• Elastic deformation
• High friction coefficient
• Reduced vibration loosening

Engineering Characteristics

Parameter	Value
Prevailing torque consistency	High
Reusability	3–5 cycles typical
Chemical resistance	Moderate
Temperature limitation	Polymer dependent
Electrical insulation	Partial

Limitations

Not suitable for:

• high-temperature flanges
• offshore fire zones
• sour service
• aggressive hydrocarbons

11.3 All-Metal Prevailing Torque Nuts

All-metal locking nuts rely on controlled plastic deformation of threads.

Locking Mechanisms

• elliptical top section
• crimped crown
• distorted pitch diameter
• slotted deformation zones

Engineering Advantages

✔ High temperature stability
✔ No polymer degradation
✔ Full reusability
✔ Resistant to oils and chemicals
✔ Compatible with offshore and petrochemical systems

11.4 Stover Type Lock Nuts

Developed for heavy-duty industrial vibration environments.

Key Geometry Feature
Top threads intentionally ovalized.

Characteristic	Value
Prevailing torque repeatability	Excellent
Impact resistance	High
Installation automation	Suitable
Heavy equipment usage	Preferred

Common in:

• mining equipment
• structural steel
• railway assemblies
• turbine equipment

11.5 Flex Lock Nuts

Flex lock nuts incorporate slots creating flexible segments.

Locking action generated by:

• elastic flexure
• distributed pressure zones

Benefits include reduced thread damage and smoother installation torque curves.

11.6 Center Lock Nuts

Locking deformation located at mid-height of nut.

Engineering benefits:

• preserves thread lead-in
• improves installation alignment
• suitable for automated assembly systems

11.7 Serrated Flange Prevailing Torque Nuts

Combine:

• integrated washer flange
• serration friction
• prevailing torque locking

Applications:

• automotive assemblies
• sheet metal structures
• machinery covers

Not suitable for hardened surfaces or sliding joints.

11.8 High-Performance PEEK Insert Lock Nuts

SM Fasteners provides engineered solutions using PEEK (Polyether Ether Ketone) inserts.

Advantages

Property	Benefit
Temperature Resistance	up to 260°C
Chemical Resistance	Excellent
Electrical Isolation	High
Radiation Stability	Excellent
Low Outgassing	Aerospace suitable

Applications include:

• LNG instrumentation
• semiconductor equipment
• electrical isolation joints
• chemical processing equipment

12. Dimensional Logic & Geometry

Prevailing torque nuts follow standard hex geometry ensuring interchangeability with standard tooling.

12.1 Basic Nut Geometry

Key dimensions:

• Width across flats (s)
• Nut height (m)
• Thread diameter (d)
• Thread pitch (P)
• Locking zone height

Standard Geometry Representation

Symbol	Description
d	Nominal diameter
P	Pitch
s	Width across flats
m	Nut height
hL	Locking zone height

12.2 Metric Dimensional Specification Table

ISO Metric Prevailing Torque Nuts

Size	Pitch (mm)	Width Across Flats (mm)	Height (mm)
M6	1.0	10	6
M8	1.25	13	8
M10	1.5	17	10
M12	1.75	19	12
M16	2.0	24	16
M20	2.5	30	20
M24	3.0	36	24
M30	3.5	46	30
M36	4.0	55	36

(Aligned with ISO 4032 dimensional envelope.)

12.3 Height Variations

Type	Height Ratio
Standard	1.0 × diameter
Thin Lock Nut	0.5 × diameter
Heavy Hex	1.2 × diameter
High Strength Lock Nut	Increased locking section

13. Thread Forms & Standards

Prevailing torque nuts must match mating bolt thread standards precisely.

13.1 Thread Standard Comparison

Standard	Region	Thread Angle
ISO Metric	Global	60°
UNC	USA	60°
UNF	USA	60°
BSW	UK	55°
BSF	UK	55°

13.2 Thread Tolerance Classes

System	External Thread	Internal Thread
ISO Metric	6g	6H
UNC/UNF	2A	2B
Precision Fit	3A	3B
Structural Bolting	As specified

Proper tolerance ensures controlled prevailing torque performance.

13.3 Metric Thread Pitch Table

Size	Coarse Pitch	Fine Pitch
M8	1.25	1.0
M10	1.5	1.25
M12	1.75	1.5
M16	2.0	1.5
M20	2.5	1.5
M24	3.0	2.0

14. Applicable International Standards

Prevailing torque nuts supplied for global projects must comply with recognized standards.

14.1 ISO Standards

Standard	Description
ISO 7040	Nylon insert hex nuts
ISO 7042	All-metal prevailing torque nuts
ISO 2320	Prevailing torque performance requirements
ISO 898-2	Mechanical properties of nuts
ISO 4032	Hex nut dimensions
ISO 965	Thread tolerances

14.2 DIN Standards

Standard	Description
DIN 985	Nylon insert lock nuts
DIN 980V	All-metal lock nuts
DIN 982	High type lock nuts
DIN 6927	Serrated flange nuts

14.3 ASTM Standards

Standard	Application
ASTM A563	Carbon steel nuts
ASTM A194	Alloy & pressure vessel nuts
ASTM F594	Stainless steel nuts
ASTM A320	Low-temperature bolting
ASTM A453	High temperature service

14.4 BS Standards

Standard	Description
BS 3692	Metric fasteners
BS 1768	Hexagon nuts
BS EN ISO 2320	Prevailing torque verification

15. Property Class Compatibility

Prevailing torque nuts must match bolt strength.

Nut Property Class	Compatible Bolt Class
Class 8	Bolt 8.8
Class 10	Bolt 10.9
Class 12	Bolt 12.9
ASTM 2H	ASTM A193 B7
ASTM 8M	Stainless B8M

Incorrect pairing causes thread stripping or preload loss.

16. Interchangeability Considerations

Global EPC projects require interchangeability between:

• ISO
• ASTM
• DIN
• BS systems

Critical parameters:

• thread pitch
• tolerance class
• proof load
• material grade
• coating thickness allowance

SM Fasteners validates interchangeability through dimensional inspection and functional prevailing torque testing under ISO 2320 requirements.

17. Engineering Selection Matrix

Application Condition	Recommended Type
High temperature	All-metal
Offshore vibration	Stover
Automotive	Nylon insert
Chemical plant	Stainless all-metal
LNG	Duplex / PEEK insert
Electrical isolation	PEEK locking nut
Heavy machinery	Center lock

18. Material Grades and Selection Criteria

Material selection for prevailing torque nuts directly influences:

• preload retention
• fatigue resistance
• corrosion performance
• temperature capability
• hydrogen embrittlement susceptibility
• lifecycle cost

SM Fasteners manufactures prevailing torque nuts across a full industrial metallurgy range suitable for EPC, offshore, petrochemical, and heavy engineering applications.

18.1 Industrial Material Families

Material Group	Typical Standards	Primary Use
Carbon Steel	ASTM A563	Structural & construction
Alloy Steel	ASTM A194 / A320	High load & pressure systems
Stainless Steel	ASTM F594	Corrosion resistance
Duplex Stainless	UNS S31803	Offshore & marine
Super Duplex	UNS S32750	Seawater service
Nickel Alloys	Inconel / Monel	Extreme environments
SMO 254	UNS S31254	Chloride resistance
Hastelloy	C276	Chemical processing
PEEK Polymer	Custom engineered	Electrical isolation & chemical resistance

18.2 Material Selection Criteria

Engineers evaluate:

Parameter	Selection Influence
Design load	Yield & proof strength
Temperature	Material stability
Corrosive environment	Alloy chemistry
H₂S exposure	NACE compliance
Maintenance interval	Corrosion allowance
Weight reduction	Advanced alloys or PEEK
Electrical isolation	Polymer solutions

19. Mechanical Properties by Material Grade

Mechanical Property Comparison

Material	Yield Strength (MPa)	UTS (MPa)	Hardness	Temperature Limit
Carbon Steel Class 8	640	800	22–32 HRC	300°C
Alloy Steel Class 10	900	1040	26–36 HRC	450°C
Alloy Steel Class 12	1100	1220	32–39 HRC	500°C
SS 304	205	515	HRB 95	400°C
SS 316	205	520	HRB 95	450°C
Duplex 2205	450	620	28 HRC	300°C
Super Duplex 2507	550	800	32 HRC	300°C
Inconel 625	460	830	—	980°C
Hastelloy C276	355	790	—	1000°C
PEEK	100	140	—	260°C

20. Material Performance vs Environment

Corrosion Resistance Matrix

Environment	Carbon Steel	SS316	Duplex	SMO 254	Nickel Alloy
Atmospheric	Medium	Excellent	Excellent	Excellent	Excellent
Marine	Poor	Good	Excellent	Excellent	Excellent
Seawater Immersion	Poor	Moderate	Excellent	Excellent	Excellent
Chloride Acid	Poor	Moderate	Good	Excellent	Excellent
Sour Gas (H₂S)	Controlled	Good	Excellent	Excellent	Excellent
High Temperature Oxidation	Moderate	Good	Good	Good	Excellent

SM Fasteners assists EPC buyers in selecting materials compliant with NACE MR0175 / ISO 15156 for sour service.

21. Property Class System (ISO 898-2)

Prevailing torque nuts must equal or exceed bolt strength.

Nut Property Class	Proof Stress (MPa)	Matching Bolt
5	500	5.8
8	800	8.8
10	1000	10.9
12	1200	12.9

22. Heat Treatment Processes

Heat treatment defines mechanical reliability and locking stability.

22.1 Typical Heat Treatment Workflow

1. Controlled heating
2. Austenitizing
3. Quenching
4. Tempering
5. Stress relief
6. Hardness verification

22.2 Heat Treatment Methods

Process	Purpose
Quench & Temper	High strength alloy nuts
Normalizing	Grain refinement
Solution Annealing	Stainless steels
Age Hardening	Nickel alloys
Stress Relieving	Distortion control

22.3 Hardness Control for Lock Nuts

Excessive hardness increases embrittlement risk.

Application	Maximum Hardness
General Service	36 HRC
High Strength	39 HRC
Sour Service	≤22 HRC (NACE)

SM Fasteners validates hardness through calibrated testing aligned with ISO 6508 and ASTM E18.

23. Hydrogen Embrittlement Control

Critical for plated high-strength prevailing torque nuts.

Preventive measures:

• controlled electroplating
• low hydrogen pick-up processes
• post-plating baking (200°C)
• hardness monitoring
• coating thickness control

24. End-to-End Manufacturing Workflow

SM Fasteners operates controlled manufacturing aligned with ISO 9001 quality systems.

24.1 Raw Material Verification

Incoming inspection includes:

• Mill Test Certificate (MTC)
• chemical composition verification
• PMI testing
• dimensional validation
• heat number traceability

24.2 Forging vs Machining

Method	Advantage	Application
Cold Forging	Grain flow strength	High-volume nuts
Hot Forging	Large diameters	Heavy hex nuts
CNC Machining	Precision alloys	Nickel & special materials

Forging enhances fatigue resistance due to continuous grain structure.

24.3 Thread Manufacturing

Thread Rolling (Preferred)

Benefits:

• increased fatigue strength
• work hardening
• improved surface finish

Thread Cutting

Used when:

• large sizes
• exotic alloys
• custom threads required

24.4 Creation of Locking Feature

Depending on nut type:

Lock Type	Manufacturing Method
Nyloc insert lock nut	Insert press fitting
Stover	Controlled top deformation
All-metal	Mechanical crimping
Flex Lock	Slot forming
Center Lock	Mid-body deformation
PEEK Insert	Precision thermal insertion

Each batch undergoes prevailing torque verification per ISO 2320.

25. Surface Engineering & Coatings

Surface treatment protects against corrosion and influences torque behavior.

25.1 Surface Finish Comparison

Coating	Corrosion Resistance	Temperature Limit	Friction Control
Zinc Plating	Moderate	120°C	Good
Hot Dip Galvanizing	High	300°C	Requires oversize tapping
Mechanical Galvanizing	High	250°C	Uniform coating
Phosphate	Low	400°C	Excellent lubrication
PTFE / Xylan	Very High	260°C	Low friction
Dacromet / Geomet	Excellent	300°C	Stable torque
Passivation	Stainless steels	High	Neutral
Nickel Plating	Chemical resistance	High	Smooth surface

25.2 Coating Selection vs Application

Industry	Recommended Finish
Structural Steel	HDG
Offshore	Duplex + PTFE
Petrochemical	Fluoropolymer coatings
Automotive	Zinc flake
LNG	Stainless passivation
Electrical Equipment	PEEK isolation nuts

26. Coating Impact on Torque & Preload

Coatings alter friction coefficient (nut factor K).

Surface Condition	Nut Factor (K)
Dry Steel	0.20–0.25
Zinc Plated	0.18
Lubricated	0.12–0.15
PTFE Coated	0.10–0.13

Proper torque adjustment is mandatory.

SM Fasteners supplies torque reference data aligned with coating selection.

27. Surface Integrity & Quality Assurance

Inspection includes:

• coating thickness measurement
• adhesion testing
• salt spray validation
• visual inspection
• dimensional verification post-coating

28. Traceability System

Each prevailing torque nut batch maintains full traceability:

• heat number marking
• manufacturing lot identification
• inspection linkage
• material certification reference

Traceability supports EPC audits and third-party inspection agencies.

29. Integration of Advanced Materials — PEEK Fasteners

SM Fasteners integrates PEEK prevailing torque nuts for specialized engineering requirements.

Functional Advantages

• non-conductive assemblies
• galvanic isolation
• chemical plant instrumentation
• cryogenic service compatibility
• weight reduction vs metal

Used in:

• LNG control panels
• electrical busbar supports
• semiconductor manufacturing
• aerospace electronics

30. Manufacturing Quality Alignment with SM Fasteners

Production workflow integrates:

• ISO 9001 process control
• UKAF accredited quality practices
• MSME-certified manufacturing infrastructure
• calibrated inspection equipment
• documented process validation

Result:

✔ Repeatable prevailing torque performance
✔ Controlled mechanical properties
✔ Global EPC supply readiness

31. Inspection & Quality Control Philosophy

Prevailing torque nuts are classified as safety-critical fastening components in pressure systems, structural assemblies, rotating equipment, and vibration-sensitive installations.

Quality assurance must verify:

• dimensional conformity
• mechanical performance
• locking effectiveness
• material integrity
• coating reliability
• traceability compliance

SM Fasteners integrates inspection procedures aligned with:

• ISO 9001 Quality Management Systems
• UKAF accredited inspection practices
• EPC project quality plans
• Third-party inspection requirements

31.1 Inspection Workflow

Stage	Inspection Activity
Raw Material	Chemical & PMI verification
Forging	Dimensional monitoring
Threading	GO/NO-GO gauge inspection
Heat Treatment	Hardness verification
Lock Formation	Deformation validation
Coating	Thickness measurement
Final QA	Prevailing torque testing
Dispatch	Documentation verification

32. Dimensional Inspection

Verification performed using:

• calibrated Vernier gauges
• micrometers
• optical comparators
• thread ring gauges
• coordinate measuring systems

Critical checks:

• thread pitch diameter
• nut height
• across flats dimension
• locking deformation geometry

33. Prevailing Torque Performance Testing

Performed according to ISO 2320.

Test Procedure

1. Nut assembled on test bolt.
2. Rotation torque measured before seating.
3. Removal torque measured after tightening cycle.
4. Repeatability validated across cycles.

Prevailing Torque Acceptance Concept

Requirement	Purpose
Minimum torque	Prevent loosening
Maximum torque	Avoid bolt overstress
Cycle performance	Reusability verification

34. Mechanical Testing

Test	Standard
Proof Load	ISO 898-2
Tensile Strength	ASTM F606
Hardness	ISO 6508
Impact Testing	ASTM A370
Load Test	Project specification

35. Non-Destructive Examination (NDT)

Applied for critical applications.

Method	Purpose
Magnetic Particle	Surface crack detection
Dye Penetrant	Stainless steel inspection
Ultrasonic	Internal discontinuities
Eddy Current	Surface integrity

36. Positive Material Identification (PMI)

PMI ensures alloy compliance.

Typical methods:

• XRF analysis
• Optical emission spectroscopy

Required for:

• duplex stainless
• nickel alloys
• sour service bolting
• refinery shutdown projects

37. Certification & Documentation

SM Fasteners supplies complete project documentation.

Standard Documentation Package

• EN 10204 3.1 Material Test Certificate
• Heat Treatment Report
• Hardness Test Report
• Dimensional Inspection Report
• Coating Certificate
• Compliance Certificate (CoC)
• Traceability Records
• Third-Party Inspection Release

Optional:

• EN 10204 3.2 certification
• NACE compliance declaration
• ITP documentation

38. Failure Modes & Engineering Prevention

Failure Mode	Cause	Prevention via Prevailing Torque Nut
Self-loosening	Vibration	Locking friction
Fatigue cracking	Preload loss	Clamp load retention
Thread stripping	Grade mismatch	Correct property class
Hydrogen embrittlement	Improper plating	Controlled processing
Stress corrosion cracking	Environment	Correct material selection

39. Industrial Applications

39.1 Construction & Structural Steel

Used in:

• high-rise structures
• bridges
• seismic connections
• wind-loaded frameworks

Benefit: vibration-resistant joint integrity.

39.2 Oil & Gas Industry

Upstream

• drilling equipment
• wellhead assemblies
• offshore platforms

Midstream

• pipeline supports
• compressor stations

Downstream

• refinery structures
• pressure equipment

All-metal prevailing torque nuts preferred for temperature and hydrocarbon exposure.

39.3 Power Generation

Applicable in:

• turbines
• boilers
• generators
• nuclear auxiliary systems

Advantages:

• thermal cycling resistance
• preload stability

39.4 Petrochemical & Chemical Processing

Critical where joint relaxation causes leakage.

Materials used:

• SS316
• Duplex
• Hastelloy
• SMO 254

39.5 LNG & Cryogenic Systems

Requirements:

• contraction resistance
• corrosion resistance
• low-temperature toughness

Duplex and nickel alloy lock nuts supplied by SM Fasteners meet LNG project demands.

39.6 Automotive & Heavy Equipment

Applications include:

• suspension assemblies
• drivetrain mounting
• engine components

Prevailing torque prevents loosening under cyclic vibration.

39.7 Railways & Infrastructure

Used in:

• rolling stock
• track equipment
• signaling structures

39.8 Shipbuilding & Offshore

Essential for:

• deck equipment
• propulsion systems
• marine structural joints

39.9 PEEK Fastener Applications

SM Fasteners provides PEEK prevailing torque solutions for:

• electrical isolation
• instrumentation mounting
• semiconductor facilities
• corrosive chemical plants
• lightweight assemblies

40. Export Capability & Global Supply

SM Fasteners supports EPC procurement worldwide.

40.1 Industrial Packaging

Method	Purpose
VCI Packaging	Corrosion prevention
Thread Protectors	Damage prevention
Moisture Barrier Bags	Export humidity protection
Batch Labeling	Traceability

40.2 Export Crating

• ISPM-15 compliant wooden crates
• palletized shipment
• container optimization
• heavy lift packaging

40.3 Logistics Documentation

Provided with shipments:

• Commercial invoice
• Packing list
• Certificate of origin
• MTC documentation
• Inspection release note
• HS code classification

41. Engineering Tables (Mandatory Reference Data)

41.1 Mechanical Properties — Grade Wise

Property Class	Proof Load (MPa)	Min Tensile Strength (MPa)
8	800	800
10	1000	1040
12	1200	1220

41.2 Tightening Torque Chart (Metric)

(Values include prevailing torque consideration — reference only)

Size	Grade 8.8 Dry (Nm)	Lubricated (Nm)
M8	25	18
M10	49	36
M12	85	63
M16	210	155
M20	410	300
M24	710	520
M30	1420	1050

41.3 Preload Calculation Formula

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

Where:

• $F_p$​ = Preload force (N)
• $T$ = Applied torque (Nm)
• $K$ = Nut factor
• $D$ = Nominal diameter (m)

Worked Example

M20 bolt
Torque = 410 Nm
K = 0.18$$F_p = \frac{410}{0.18 \times 0.020}$$

$$F_p ≈ 114\,000\,N$$

41.4 Thread Standards & Tolerances

Thread Type	External	Internal
ISO Metric	6g	6H
UNC	2A	2B
UNF	2A	2B
BSW	Medium Fit	Medium Fit
BSF	Medium Fit	Medium Fit

41.5 Surface Finish Performance Comparison

Finish	Corrosion Resistance	Typical Use
Zinc	Indoor structures
HDG	Structural steel
PTFE	Offshore
Zinc Flake	Automotive
Passivation	Stainless
Nickel	Chemical plants

41.6 Material Comparison Table

Material	Strength	Corrosion Resistance	Relative Cost	Typical Industry
Carbon Steel	High	Low	Low	Construction
Alloy Steel	Very High	Moderate	Medium	Power
SS316	Medium	High	Medium	Petrochemical
Duplex	High	Very High	High	Offshore
SMO 254	High	Excellent	Very High	Seawater
Inconel	Very High	Excellent	Premium	LNG/Aerospace
PEEK	Medium	Excellent	Premium	Electrical

41.7 Weight Chart — Prevailing Torque Nuts

(Aligned with SM Fasteners production references)

Size	Weight / Piece (kg)	Weight / 100 pcs (kg)
M6	0.003	0.30
M8	0.007	0.70
M10	0.012	1.20
M12	0.020	2.00
M16	0.050	5.00
M20	0.090	9.00
M24	0.160	16.0
M30	0.350	35.0
M36	0.650	65.0

42. Engineering Procurement Checklist

EPC buyers typically verify:

• applicable standard compliance
• property class compatibility
• prevailing torque certification
• coating specification
• MTC traceability
• inspection release
• packaging suitability

SM Fasteners supports full procurement lifecycle integration.

43. SM Fasteners — Engineering Manufacturing Capability

Prevailing Torque Nuts are produced within a certified manufacturing ecosystem:

• ISO 9001 Quality Management System
• UKAF accredited quality assurance
• MSME recognized manufacturing infrastructure
• Advanced alloy machining capability
• Custom fastener engineering
• PEEK fastener manufacturing expertise
• Global EPC supply readiness