Metric Class 6 Spring-Stop Lock Nuts – Fuji-Style Metal Spring Locking

Metric Class 6 Spring-Stop Lock Nuts

Fuji-Style Metal Spring Locking Nuts for Industrial Equipment, Repeated Assembly and OEM Fastener Manufacturing

JUXIN FASTENERS manufactures standard and custom industrial fasteners for international OEM manufacturers, 

Tier-1 and Tier-2 suppliers, mechanical engineers, manufacturing engineers, production managers, procurement development teams and supply chain managers.

Our metric Class 6 Spring-Stop Lock Nuts use a metal spring locking element to create prevailing torque when installed on a compatible threaded bolt or stud.

This product is also described in the North American industrial fastener market as a Fuji-style locknut.

Unlike nylon-insert locknuts, Spring-Stop Lock Nuts do not depend on a polymer locking ring.

Unlike DIN 935 castle nuts, they do not require a separate cotter pin and a transverse hole through the mating bolt.

The featured product combines a carbon steel nut body with a spring locking component and is intended for applications 

where the locking performance has been validated for the actual assembly conditions.

JUXIN supports OEM/ODM manufacturing, custom thread configurations, customer-defined surface finishes and industrial batch production.

Product Specifications

The exact spring design, nut dimensions, material properties, finish and functional performance must be confirmed for each production order.

What Is a Spring-Stop Lock Nut?

A Spring-Stop Lock Nut is a prevailing-torque fastener with a metal spring locking feature.

During installation, the mating bolt engages the spring element.

Elastic deflection of the locking feature produces resistance to rotation.

The resulting prevailing torque helps resist unintended loosening when the complete joint has been correctly designed and installed.

The locking action differs from a conventional free-spinning hex nut.

How the Spring Locking Element Works

The locking system generally involves three functional elements:

• A threaded metal nut body.

• A retained spring locking feature.

• A compatible externally threaded bolt or stud.

As the mating thread passes through the locking region, the spring element deflects and develops contact force.

That contact force creates prevailing torque.

The locking performance depends on the actual spring geometry, material, mating thread and operating conditions.

The spring feature should not be confused with a separate helical spring washer.

Spring-Stop Lock Nuts vs. Other Locking Nuts

These systems are not automatically interchangeable.

Selection must be based on the equipment manufacturer's approved design.

Why Engineers Select Spring-Stop Lock Nuts

A metal spring locking feature may offer useful design characteristics in applications involving:

• Repeated equipment servicing.

• Installation and removal.

• Machinery vibration.

• Metal-only locking features.

• Industrial assembly operations.

• Controlled prevailing-torque performance.

However, these benefits depend on the actual finished product and its tested performance.

Repeated-use capability must not be assumed without appropriate testing.

Class 6 Carbon Steel: Mechanical Property Considerations

The featured product is identified as a metric Class 6 steel locknut.

Class 6 is a mechanical property classification associated with specified steel nut performance.

It must not be confused with DIN dimensional product grade A or B.

A Class 6 locknut should not automatically replace a higher-strength nut.

The engineering team should review:

• Mating bolt property class.

• Required joint preload.

• Nut proof-load requirements.

• Thread engagement.

• Prevailing-torque performance.

• Installation torque.

• Operating loads.

The selected nut must be compatible with the complete fastening system.

Metric Size and Thread Reference Table

JUXIN's listed Spring-Stop Lock Nut family includes M5, M6, M8, M10 and M12.

These are standard ISO metric coarse-thread references.

The actual product thread designation, tolerance and availability must be confirmed.

The inch values are nominal diameter conversions only.

They are not equivalent American thread designations.

Metric Fine-Thread Customization

Some equipment requires metric fine threads.

Examples for engineering review include:

These are possible thread designations for custom engineering review, not confirmed standard stock configurations.

A fine-thread design must be evaluated for spring-element compatibility and prevailing-torque performance.

American UNC and UNF Thread References

American OEM buyers may require inch-series spring locking nuts.

JUXIN can review custom UNC and UNF designs against approved drawings.

These are standard Unified thread references.

They do not establish stock availability or confirm that every size is suitable for the featured spring locking geometry.

Custom inch-series manufacturing requires separate engineering validation.

Metric and Inch Threads Are Not Interchangeable

Metric, UNC, UNF, BSW and BSF are different thread systems.

Examples:

• M6 × 1.0 is not interchangeable with 1/4-20 UNC.

• M8 × 1.25 is not interchangeable with 5/16-18 UNC.

• M10 × 1.5 is not interchangeable with 3/8-16 UNC.

• M12 × 1.75 is not interchangeable with 1/2-13 UNC.

Thread diameter similarity does not establish compatibility.

The exact thread designation and tolerance must be verified.

Prevailing Torque: What Should Be Tested?

The functional performance of a Spring-Stop Lock Nut should be evaluated using the applicable product specification.

Relevant measurements may include:

• First-installation prevailing torque.

• First-removal prevailing torque.

• Subsequent installation torque.

• Subsequent removal torque.

• Spring element retention.

• Thread condition after cycling.

• Proof-load performance.

• Functional performance after environmental exposure.

The required values must be defined by the applicable standard or customer-approved test specification.

Important Difference Between Prevailing Torque and Tightening Torque

Prevailing torque is the rotational resistance generated by the locking feature before the nut seats against the joint.

Tightening torque is the torque applied during final joint assembly.

The total installation torque includes resistance from the locking feature and the torque required to generate joint preload.

Therefore, tightening values developed for a free-spinning nut should not automatically be used for a Spring-Stop Lock Nut.

Repeated Installation and Removal

One commercial advantage often associated with metal spring locking designs is their potential suitability for repeated assembly.

However, reuse performance is not unlimited.

The actual locking torque may change because of:

• Spring-element deformation.

• Thread wear.

• Coating wear.

• Lubrication changes.

• Installation conditions.

• Corrosion.

• Temperature exposure.

An OEM reuse specification should identify the number of installation cycles and the minimum acceptable removal prevailing torque.

Vibration Resistance and Joint Reliability

A spring locking feature can generate resistance to unintended nut rotation.

However, the presence of prevailing torque does not automatically prevent loss of joint preload under all vibration conditions.

The complete assembly may require testing for:

• Transverse vibration.

• Joint relaxation.

• Repeated dynamic loading.

• Thermal cycling.

• Fatigue.

• Installation variation.

Application-specific testing should determine suitability.

Temperature Considerations

Metal spring locking features avoid the nylon insert used in conventional nylon-insert locknuts.

This may be relevant to applications where polymer temperature limits are a concern.

Nevertheless, the operating temperature range of the finished Spring-Stop Lock Nut depends on:

• Spring material.

• Spring heat treatment.

• Nut material.

• Coating system.

• Required prevailing torque.

• Service environment.

No universal maximum temperature should be assumed.

Surface Treatment Options

The terms "environmentally friendly zinc" and "environmentally friendly blue zinc" should be replaced with precise coating specifications.

Where required, customers should identify coating thickness, passivation, corrosion testing and restricted-substance requirements.

Dacromet is a trade-related coating designation and should not automatically be treated as equivalent to every zinc flake coating.

Coating Effects on Spring Locking Performance

Surface treatment may influence:

• Thread friction.

• Installation prevailing torque.

• Removal prevailing torque.

• Spring-element movement.

• Corrosion resistance.

• Repeated-use performance.

A coating change may therefore require renewed functional testing.

Industrial Machinery Manufacturing

Spring-Stop Lock Nuts may be evaluated for suitable machinery fastening applications where a metal prevailing-torque locking feature is required.

Potential uses include:

• Equipment brackets.

• Machinery covers.

• Mechanical accessories.

• Serviceable equipment assemblies.

• Auxiliary mounting systems.

The complete joint must be validated for loading and vibration.

Industrial Maintenance and Repair

Maintenance teams may need fasteners that can be removed during servicing and replaced or reused according to approved procedures.

Spring-Stop Lock Nuts may be relevant where:

• Access is limited.

• A cotter pin arrangement is undesirable.

• The mating bolt is not cross-drilled.

• Prevailing-torque locking is specified.

• Repeated assembly performance is validated.

Reuse must follow the applicable maintenance requirements.

Agricultural Machinery Manufacturing

Potential applications include suitable:

• Equipment brackets.

• Protective covers.

• Auxiliary mechanisms.

• Serviceable mechanical connections.

Dust, moisture, shock loading and corrosion must be considered.

Construction Equipment Manufacturing

Potential applications include approved non-critical machinery assemblies and equipment accessories.

Heavy equipment may experience severe shock and vibration.

Fasteners must be selected and tested for the actual loading conditions.

Automotive Manufacturing

Potential applications include suitable non-critical auxiliary assemblies.

Automotive fastening programs may require:

• Prevailing-torque testing.

• Coating validation.

• Mechanical property verification.

• Production traceability.

• Assembly process control.

Safety-critical automotive joints require application-specific qualification.

Motorcycle Manufacturing

Potential applications include selected non-critical equipment brackets and auxiliary assemblies.

The nut must not automatically replace a specified high-strength or safety-critical motorcycle fastener.

Robotics and Industrial Automation

Potential applications include:

• Sensor brackets.

• Auxiliary equipment mounts.

• Machine covers.

• Serviceable automation fixtures.

• Instrumentation supports.

Dynamic loading and vibration requirements must be reviewed.

Electrical Power Equipment Manufacturing

Potential applications include auxiliary equipment brackets and mechanical enclosure hardware.

Electrical bonding and grounding requirements must be assessed separately.

AI Data Center Cabinet Manufacturing

Potential applications include suitable non-critical cabinet accessories, instrumentation supports and mechanical brackets.

Installation access, vibration, grounding and sheet metal thickness must be considered.

For thin sheet metal requiring captive threads, self-clinching nuts or rivet nuts may be more appropriate.

Telecommunications Equipment Manufacturing

Potential applications include enclosure accessories, equipment brackets and auxiliary mounting hardware.

Outdoor exposure may require additional corrosion and environmental qualification.

Rail Transit Equipment Manufacturing

Potential applications include approved non-critical equipment assemblies.

Rail projects may require vibration, fatigue, fire-related and traceability requirements depending on the application.

Marine Equipment Manufacturing

Spring-Stop Lock Nuts may be evaluated for selected marine equipment applications.

Saltwater exposure, coating performance and spring-element corrosion must be reviewed.

HVAC Equipment Manufacturing

Potential applications include suitable mechanical equipment brackets, auxiliary fixtures and serviceable assemblies.

Vibration, moisture and thermal cycling should be evaluated.

Medical and Food Machinery Manufacturing

Potential applications are limited to suitable external or non-contact mechanical assemblies where material and environmental requirements have been verified.

No medical-device or food-contact certification is claimed.

Semiconductor Equipment Manufacturing

Potential applications include suitable external equipment fixtures.

The spring locking design must be evaluated for cleanliness, contamination, vacuum and temperature requirements.

Common Engineering Problems and Corrective Actions

JUXIN FASTENERS Manufacturing Capabilities

JUXIN FASTENERS is an industrial fastener manufacturer supporting OEM/ODM projects and industrial batch production.

Our broader manufacturing capabilities include:

• Cold heading and cold forming.

• CNC machining.

• Internal threading.

• Product-specific secondary operations.

• Metal locking feature manufacturing review.

• Customer-defined surface treatment.

• Made-to-drawing fastener production.

• Industrial batch manufacturing.

The actual manufacturing route for Spring-Stop Lock Nuts depends on the nut geometry, spring design, material and production requirements.

OEM/ODM Customization

JUXIN can review:

• M5, M6, M8, M10 and M12 configurations.

• Custom metric thread sizes.

• Metric fine threads.

• UNC and UNF inch-series designs.

• Carbon steel material requirements.

• Spring locking element geometry.

• Prevailing-torque requirements.

• Reuse-cycle requirements.

• Custom nut height and hex dimensions.

• Zinc and zinc-nickel finishes.

• Black zinc and zinc flake coatings.

• OEM packaging and identification.

• Annual purchasing programs.

All custom requirements are subject to engineering feasibility review.

Quality Inspection and Functional Verification

Depending on the agreed quality plan, inspection may include:

• Material verification.

• Mechanical property testing.

• Thread gauge inspection.

• Width-across-flats measurement.

• Nut height inspection.

• Spring-element retention inspection.

• First-installation prevailing-torque testing.

• Removal prevailing-torque testing.

• Reuse-cycle testing.

• Surface treatment inspection.

• Coating thickness verification.

• Packaging and traceability review.

Acceptance limits must be specified by the applicable product standard or customer-approved engineering documentation.

Applicable Standards and Engineering References

ISO 7719

ISO 7719:2025 specifies regular all-metal prevailing-torque hexagon nuts in steel and stainless steel with metric coarse threads.

Its applicability depends on the exact product geometry and performance requirements.

ISO 7042

ISO 7042:2025 specifies high all-metal prevailing-torque hexagon nuts with metric coarse threads.

A high-nut design must not be confused with a regular-height Spring-Stop Lock Nut.

ISO 7720

ISO 7720:2025 addresses high all-metal prevailing-torque hexagon nuts with slot features.

The slot geometry and locking principle must be compared with the actual product design before claiming conformity.

ISO 10513

ISO 10513 addresses specified all-metal prevailing-torque hexagon nuts with metric fine threads.

It is not the primary dimensional reference for this metric coarse-thread Class 6 product.

DIN 980 and DIN 6925

DIN 980 and DIN 6925 are associated with specified all-metal prevailing-torque nut designs.

They should not automatically be treated as interchangeable with a Spring-Stop or Fuji-style locking structure.

ASME B18.16.6

ASME B18.16.6 covers defined inch-series prevailing-torque locknut configurations and performance requirements.

Custom UNC/UNF Spring-Stop Lock Nuts must be evaluated against the applicable requirements and approved drawings.

ASME B1.1

ASME B1.1 provides Unified inch screw-thread requirements, including UNC and UNF.

ISO Metric Thread References

Relevant thread standards include ISO 261, ISO 724 and the ISO 965 series.

Customer-Specific Product Qualification

The final standard claim must be based on the actual product design, material, dimensional inspection and functional testing.

Listing a standard as a technical reference does not establish product certification or conformity.

Frequently Asked Questions

What is a Spring-Stop Lock Nut?

It is a prevailing-torque locknut that uses a metal spring locking feature.

Are Spring-Stop Lock Nuts also called Fuji locknuts?

The term Fuji locknut is used in the North American market for a related spring-locking design. JUXIN describes this product as Fuji-style without claiming brand authorization.

Does this product contain nylon?

No. The featured locking mechanism uses a metal spring element.

What metric sizes are listed?

M5, M6, M8, M10 and M12.

What is Class 6?

Class 6 identifies the specified mechanical property class of the steel nut, subject to applicable verification.

Can Spring-Stop Lock Nuts be reused?

Reuse depends on the actual locking performance after repeated installation and removal.

Are they suitable for vibration?

They may be considered for applications requiring prevailing-torque locking, but vibration resistance must be validated for the complete joint.

Are these suitable for high temperatures?

Only where the spring material, coating and finished fastener have been validated for the operating temperature.

Can JUXIN manufacture UNC or UNF versions?

Custom inch-series designs can be evaluated against customer drawings.

Are metric and UNC/UNF threads interchangeable?

No. They are separate thread systems.

Are Spring-Stop Lock Nuts the same as DIN 980 nuts?

Not necessarily. The locking geometry and applicable standards must be reviewed.

Does JUXIN support OEM manufacturing?

Yes. JUXIN supports made-to-drawing custom fastener manufacturing and industrial batch production.

Request a Quote — Metric Class 6 Spring-Stop Lock Nut Manufacturer

JUXIN FASTENERS manufactures standard and custom industrial fasteners for international OEM, Tier-1 and Tier-2 manufacturing programs.

We support metric Spring-Stop Lock Nut projects involving metal spring locking features, custom thread requirements, customer-defined coatings and industrial batch quantities.

Our broader manufacturing capabilities include cold heading, cold forming, CNC machining, threading, secondary processing and surface treatment.

Recommended RFQ Information

• Required metric or UNC/UNF thread.

• Nut dimensions and height.

• Carbon steel mechanical property class.

• Spring locking element requirements.

• Mating bolt grade and thread tolerance.

• First-installation prevailing-torque limits.

• Removal prevailing-torque limits.

• Required reuse-cycle performance.

• Operating temperature.

• Vibration and service environment.

• Surface treatment specification.

• Applicable product standard.

• Inspection and testing requirements.

• 2D drawing or 3D CAD model.

• Order quantity and annual forecast.

• Packaging and delivery requirements.

Please send your technical drawings, specifications and purchasing requirements for engineering review and quotation.

JUXIN FASTENERS

Industrial Fastener Manufacturer | OEM/ODM Custom Fastener Manufacturing | Cold Heading, Cold Forming and CNC Machining

Website: https://www.juxinfasteners.com/

Email: info@juxinfasteners.com

Packaging Standard

At Juxin Fasteners, we apply standardized export packaging to ensure product protection, traceability, and compliance with international logistics requirements.

1. Standard Export Packaging

Unless otherwise specified, all products will be packed according to our factory standard export packaging, which includes:

Moisture-resistant inner protection

Poly bag or small box packing as required

Reinforced export cartons

Clear labeling with part number, specification, batch number, and quantity

Palletizing for sea or air shipment when necessary

Our standard packaging is designed to ensure safe transportation, efficient warehousing, and long-distance international shipping.

2. Customized Packaging Options

We also provide customized packaging solutions according to customer requirements, including but not limited to:

Private labeling

Customized barcodes

Specific carton dimensions

Retail packaging

Special pallet configuration

Customer-specific marking and identification

So that you know, customized packaging may involve additional costs and extended lead time depending on the complexity of the requirements.

3. Compliance & Quality Assurance

All packaging processes are controlled under our ISO 9001 quality management system to ensure consistency, traceability, and product integrity throughout the supply chain.