Flat Washer vs. Split Lock Washer: Functions, Differences and Engineering Selection

Flat Washer vs. Split Lock Washer: Differences, Functions and Engineering Selection

Flat washers and split lock washers are among the most recognizable components used in threaded fastening assemblies.

Although both are installed with bolts, screws or nuts, they perform different mechanical functions and should not be treated as interchangeable.

A flat washer, also called a plain washer, primarily provides a bearing surface between a fastener and the joint material.

A split lock washer, also known as a helical spring lock washer or split spring washer, has a split, twisted-ring geometry intended to provide spring action and frictional interaction.

However, a split lock washer should not automatically be considered an effective solution for preventing bolt loosening under severe vibration.

For mechanical engineers, production managers and OEM procurement teams, understanding these differences is essential for selecting suitable components and avoiding fastening failures.

This guide examines:

• Flat washer functions and geometry

• Split lock washer design and limitations

• Load distribution and surface protection

• Bolt preload and joint relaxation

• Vibration-induced loosening

• Washer material and hardness

• Installation and production consistency

• Alternative locking methods

• Applicable washer standards

• OEM washer selection and procurement

What Is a Flat Washer?

A flat washer is a thin, generally circular component with a central hole.

It is placed between the bearing surface of a bolt head or nut and the joint material.

Flat washers are available in different:

• Outside diameters

• Inside diameters

• Thicknesses

• Materials

• Hardness levels

• Surface finishes

The correct dimensions depend on the fastener size, joint material and applicable standard.

Main Functions of Flat Washers

Load Distribution

A flat washer provides a larger bearing surface than some bolt heads or nuts.

This may reduce localized bearing stress on the joint material.

However, actual load distribution depends on washer stiffness, geometry and the properties of the clamped material.

Surface Protection

A washer can reduce direct contact between a rotating fastener bearing surface and the joint material.

This may help protect coatings or softer surfaces during tightening.

However, a washer does not guarantee that a surface will remain undamaged.

Bearing Surface Control

A washer can provide a more consistent bearing interface when the joint design requires one.

This may be particularly relevant when the underlying material has a rough or uneven surface.

Hole Coverage

A properly selected washer may provide bearing support around a clearance hole.

However, washer dimensions must be compatible with the hole geometry.

A standard flat washer should not automatically be used as a substitute for a specially designed plate washer or structural washer.

Engineering Information Gain: A Larger Washer Is Not Always Better

Increasing the outside diameter of a washer can increase its potential bearing area, 

but the actual load transfer depends on washer stiffness and the supporting material.

A washer that is too thin or insufficiently hard may deform under high bolt preload.

Engineering Recommendation

Select washer diameter, thickness and hardness according to the joint requirements rather than choosing the largest available washer.

What Is a Split Lock Washer?

A split lock washer is a helical ring with a split and an offset between its ends.

The geometry allows the washer to compress as the fastener is tightened.

Split lock washers are commonly referred to as:

• Split Lock Washers

• Helical Spring Lock Washers

• Spring Lock Washers

• Split Spring Washers

In American industrial terminology, split lock washer is generally more specific than the broad term spring washer.

How Does a Split Lock Washer Work?

During tightening, the helical washer is compressed between the fastener and the joint surface.

Depending on the washer design and joint conditions, the washer may create spring action and local surface interaction.

However, this behavior does not guarantee that the fastener will resist loosening under transverse vibration.

Important Engineering Limitation

In a highly preloaded joint, a conventional split lock washer may become nearly or fully flattened.

Its remaining elastic travel may be small compared with the stiffness and preload of the bolted joint.

Therefore, the washer should not be assumed to maintain significant preload after joint relaxation.

Do Split Lock Washers Prevent Bolts From Loosening?

Not reliably in every application.

Vibration-induced loosening depends on factors such as:

• Bolt preload

• Joint stiffness

• Transverse movement

• Friction

• Joint geometry

• Material settlement

• Fastener engagement

Split lock washers have historically been used as anti-loosening devices, but their effectiveness under severe vibration is limited and application-dependent.

Engineering Recommendation

For vibration-critical joints, select a locking method based on validated performance rather than assuming that a split lock washer alone provides sufficient protection.

Flat Washer vs. Split Lock Washer: Key Differences

Engineering Information Gain: Neither Washer Automatically Solves Every Joint Problem

A flat washer is not a locking device.

A split lock washer is not a substitute for proper joint preload, suitable fastener selection or validated anti-loosening measures.

The correct design depends on the failure mechanism that must be prevented.

Understanding Bolt Preload

Bolt preload is the tensile force developed in a bolt during tightening.

The corresponding clamping force holds the joint components together.

Preload affects:

• Joint separation resistance

• Fatigue performance

• Frictional load transfer

• Resistance to relative movement

• Joint reliability

A washer can influence the bearing interface, but it does not independently establish the required preload.

Torque Is Not the Same as Preload

Installation torque is affected by friction in the threads and beneath the bolt head or nut.

Two assemblies tightened to the same torque may develop different preload if their friction conditions differ.

Washer material, coating and surface finish can affect the torque-tension relationship.

Engineering Recommendation

When preload is critical, validate the tightening process for the actual bolt, nut, washer, coating and joint materials.

Washer Hardness and Bearing Surface Deformation

Washer hardness is an important consideration in high-strength bolted joints.

An unsuitable washer may experience:

• Plastic deformation

• Indentation

• Cupping

• Surface damage

• Loss of bearing support

These effects can contribute to joint settlement.

Engineering Information Gain: A Soft Washer Can Reduce Joint Reliability

A flat washer may increase nominal bearing area, but if it plastically deforms under load, the resulting settlement can reduce bolt preload.

Engineering Recommendation

Match washer hardness and geometry to the fastener strength and joint design.

Do not assume that a generic commercial flat washer is suitable for every high-strength bolt assembly.

Joint Settlement and Embedment

After tightening, small surface irregularities at contact interfaces may flatten under load.

This process is often described as embedment or joint settlement.

Potential sources include:

• Rough contact surfaces

• Soft coatings

• Washer deformation

• Joint material compression

• Gasket relaxation

Settlement can reduce bolt preload.

Can a Split Lock Washer Compensate for Settlement?

A conventional split lock washer has limited elastic travel once compressed.

It should not be relied upon to compensate for significant settlement in a highly preloaded joint.

Engineering Recommendation

If settlement is a concern, address the underlying joint design and consider an appropriately 

engineered spring element only where its load-deflection behavior has been validated.

Vibration-Induced Bolt Loosening

Threaded fasteners may loosen when repeated transverse movement occurs between joint components.

The risk depends on the joint design and loading conditions.

Potential contributing factors include:

• Insufficient preload

• Joint slip

• Repeated transverse displacement

• Friction changes

• Inadequate fastener engagement

• Settlement

• Fatigue

Why a Split Lock Washer May Be Insufficient

The helical shape alone does not prevent all relative movement or rotation.

Under certain conditions, the washer may flatten and lose much of its spring action.

Engineering Recommendation

For severe vibration, evaluate suitable locking technologies and validate the complete joint under representative conditions.

Alternative Anti-Loosening Solutions

Depending on the application, alternatives may include:

Nylon-Insert Lock Nuts

Nylon-insert lock nuts develop prevailing torque through interaction between the nylon insert and mating bolt thread.

Potential considerations include:

• Temperature

• Chemical exposure

• Reuse

• Prevailing torque

• Thread compatibility

All-Metal Prevailing-Torque Lock Nuts

These nuts use a metallic locking feature to generate prevailing torque.

Potential advantages may include suitability for certain higher-temperature applications, depending on the design and material.

Wedge-Locking Washers

Wedge-locking washer systems use paired components with a specialized geometry.

Their effectiveness depends on correct product selection, installation and application conditions.

Thread-Locking Adhesives

Thread-locking compounds may help resist loosening in suitable threaded assemblies.

Performance depends on:

• Material compatibility

• Surface condition

• Thread clearance

• Curing conditions

• Temperature

• Service requirements

Engineered Spring Washer Systems

Certain conical spring washers or disc springs may be used where controlled load-deflection characteristics are required.

These are not mechanically equivalent to conventional split lock washers.

Engineering Recommendation

Choose a locking solution based on the joint's actual failure mode and validation requirements.

Can Flat Washers and Split Lock Washers Be Used Together?

They may be used together in some existing assembly designs.

However, combining a flat washer with a split lock washer does not automatically create a reliable vibration-resistant joint.

The additional interfaces may affect:

• Friction

• Surface settlement

• Joint stiffness

• Installation behavior

Engineering Recommendation

Do not add a split lock washer to an existing joint without evaluating its effect on preload, joint geometry and assembly performance.

Flat Washer and Split Lock Washer Installation

Correct installation depends on the joint design and applicable specification.

Flat Washer Installation

A flat washer is typically placed beneath the bearing surface of a bolt head or nut where required by the assembly design.

The washer should:

• Match the fastener size

• Provide the specified bearing area

• Have the required hardness

• Be compatible with the joint material

• Be free from unacceptable defects

Split Lock Washer Installation

Where specified, the split lock washer is installed in the orientation and location required by the approved assembly design.

However, simply compressing the washer does not verify vibration resistance.

Engineering Recommendation

Follow the approved assembly procedure and specified tightening method.

Do not apply a universal installation rule to all washer and fastener combinations.

Washer Materials and Surface Treatments

Washer material selection affects strength, corrosion resistance and cost.

Carbon Steel Washers

Carbon steel washers are commonly used in industrial fastening applications.

Relevant considerations include:

• Material grade

• Hardness

• Surface treatment

• Corrosion exposure

• Fastener strength

Stainless Steel Washers

Stainless steel washers may be considered where corrosion resistance is important.

However, stainless steel grades differ in mechanical properties and environmental suitability.

Engineering Recommendation

Specify the actual stainless steel grade and required mechanical properties.

Other Washer Materials

Depending on the application, washers may also be made from:

• Brass

• Aluminum

• Engineering plastics

• Other specified materials

These materials should not be substituted for steel washers without evaluating bearing strength, temperature and other joint requirements.

Surface Treatments

Potential finishes include:

• Zinc plating

• Other specified protective coatings

• Passivation for suitable stainless steel components

• Application-specific surface treatments

Coating thickness and friction characteristics may affect assembly performance.

Engineering Recommendation

Confirm that the coating is compatible with the fastener and joint requirements.

Relevant International Washer Standards

Different standards define different washer geometries, dimensions or performance requirements.

The correct standard must be selected according to the specific product.

Metric Flat Washers

Relevant standards may include:

• ISO 7089 — Plain washers, normal series, product grade A

• ISO 7090 — Plain washers, chamfered, normal series, product grade A

• DIN 125 — Common legacy designation for certain plain washer configurations

Metric Split Lock Washers

DIN 127 is a familiar legacy reference for split spring lock washers.

However, its applicability and current specification status should be checked against the customer's drawing and purchasing requirements.

American Washer Specifications

Relevant American standards may include:

• ASME B18.21.1 — Lock washers

• ASME B18.21.2 — Plain washers

Specific dimensions and configurations must be selected according to the applicable standard edition.

SAE and USS Flat Washers

In the U.S. market, SAE and USS are common commercial descriptions for different flat washer dimensional series.

The precise dimensions should be verified against the specified standard or supplier drawing.

Metric vs Inch Fasteners

Metric washer dimensions and inch-series washer dimensions are not automatically interchangeable.

Similarly, metric bolt threads and UNC/UNF threads are different systems.

A washer's nominal hole size does not establish compatibility with every bolt or joint.

Engineering Recommendation

Specify the required washer dimensions and applicable standard rather than relying only on a nominal size description.

Industrial Applications

Automotive Manufacturing

Washers may be used in selected vehicle assemblies where bearing support, surface protection or specified locking arrangements are required.

Potential applications include:

• Equipment brackets

• Noncritical mounting assemblies

• Accessory components

• Vehicle manufacturing fixtures

• Industrial automotive production equipment

Safety-critical vehicle joints require approved fastening specifications and validation.

Industrial Machinery

Flat washers may provide bearing support in:

• Machine frames

• Equipment enclosures

• Conveyor systems

• Industrial brackets

• Maintenance assemblies

Where vibration resistance is required, the locking method should be selected independently of the flat washer's bearing function.

Electrical Equipment

Washers may be used in:

• Control cabinets

• Electrical enclosures

• Equipment mounting

• Support brackets

• Industrial power equipment

For electrical connections, contact resistance, material compatibility and the approved connection design may impose additional requirements.

A general-purpose washer should not be assumed to meet electrical contact requirements.

Structural and Mechanical Assemblies

Flat washers may be specified for structural and mechanical connections where suitable bearing support is required.

High-strength structural bolting systems may require particular washer grades and installation procedures.

Engineering Recommendation

Use the specified structural fastener system rather than substituting general-purpose washers.

Manufacturing and Production-Line Considerations

Production managers and manufacturing engineers must evaluate washer selection in the context of assembly efficiency and quality consistency.

Relevant factors include:

• Washer feeding

• Part orientation

• Installation cycle time

• Fastener engagement

• Torque control

• Component identification

• Inspection

• Rework

• Packaging

Automated Assembly

Washers used in automated assembly may require controlled:

• Outside diameter

• Inside diameter

• Thickness

• Flatness

• Burr condition

• Surface finish

Dimensional inconsistency can create feeding or assembly problems.

Engineering Recommendation

Include production equipment requirements when selecting washers for automated fastening systems.

Avoiding Washer Mix-Ups

Similar-looking washers may differ in:

• Hardness

• Thickness

• Material

• Coating

• Dimensional series

Incorrect substitution can affect joint performance.

Engineering Recommendation

Use clear part identification, controlled packaging and appropriate incoming inspection.

Common Washer Selection Mistakes

Quality Inspection and OEM Washer Specifications

An appropriate inspection plan depends on the washer type and application.

Dimensional Inspection

Potential characteristics include:

• Inside diameter

• Outside diameter

• Thickness

• Flatness

• Chamfer

• Split geometry where applicable

Material and Hardness Inspection

Depending on the drawing, requirements may include:

• Material grade

• Hardness

• Mechanical properties

• Surface treatment

• Material documentation

Visual Inspection

Potential defects include:

• Burrs

• Cracks

• Corrosion

• Coating defects

• Deformation

Functional Evaluation

For critical bolted joints, washer inspection alone may be insufficient.

Joint-level validation may be needed to evaluate:

• Tightening behavior

• Preload

• Settlement

• Vibration

• Fatigue

• Environmental exposure

Engineering Recommendation

Define acceptance criteria according to the component's actual function.

OEM Washer Sourcing and Supplier Qualification

Purchasing and supplier development teams should evaluate both technical requirements and commercial supply conditions.

Technical Requirements

• Applicable standard

• Material

• Dimensions

• Hardness

• Surface finish

• Inspection criteria

• Approved drawing

Manufacturing Requirements

• Dimensional consistency

• Process feasibility

• Surface treatment

• Quality control

• Packaging

• Part identification

Commercial Requirements

• Prototype quantity

• MOQ

• Annual demand

• Unit price

• Lead time

• Delivery schedule

• Supply continuity

Engineering Recommendation

Request quotations using the approved washer specification rather than only the general terms flat washer or spring washer.

OEM RFQ Checklist for Flat Washers and Split Lock Washers

Frequently Asked Questions

What is the main difference between a flat washer and a split lock washer?

A flat washer primarily provides a bearing surface and load distribution.

 A split lock washer has a helical geometry intended to provide spring action and surface interaction.

Is a spring washer the same as a split lock washer?

The term spring washer is broad. A split lock washer is one particular type of spring washer.

Do split lock washers prevent vibration loosening?

Not reliably in every application. Their effectiveness is limited and depends on the joint design and loading conditions.

Can a flat washer prevent a bolt from loosening?

A conventional flat washer does not provide an inherent locking mechanism.

Should flat washers and split lock washers be used together?

Only where the joint design specifies and validates that arrangement.

Are hardened flat washers necessary?

They may be required where high bearing loads or particular fastener strength levels make washer hardness important.

What is the difference between SAE and USS flat washers?

They are common U.S. commercial dimensional series. 

The applicable dimensions should be verified against the specified standard or drawing.

Can metric and inch washers be substituted?

Not automatically. Dimensional compatibility and the joint requirements must be checked.

What alternatives are available for vibration-resistant joints?

Depending on the application, alternatives may include prevailing-torque lock nuts, wedge-locking washers, 

thread-locking adhesives or other validated locking methods.

Can Juxin Fasteners review custom washer drawings?

Juxin Fasteners welcomes drawing-based industrial fastening inquiries for technical and commercial review.

Why Work With Juxin Fasteners?

Juxin Fasteners has more than 20 years of experience serving international industrial fastener customers.

Our broader fastening product portfolio includes:

• Nylon-insert lock nuts

• All-metal locking nuts

• Industrial bolts and nuts

• Threaded fastening components

• Custom OEM fasteners

• Other drawing-based industrial fastening solutions

For flat washers, split lock washers and custom washer requirements, specific product availability, 

manufacturing feasibility, materials, MOQ, lead times and quality documentation must be confirmed during inquiry review.

Request a Quotation for Industrial Washers and Fastening Solutions

Are you sourcing flat washers, split lock washers or complete fastening components for OEM manufacturing?

Juxin Fasteners welcomes technical inquiries from mechanical engineers, design engineers, structural engineers, 

manufacturing engineers, production managers, purchasing specialists, procurement managers, supplier development teams and supply chain professionals.

For technical review and quotation, please provide:

2D/3D drawings, washer type, applicable standard, material, hardness, dimensions, surface treatment, 

mating bolt or nut specification, joint application, operating environment, inspection requirements, prototype quantity, annual demand, packaging and delivery schedule.

Our team can review your requirements and confirm suitable product options, technical feasibility and commercial quotation conditions.

Company: Juxin Fasteners

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

Email: info@juxinfasteners.com

Send your washer specifications, fastening drawings and OEM RFQ to Juxin Fasteners for technical review and quotation.