Why Are Washers Used with Bolts? Functions, Selection and OEM Fastening Applications

Why Are Washers Used with Bolts? Functions, Engineering Selection and OEM Applications

Washers are widely used in industrial bolted connections, but their purpose is often misunderstood.

A washer is not simply an additional component placed beneath a bolt head or nut. 

Depending on its design, material and installation position, it can influence the bearing interface, local contact pressure, surface protection and assembly behavior of a threaded joint.

The most common question is straightforward:

Why are washers used with bolts?

Washers are used when a bolted joint requires a particular bearing surface, load distribution, surface protection, hole coverage or specialized function such as sealing.

However, not every bolt requires a washer, and installing one does not automatically improve joint reliability.

A properly designed connection must account for the bolt, nut, washer, clamped components and installation process as a complete system.

For OEM design engineers, mechanical engineers, manufacturing engineers and production managers, the important question is not simply whether a washer should be installed.

It is whether the washer performs a necessary engineering function in the specific joint.

This guide explains how washers work, when they are required, when they may be unnecessary and how to select suitable washer specifications for industrial production.

What Is a Washer in a Bolted Joint?

A washer is a component positioned at a fastening interface to perform a defined mechanical or functional purpose.

A conventional flat washer is typically a thin circular part with a central hole.

It may be installed beneath:

• A bolt head

• A screw head

• A nut

Other washer designs include:

• Hardened flat washers

• Split lock washers

• Conical spring washers

• Wedge-locking washer systems

• Sealing washers

• Bonded sealing washers

• Specialized structural washers

These products do not perform identical functions.

The correct washer must be selected according to the joint design and applicable specification.

Main Functions of Washers Used with Bolts

Load Distribution and Bearing Support

One of the primary functions of a flat washer is to provide a suitable bearing surface between the fastener and the connected component.

When a bolt is tightened, the bolt head or nut applies compressive force to the joint.

A properly selected washer can increase the effective bearing area compared with direct contact from certain fastener heads or nuts.

This may reduce localized bearing stress.

Potential benefits include:

• Reduced local indentation

• Improved support around a clearance hole

• Better compatibility with selected joint materials

• A more suitable fastener bearing interface

However, actual pressure distribution depends on washer stiffness, geometry and the supporting material.

Engineering Information Gain: A Larger Washer Does Not Always Distribute Load Uniformly

A washer's full annular area does not necessarily carry uniform contact pressure.

The actual load path depends on the stiffness of the washer and the underlying component.

A washer that is too thin may bend under load, while a washer that is insufficiently hard may experience indentation.

Engineering Recommendation

Select washer outside diameter, thickness and hardness together rather than using outside diameter as the only selection criterion.

Surface Protection During Tightening

A washer may help protect the joint surface from direct contact with a rotating bolt head or nut.

This can be relevant for:

• Painted metal panels

• Coated steel parts

• Aluminum components

• Selected plastic housings

• Precision equipment surfaces

During tightening, the fastener bearing surface may rotate against the washer rather than directly against the connected material.

However, friction can occur at either washer interface, and the washer itself may leave marks or cause indentation.

Engineering Recommendation

For appearance-sensitive or coated components, validate the complete tightening process and inspect the resulting surface condition.

Do not assume that adding a washer guarantees a damage-free finish.

Supporting the Area Around Clearance Holes

Bolted joints require clearance between the fastener shank and the hole in many conventional assembly designs.

If the bearing surface of the bolt head or nut is too small for the hole geometry, a washer may provide additional support.

This is particularly relevant when the joint includes:

• Oversized clearance holes

• Slots

• Thin sheet components

• Certain fabricated brackets

However, not every flat washer is suitable for an oversized hole or slot.

Some joints require specially designed plate washers or structural washers.

Engineering Recommendation

Confirm the required bearing overlap and applicable washer geometry.

Providing a Suitable Interface for High-Strength Bolts

High-strength bolts can generate substantial preload.

The bearing interface must be able to withstand the resulting compressive loading without unacceptable deformation.

A hardened washer may be specified to provide suitable bearing support.

Important characteristics include:

• Material

• Hardness

• Thickness

• Flatness

• Inside diameter

• Outside diameter

• Surface finish

Engineering Information Gain: Washer Hardness and Washer Stiffness Are Different

Hardness influences resistance to indentation.

Thickness and geometry influence bending stiffness.

A washer can be hard enough to resist local indentation but still bend or cup if its geometry is unsuitable.

Engineering Recommendation

For highly preloaded joints, verify the complete washer specification instead of selecting a washer based only on nominal bolt diameter.

Supporting Specific Sealing Functions

Certain washers are designed to provide a sealing function.

Examples include:

• Copper sealing washers

• Aluminum sealing washers

• Bonded sealing washers

• Other engineered sealing washer designs

These components may be used in suitable fluid or pneumatic connections.

However, sealing washers differ from conventional flat washers.

Their performance depends on factors such as:

• Sealing material

• Contact surface finish

• Fluid compatibility

• Pressure

• Temperature

• Compression

• Reuse requirements

Engineering Recommendation

Use a washer specifically designed and validated for the sealing application.

A standard steel flat washer should not be treated as a fluid seal.

Do All Bolts Need Washers?

No.

Some bolted joints are designed to operate without separate washers.

For example, a fastener may have an integrated flange or another bearing geometry that provides the required interface.

In other assemblies, a separate washer may be unnecessary because the fastener and joint materials already satisfy the design requirements.

Whether a washer is needed depends on:

• Fastener head or nut geometry

• Joint material

• Clearance hole size

• Required preload

• Surface condition

• Bearing stress

• Installation method

• Applicable specification

When Is a Washer Commonly Considered?

A washer may be appropriate when:

• The joint requires a larger bearing surface.

• The connected material is relatively soft.

• The clearance hole requires additional bearing support.

• A hardened bearing interface is specified.

• Surface protection is needed.

• A specialized sealing or locking function is required.

• The governing standard or OEM drawing specifies a washer.

When Might a Washer Be Unnecessary?

A washer may be unnecessary when:

• The fastener has an appropriate integrated bearing flange.

• The joint design already provides adequate bearing support.

• The approved fastening specification does not require one.

• Additional joint stack thickness would be undesirable.

• The washer could interfere with the intended fastener seating geometry.

Engineering Information Gain: Adding a Washer Changes the Joint

Adding a washer changes the assembly stack.

It may affect:

• Bolt grip length

• Thread engagement

• Joint stiffness

• Bearing friction

• Tightening behavior

• Assembly height

• Settlement interfaces

Engineering Recommendation

Do not add or remove washers from an approved assembly without reviewing the resulting joint conditions.

How Washers Affect Bolt Preload

Bolt preload is the tensile force generated in the bolt during tightening.

The corresponding clamping force holds the connected components together.

Preload is important for:

• Resistance to joint separation

• Frictional load transfer

• Fatigue performance

• Limiting relative movement

• Maintaining joint integrity

A washer can influence preload indirectly through its bearing characteristics.

However, a flat washer does not independently create or maintain a specified preload.

The preload depends on the complete fastening system and installation process.

Torque Is Not the Same as Preload

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

Changes in washer material, coating or surface condition can alter bearing friction.

As a result, identical tightening torque does not necessarily produce identical preload.

Engineering Recommendation

Where preload accuracy is important, validate the torque-preload relationship using the actual production fasteners, washers, coatings and joint materials.

Washer Deformation and Joint Settlement

When a joint is tightened, its contact interfaces may experience small permanent changes.

These changes can contribute to joint settlement.

Potential sources include:

• Washer indentation

• Washer cupping

• Surface roughness

• Coating compression

• Joint material deformation

• Gasket relaxation

Settlement may reduce the effective joint stack thickness and contribute to preload loss.

Engineering Information Gain: A Washer Can Reduce One Problem and Create Another

A properly selected washer may help protect a soft bearing surface.

However, an unsuitable washer can introduce additional deformation or settlement.

For example:

• A thin washer may bend over a large clearance hole.

• A soft washer may indent beneath a high-strength bolt.

• A hard washer may remain stable while the softer clamped material deforms.

Engineering Recommendation

Evaluate washer geometry, hardness and the underlying material together.

Can Washers Prevent Bolts From Loosening?

Some specialized washer systems are designed to contribute to locking performance.

However, a conventional flat washer does not have an inherent anti-loosening mechanism.

A split lock washer should also not be assumed to prevent loosening under severe vibration.

Vibration-induced loosening depends on factors including:

• Bolt preload

• Transverse joint movement

• Friction

• Joint stiffness

• Fastener engagement

• Settlement

• Loading conditions

Split Lock Washers

Split lock washers have a helical geometry intended to provide spring action and surface interaction.

Their effectiveness is application-dependent and may be limited in highly preloaded or vibration-critical joints.

Wedge-Locking Washer Systems

Certain paired wedge-locking washer systems are designed to resist rotational loosening through specialized geometry.

Their suitability depends on the actual joint and correct installation.

Other Locking Solutions

Depending on the application, engineers may consider:

• Nylon-insert lock nuts

• All-metal prevailing-torque lock nuts

• Suitable thread-locking adhesives

• Other validated locking arrangements

Engineering Recommendation

Select a locking method based on the joint's actual loosening mechanism.

Do not rely on a generic washer description as proof of vibration resistance.

Common Types of Washers and Their Functions

Choosing the Right Washer for Different Joint Materials

Steel Components

Steel joints may use conventional or hardened washers depending on:

• Fastener strength

• Required preload

• Bearing area

• Surface finish

• Governing specification

For highly loaded joints, washer hardness and geometry may be particularly important.

Aluminum Components

Aluminum components may be more susceptible to local indentation than some steel components.

A suitable washer may help distribute bearing load.

However, engineers must also consider:

• Aluminum alloy properties

• Joint thickness

• Surface treatment

• Galvanic corrosion

• Contact pressure

Engineering Recommendation

Check the bearing capacity of the aluminum component rather than assuming a hardened steel washer eliminates deformation risk.

Plastic and Polymer Components

Plastic components may exhibit:

• Local compression

• Creep

• Temperature-dependent behavior

• Stress relaxation

A larger washer may reduce nominal bearing pressure, but it does not eliminate long-term deformation.

Engineering Recommendation

Evaluate polymer behavior under the expected preload, temperature and service duration.

Composite Components

Composite materials may require specialized bearing arrangements.

Potential concerns include:

• Local crushing

• Delamination

• Hole damage

• Uneven contact pressure

Engineering Recommendation

Follow the approved joint design rather than substituting general-purpose washers.

How to Select Washer Size

Washer selection involves more than matching the nominal bolt diameter.

Key dimensions include:

• Inside diameter

• Outside diameter

• Thickness

• Chamfer

• Flatness

Inside Diameter

The washer hole must be compatible with the fastener and the assembly geometry.

Outside Diameter

The outside diameter influences the available bearing area.

However, a larger outside diameter does not guarantee uniform pressure distribution.

Thickness

Thickness influences washer stiffness and resistance to bending.

Chamfer

Certain washer designs include a chamfer to accommodate fastener bearing geometry.

Engineering Recommendation

Use the dimensions defined by the applicable washer standard or approved engineering drawing.

Metric and Inch Washer Specifications

Metric and inch-series washers may have different dimensional requirements.

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

Metric washer specifications may be based on ISO or DIN requirements.

A washer's nominal size does not establish complete dimensional equivalence.

Important Compatibility Note

Metric, UNC, UNF, BSW and BSF thread systems are different and must not be treated as directly interchangeable.

Although a washer is not itself threaded, its dimensions and intended fastener system still require verification.

Engineering Recommendation

Specify the complete washer dimensions and applicable standard.

Relevant International Washer Standards

Different washer standards define different product requirements.

The correct standard depends on the specific washer type.

ISO 7089

ISO 7089 specifies plain washers of the normal series, product grade A.

ISO 7090

ISO 7090 specifies chamfered plain washers of the normal series, product grade A.

ISO 898-3

ISO 898-3 addresses mechanical properties of flat washers made of carbon steel and alloy steel for specified metric fastening applications.

DIN 125

DIN 125 is a familiar legacy designation for certain plain washer configurations.

When reviewing an older drawing, confirm the applicable dimensions and whether a corresponding current specification is required.

ASME B18.21.2

ASME B18.21.2 covers plain washers.

ASTM F436/F436M

ASTM F436/F436M addresses hardened steel washers for specified structural bolting applications.

It should not be treated as automatically equivalent to generic ISO washer hardness categories.

Engineering Recommendation

Use the applicable standard and edition stated in the customer drawing or engineering specification.

Industrial Applications of Washers with Bolts

Automotive Manufacturing

Washers may be specified in selected vehicle assemblies and automotive manufacturing equipment.

Potential applications include:

• Equipment mounting brackets

• Vehicle body hardware

• Mechanical supports

• Production fixtures

• Accessory assemblies

Critical vehicle connections require approved fastening specifications and validation.

Industrial Machinery

Washers may be used in:

• Machine frames

• Conveyor systems

• Equipment housings

• Gearbox mounting arrangements

• Maintenance assemblies

Selection depends on preload, joint material, vibration and service conditions.

Electrical Equipment and Power Distribution

Washers may provide mechanical bearing support in:

• Electrical enclosures

• Control cabinets

• Equipment frames

• Industrial power equipment

• Mounting brackets

However, electrical current-carrying connections may require specific contact and conductivity characteristics.

A general-purpose washer should not be assumed to satisfy electrical connection requirements.

Structural Steel and Construction Equipment

Structural connections may require washers as part of a specified fastener assembly.

In high-strength structural bolting, washer dimensions, hardness and installation procedures may be governed by the applicable structural specification.

Engineering Recommendation

Do not substitute commercial flat washers for specified structural washers without approval.

Automation and Precision Equipment

Washers may be used in:

• Robotic equipment frames

• Precision machinery

• Industrial automation systems

• Sensor mounting brackets

• Equipment enclosures

Where dimensional consistency and controlled assembly are important, washer geometry and finish may influence production results.

Production-Line Considerations for Washer Selection

For manufacturing engineers and production managers, washer selection affects more than the finished joint.

It can also influence assembly efficiency.

Manual Assembly

Potential concerns include:

• Missing washers

• Incorrect washer size

• Incorrect washer type

• Assembly sequence errors

• Additional handling time

Automated Assembly

Automated production may require consistent:

• Washer dimensions

• Flatness

• Burr condition

• Surface finish

• Feeding behavior

Washer stacking or dimensional inconsistency may create feeding problems.

Engineering Information Gain: The Cheapest Washer May Not Produce the Lowest Assembly Cost

A lower unit price does not necessarily reduce total manufacturing cost.

Additional washer handling, feeding interruptions, inspection and rework may offset the initial price difference.

Engineering Recommendation

Evaluate the total installed cost, not only the washer purchase price.

Washer Quality Inspection

Inspection requirements should follow the applicable standard and customer drawing.

Dimensional Inspection

Potential characteristics include:

• Inside diameter

• Outside diameter

• Thickness

• Flatness

• Chamfer geometry

Material and Hardness

Depending on the application, requirements may include:

• Material grade

• Hardness

• Heat treatment

• Surface treatment

Visual Inspection

Potential defects include:

• Burrs

• Cracks

• Deformation

• Corrosion

• Coating defects

Functional Validation

For critical joints, component inspection alone may be insufficient.

Joint-level testing may be needed to evaluate:

• Tightening behavior

• Preload

• Settlement

• Vibration

• Fatigue

• Environmental exposure

Engineering Recommendation

Define acceptance criteria according to the washer's intended function.

Common Mistakes When Using Washers with Bolts

OEM Washer Sourcing and Supplier Qualification

Purchasing managers, procurement specialists and supplier development teams should evaluate washer requirements using a complete technical specification.

Engineering Requirements

• Washer type

• Applicable standard

• Material

• Dimensions

• Hardness

• Surface finish

• Application requirements

Manufacturing Requirements

• Dimensional consistency

• Material control

• Surface treatment

• Inspection criteria

• Packaging

• Part identification

Commercial Requirements

• Prototype quantity

• MOQ

• Annual demand

• Unit price

• Lead time

• Delivery schedule

• Long-term supply requirements

Engineering Recommendation

Avoid sourcing critical washers using only a generic description such as M8 washer or flat washer for bolts.

A complete specification reduces the risk of incorrect substitutions.

OEM RFQ Checklist for Industrial Washers

Frequently Asked Questions

Why are washers used with bolts?

Washers may provide bearing support, load distribution, surface protection, hole coverage or specialized sealing and locking functions.

Do all bolts require washers?

No. The need for a washer depends on the fastener design, joint materials, loading and applicable specification.

Can bolts be installed without washers?

Yes, when the joint has been designed and approved for that arrangement.

Do washers prevent bolts from loosening?

Conventional flat washers do not provide an inherent locking mechanism. Specialized locking systems must be selected and validated for the application.

Should a washer go under the bolt head or nut?

The correct location depends on the joint design and applicable assembly specification. Some joints use washers on one side, both sides or neither side.

Can washers improve bolt preload?

A suitable washer can provide a more appropriate bearing interface, but preload depends on the complete joint and tightening process.

Why are hardened washers used with high-strength bolts?

They may be specified to resist indentation and provide suitable bearing support under high compressive loading.

Can a washer damage an aluminum or plastic component?

Yes. An unsuitable washer may still cause indentation or local deformation.

Are sealing washers the same as flat washers?

No. Sealing washers are designed for particular sealing functions and require material and application compatibility.

Can metric washers replace SAE or USS washers?

Not automatically. Dimensions and specification requirements must be verified.

What information should an OEM provide when sourcing washers?

The approved drawing, standard, material, dimensions, hardness, finish, application, inspection requirements and purchasing quantity.

Industrial Fastening Solutions from Juxin Fasteners

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

Our broader fastening product portfolio includes:

• Nylon-insert lock nuts

• All-metal prevailing-torque lock nuts

• Industrial nuts and threaded fasteners

• Self-clinching fasteners

• Blind rivet nuts

• Rivets

• Threaded inserts

• Plastic fastening components

• Custom drawing-based OEM fasteners

For flat washers, hardened washers, sealing washers and other washer requirements, product availability, materials, manufacturing feasibility, MOQ,

 lead times and inspection documentation must be confirmed during inquiry review.

Request an OEM Washer or Industrial Fastener Quotation

Are you evaluating washers for an industrial bolted joint, sourcing replacement fastening components or preparing a new OEM manufacturing project?

Juxin Fasteners welcomes inquiries from design engineers, structural engineers, mechanical engineers, manufacturing engineers, production managers,

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

To support technical review and quotation, please provide:

2D/3D drawings, washer type, applicable standard, dimensions, material, hardness, surface treatment, mating bolt or nut specifications, 

joint materials, hole geometry, required preload, operating environment, inspection criteria, prototype quantity, annual demand, packaging and delivery requirements.

Our team can review the submitted information and confirm relevant fastening options, technical feasibility and commercial quotation conditions.

Company: Juxin Fasteners

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

Email: info@juxinfasteners.com

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