Flat Washer Selection Guide: Materials, Dimensions and Engineering Considerations

Flat washers—also called plain washers—are among the simplest components in a bolted joint, but their dimensions, material, 

hardness and surface condition can influence how load is transferred from a bolt head or nut into the assembled component.

A flat washer is typically positioned beneath a bolt head, screw head or nut to provide a defined bearing surface.

 Depending on the joint design, it may help distribute bearing pressure over a larger area, protect the mating surface,

 bridge an oversized or irregular clearance hole, or provide a suitable interface between the fastener and component.

Flat washers should not, however, be treated as universal anti-loosening devices.

A conventional flat washer does not by itself guarantee resistance to vibration loosening. If joint locking is required, the complete bolted-joint design and an appropriate locking method should be evaluated separately.

For engineers and sourcing teams, flat washer selection should therefore follow:

fastener size → clearance hole → washer ID → washer OD → thickness → material → hardness → finish → mating surface → environment → validation

What Is a Flat Washer?

A flat washer is generally a thin component with a central hole installed between a threaded fastener and the surface of the assembled part.

Common international dimensional references for metric plain washers include ISO 7089 and related ISO washer standards. DIN 125 is also widely recognized in legacy drawings and commercial specifications.

The applicable standard should be confirmed from the customer's drawing, specification or approved part rather than assumed from the description “flat washer.”

Flat washers may be produced as:

• Standard metric flat washers

• Inch-series flat washers

• Large-OD washers

• Small-OD washers

• Thick washers

• Thin washers

• Hardened washers

• Stainless steel washers

• Non-ferrous metal washers

• Nylon and plastic washers

• Drawing-controlled custom washers

Although these products can appear visually similar, they are not necessarily interchangeable.

What Does a Flat Washer Actually Do?

The engineering function depends on the joint.

1. Distribute Bearing Load

A washer increases the bearing area beneath a bolt head or nut.

This can be useful where the mating material could otherwise experience excessive local bearing stress.

The benefit depends on:

• Washer outside diameter

• Washer thickness

• Washer stiffness

• Washer hardness

• Mating material

• Fastener preload

• Hole geometry

A larger washer is not automatically better. The surrounding component must have sufficient supporting area.

2. Protect the Mating Surface

A washer can provide an intermediate bearing surface between a rotating fastener and the assembled component.

This may help reduce direct marking or localized surface damage during tightening, depending on material, finish and assembly conditions.

This consideration can be important for:

• Coated sheet metal

• Painted components

• Aluminum components

• Electrical enclosures

• Finished equipment surfaces

However, whether surface damage is acceptable should be verified under the actual tightening process.

3. Bridge a Clearance Hole

A washer can provide additional bearing area around a clearance hole.

Selection must consider the relationship between:

bolt diameter → clearance-hole diameter → washer ID → washer OD

The washer should provide adequate support without interfering with nearby features.

4. Provide a Controlled Bearing Interface

In engineered bolted joints, washer hardness, flatness and geometry may be important to maintaining a suitable bearing interface.

This becomes increasingly relevant when the joint uses higher-strength fasteners or relatively soft mating materials.

What a Flat Washer Does Not Automatically Do

A common engineering mistake is assigning functions to a plain washer that the component cannot independently guarantee.

A standard flat washer does not automatically:

• Prevent vibration loosening

• Lock a threaded joint

• Seal against liquids or gases

• Electrically insulate a joint

• Provide a defined spring force

• Compensate for every misalignment condition

These functions may require different components or engineered systems.

Examples include:

• Locking fasteners for loosening resistance

• Sealing washers for fluid sealing

• Spring elements for controlled elasticity

• Insulating washers for electrical isolation

Correct technology selection begins by defining the actual joint problem.

Critical Flat Washer Dimensions

Procurement teams should avoid specifying a washer only as “M6 washer” or “1/4-inch washer.”

Several dimensions define whether the washer will fit and function correctly.

Inside Diameter — ID

The washer inside diameter must provide the required relationship with the bolt or screw.

Too small, and assembly may be impossible.

Too large, and the available bearing area around the hole may be reduced.

Outside Diameter — OD

The outside diameter controls much of the available bearing footprint.

OD selection should consider:

• Component geometry

• Edge distance

• Nearby features

• Clearance

• Bearing-pressure requirements

• Appearance requirements

Thickness

Thickness influences washer stiffness and resistance to deformation.

A thin washer over a large clearance hole may deform differently from a thicker washer of the same ID and OD.

Thickness therefore should not be changed casually during second-source qualification.

Flatness and Form

For precision assemblies, washer form and flatness may also affect the bearing interface.

Drawing-controlled applications should define the required dimensional tolerances rather than relying solely on a generic commercial description.

Carbon Steel Flat Washers

Carbon steel is widely used for industrial flat washers.

Depending on the application, carbon steel washers can be supplied with different:

• Material requirements

• Hardness requirements

• Dimensions

• Surface finishes

• Corrosion-protection systems

Common applications include:

• Industrial machinery

• Automotive assemblies

• Equipment frames

• Electrical equipment

• Commercial equipment

• General OEM assemblies

The steel grade and hardness should be selected according to the joint requirements rather than assuming that all carbon steel washers have the same mechanical properties.

Why Washer Hardness Matters

Washer hardness is an important but frequently overlooked parameter.

Consider a highly loaded bolted joint using a relatively soft washer.

Under the bearing pressure generated by the fastener, the washer may experience excessive indentation or deformation.

That can affect the bearing interface and potentially influence joint behavior.

For this reason, engineers should consider the relationship between:

bolt property class → nut → washer hardness → mating material → preload

A generic low-hardness commercial washer should not automatically replace a washer specified for a mechanically demanding bolted joint.

Where hardness is functionally important, it should be defined on the drawing or purchasing specification.

Zinc-Plated Flat Washers

Zinc plating is commonly specified for carbon steel washers where a metallic corrosion-protection finish is required.

However, “zinc plated” alone may not completely define the customer's requirement.

Depending on the project, sourcing specifications may also need to identify:

• Finish type

• Appearance

• Passivation

• Coating requirement

• Corrosion-performance requirement

• Compatibility with mating fasteners

The washer finish should also be considered together with the bolt, nut and assembled component.

For more detail, see our Fastener Surface Treatments engineering guide.

Stainless Steel Flat Washers

Stainless steel flat washers are commonly selected where corrosion resistance, material compatibility or appearance is important.

Potential applications include:

• Food-service equipment

• HVAC equipment

• Outdoor equipment

• Industrial machinery

• Commercial equipment

• Marine-related equipment

• Electrical enclosures

However, the statement “stainless steel washer” is not a complete engineering specification.

The required stainless steel grade should be defined according to:

• Environment

• Mating fastener

• Mechanical requirements

• Corrosion conditions

• Customer specification

Stainless steel washers should not automatically be classified as suitable only for light loads.

 Mechanical suitability depends on the specified material, dimensions, hardness or property requirements and the complete bolted joint.

Copper and Copper-Alloy Washers

Copper and copper-alloy washers can be considered where their specific material characteristics are useful.

Depending on the design, potential considerations include:

• Electrical conductivity

• Thermal conductivity

• Material compatibility

• Conformability

• Corrosion environment

Applications can include certain:

• Electrical assemblies

• Power equipment

• Thermal systems

• Automotive components

• Industrial equipment

However, a conventional copper flat washer should not automatically be described as a sealing washer.

If sealing is required, the washer geometry, material condition, mating surfaces, pressure, temperature and medium must be engineered for that purpose.

Nylon and Plastic Flat Washers

Plastic washers are a separate engineering category from metallic flat washers.

Materials can include, depending on application and customer specification:

• PA / Nylon

• Glass-fiber-reinforced nylon

• PC

• PP

• PPS

• PVC

• PVDF

• Other engineering polymers

Potential reasons for selecting a plastic washer include:

• Electrical insulation

• Reduced weight

• Separation of dissimilar materials

• Protection of finished surfaces

• Chemical compatibility

• Non-metallic component requirements

But the selection cannot be based simply on “plastic versus metal.”

Engineering Considerations for Plastic Washers

Polymer behavior differs significantly from steel.

Engineers should consider:

• Creep

• Stress relaxation

• Moisture absorption

• Temperature

• Chemical exposure

• Washer thickness

• Bearing pressure

• Assembly torque

• Long-term clamping requirements

For example, nylon can absorb moisture, which can affect dimensions and mechanical behavior.

A plastic washer subjected to sustained compression can also exhibit creep or stress relaxation.

Therefore, a plastic washer should not automatically replace a metal washer in a preload-critical joint.

For polymer applications, see our Nylon and Plastic Washers material-selection guide.

Washer Material Selection Decision Path

A practical engineering selection process can begin with the assembly environment.

General Industrial Indoor Assembly

Consider:

• Carbon steel

• Required hardness

• Suitable surface finish

Then evaluate the joint load and corrosion conditions.

Corrosive or Moist Environment

Evaluate:

• Stainless steel

• Coated carbon steel

• Other corrosion-compatible materials

Material selection should consider the entire assembly rather than the washer alone.

Electrical Insulation Required

Evaluate:

• Nylon

• Engineering plastics

• Application-specific insulating materials

Then verify temperature, voltage-related design requirements, creep and assembly pressure.

Electrical or Thermal Function Required

Evaluate whether a suitable copper or other conductive washer is appropriate for the actual design.

Do not infer electrical performance from appearance alone.

High Joint Load

Prioritize:

• Mechanical properties

• Washer hardness

• Thickness

• Bearing area

• Mating-material strength

Do not select only by corrosion resistance.

Washer OD vs Bearing Pressure

One of the most useful washer-selection principles is understanding the relationship between washer OD and the supporting component.

Increasing washer OD can increase the nominal bearing area.

But that benefit is only useful if the underlying component can support the washer.

If part of a large washer extends over:

• An unsupported opening

• A thin edge

• A slot

• A flexible flange

the expected load-distribution benefit may not be achieved.

Washer geometry should therefore be evaluated together with the component underneath it.

Washer Thickness vs Deformation

Thickness is sometimes treated as a purchasing detail rather than a functional dimension.

That can be a mistake.

Under load, washer deformation depends on factors including:

• Thickness

• OD

• ID

• Material

• Hardness

• Bearing pressure

• Support beneath the washer

Changing washer thickness during cost reduction or second-source development can therefore change joint behavior.

For drawing-controlled components, thickness tolerance should be included in supplier comparison.

Flat Washer Selection for Slotted Holes

Slotted holes require additional attention.

The washer must provide sufficient bearing coverage throughout the intended assembly position.

Engineers should evaluate:

• Slot width

• Slot length

• Fastener position

• Washer OD

• Washer thickness

• Edge support

• Expected load direction

A standard washer may not provide sufficient coverage for every slotted-hole geometry.

A larger OD or drawing-controlled washer may be more appropriate depending on the joint.

Flat Washers in Automotive Assemblies

Automotive applications can involve:

• Body structures

• Chassis-related assemblies

• Battery-system components

• Thermal-management equipment

• Brackets

• Electrical equipment

• Interior structures

Washer selection may need to account for:

• Fastener property class

• Sheet thickness

• Coatings

• Corrosion environment

• Vibration

• Assembly torque

• Automated installation

• Joint service conditions

The washer should be validated as part of the complete bolted joint.

Flat Washers for Electrical and Power Equipment

Flat washers are widely used in:

• Electrical cabinets

• Power distribution equipment

• Switchgear

• UPS systems

• Energy-storage equipment

• Data center infrastructure

• Industrial control equipment

Material selection may be driven by:

• Mechanical loading

• Corrosion environment

• Electrical isolation

• Conductivity requirements

• Component finish

• Service temperature

Mechanical and electrical requirements should be defined separately.

A metallic washer should not automatically be assumed to provide a controlled electrical connection, 

and a plastic washer should not automatically be assumed to meet a specific electrical insulation requirement without validation.

Flat Washers for Industrial Machinery

Industrial machinery often contains both general-purpose and highly loaded bolted joints.

Applications can include:

• Equipment frames

• Covers

• Mounting brackets

• Machine guards

• Motor assemblies

• Automation equipment

• Fixtures

The selection process should distinguish between a general assembly washer and a washer forming part of a mechanically critical joint.

When Is a Custom Washer Better Than a Standard Washer?

A standard ISO, DIN, ASME/ANSI or other established washer may be appropriate when its dimensions and properties meet the assembly requirement.

A custom washer may be considered when the design requires:

• Non-standard ID

• Non-standard OD

• Special thickness

• Special material

• Special hardness

• Special surface finish

• Unusual shape

• Tight dimensional tolerance

• Drawing-controlled geometry

The decision should be based on functional requirements rather than making a standard washer custom without engineering benefit.

Common Flat Washer Selection Mistakes

Mistake 1 — Specifying Only the Bolt Size

“M8 washer” may not completely define ID, OD, thickness, hardness or standard.

Better approach: specify the applicable standard or complete dimensions.

Mistake 2 — Assuming Every Washer Prevents Loosening

A plain flat washer is not inherently a locking fastener.

Better approach: evaluate the actual loosening mechanism and select an appropriate locking system if required.

Mistake 3 — Ignoring Washer Hardness

This can create problems in highly loaded bolted joints.

Better approach: evaluate washer mechanical properties relative to the bolt and mating material.

Mistake 4 — Selecting Stainless Steel Only by Appearance

Different stainless materials and mechanical requirements are not interchangeable.

Better approach: define the required grade or specification.

Mistake 5 — Replacing Metal with Plastic Without Joint Analysis

Plastic washers behave differently under sustained compression and temperature.

Better approach: evaluate creep, stress relaxation, moisture and service environment.

Mistake 6 — Changing Thickness During Second Sourcing

A washer that looks identical may have a different thickness or mechanical behavior.

Better approach: compare the complete drawing and functional specification.

Flat Washer Failure and Assembly Troubleshooting

When a washer-related assembly shows unexpected behavior, inspect the complete interface.

Washer Cupping or Bending

Check:

• Washer thickness

• Unsupported area

• Clearance-hole size

• Material

• Hardness

• Fastener load

Washer Embedding Into the Surface

Check:

• Mating-material hardness

• Washer OD

• Washer hardness

• Applied preload

• Surface condition

Corrosion Around the Washer

Check:

• Washer material

• Finish

• Mating materials

• Moisture exposure

• Chemical environment

• Coating damage

Plastic Washer Deformation

Check:

• Bearing pressure

• Temperature

• Creep

• Material selection

• Washer thickness

• Assembly torque

The correct solution should address the failure mechanism rather than simply changing washer size.

Second-Source Qualification for Flat Washers

Flat washers are often considered simple commodity components, but uncontrolled substitution can create dimensional or joint-performance differences.

For second-source development, compare:

existing drawing → approved sample → standard → ID → OD → thickness → material → hardness → finish → functional requirement

Also confirm:

• Dimensional tolerances

• Surface condition

• Mating bolt or screw

• Mating nut

• Component material

• Assembly process

• Packaging requirements

• Annual volume

A visually similar washer should not automatically be considered equivalent.

RFQ Checklist for Flat Washers

For a standard washer, provide:

• Applicable ISO, DIN, ASME/ANSI or other required standard

• Fastener size

• Material

• Finish

• Required quantity

• Annual demand

For a drawing-controlled washer, provide:

• 2D drawing

• Inside diameter

• Outside diameter

• Thickness

• Dimensional tolerances

• Material

• Hardness where required

• Surface finish

• Quantity

• Annual demand

For a new engineering application, provide:

• Mating bolt or screw

• Clearance-hole dimensions

• Component material

• Expected joint loading

• Required bearing area

• Environment

• Corrosion requirement

• Electrical requirements where applicable

• Production quantity

For second-source development, provide:

• Existing drawing

• Approved sample

• Current standard/specification

• Material

• Hardness requirement

• Surface treatment

• Mating fastener

• Application information

• Annual demand

Related Fastening Solutions

Engineers and procurement teams evaluating washer systems may also need to consider:

• Stainless Steel Fasteners

• Custom Washers

• Nylon and Plastic Washers

• High-Strength Bolts

• Fastener Surface Treatments

• Automotive Fasteners

• Custom Fasteners

• Custom Stamped Components

• CNC Machined Components

These technologies should be selected according to the complete assembly rather than treated as interchangeable components.

JUXIN FASTENERS Flat Washer and Custom Washer Support

JUXIN FASTENERS supports standard and drawing-controlled fasteners for industrial OEM, procurement and supplier-development projects.

Relevant sourcing capabilities include:

• Standard flat washers

• Stainless steel washers

• Carbon steel washers

• Nylon and plastic washers

• Drawing-controlled custom washers

• Custom stamped components

• Standard fasteners

• Custom fasteners

• Automotive fasteners

• CNC machined components

Projects can begin from:

• International standard

• Customer drawing

• Existing sample

• Application requirements

• Existing production part

• Second-source project

For a standard washer, specify the required standard, size, material and finish.

For a custom or drawing-controlled washer, define the ID, OD, thickness, tolerances, material, hardness where applicable and surface treatment.

Specify the Washer as Part of the Joint

For design engineers:

fastener → clearance hole → washer ID → washer OD → thickness → material/hardness → finish → mating surface → load → validation

For procurement and supplier-development teams:

standard/drawing → dimensions → material → hardness → finish → approved sample → quantity → annual demand → qualification

The key sourcing principle is simple:

Do not specify a flat washer by bolt size alone when dimensions or mechanical properties matter to the joint.

A reliable washer specification should define the geometry, material and functional requirements necessary for the actual assembly.

For standard flat washers, stainless steel washers, plastic washers or custom drawing-controlled washers, send JUXIN FASTENERS your specification, drawing, sample or application requirements.

For second-source development, include the existing drawing or standard, approved sample, material, hardness where required, finish, mating fastener and annual demand.

Email: info@juxinfasteners.com
Website: www.juxinfasteners.com