Flat Washer vs Spring Washer vs Lock Washer: Functions, Differences & Selection Guide

Flat washers, spring washers and products commonly called lock washers are used throughout industrial fastening systems, 

but they should not be treated as three equivalent washer categories with simply increasing levels of locking performance.

They solve different engineering problems.

A flat washer is primarily a bearing-interface component.

A spring washer uses elastic geometry to provide spring behavior.

A locking washer system is intended to resist unwanted fastener rotation through a defined mechanical or frictional mechanism.

The terminology becomes confusing because some traditional spring washers are themselves historically called spring lock washers.

Therefore, the better engineering question is not:

“Which washer is strongest?”

It is:

“What function does this bolted joint require?”

A practical selection path is:

Joint Requirement → Bearing / Elastic / Locking Function → Washer Mechanism → Fastener Interface → Parent Material → Dimensions → Material → Finish → Validation

JUXIN FASTENERS supplies industrial washers, spring washers, screws, bolts, nuts, locking fasteners and custom fastening components for OEM and drawing-controlled industrial applications.

Quick Comparison: Flat Washer vs Spring Washer vs Locking Washer

This comparison is more useful than ranking washers as having “low,” “medium” or “high” locking ability.

Different washer designs operate through different physical mechanisms.

Flat Washer, Spring Washer and Lock Washer Are Not Three Parallel Categories

This is an important terminology issue.

The term flat washer generally describes a plain washer geometry.

The term spring washer describes a broad family of washers designed to provide elastic behavior.

The term lock washer is a functional or historical product description that can include different geometries.

For example, ASME B18.21.1 covers applicable inch-series:

• helical spring-lock washers;

• tooth-lock washers;

• plain washers.

This means a helical spring washer may itself be called a spring-lock washer.

Therefore:

Spring Washer vs Lock Washer

is not always a comparison between two mutually exclusive product families.

The exact washer geometry must be identified.

What Is a Flat Washer?

A flat washer is generally a flat annular component installed under a bolt head, screw head or nut.

Its principal function is usually related to the bearing interface rather than locking.

Depending on the joint, a flat washer may:

• distribute bearing load;

• protect a mating surface;

• reduce localized indentation;

• bridge an appropriate clearance hole;

• provide a controlled bearing surface;

• support a specified bolted-joint configuration.

A flat washer should not automatically be treated as an anti-loosening component.

How Does a Flat Washer Distribute Load?

When a bolt or nut is tightened, clamp load is transferred through its bearing surface into the connected component.

A suitably selected washer can increase the effective bearing area.

This can be useful when the parent material is:

• relatively soft;

• thin;

• coated;

• aluminum;

• polymeric;

• otherwise sensitive to localized bearing pressure.

However:

Larger Washer ≠ Automatically Stronger Joint

Engineers must also consider:

• washer thickness;

• hole diameter;

• edge distance;

• parent-material strength;

• fastener preload;

• surrounding geometry.

Flat Washers and High-Strength Bolts

High-strength fasteners can develop substantial clamp loads.

The bearing interface therefore becomes an important part of the joint.

Depending on the applicable fastener system, engineers may need to specify:

• washer standard;

• dimensions;

• hardness;

• material;

• surface treatment.

A general-purpose flat washer should not automatically be substituted for a specified hardened washer.

For more detail, see Flat Washer Hardness & Bolt Grade Selection Guide.

What Is a Spring Washer?

A spring washer uses elastic geometry to deform under load.

However, spring washer is a broad term rather than one single product.

Examples can include:

• split/helical spring washers;

• wave washers;

• curved spring washers;

• disc springs;

• other engineered spring washer geometries.

These products do not all perform the same function.

Split Spring Washers

The familiar split spring washer has a helical ring geometry with a split.

During tightening, it compresses toward a flatter condition.

Historically, it has been used in assemblies where a spring-lock washer is specified.

However, the existence of spring force does not mean the washer can automatically maintain the full preload of a bolted joint.

This distinction is important:

Spring Washer Force ≠ Bolt Preload

Why Split Spring Washers Are Called Lock Washers

Split/helical spring washers are frequently called:

• spring lock washers;

• split lock washers;

• helical spring-lock washers.

The word lock can create an expectation that the washer guarantees resistance to loosening.

That is not the correct engineering interpretation.

ASME B18.21.1 notes that the word “lock” in these product names is a generic term historically associated with product identification and does not imply indefinite permanency of the attachment.

Therefore:

Product Name ≠ Guaranteed Locking Performance

Can a Split Spring Washer Prevent Bolt Loosening?

Not reliably in every joint.

Its behavior depends on:

• preload;

• washer geometry;

• mating surfaces;

• joint stiffness;

• external loading;

• transverse movement;

• friction;

• installation condition.

For severe vibration or safety-critical applications, engineers should not assume that a conventional split washer alone provides sufficient rotational locking.

For a deeper analysis, see Do Spring Washers Prevent Bolt Loosening?.

What NASA Guidance Says About Split Lock Washers

Engineering guidance for high-consequence fastening systems provides a useful warning against relying on product names.

NASA fastener guidance states that free-spinning split lock washers provide minimal, if any, locking and should not be used as secondary locking features.

That guidance applies to NASA's own high-reliability context rather than every commercial machine.

However, it illustrates an important engineering principle:

Do not assume locking performance solely because a washer is called a “lock washer.”

What Is a Lock Washer?

The term lock washer can describe different products intended to resist unwanted fastener rotation.

There is no single universal lock-washer mechanism.

Depending on the design, locking can involve:

• surface interaction;

• teeth;

• serrations;

• wedge geometry;

• elastic geometry;

• other engineered features.

This means a purchasing specification such as:

“M8 lock washer”

may be incomplete.

The buyer should identify the actual locking-washer type.

Toothed Lock Washers

Toothed lock washers use teeth that interact with adjacent bearing surfaces.

Depending on the design, teeth can be:

• internal;

• external;

• arranged according to the specific product geometry.

Their function depends on interaction between the teeth and mating surfaces.

This creates important selection questions.

Surface Hardness Matters for Toothed Washers

A toothed washer can only interact with the bearing surface according to the mechanical properties of that interface.

Engineers should consider:

• mating-surface hardness;

• washer hardness;

• coating;

• paint;

• plating;

• parent material;

• surface-finish requirements.

If the teeth damage a functional coating or decorative surface, the washer may create a new problem while attempting to solve loosening.

Toothed Washer ≠ Universal Vibration Solution

A toothed washer may increase resistance to relative rotation in an appropriate joint.

However, it should not automatically be described as a universal high-vibration solution.

Locking performance depends on the complete joint.

If the joint itself slips under transverse loading, engineers need to understand that movement rather than relying solely on washer teeth.

What Is a Wedge-Locking Washer?

Wedge-locking washers use a fundamentally different mechanism from conventional split spring washers.

A typical wedge-locking system consists of a matched pair of washers.

The washers use engineered cam geometry on their mating faces and surface features on the external faces.

During attempted loosening, relative movement across the cam interface can create a wedge effect that opposes fastener rotation.

This is a dedicated locking mechanism rather than simply spring action.

Split Spring Washer vs Wedge-Locking Washer

These products should not be grouped together merely because both can be marketed as locking washers.

Split Spring Washer

Uses a helical elastic washer geometry.

Wedge-Locking Washer Pair

Uses engineered cam geometry between two matched washers.

The mechanical principles are different.

Therefore:

Split Washer ≠ Wedge-Locking Washer

Wedge-Locking Systems and Severe Vibration

Commercial wedge-locking systems are specifically engineered for bolted joints exposed to vibration and dynamic loading.

For example, established wedge-locking technology uses paired cam washers where the cam angle is designed relative to the thread geometry so attempted loosening creates a wedge effect.

This type of system should be evaluated according to the specific manufacturer's technical requirements, joint geometry and validation data.

JUXIN FASTENERS should not claim that an unspecified generic washer automatically provides equivalent performance to a proprietary engineered wedge-locking system.

Why We Do Not Rank Washers as Low, Medium and High Locking

The original version of this article used a simple comparison:

Flat Washer → Low

Spring Washer → Medium

Lock Washer → High

That model is misleading.

A flat washer may not be intended to lock at all.

A split spring washer uses one mechanism.

A toothed washer uses another.

A wedge-locking pair uses another.

A sealing washer solves a different problem entirely.

The better comparison is:

Function + Mechanism + Joint Condition + Validation

not:

Low + Medium + High

Flat Washer vs Spring Washer

Use this comparison when the decision is primarily between bearing support and elastic washer behavior.

Flat Washer

Typically selected for:

• bearing load distribution;

• surface protection;

• controlled bearing interface.

Spring Washer

Selected where the particular spring geometry provides a required elastic function or where an approved drawing specifies that washer.

The choice depends on function rather than vibration alone.

Flat Washer vs Lock Washer

These products solve different problems.

A flat washer primarily manages the bearing interface.

A locking washer is intended to resist unwanted rotation through a specific mechanism.

In some assemblies, both bearing and locking functions may be required.

However, that does not automatically mean two washers should be stacked together.

Spring Washer vs Lock Washer

This comparison requires careful terminology.

A split spring washer can itself be a type of lock washer.

Therefore, engineers should instead ask:

Split Spring Washer vs Toothed Lock Washer?

or:

Split Spring Washer vs Wedge-Locking Washer?

That produces a technically meaningful comparison.

Should a Flat Washer and Spring Washer Be Used Together?

Traditional assemblies sometimes place a flat washer and spring washer in the same fastener stack.

But:

More Washers ≠ Better Joint

Adding components can change:

• stack height;

• thread engagement;

• bearing interfaces;

• friction;

• settlement behavior;

• installation consistency.

If an existing approved drawing requires both, follow the specification.

For a new design, the washer stack should have a defined engineering purpose.

Washer Stacking Can Introduce More Interfaces

Every additional component introduces another interface into the joint.

These interfaces can affect:

• friction;

• embedment;

• settlement;

• tolerance stack-up;

• assembly height.

Therefore, washer stacking should not be treated as a universal method for improving locking.

What Is a Wave Washer?

A wave washer is another type of spring washer.

Its function is usually different from that of a split spring-lock washer.

Wave washers can provide axial compliance in applications such as:

• bearings;

• electric motors;

• shaft assemblies;

• compact mechanical mechanisms.

Selection is commonly based on required load-deflection behavior and installation geometry.

Wave Washer ≠ Lock Washer

A wave washer should not automatically be selected because an assembly experiences vibration.

Its primary purpose may be:

• axial preload;

• clearance compensation;

• tolerance management.

This demonstrates why spring washer is too broad a term for precise engineering selection.

What Is a Disc Spring?

A disc spring is a conical spring component capable of generating defined axial force through controlled deflection.

Selection can involve:

• outside diameter;

• inside diameter;

• thickness;

• free height;

• installed height;

• load;

• deflection;

• stacking arrangement;

• fatigue requirement.

Disc springs are engineered spring elements and should not be treated as ordinary lock washers.

Washer Function Map

A useful way to classify washers is by required function.

This is more useful than choosing a washer by appearance.

Locking Washer vs Locknut

A washer-based locking system is not the only way to resist unwanted fastener rotation.

Prevailing-torque nuts can provide rotational resistance through the threaded interface.

These can include:

• nylon-insert locknuts;

• all-metal prevailing-torque nuts.

The correct choice depends on:

• temperature;

• thread size;

• bolt specification;

• environment;

• disassembly;

• reuse;

• joint requirements.

For polymer-insert locking systems, see Nylon Insert Locknuts for Anti-Vibration Applications.

Locking Washer vs Thread-Locking Compound

Thread-locking compounds operate at the thread interface rather than through washer geometry.

Selection can depend on:

• required locking strength;

• temperature;

• chemical exposure;

• surface condition;

• production process;

• serviceability.

A chemical thread-locking system and a mechanical washer-based system should not be treated as identical solutions.

Locking Washer vs Positive Mechanical Locking

Some high-consequence applications use positive mechanical locking.

Examples can include:

• castellated nuts with cotter pins;

• locking plates;

• tab washers;

• safety wire in appropriate controlled applications.

These physically restrict fastener rotation or disengagement.

They operate differently from friction-based or washer-based systems.

Why Bolts Loosen Under Vibration

Rotational self-loosening can occur when cyclic loading creates sufficient relative movement in the joint.

Transverse movement is particularly important.

Therefore, when a bolt loosens, engineers should evaluate:

• preload;

• joint stiffness;

• clamp length;

• external load;

• interface slip;

• bearing surfaces;

• friction;

• installation consistency.

Simply changing washer type may not solve the root cause.

Preload Loss Is Not Always Rotational Loosening

A joint can lose clamp force without the nut or bolt rotating.

Possible causes include:

• embedment;

• material relaxation;

• creep;

• thermal effects;

• gasket compression;

• parent-material deformation.

If preload is being lost without rotation, installing a more aggressive “lock washer” may not address the failure mechanism.

Washer Selection for Soft Materials

When the parent material is relatively soft, a flat washer may be useful for distributing bearing load.

Examples can include:

• aluminum;

• polymers;

• thin sheet;

• coated surfaces.

However, the washer does not eliminate the need to evaluate:

• bearing pressure;

• material creep;

• local deformation;

• edge distance;

• hole geometry.

Washer Selection for Painted or Coated Surfaces

Toothed or serrated locking washers may interact aggressively with a coated surface.

This can:

• damage paint;

• penetrate plating;

• expose substrate material;

• affect corrosion protection;

• change electrical contact.

This may be intentional in a specifically designed system, or it may be unacceptable.

The surface requirement must therefore be known before selecting the locking mechanism.

Washer Selection for Stainless Steel Assemblies

Stainless steel washers can be appropriate for corrosion-sensitive applications, but stainless steel is not universally corrosion-proof.

Selection should consider:

• stainless grade;

• chlorides;

• humidity;

• chemicals;

• temperature;

• mating materials;

• galvanic compatibility.

Material grade should be specified rather than simply requesting “stainless washer.”

Washer Selection for Carbon Steel Assemblies

Carbon steel washers are widely used in industrial equipment.

Depending on the project, they may require:

• zinc-based coating;

• trivalent chromium passivation;

• other customer-specified surface treatments.

For applicable JUXIN FASTENERS projects, environmentally compliant trivalent chromium zinc systems can be evaluated where required.

Corrosion Testing

Where salt spray/fog testing is specified, ASTM B117 may be used as a test method.

However:

Salt Spray Hours ≠ Direct Service-Life Prediction

The project should define:

• coating;

• test duration;

• acceptance criteria.

A corrosion-test method alone does not establish field durability.

DIN 127 in Legacy Drawings

DIN 127 spring lock washers continue to appear in older industrial drawings and replacement-part requirements.

However, DIN 127 is a withdrawn standard.

That creates an important sourcing distinction:

Legacy Replacement ≠ New Design Recommendation

If an existing customer drawing specifies DIN 127, procurement should determine whether the requirement is:

• exact legacy replacement;

• compatibility with existing equipment;

• or redesign using a current fastening strategy.

The original drawing should not be silently changed by the supplier.

ASME B18.21.1 Washer Requirements

For applicable inch-series products, ASME B18.21.1 covers dimensional requirements, physical properties and related test methods for:

• helical spring-lock washers;

• tooth-lock washers;

• plain washers.

The standard also provides an important terminology warning: the word “lock” is historically associated with product identification and does not imply indefinite permanency of the attachment.

This makes ASME B18.21.1 especially relevant to understanding why washer names and actual joint performance must be separated.

Automotive Applications

Automotive equipment contains many different washer applications.

Flat washers may be used where a controlled bearing interface is required.

Spring washers may appear in existing equipment designs.

Dedicated locking systems may be required where rotational self-loosening is a concern.

Potential applications can include:

• brackets;

• electrical equipment;

• auxiliary systems;

• equipment mounts;

• serviceable assemblies.

Safety-critical wheel, brake, steering, suspension or crash-related joints require application-specific engineering and validation.

EV Battery and Power Electronics Equipment

EV and power electronics assemblies can contain washers in:

• electronics housings;

• auxiliary brackets;

• power-conversion equipment;

• service structures;

• thermal-management equipment;

• enclosure hardware.

The washer function should be explicitly defined.

A generic washer should not automatically be described as providing:

• IP sealing;

• grounding;

• EMI/RFI shielding;

• battery enclosure structural integrity.

Data Center and HPC Equipment

AI data centers, HPC infrastructure and power-conversion systems contain numerous bolted mechanical assemblies.

Potential washer applications include:

• server and equipment racks;

• power distribution systems;

• UPS equipment;

• cooling equipment;

• electrical enclosures;

• auxiliary mounting structures.

Selection should follow the actual mechanical requirement rather than simply specifying a washer because vibration may exist.

HVAC Equipment

HVAC equipment can experience vibration from:

• motors;

• fans;

• compressors;

• pumps.

The presence of vibration does not automatically mean a split spring washer is the correct solution.

Engineers should determine:

• whether the joint is slipping;

• whether the fastener is rotating;

• whether preload is being lost;

• whether frequent service is required.

Then the locking strategy can be selected.

Industrial Machinery

Machinery applications include:

• motors;

• pumps;

• compressors;

• gearboxes;

• equipment frames;

• guards;

• brackets;

• service panels.

Different locations within the same machine may require different washer functions.

There is no universal “machine washer.”

Industrial Automation and Robotics

Automation equipment can contain:

• servo systems;

• actuators;

• machine frames;

• sensors;

• equipment covers;

• control cabinets.

Some joints require frequent adjustment or disassembly.

Others remain fixed for long service periods.

Serviceability should therefore be included in locking-method selection.

Railway Equipment

Railway equipment includes ordinary mechanical assemblies as well as highly controlled safety-related joints.

A generic washer should not be represented as suitable for every railway fastening application.

The exact railway project specification and joint classification should control selection.

Heavy Equipment

Construction, agricultural and mining machinery can expose bolted joints to:

• shock;

• vibration;

• contamination;

• outdoor exposure;

• repeated service.

These conditions make joint analysis particularly important.

A locking washer should not be selected by appearance alone.

Electrical Cabinets and Power Equipment

Flat washers and other washer types may be used in:

• electrical cabinets;

• UPS systems;

• power distribution equipment;

• control enclosures;

• mounting structures.

Where electrical bonding or grounding is required, the electrical interface must be separately specified.

A generic toothed washer should not automatically be represented as a qualified grounding solution.

Medical and Laboratory Equipment

Washers may appear in mechanical assemblies for:

• diagnostic equipment;

• laboratory equipment;

• carts;

• housings;

• internal brackets;

• service panels.

The applicable equipment specification should determine material, finish and cleaning requirements.

A generic stainless washer should not automatically be described as medically certified or sterile-compatible.

Engineer Search Intent: What Are They Actually Trying to Decide?

An engineer searching:

flat washer vs spring washer

is usually not looking for three product definitions.

They are trying to answer:

Which mechanism does my joint need?

Other engineering searches can include:

• spring washer vs lock washer;

• lock washer vs flat washer;

• split washer vs wedge lock washer;

• do lock washers work;

• washer for vibration;

• flat washer under spring washer;

• washer for high-strength bolt;

• best locking method for vibrating bolt.

These are decision-stage searches.

Procurement Search Intent

Procurement searches can include:

• flat washer supplier;

• spring washer manufacturer;

• lock washer supplier;

• split lock washer manufacturer;

• industrial washer supplier;

• custom washer manufacturer;

• OEM washer supplier;

• anti-loosening fastener supplier.

The buyer still needs engineering to define which washer mechanism has been approved.

“Lock Washer” Is Not a Complete RFQ

A purchasing request such as:

“Need 50,000 pcs M8 lock washers”

still leaves important questions unanswered.

Which type?

• split spring?

• toothed?

• another locking geometry?

Which standard?

Which material?

Which finish?

Which application?

Which mating fastener?

A professional RFQ should eliminate this ambiguity.

Washer Selection Decision Matrix

Industrial Washer RFQ Checklist

For technical evaluation by JUXIN FASTENERS, provide where applicable:

• 2D drawing;

• 3D model where relevant;

• physical sample;

• customer part number;

• existing/reference part number;

• washer type;

• required washer function;

• applicable ISO, DIN, ASME/ANSI, ASTM, SAE, EN, BS or customer specification;

• metric or inch system;

• mating bolt or screw size;

• thread size;

• bolt property class or specification;

• mating nut specification;

• washer inside diameter;

• washer outside diameter;

• washer thickness;

• spring geometry where applicable;

• material;

• material grade;

• hardness requirement;

• surface treatment;

• coating thickness where specified;

• trivalent chromium zinc requirement where applicable;

• RoHS/REACH requirement where applicable;

• corrosion-test requirement;

• parent material;

• parent-material thickness;

• mating-surface hardness where relevant;

• painted or coated surface condition;

• hole diameter;

• slot dimensions where applicable;

• expected vibration;

• expected joint movement;

• tightening requirement;

• operating temperature;

• corrosion environment;

• locking requirement;

• disassembly requirement;

• reuse requirement where applicable;

• safety classification where applicable;

• sample quantity;

• production quantity;

• annual demand;

• packaging requirements;

• labeling requirements;

• inspection requirements;

• customer-specific requirements.

Frequently Asked Questions

What is the difference between a flat washer and a spring washer?

A flat washer primarily provides a bearing interface and can distribute load.

A spring washer uses elastic geometry and is selected for a defined spring-related function.

Is a spring washer the same as a lock washer?

Sometimes the terminology overlaps.

A split/helical spring washer is commonly called a spring lock washer.

However, other lock washers use completely different mechanisms, such as teeth or wedge-locking geometry.

Which is better: a flat washer or spring washer?

Neither is universally better.

They serve different functions.

The correct selection depends on the joint requirement.

Which washer is best for vibration?

There is no universal best washer for every vibration environment.

First determine whether the joint experiences rotational self-loosening, preload loss, slip or another failure mechanism.

Then select and validate the appropriate locking strategy.

Does a spring washer prevent loosening?

A conventional split spring washer should not automatically be relied upon as a robust locking solution for severe vibration.

Its suitability depends on the joint and application.

Is a wedge-locking washer the same as a split lock washer?

No.

A split washer uses helical spring geometry.

A wedge-locking system uses paired washers with engineered cam geometry to resist rotational loosening.

Can I use a flat washer with a lock washer?

Some approved designs use washer combinations.

However, stacking washers should have a defined engineering purpose.

More washers do not automatically improve joint reliability.

Should I use a spring washer under a nylon locknut?

Not automatically.

A nylon-insert locknut already uses a prevailing-torque locking mechanism.

Additional components should only be added when required by the approved joint design.

Do toothed lock washers damage surfaces?

They can interact with or penetrate mating surfaces depending on material hardness, coating and installation conditions.

Surface-protection and corrosion requirements should therefore be considered before selection.

What should procurement specify instead of “lock washer”?

Specify the exact washer type, standard or drawing, dimensions, material, hardness where applicable, finish, mating fastener and application requirement.

From Washer Comparison to an Approved Fastening System

A search may begin with:

“Spring washer vs flat washer?”

But a professional sourcing process should continue:

What Is the Joint Problem? → What Function Is Required? → Which Mechanism Solves It?

 → Which Washer/Fastener Type? → Which Standard? → Which Material & Finish? → How Will It Be Validated? → Approved Specification → Supplier RFQ

That is a more reliable process than choosing a washer from a generic product comparison chart.

Industrial Washer and Locking Fastener Solutions from JUXIN FASTENERS

JUXIN FASTENERS supports OEM sourcing of flat washers, spring washers, industrial washers, locking nuts, screws,

 bolts and custom fastening components for machinery, automotive equipment, EV systems, electrical equipment, power electronics, HVAC, industrial automation, heavy equipment and other engineered assemblies.

For the complete washer family, see Industrial Washers: Types, Functions & Selection Guide.

For spring-washer loosening analysis, see Do Spring Washers Prevent Bolt Loosening?.

For complete bolted-joint washer selection, see Washers and Bolts: Fastening Systems Selection Guide.

For prevailing-torque locking alternatives, see Nylon Insert Locknuts for Anti-Vibration Applications.

For OEM washer and locking-fastener RFQs, send your drawing, washer function, applicable standard, dimensions, mating fastener, material, surface treatment, application and estimated demand to:

info@juxinfasteners.com

The right question is not simply:

“Flat washer, spring washer or lock washer?”

The right question is:

“Which mechanical function does this joint actually require?”