What Is an Internal Tooth Lock Washer? Working Principle, Selection & Applications

An internal tooth lock washer, also known as an internal star washer, internal toothed washer, or internal serrated lock washer, 

is a mechanical fastening component with multiple teeth formed around its inside diameter.

When compressed beneath a screw head, bolt head, or nut, the teeth create localized contact with the adjoining surfaces. 

Depending on the materials, hardness, coatings, and tightening conditions,

this tooth engagement can increase resistance to rotational movement and may also create localized metal-to-metal contact in suitable electrical assemblies.

Internal tooth lock washers are commonly considered for applications involving:

• Electrical equipment

• Industrial control systems

• Automotive electrical assemblies

• Small motors

• Appliances

• Instrumentation

• Electronic equipment

• Industrial machinery

• Automation systems

• Compact mechanical assemblies

Their primary geometric advantage is the location of the teeth. Because the teeth project inward rather than outward, 

most of the toothed area remains underneath the fastener's bearing surface, making internal tooth washers useful where radial installation space is limited.

However, an internal tooth lock washer should not be considered a universal guarantee against fastener loosening. Joint performance depends on the entire bolted connection, not the washer alone.

How Does an Internal Tooth Lock Washer Work?

The working principle is based on localized tooth engagement at the bearing surfaces.

During tightening, the washer is compressed between the fastener and the mating component.

The internal teeth may deform and engage the adjacent surfaces, producing several possible effects:

• Localized contact pressure

• Increased resistance to rotation

• Mechanical engagement with the bearing surfaces

• Penetration of certain surface films or coatings

• Localized conductive contact in appropriate electrical assemblies

This is different from the function of a conventional flat washer.

A flat washer is primarily used to increase bearing area and distribute load. An internal tooth washer intentionally introduces multiple serrated contact points around the inside diameter.

The effectiveness of those contact points depends heavily on the materials and surfaces being joined.

What Happens to the Teeth During Tightening?

The teeth are not simply decorative serrations.

As tightening load develops, the teeth interact with the fastener bearing surface and the mating surface.

The actual behavior depends on factors such as:

• Washer material

• Washer hardness

• Fastener material

• Mating material

• Surface hardness

• Coating type

• Coating thickness

• Surface roughness

• Applied preload

On relatively soft surfaces, tooth penetration may be more pronounced.

On harder surfaces, engagement behavior can be different.

This means two assemblies using the same internal tooth washer can perform differently if the mating materials or surface treatments change.

For engineers, this is an important reason not to select a lock washer from nominal screw size alone.

Does an Internal Tooth Lock Washer Prevent Bolts From Loosening?

Internal tooth washers can increase resistance to rotational movement in suitable assemblies, but they should not automatically be described as preventing loosening under every vibration condition.

Self-loosening of threaded fasteners is influenced by the complete joint, including:

• Fastener preload

• Joint stiffness

• Fastener stiffness

• Transverse movement

• Surface friction

• Lubrication

• Temperature cycling

• External loading

• Mating-material hardness

• Coating behavior

• Assembly method

If a joint is exposed to severe vibration, transverse displacement, repeated shock or safety-critical loading, engineers should validate the complete locking system under representative service conditions.

Depending on the application, another locking method may be required.

Internal Tooth Lock Washer vs External Tooth Lock Washer

This is one of the most important selection decisions within the toothed washer family.

Both designs use teeth, but their geometry is different.

Internal Tooth Lock Washer

The teeth project inward around the washer's inside diameter.

Typical reasons for selecting an internal tooth design include:

• Limited radial space

• Compact fastener-head geometry

• Preference for teeth largely underneath the fastener head

• Smaller external washer envelope

• Electrical or electromechanical assemblies

• Appearance considerations

External Tooth Lock Washer

The teeth project outward around the washer's outside diameter.

This provides a larger effective tooth-contact diameter and can be useful where greater radial space is available or a broader serrated contact pattern is desired.

External tooth washers are also frequently considered for grounding and bonding applications where multiple outward-facing contact points are beneficial.

The correct question is therefore not:

Which washer is stronger?

The more useful engineering question is:

Which tooth geometry interacts correctly with the available bearing surface, mating material and functional requirement?

For a detailed comparison of the outward-tooth design, review our External Tooth Lock Washer Solutions.

Internal Tooth Lock Washer vs Flat Washer

These components perform different primary functions.

Flat Washer

A flat washer is generally used to:

• Increase bearing area

• Distribute clamp load

• Reduce localized indentation

• Protect the mating surface

• Provide spacing where appropriate

Internal Tooth Lock Washer

An internal tooth washer is selected primarily for:

• Serrated surface engagement

• Increased rotational resistance

• Compact toothed contact

• Potential electrical contact where properly engineered

Replacing one with the other without reviewing the joint can change its behavior.

In some assemblies, load distribution and locking functions may both be required, and the washer arrangement should be determined from the complete joint design.

Internal Tooth Lock Washer vs Split Lock Washer

A split helical spring washer and an internal tooth washer should not be treated as interchangeable simply because both are commonly called "lock washers."

They use different geometries and engage the joint differently.

Selection should consider:

• Fastener size

• Fastener head geometry

• Mating material

• Surface hardness

• Available radial space

• Surface coating

• Required contact pattern

• Assembly process

• Service loading

The appropriate design depends on the application rather than on a universal ranking between washer types.

Internal Tooth Washer vs Serrated Flange Fastener

Serrated flange nuts and serrated flange screws incorporate the serrations directly into the fastener.

An internal tooth washer is a separate component.

Integrated serrations can reduce component count, but a separate washer may be appropriate when:

• The existing screw or nut must remain unchanged

• A customer drawing specifies a separate washer

• A different washer material is required

• Service replacement is necessary

• Electrical contact requirements affect washer selection

• Existing equipment architecture already uses a washer

OEM engineers should compare the complete assembly rather than evaluating the washer independently.

Why Internal Tooth Washers Are Useful in Compact Assemblies

One of the most useful characteristics of an internal tooth washer is often overlooked:

its teeth are contained within the external washer envelope.

That can be valuable in assemblies with limited radial clearance.

Examples include:

• Electrical terminals

• Instrument housings

• Small motors

• Compact brackets

• Control devices

• Sensors

• Appliance assemblies

• Electronic equipment

In tightly packaged equipment, simply changing from an internal tooth washer to a larger external tooth design may interfere with nearby components.

This is why engineers should consider the complete installation envelope.

Internal Tooth Lock Washers in Electrical Applications

Internal star washers are frequently used in electrical and electromechanical assemblies because the teeth can create localized contact points.

Potential applications include:

• Control cabinets

• Electrical panels

• Terminal assemblies

• Electrical enclosures

• Instrumentation

• Appliances

• Small motors

• Power supplies

• Automation equipment

Where electrical continuity is part of the functional requirement, the tooth geometry may help penetrate certain surface films or coatings and establish localized conductive contact.

However, an internal tooth washer should not automatically be described as providing a guaranteed grounding connection.

Electrical performance depends on:

• Washer material

• Fastener material

• Mating material

• Coating system

• Surface contamination

• Contact pressure

• Corrosion

• Environmental exposure

Where grounding, bonding or electrical safety is critical, the completed assembly should be validated against the applicable electrical requirements.

Why Coating Selection Matters in Electrical Assemblies

A common sourcing mistake is to specify a coating based only on corrosion resistance or appearance.

For a toothed washer, the coating can affect the functional interface.

For example, engineers may need to evaluate:

Tooth → Fastener Surface → Coating → Mating Surface

A thick or mechanically resistant coating may interact differently with the teeth than a thin surface film.

The same applies to painted or powder-coated assemblies.

If electrical continuity depends on tooth penetration, the coating system must be considered during joint validation.

This is especially important in:

• Electrical enclosures

• Control cabinets

• Power equipment

• Automotive electrical systems

• Telecom equipment

• Industrial automation

Automotive and EV Applications

Modern vehicles contain a growing number of electronic and electromechanical systems.

Potential applications for internal tooth washers can include:

• Electrical modules

• Sensors

• Actuators

• Small motors

• Instrumentation

• Brackets

• Interior mechanisms

• Auxiliary electrical systems

• Control assemblies

EV and hybrid vehicles also contain extensive power-electronics and electrical architectures.

Where a toothed washer is specified for an automotive program, the correct component should be defined by the applicable OEM or Tier supplier drawing and validation requirements.

A washer should not be assumed suitable for automotive use simply because its general geometry is similar to an existing component.

Industrial Automation and Control Equipment

Automation equipment combines mechanical fastening with extensive electrical and control architecture.

Internal tooth washers may be used in assemblies involving:

• Sensors

• Controllers

• Electrical cabinets

• Terminal components

• Instrumentation

• Relays

• Small motors

• Mounting brackets

• Machine enclosures

In high-volume production, consistency is important.

Variation in:

• Tooth geometry

• Material hardness

• Washer thickness

• ID and OD

• Coating thickness

• Forming quality

can influence assembly behavior.

For OEM sourcing, even a small washer may therefore require controlled dimensional and material specifications.

Machinery and Mechanical Equipment

Internal tooth washers may also be used in general industrial machinery where a compact toothed interface is required.

Potential applications include:

• Motors

• Pumps

• Fans

• Instrument housings

• Machine guards

• Equipment brackets

• Auxiliary mechanisms

• Electrical boxes

For equipment exposed to continuous vibration or repeated dynamic loading, the complete joint should be evaluated rather than relying on the washer name as evidence of locking performance.

Appliances and HVAC Equipment

Internal star washers can also appear in:

• Washing machines

• Dryers

• HVAC equipment

• Fans

• Blowers

• Compressors

• Food-service equipment

• Household and commercial appliances

Potential functions can include mechanical retention and electrical contact, depending on the assembly design.

Where moisture, condensation or cleaning chemicals are present, corrosion resistance should be considered during material and coating selection.

Materials for Internal Tooth Lock Washers

The correct material depends on mechanical performance, corrosion exposure, temperature and electrical requirements.

Carbon and Spring Steel

Steel internal tooth washers are widely used for general industrial applications.

Potential advantages include:

• Suitable tooth-forming characteristics

• Mechanical strength

• Cost efficiency

• Compatibility with multiple surface finishes

Typical applications include:

• Machinery

• Appliances

• Electrical equipment

• Automotive components

• General industrial assemblies

A protective finish may be required depending on the environment.

Stainless Steel

Stainless steel may be selected where corrosion resistance is important.

Potential applications include:

• Outdoor equipment

• Food-service equipment

• Medical equipment

• HVAC systems

• Marine-related environments

• Humid industrial installations

The required stainless grade should be specified according to the application rather than using "stainless steel" as a complete material specification.

Specialty Materials

Some applications may require alternative materials for:

• Electrical conductivity

• Corrosion resistance

• Temperature capability

• Magnetic properties

• Customer-specific requirements

Where a specialty material is required, the exact material specification should be included in the drawing or RFQ.

Surface Finishes for Internal Tooth Washers

Depending on material and application, possible surface treatments can include:

• Zinc-based coatings

• Zinc-nickel systems

• Zinc-flake systems

• Black oxide

• Phosphate-based finishes

• Passivation for appropriate stainless materials

• Customer-specified finishes

Finish selection should consider more than corrosion resistance.

Engineers and procurement teams should also evaluate:

• Coating thickness

• Tooth definition

• Electrical conductivity

• Mating material

• Fastener coating

• Friction

• Environmental exposure

• Hydrogen-embrittlement risk where applicable

The correct finish is therefore application-dependent.

Can Internal Tooth Lock Washers Be Reused?

They should not automatically be treated as reusable components.

During tightening, the teeth can deform and create impressions in the adjoining surfaces.

After disassembly:

• Tooth geometry may have changed

• The original engagement points may be disturbed

• Surface coatings may already be penetrated

• Contact conditions may change

• Electrical performance may change

For controlled or critical joints, replacing the washer after disassembly may be appropriate unless reuse has been specifically validated.

This is a significant difference from treating the washer as a simple reusable spacer.

Can Internal Tooth Washers Damage Painted or Coated Surfaces?

Yes.

The teeth are designed to engage the surfaces around them.

This can produce:

• Surface marking

• Coating penetration

• Scratching

• Localized indentation

That behavior may be useful when mechanical tooth engagement or electrical contact is required.

It may be unacceptable for:

• Decorative surfaces

• Visible painted panels

• Soft materials

• Cosmetic assemblies

• Surfaces requiring uninterrupted corrosion protection

The functional benefit of tooth penetration therefore has to be balanced against surface-protection requirements.

How to Select an Internal Tooth Lock Washer

A reliable selection process starts with the joint rather than the washer catalog.

1. Identify the Fastener

Confirm:

• Metric or inch-series thread

• Nominal fastener size

• Head or nut geometry

• Bearing-surface diameter

2. Determine the Available Space

Check:

• Maximum washer outside diameter

• Nearby components

• Recess dimensions

• Counterbores

• Housing geometry

This is especially important in compact assemblies.

3. Evaluate the Mating Material

Consider:

• Steel

• Stainless steel

• Aluminum

• Copper alloy

• Polymer

• Coated sheet metal

The material influences tooth engagement.

4. Evaluate Surface Hardness

Tooth interaction changes with surface hardness.

A washer suitable for relatively soft sheet metal may behave differently against a hardened component.

5. Review Surface Treatments

Determine whether the washer must interact with:

• Zinc plating

• Zinc-nickel

• Zinc-flake coating

• Paint

• Powder coating

• Conversion coating

• Stainless passive surfaces

6. Define Environmental Requirements

Consider:

• Humidity

• Corrosion

• Temperature

• Chemical exposure

• Outdoor service

• Electrical requirements

7. Determine the Actual Functional Requirement

Ask what the washer is expected to do:

• Increase rotational resistance?

• Create electrical contact?

• Fit a restricted installation envelope?

• Meet an existing customer drawing?

• Replace an approved component?

• Support a specific assembly process?

This prevents the washer from being selected simply because it is traditionally called a "lock washer."

Standards and Specification Considerations

Internal tooth lock washers may be manufactured according to recognized standards or customer-specific drawings.

For North American inch-series applications, ASME B18.21.1 is an important sourcing reference.

Where the application specifically requires ASME B18.21.1, engineers and buyers should review our dedicated ASME B18.21.1 Internal Tooth Lock Washer Guide.

For metric or customer-specific applications, the applicable drawing or standard should be clearly identified.

A visually similar washer should not automatically be treated as dimensionally interchangeable.

Standard Internal Tooth Washer or Custom Washer?

A standard component is appropriate when the assembly is designed around an established specification.

A custom internal tooth washer may be required when the application needs:

• Non-standard inside diameter

• Restricted outside diameter

• Special tooth geometry

• Special thickness

• Different material

• Specific hardness

• Special coating

• Proprietary assembly dimensions

• Customer-defined electrical contact characteristics

For OEM projects, the drawing should define the required geometry whenever the component differs from a recognized standard.

Common Selection Mistakes

Several mistakes repeatedly create sourcing or engineering problems.

Selecting Only by Screw Size

Two washers intended for the same nominal screw size may have different:

• OD

• Thickness

• Tooth geometry

• Material

• Hardness

• Finish

The nominal screw size alone is not a complete specification.

Assuming All Star Washers Are Equivalent

"Star washer" is a generic description.

The teeth may be:

• Internal

• External

• Differently formed

• Designed around different dimensional standards

The exact configuration matters.

Assuming the Washer Guarantees Vibration Resistance

The washer is only one part of the joint.

Severe self-loosening conditions require assembly-level evaluation.

Ignoring Surface Coating

A coating can significantly influence tooth engagement, friction and electrical contact.

Assuming Reuse Is Always Acceptable

The teeth and mating surfaces may change after the first installation.

Reuse should be based on application requirements and validation.

What Engineers Should Put on the Drawing

A useful engineering specification may include:

• Applicable standard

• Washer type

• Nominal fastener size

• Inside diameter

• Outside diameter

• Thickness

• Tooth geometry

• Material

• Hardness where required

• Surface finish

• Coating requirement

• Corrosion requirement

• Electrical requirements where applicable

• Dimensional tolerances

• Inspection requirements

For a custom washer, a controlled 2D drawing is the preferred basis for supplier evaluation.

What Procurement Teams Should Verify

Procurement and supplier-development teams should evaluate more than price.

Important questions include:

• Is the washer standard or drawing-controlled?

• Is the material clearly defined?

• Is hardness controlled where required?

• Is tooth geometry consistent?

• Are ID, OD and thickness controlled?

• Is the specified coating available?

• Can corrosion requirements be supported?

• Are electrical requirements understood?

• Can samples be supplied for qualification?

• Can production lots be supplied consistently?

• What inspection documentation is required?

This is especially important for OEM and Tier supply chains where the washer is part of a validated assembly.

Second-Source Qualification

Internal tooth washers are frequently involved in cost-reduction and second-source projects.

A structured qualification process can follow:

Existing Component → Drawing Review → Standard Identification → Material Review 

→ Dimensional Comparison → Finish Review → Sample Production → Assembly Validation → Production Approval

If only an existing sample is available, dimensional inspection can help establish the geometry.

However, sample measurement alone may not identify:

• Material grade

• Hardness

• Coating specification

• Functional requirements

• Original standard

These requirements should be confirmed before mass production.

RFQ Checklist for Internal Tooth Lock Washers

For an efficient quotation, provide as much of the following information as possible:

• Standard, if applicable

• Drawing

• Metric or inch fastener size

• Inside diameter

• Outside diameter

• Thickness

• Tooth configuration

• Material

• Hardness requirement

• Surface treatment

• Corrosion requirement

• Mating material

• Application

• Electrical requirement, if applicable

• Prototype quantity

• Production quantity

• Estimated annual demand

• Inspection requirements

• Packaging requirements

For replacement or second-source projects, an approved existing sample can also help with technical evaluation.

Engineering Search vs Procurement Search

An engineer researching this product may search for:

• What is an internal tooth lock washer?

• How does an internal tooth lock washer work?

• Internal star washer

• Internal vs external tooth lock washer

• Internal tooth washer for grounding

• Internal tooth washer material

• Internal tooth washer dimensions

• Star washer electrical contact

A procurement or supplier-development professional may search for:

• Internal tooth lock washer manufacturer

• Internal star washer supplier

• Toothed lock washer manufacturer

• Stainless steel internal tooth washer supplier

• Custom internal tooth washer

• OEM lock washer supplier

• ASME B18.21.1 washer supplier

• Lock washer second source

The engineer needs to determine whether the component works for the joint.

Procurement needs to determine whether the supplier can manufacture it consistently to the required specification.

A successful OEM sourcing process has to answer both questions.

Related Washer Solutions

For a larger effective tooth-contact diameter, review our External Tooth Lock Washer Solutions.

For North American inch-series requirements, review our ASME B18.21.1 Internal Tooth Lock Washer Guide.

For basic load distribution without serrated engagement, review our Flat Washer Solutions.

For compact axial spring preload, review our Wave Spring Washer Solutions.

For curved elastic preload applications, review our Curved Spring Washer Solutions.

For angular misalignment compensation, review our Spherical Washer Solutions.

Internal links should connect these pages according to the engineering problem being solved rather than merely because all of the products are washers.

Internal Tooth Lock Washer Manufacturing and OEM Supply

JUXIN FASTENERS supports sourcing projects involving:

• Internal tooth lock washers

• Internal star washers

• Internal toothed washers

• Internal serrated washers

• Carbon steel toothed washers

• Stainless steel star washers

• ASME B18.21.1 internal tooth washers

• Custom toothed washers

• Drawing-based washer components

Projects can be reviewed from:

• International or industry standards

• Customer drawings

• Existing samples

• Material specifications

• Surface-treatment requirements

• Application conditions

For a standard component, providing the applicable standard and fastener size helps avoid ambiguity.

For a custom component, a 2D drawing is the preferred basis for evaluating dimensions, material, hardness, tooth geometry and finish.

From Engineering Selection to Production RFQ

For a new application, the sourcing process can be structured as:

Joint Function → Washer Type → Standard or Drawing → Fastener Size → Mating Surface → Material → Finish → Prototype → Assembly Validation → Production

For an existing second-source project:

Existing Part + Drawing → Technical Comparison → Material/Finish Review → Samples → Validation → Production Approval

This approach is more reliable than sourcing an internal star washer by appearance or nominal screw size alone.

If you are sourcing internal tooth lock washers, internal star washers, ASME B18.21.1 washers, 

stainless steel toothed washers or custom washer components, send your drawing, standard, existing sample or application requirements to JUXIN FASTENERS.

For faster technical review and quotation, include:

• Standard or drawing

• Fastener size

• Material

• Hardness, if applicable

• Surface treatment

• Mating material

• Application

• Electrical requirements, if applicable

• Corrosion requirements

• Prototype quantity

• Production quantity

• Estimated annual demand

• Inspection and documentation requirements

Email: info@juxinfasteners.com

JUXIN FASTENERS can review the engineering and commercial requirements and evaluate a standard or custom internal tooth lock washer solution for OEM and industrial production.