What Is the Difference Between Internal and External Tooth Washers?

What Is the Difference Between Internal and External Tooth Washers?

Tooth washers are widely used fastening components designed to improve joint stability and reduce the risk of loosening in mechanical assemblies. Although they may appear simple, tooth washers play an important role in vibration resistance, electrical grounding, and maintaining consistent fastening pressure. Among the various types available, internal tooth washers and external tooth washers are the two most commonly used designs.

Understanding the difference between these two washer types is important when selecting the correct fastening solution for industrial equipment, electrical systems, automotive assemblies, and machinery applications.

What Is a Tooth Washer?

A tooth washer, also known as a lock washer, is a washer with serrated teeth designed to bite into the mating surface during tightening. These teeth create additional friction between the fastener and the contact surface, helping resist loosening caused by vibration or movement.

Unlike plain flat washers, tooth washers are specifically engineered for:

▶  Anti-loosening performance

▶  Increased grip strength

▶  Electrical grounding contact

▶  Improved joint stability

Tooth washers are commonly manufactured from:

▶  Carbon steel

▶  Stainless steel

▶  Brass

▶  Copper

Different materials are selected depending on corrosion resistance, conductivity, and mechanical strength requirements.

What Are Internal Tooth Washers?

Internal tooth washers feature teeth located around the inner diameter of the washer. The teeth point inward toward the bolt or screw head.

When the fastener is tightened, the inner teeth dig into the underside of the screw head or nut, creating friction that helps prevent rotation.

Main Characteristics of Internal Tooth Washers

▶  Clean outer edge appearance

▶  Compact design for limited installation space

▶  Uniform pressure around the fastener

▶  Reduced risk of damaging outer surfaces

Because the teeth remain inside the washer diameter, internal tooth washers are often preferred for:

▶  Electrical equipment

▶  Consumer electronics

▶  Decorative assemblies

▶  Precision instruments

In many compact products, engineers prefer internal tooth washers because they provide locking performance without extending beyond the edge of the fastener.

What Are External Tooth Washers?

External tooth washers have teeth positioned around the outer edge of the washer. The teeth extend outward and create a larger contact radius.

This larger engagement area allows external tooth washers to generate stronger gripping force and higher resistance to loosening.

Main Characteristics of External Tooth Washers

▶  Higher locking strength

▶  Better resistance to vibration

▶  Larger friction contact area

▶  Stronger grip on softer materials

External tooth washers are widely used in:

▶  Automotive systems

▶  Industrial machinery

▶  Heavy equipment

▶  Structural assemblies

Because the outer teeth bite more aggressively into the contact surface, they are generally considered more effective in high-vibration environments.

Internal vs External Tooth Washers

The main difference between these two washer types lies in the position of the teeth and the resulting locking behavior.

Which Tooth Washer Should You Choose?

The correct choice depends on the application environment and fastening requirements.

If appearance, compactness, and controlled surface contact are important, internal tooth washers are often the better solution. They work well in electrical components and smaller assemblies where space is limited.

On the other hand, external tooth washers are usually preferred when stronger locking performance is required. In industrial machinery exposed to constant vibration, they typically provide more reliable anti-loosening capability.

For example, a customer assembling a small control panel may initially select external tooth washers because of their stronger grip. However, after installation, the exposed teeth may interfere with nearby components or create an uneven appearance. Switching to internal tooth washers can solve the issue while still maintaining sufficient locking force.

In contrast, for vibrating industrial motors or pumps, external tooth washers are generally more effective because they create stronger friction against the mounting surface.

Simple Q&A

Q: Can tooth washers replace spring washers?

In some applications, yes. Tooth washers provide excellent friction locking, but spring washers offer elastic preload. The choice depends on the working condition.

Q: Which washer is better for vibration resistance?

External tooth washers usually provide stronger vibration resistance due to their larger contact radius.

Q: Are stainless steel tooth washers corrosion resistant?

Yes. Stainless steel tooth washers are commonly used in outdoor or humid environments because of their excellent corrosion resistance.

Q: Can tooth washers damage the mounting surface?

Because the teeth bite into the surface, minor marking may occur. This is normal and helps improve locking performance.

Final Thoughts

Internal and external tooth washers are both effective locking components, but they are designed for different fastening priorities. Internal tooth washers provide a compact and visually clean solution for precision assemblies, while external tooth washers deliver stronger locking performance for heavy-duty and vibration-prone applications.

Choosing the correct washer type can improve fastening reliability, reduce maintenance frequency, and extend the service life of mechanical assemblies. Even though tooth washers are small components, their impact on overall joint performance is significant in many engineering and industrial systems.