Tooth Lock Washer Guide: Internal vs External Types, Uses, and Selection Tips for Mechatronic Assemblies

Quick Answer

A tooth lock washer is a locking washer with small teeth formed around its inner or outer edge. When the fastener is tightened, the teeth bite into the underside of the screw head or nut and into the mating surface. This creates multiple contact points that increase friction and help resist rotational movement.

The two most common forms are:

· Internal tooth lock washer: teeth are located around the inner diameter.

· External tooth lock washer: teeth are located around the outer diameter.

In general, internal tooth lock washers are better for compact spaces, cleaner appearance, and grounding points. External tooth lock washers usually provide stronger surface grip because their teeth act on a larger diameter.

Tooth lock washers are widely used in sensor brackets, control panels, grounding screws, sheet metal covers, motor accessories, aluminum extrusion frames, and compact mechatronic assemblies.

For readers who need a broader overview of locking washer structures, the JLCMC Lock Washer Guide for Mechatronics is a useful starting point. This article focuses specifically on tooth lock washers and how to choose between internal and external tooth designs.

1. What Is a Tooth Lock Washer?

A tooth lock washer, also called a toothed lock washer or serrated lock washer, is a thin washer with multiple teeth designed to bite into contact surfaces after tightening.

Unlike a flat washer, which mainly spreads load, a tooth lock washer is mainly used to improve resistance to loosening. Its teeth create mechanical engagement with the fastener head or nut and the mating surface. This helps increase friction and reduce the chance of rotational loosening under light vibration or small joint movement.

Tooth lock washers are commonly used with small and medium metric screws, especially:

In mechatronic design, M3 to M6 are especially common. These sizes frequently appear around sensors, brackets, panels, cable holders, motor accessories, and compact mechanical modules.

A tooth lock washer is small, but it has a clear job: help the fastener stay where it was tightened. Small teeth, serious responsibility.

2. How Does a Tooth Lock Washer Work?

A tooth lock washer mainly works through two effects:

1.Mechanical interlocking from tooth engagement

2.Increased friction at the contact surface

When the fastener is tightened, the washer is compressed. The teeth bite into the underside of the screw head or nut and into the mating surface, creating multiple contact points that increase friction and help resist rotational movement.

The washer also provides limited elastic compliance as the teeth deform during tightening. However, the primary locking effect comes from tooth engagement with the mating surfaces rather than from spring action alone.

This distinction matters. A tooth lock washer should not be described as working mainly like a spring washer. Its locking behavior is more closely related to surface bite and friction.

The locking effect depends on several factors:

· Tooth form

· Washer material

· Fastener size

· Tightening torque

· Surface hardness

· Surface coating

· Joint stiffness

· Vibration level

· Whether surface marking is acceptable

Tooth lock washers usually perform best on metal surfaces that allow the teeth to engage, such as bare steel, zinc-plated steel, conductive metal panels, and general machine brackets.

They are less suitable for surfaces that should not be scratched, such as decorative aluminum, painted panels, plastic surfaces, precision-ground parts, and sealing faces.

⚠️ Engineering Note on Material Hardness:

For the teeth to successfully mechanically interlock, the washer's hardness must significantly exceed that of both the screw head and the mating surface. Tooth lock washers will flatten or fracture against hardened steel fasteners (e.g., Class 10.9 or 12.9 bolts). Conversely, when used on soft aluminum or plastics, the teeth can cause local material creep and gradual joint relaxation under heavy dynamic loads.

The key point is simple:

A tooth lock washer helps a properly tightened joint resist loosening. It does not fix poor joint design, weak thread engagement, or incorrect torque.

3. Internal Tooth vs External Tooth Lock Washer

The most important selection question is usually this:

Should you choose an internal tooth lock washer or an external tooth lock washer?

Both types work by tooth engagement, but the tooth position changes their performance, appearance, clearance requirement, and best-use scenario.

Internal Tooth Lock Washer

An internal tooth lock washer has teeth around the inner diameter. The outer edge remains smooth, so the washer looks cleaner after installation.

It is commonly used when:

· Space around the screw head is limited

· A cleaner appearance is preferred

· The washer should stay mostly hidden

· The application involves grounding or electrical bonding

· The fastener is small

· External teeth may interfere with nearby parts

Because the teeth are closer to the screw centerline, the locking radius is smaller. This means internal tooth washers usually provide less rotational resistance than external tooth washers of the same nominal size. However, they are compact, neat, and very practical in electrical and space-limited assemblies.

Internal tooth lock washers are commonly used in:

· Grounding screws

· Control panels

· Instrument covers

· Sensor brackets

· PCB support plates

· Terminal blocks

· Small enclosure hardware

· Compact motor covers

External Tooth Lock Washer

An external tooth lock washer has teeth around the outer diameter. These teeth act farther from the screw centerline, which usually gives stronger resistance to rotational loosening.

It is commonly used when:

· Stronger surface bite is needed

· There is enough radial clearance

· Surface marks are acceptable

· Appearance is not the main concern

· The washer is used on brackets, covers, or general machine parts

External tooth lock washers are common in:

· Sheet metal covers

· Machine guards

· Cable clamp brackets

· Motor accessory plates

· Aluminum extrusion accessories

· Light-duty frame hardware

· General automation brackets

The main advantage is stronger grip. The main limitation is that the teeth are visible and may scratch the surrounding surface.

Internal vs External Tooth Washer Comparison

A practical rule:

Use internal tooth lock washers for compact, clean, or grounding-related joints. Use external tooth lock washers when stronger bite is needed and enough space is available.

4. Standards and Specification Notes

Tooth lock washers are often described by standard references, tooth type, nominal screw size, material, and finish.

Common examples include:

· Internal Tooth Lock Washer: DIN 6797 J / GB/T 861.1

· External Tooth Lock Washer: DIN 6797 A / GB/T 862.1

In engineering drawings or product catalogs, tooth lock washers may be specified by:

· Standard reference

· Nominal screw size

· Tooth type

· Inner diameter

· Outer diameter

· Thickness

· Material

· Surface finish

Typical descriptions may look like:

· M3 internal tooth lock washer

· M4 external tooth lock washer

· M5 stainless steel toothed lock washer

· DIN 6797 J internal tooth lock washer

· DIN 6797 A external tooth lock washer

· GB/T 862.1 internal tooth lock washer

· GB/T 862.2 external tooth lock washer

When selecting a washer, do not rely only on the nominal screw size. Always check the actual dimensions.

Important dimensions include:

· Inner diameter

· Outer diameter

· Thickness

· Tooth height

· Tooth direction

· Suitable screw size

· Available bearing area

This is especially important in compact mechatronic assemblies. A washer may fit an M4 screw but still be too large for a narrow sensor slot. An external tooth washer may interfere with a nearby enclosure rib, cable clamp, or linear guide accessory.

Small washers can create very real layout problems. They are tiny, but they are not shy.

5. Special Structure Note: Countersunk Tooth Lock Washers

A countersunk tooth lock washer is a special structure used with flat head or countersunk screws. It is designed to fit the geometry of a countersunk screw head while still providing tooth engagement.In industrial standards, these are typically referenced as DIN 6797 V or DIN 6798 V types.

This type should not be grouped directly with internal tooth, external tooth, and internal-external tooth washers, because the classification logic is different.

· Internal and external tooth washers are classified by tooth position.

· Countersunk tooth washers are classified by screw head compatibility and installation structure.

Countersunk tooth lock washers are less common than standard internal or external tooth washers. They may be used in low-profile covers, flush panels, compact enclosures, and special fixture designs where a flat head screw is required.

For most standard mechatronic fastening points, internal tooth and external tooth lock washers are the more common choices.

6. Applications in Mechatronic Assemblies

Tooth lock washers are useful in mechatronic systems because these systems often combine motion, vibration, sensors, electrical contact, and compact mechanical layouts.

Sensor Brackets

Sensors are often mounted near moving axes, conveyors, grippers, cylinders, and rotating shafts. Even small vibration can shift a sensor if the bracket is not secured properly.

Common sensor bracket screws include M3, M4, and M5.

Internal tooth washers are useful when the bracket is compact or visible. External tooth washers are better when stronger bite is needed and surface marks are acceptable.

Typical applications include:

· Photoelectric sensor brackets

· Proximity sensor brackets

· Limit switch plates

· Safety light curtain brackets

· Encoder reference sensor mounts

· Adjustable detection brackets

Control Panels and Grounding Points

Tooth lock washers are often used in control cabinets and electrical panels, especially for grounding screws. The teeth can help break through light oxide layers or thin coatings, improving metal-to-metal contact.

Common uses include:

· Grounding screws

· Terminal blocks

· DIN rail mounting points

· Shielding connections

· Control cabinet panels

· Metal enclosure bonding

Internal tooth lock washers are especially common here because they fit neatly under screw heads and bite near the screw hole.

For grounding applications, always verify electrical continuity after assembly.

Motor Accessories

Tooth lock washers can be used around motor covers, encoder covers, terminal boxes, and cable clamps. Common motor references in automation include NEMA 17, NEMA 23, 57 mm stepper motors, and 86 mm stepper motors.

For accessory screws, M3 to M5 tooth washers can be practical. For main motor mounting bolts, especially in high-vibration axes, choose carefully. A tooth lock washer may not be enough for dynamic motor base connections.

Linear Motion Accessories

Linear motion systems often include guide rails, sliders, ball screws, shaft supports, and actuator accessories. Popular model-style references include MGN12H, MGN15H, HGR15, HGR20, SFU1605, SFU1610, SBR12, and SBR16.

Tooth lock washers may be suitable for:

· Limit switch brackets

· Cable holders

· Protective covers

· Sensor plates

· Lightweight side brackets

· Non-precision accessory mounts

⚠️ CRITICAL WARNING FOR LINEAR RAILS:

Never use tooth lock washers directly under the structural mounting bolts of precision linear guide rails (such as HGR or MGN series). Linear rails rely on perfect, high-rigidity metal-to-metal seating against a ground flat surface to maintain their straightness. Introducing a toothed washer will unevenly distort the rail profile, damage the precision mounting holes, and instantly destroy the motion accuracy or cause the carriage to jam. (They should only be limited to external, non-precision side brackets or sensor plates).

Aluminum Extrusion Frames

Aluminum extrusion frames commonly use profiles such as 2020, 3030, 4040, and 4080. Tooth lock washers can be used on light brackets, cover plates, cable duct supports, and grounding points.

Internal tooth washers are better when appearance matters. External tooth washers are useful in hidden areas where stronger bite is needed.

Be careful with anodized aluminum. Tooth washers can scratch the finish, which may be acceptable inside a machine frame but not on visible product surfaces.

7. Material and Finish Selection

The material and finish of a tooth lock washer affect corrosion resistance, tooth strength, electrical contact, and compatibility with the mating surface.

Carbon Steel

Carbon steel tooth lock washers are widely used in general industrial applications. They are economical and provide good mechanical strength. They are often zinc-plated for basic corrosion resistance.

Suitable for:

· Indoor machinery

· Sheet metal covers

· General brackets

· Control panels

· Cost-sensitive assemblies

Stainless Steel

Stainless steel tooth lock washers offer better corrosion resistance. They are suitable for humid environments, clean equipment, stainless panels, and corrosion-sensitive machinery.

Suitable for:

· Outdoor cabinets

· Stainless assemblies

· Clean production equipment

· Humid environments

· Corrosion-sensitive panels

Spring Steel

Spring steel tooth washers are often selected for stronger elastic behavior and tooth recovery. They can provide good tooth engagement when properly specified.

Suitable for:

· Industrial fastening points

· Repeated light vibration

· General machine assemblies

· Applications needing stronger tooth engagement

Zinc-Plated Finish

Zinc plating is common for steel washers. It provides practical corrosion protection for indoor use and pairs well with many zinc-plated screws and nuts.

Surface Compatibility

Tooth lock washers are designed to bite. That means they may damage:

·Painted surfaces

· Powder-coated panels

· Anodized aluminum

· Soft aluminum

· Plastic parts

· Precision-ground surfaces

· Sealing faces

Before selecting a tooth lock washer, ask one simple question:

Is surface marking acceptable here?

If the answer is no, consider another locking method.

8. Installation Tips

Correct installation is simple, but placement matters.

Place the Washer Directly Under the Fastener

For a screw threaded into a tapped hole, place the tooth lock washer directly under the screw head.

For a bolt and nut assembly, the washer is often placed under the nut if the nut is being tightened. It may also be placed under the bolt head depending on the joint design.

The washer should directly engage the surface where locking action is needed.

Match the Washer to the Screw Head

An internal tooth washer should sit neatly under the screw head. An external tooth washer should have enough surface area and should not hang over an edge or slot.

If the washer is too large, it may interfere with surrounding parts. If it is too small, it may not engage properly.

Avoid Random Washer Stacking

Do not casually place a tooth lock washer over a flat washer. If the teeth bite only into another washer, the actual clamped part may still slip.

If you need both load spreading and anti-loosening performance, evaluate the full joint design instead of stacking washers by habit.

Tighten Properly

A tooth lock washer needs compression to work. If the screw is too loose, the teeth cannot bite. If the screw is over-tightened, the washer may flatten too much or damage the surface.

Use an appropriate torque based on screw size, thread engagement, base material, and surface condition.

Inspect After Assembly

After tightening, check that:

· The washer is seated flat

· The teeth are not hanging over an edge

· The screw head fully contacts the washer

· The washer is not cracked

· The surface is not excessively damaged

· Grounding points pass continuity checks

9. When Not to Use a Tooth Lock Washer

Tooth lock washers are useful, but they are not suitable for every joint.

Precision Mounting Surfaces

Do not use tooth lock washers under linear guide rails, bearing seats, ground plates, precision spacer blocks, or alignment-critical fixtures. The teeth can create uneven contact and affect alignment.

Decorative or Soft Surfaces

Tooth washers can scratch anodized aluminum, painted panels, and soft metals. Internal tooth washers hide marks better, but they still bite into the surface.

Severe Vibration Joints

Tooth lock washers are suitable for light to moderate vibration. They should not be the only locking method for servo motor bases, robot joint plates, heavy actuator supports, belt drive motor mounts, or CNC machine structures.

Sealing Interfaces

Do not use tooth washers on gasketed or sealed surfaces unless the design specifically allows it. The teeth may damage the sealing face.

Electrical Isolation Areas

Tooth washers improve metal contact. Do not use them where electrical insulation is required.

10. Tooth Lock Washers and JLCMC Mechatronic Component Selection

In real machine design, tooth lock washers are usually selected together with screws, nuts, brackets, structural parts, motion components, and electrical hardware.

On JLCMC.com, tooth lock washers and related fastening accessories can be considered alongside:

· Hex socket screws

· Phillips machine screws

· Nuts and washer accessories

· Sensor brackets

· Aluminum extrusion accessories

· Linear guide components

· Ball screw accessories

· Motor mounting hardware

· Positioning parts

· Control cabinet hardware

For example:

· Use M3 internal tooth lock washers for compact sensor brackets.

· Use M4 internal tooth lock washers for grounding screws in control panels.

· Use M4 external tooth lock washers for sheet metal covers.

· Use M5 external tooth lock washers for cable brackets on aluminum extrusion frames.

· Use M6 external tooth lock washers for light actuator accessories.

· Use stainless steel tooth lock washers for corrosion-sensitive panels.

The goal is not to add washers everywhere. The goal is to choose the right locking method for each joint.

A practical selection workflow:

1.Identify the screw size.

2.Check available space.

3.Decide whether surface bite is acceptable.

4.Choose internal or external tooth type

5.Select material and finish.

6.Confirm washer dimensions.

7.Tighten correctly.

8.Test the joint under real operating conditions.

A washer that looks perfect in a catalog still needs to survive the machine.

11. Tooth Lock Washer Selection Table

12. Common Mistakes to Avoid

Using External Tooth Washers Where Space Is Too Tight

External tooth washers need enough radial clearance. If the teeth touch a wall, slot, or bracket edge, the washer may not seat correctly.

Choosing Internal Tooth Washers When Stronger Bite Is Needed

Internal tooth washers are neat and compact, but external tooth washers usually provide stronger rotational resistance. If appearance is not important and space is available, external tooth may be better.

Ignoring Surface Damage

Both internal and external tooth washers can leave marks. External tooth washers usually leave more visible marks. Always check whether the surface can accept tooth bite.

Using Tooth Washers on Precision Components

Avoid tooth washers under precision linear guides, bearing seats, and alignment-critical parts.

Stacking With Flat Washers Without a Clear Reason

If the tooth washer bites only into a flat washer, it may not lock the actual clamped part. Washer stacking should be intentional.

Expecting Too Much From a Small Washer

A tooth lock washer is useful, but it is not a high-vibration cure-all. For demanding joints, choose a stronger locking strategy.

13. FAQ About Tooth Lock Washers

What is the purpose of a tooth lock washer?

A tooth lock washer helps resist fastener loosening by biting into the fastener head or nut and the mating surface. This tooth engagement increases friction and helps resist rotational movement.

What is the difference between internal and external tooth lock washers?

An internal tooth lock washer has teeth around the inner diameter and is better for compact spaces, cleaner appearance, and grounding points. An external tooth lock washer has teeth around the outer diameter and usually provides stronger grip.

Which is stronger, internal tooth or external tooth lock washer?

External tooth lock washers usually provide stronger rotational resistance because the teeth act on a larger diameter. Internal tooth washers are better when space, appearance, or grounding layout matters.

When should I use an internal tooth lock washer?

Use an internal tooth lock washer for grounding screws, control panels, compact brackets, instrument covers, sensor mounts, and applications where the washer should remain mostly hidden.

When should I use an external tooth lock washer?

Use an external tooth lock washer for sheet metal covers, brackets, machine guards, cable clamps, and applications where stronger surface bite is needed and visible teeth are acceptable.

Can tooth lock washers be used for electrical grounding?

Yes. They are often used for grounding because the teeth can help create metal-to-metal contact. Always verify electrical continuity after assembly.

Can I use a tooth lock washer on aluminum?

Yes, but be careful. Tooth washers can scratch aluminum, especially anodized aluminum. Use them only where marks are acceptable or functionally useful.

Are tooth lock washers suitable for high vibration?

They are suitable for light to moderate vibration. For severe vibration, shock loads, or safety-critical joints, use stronger locking methods.

Final Takeaway

A tooth lock washer is a small but useful locking component. It uses sharp teeth to bite into the fastener and mating surface, increasing friction and helping resist loosening in compact mechanical and electrical assemblies.

The most important choice is usually between internal tooth and external tooth designs.

Choose an internal tooth lock washer when you need compact size, clean appearance, or grounding contact near the screw hole.

Choose an external tooth lock washer when you need stronger surface bite and have enough clearance around the fastener.

In mechatronic assemblies, tooth lock washers are especially useful for sensor brackets, control panels, grounding screws, sheet metal covers, small motor accessories, and aluminum frame add-ons. They are less suitable for precision mounting surfaces, decorative finishes, sealing interfaces, and severe vibration joints.

The best question is not simply:

“Do I need a lock washer?”

A better question is:

“Should this joint use internal tooth bite, external tooth bite, or a stronger locking method?”

Answer that correctly, and a tiny washer can prevent a very real maintenance headache.