Stainless Steel Star Washer

Choosing the Right Star Washer

(Internal Tooth / External Tooth Toothed Lock Washer)

1) What does a star washer do & when should you use it?

A star washer (toothed lock washer) is a washer with multiple sharp teeth around its inner or outer edge.
It is designed to:

Increase friction between the nut/bolt and the surface

Provide a mild spring effect

Help resist loosening from vibration and movement

In electrical applications, break through paint/oxide to ensure good grounding/earthing contact

Use a star washer when:

You need better anti-rotation than a plain washer alone

You want the teeth to bite into the surface or the nut/bolt

You need good electrical continuity through painted or plated surfaces (earthing lugs, busbars, panels)

2) Internal tooth vs external tooth – which one to choose?

Internal tooth star washer

Teeth are on the inner diameter.

Best when you want to bite against the shaft or under the head of a screw, especially under pan/head screws in electrical and electronic assemblies.

Offers a cleaner outer profile, better when space is limited or you don’t want teeth visible at the edge.

External tooth star washer

Teeth are on the outer diameter.

Provides a larger outside diameter and generally stronger locking action because teeth have a bigger lever arm.

Ideal under hex heads and nuts where there is enough outer space and you want maximum bite into the surface.

Simple selection:

Under screw heads (pan, round, cheese, small heads), and in electrical lugs, terminals → Internal tooth.

Under nuts and larger bolt heads, especially on flat surfaces → External tooth.

3) Environment & corrosion – material & finish

Indoor / dry / electrical & light mechanical use:

Use MS (mild steel) star washers with zinc plating or phosphated/black finish.

Common in control panels, enclosures, electronics, instrument panels.

Outdoor / mildly corrosive / industrial use:

Use zinc-plated MS or thicker coatings where some corrosion protection is needed.

For harsher outdoor use, prefer stainless.

Stainless / food / pharma / marine / aggressive environments:

Use SS 304 (A2) star washers for general stainless assemblies.

Use SS 316 (A4) star washers for coastal, marine, and chemical environments.

As always, best practice is to match washer material with the bolt/nut and environment to avoid galvanic corrosion and staining.

Mechanical Properties (Guide) – Star Washer

For star washers, the most important properties are material strength, hardness, and the spring/biting ability of the teeth. They are thin, so they don’t carry significant structural load – they are mainly locking and electrical contact components.

1) Mild steel star washers (standard)

Typical characteristics:

Material: low/medium carbon steel, often hardened or work-hardened at the tooth edges.

Teeth are formed and then heat-treated or strain-hardened to maintain springiness and bite.

Functional behaviour:

Teeth provide a small spring deflection as the nut/bolt is tightened.

As tightening torque increases, teeth dig into:

The underside of the nut/bolt head, and

The surface of the joint member.

This increases friction and resistance to rotation, and can slightly compensate for minor relaxation in the joint.

Limitations:

Being thin, they are not designed to carry major bearing loads.

Over-tightening can flatten the teeth completely, reducing their locking ability.

2) Stainless steel star washers (A2 / A4)

Typical characteristics:

Material: A2 (304) or A4 (316) stainless.

Provide corrosion-resistant tooth washers for stainless and outdoor assemblies.

Functional behaviour:

Similar spring and biting action to carbon steel, but with excellent corrosion resistance.

Ideal for stainless electrical panels, outdoor enclosures, marine-duty equipment, etc.

Because stainless is tougher, teeth may not be quite as sharp as hardened carbon steel, but still give effective bite in many applications.