Tooth Lock Washer

What a star washer does

A tooth lock washer, better known as a star washer, has teeth that stick out radially from its inner or outer edge. Hobson's Types of Washers sheet explains that the flexed teeth absorb shock, vibration and slipping, which keeps tension in the joint and resists the fastener loosening. They work by raising the friction between the fastener and the surface it sits on, through the teeth biting in. They also add some spring like a spring washer, but far less.

Common uses are machine screws and small bolts on electrical enclosures, appliances, light fittings, panels and machinery guards. They are also called shakeproof washers, and the finer toothed versions serrated washers.

Internal, external or serrated star washers?

• •Internal tooth: the teeth are on the inner edge, around the bolt hole, so they sit under the head or nut and stay out of sight. Hobson's stainless versions are DIN 6797 type J.
• •External tooth: the teeth are on the outer edge, so the washer bites over a wider ring. Hobson's stainless versions are DIN 6797 type A.
• •Serrated (DIN 6798, internal type J or external type A): more, finer teeth than the plain tooth washers. An M6 304 external washer has 8 teeth as a DIN 6797 tooth washer and 12 as a DIN 6798 serrated one, per Hobson's dimension data.
• •The zinc plated metric washers are made to IFI 532 type A and the imperial ones to ASME B18.21.1 type A.

Do star washers stop bolts loosening?

They help in the right joint. Hobson's technical paper on vibration loosening makes these points about tooth lock and serrated washers:

• •They need pre-tension: they only work while the bolt is tightened, and they are not suitable for a joint that does not need pre-tension.
• •They are not suitable for joints subject to severe vibration.
• •Serrations embed into the joint surface, so consider whether that surface can take the marks.
• •The usual torque to tension figures do not apply when they are fitted, because they change the friction under the head or nut.
• •For a joint under significant vibration, the paper also says a larger diameter fastener is better. For severe vibration, look at two-piece wedge locking safety washers, which Hobson describes as designed to prevent bolt assemblies loosening due to vibration.

Star washer sizes and dimensions

Every tooth lock and serrated washer we carry, grouped by type and material, smallest to largest. Choose the washer by the bolt or screw size it goes on.

Zinc plated, 304 or 316 stainless

Hobson's corrosion guide for metal connectors puts zinc plated only in enclosed areas of low corrosion; anything exposed to the weather needs stainless (or hot dip galvanised, which these washers do not come in), with stainless in the sea spray zone and 316 within 2m of a pool edge. Match the washer to the screw or bolt.

• •Zinc plated steel: a 5 micron zinc coating, for indoor and non-corrosive conditions. Metric M3 to M20 and imperial 1/8 to 1 inch, internal and external tooth.
• •304 stainless: for general outdoor use. Internal and external tooth and serrated.
• •316 stainless: for coastal, pool and marine work (not for immersion in seawater). External serrated, M5 to M16.

Fitting a star washer

1. Pick the washer for the screw size: an M6 washer for an M6 screw.
2. Fit it between the head or nut and the joint surface, so the teeth can bite into both.
3. Tighten the fastener fully. The washer needs the bolt's tension to work.
4. Expect the teeth to mark the surface, and check the joint can accept that.

Star washers in Adelaide

Allmat supplies star washers, spring washers, flat washers, nuts and bolts to electricians, fabricators, mechanics and trade across Adelaide and South Australia. Add the washers to a quote with the screws they go on and we will price the lot.

Questions