The most common washer style, flat washers install between the fastened material's surface and a nut or bolt head to help distribute the load or reduce friction.
These washers are either thicker than standard for leveling or spacing, or they have a larger outer diameter to cover enlarged holes. Those with an oversized OD are also known as fender washers.
Fit these undersized washers into tight spaces where standard flat washers are too wide. They have narrow outside diameters so they work well on screw holes that are close to an edge or corner.
These tight-fitting SAE washers have a smaller outside diameter and thinner gauge than USS washers. Originally developed for automotive applications, they're now a common choice for everyday use.
USS washers have a larger outside diameter and thicker gauge than SAE washers for industrial use.
Demanding applications call for quality washers. This group includes washers that meet MS 15795 or AN 960 military specifications, or NAS 620 or NAS 1149 aerospace specifications.
Use a square washer to prevent rotation or in narrow applications where a round washer won't fit.