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Washer Wisdom: Flat Washers

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Originally Posted On: Washer Wisdom: Flat Washers (


For the next installment of our Everything but the Bolt educational series, we’ll be digging into the various types of washers available for industrial uses. Some applications will only require a basic flat washer, while some applications will require a very specific type of washer, either for functionality or to adhere to certain code requirements.

The most common type of washer is a basic round flat washer, also known as a plain washer. Flat washers used in conjunction with bolts provide a smoother bearing surface on the underside of the bolt head or nut.

USS Flat Washers

USS flat washers were developed as a standard (the USS is the abbreviation for United States Standard) for basic engineering requirements in most industrial applications. The specifications of Outer Diameter (OD), Inner Diameter(ID) and Thickness (Th) are made to appeal to the widest use and highest stability, allowing users to create extreme tightness at the joint in which they are being used.

SAE Flat Washers

SAE flat washer specifications were developed specifically by the Society of Automotive Engineers to meet automotive manufacturing requirements in a broad sense. They typically have smaller outer diameters and less thickness than their USS counterparts. Although SAE washers are mostly used in automotive applications, they are also recommended for use any time the assembly includes Grade 5 or Grade 8 hex cap screws.

Fender Washers

Fender washers offer an outside diameter, and therefore bearing surface, that is much larger than that of USS flat washers. The large outside diameter design helps to better distribute applied forces when tightening an assembly. Fender washers are most commonly used in sheet metal, plumbing, automotive and electrical applications.

Thru Hardened Washers

Thru hardened washers provide additional strength and protection by going through a process of hardening or carburizing. While typically of a smaller size than standard USS washers, hardened washers are recommend for us with Grade 8 bolts, as well as other heat-treated fasteners.

ANSI Washers

ANSI washers are flat washers that meet standard provided by the American National Standards Institute (ANSI). Type A washers take into consideration both narrow and wide sizes, while Type B washers come in narrow, regular and wide sizes. Though the difference between the two types can be very minimal, the precise sizing can come in handy for applications where the washer dimensions are critical to an assembly’s functionality.

If you have further questions about flat washers or plain washers, please don’t hesitate to contact us for more information.

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