ASME/ANSI B 18.21.1 (USS) Flat Washers

When a bolted joint is made through thin sheet, oversized holes, or materials with a relatively limited bearing area, the contact pressure beneath the bolt head or nut can become concentrated around the fastening hole. ASME/ANSI B18.21.1 (USS) Flat Washers address this requirement with a larger outside diameter that spreads the clamping force across a wider portion of the workpiece.

The USS washer is therefore more than a simple spacer between a fastener and the assembled component. Its relatively large bearing surface helps support the material surrounding the hole and can reduce localized indentation or deformation during tightening. The inside diameter provides clearance for the bolt or screw, while the larger outside diameter increases the area available to transfer the load into the connected material.

Specification:

USS Flat Washers are identified primarily by their inch nominal size and USS dimensional pattern. Unlike washers that use the same outside diameter relationship across different series, the USS pattern combines a relatively large outside diameter with the corresponding fastener opening, creating a specific washer geometry for inch-series fastening. When ordering, the nominal fastener size should therefore be confirmed first, followed by the required washer dimensions.

The main specification points are the inside diameter, outside diameter, and washer thickness. The inside diameter determines whether the washer can be fitted over the selected bolt or screw, while the outside diameter defines the physical footprint of the washer on the component. Thickness is relevant when the washer is part of a controlled fastener stack or when the available clearance beneath the fastener head is limited.

Material is specified separately from the USS dimensional pattern. Carbon steel is commonly used for standard industrial fastening, while stainless steel 304 or 316 can be selected when the complete assembly requires improved corrosion resistance. For carbon steel washers, coatings such as zinc plating or hot-dip galvanizing can be specified according to the required corrosion protection.