Flat Washers ASME B18.21.1 Type A and Type B

ASME B18.21.1 flat washers, also known as plain washers, are non-locking washers used in inch-series bolted joints to distribute bearing loads and help protect contact surfaces. The standard distinguishes Type A and Type B plain washers by their intended functions and dimensional series.

Type A flat washers serve general applications where minimizing embedment and aiding tightening are important. They are available in narrow and wide series, with dimensions specified for the corresponding nominal fastener sizes.

Type B flat washers serve the general purpose of distributing loads over larger areas of lower-strength materials. They are available in narrow, regular, and wide series, offering additional dimensional choices for different joint configurations and bearing surfaces.

The two types should not be treated as interchangeable solely because they fit the same nominal bolt size. Their inside diameter, outside diameter, and thickness must match the requirements of the bolted joint. Type A narrow and wide washers are sometimes informally called SAE and USS washers, respectively, but these names are not formal equivalents under ASME B18.21.1. Specifying the actual type and series is important when dimensional conformity matters.

These washers are commonly used in machinery, equipment assembly, general construction, and other mechanical fastening applications. Selection depends on the fastener size, available bearing area, contact material, and required washer dimensions.