USS Flat Washer Dimensions

History

• At the start of the Industrial Revolution, maintaining machinery and equipment was fairly difficult because of a lack of standardization for fasteners, washers and other critical components. Toolmaker William Sellers devised a system to standardize fasteners based on a 60-degree thread angle in the threads of bolts. He took his system to the Franklin Institute, a Philadelphia-based mechanical engineering society, which adopted his system in 1864. The Sellers Standard, or Franklin Institute Standard, set consistent sizes for fasteners, washers and related components. Several branches of the military and major railroad companies soon adapted this standard, and the Sellers system eventually became known as the United States Standard. While the USS eventually was incorporated into the Unified Thread Standard, the USS designation still is used to designate the size of flat washers in the United States.

USS Washer Dimensions

• To choose USS washer sizes, start with the size of the bolt you plan to use with the washer. Each washer is designed to work with a specific bolt diameter. The USS standard generally covers washers designed to work with bolts ranging from 3/16 to 3 inches. The smallest USS washers, designed to work with 3/16-inch bolts, feature an inner diameter of 1/4 inch and an outer diameter of 9/16 inch. The largest USS washers, designed to work with 3-inch bolts, have an inner diameter of 1/8 inch and an outer diameter of 5 1/2 inches. Many washer manufacturers provide USS tables to help you determine the correct flat washer based on bolt size.

Washer Gages

• The thickness of a USS washer may be given in inches or using gage. The higher the gage, the thinner the washer. A washer marked as 00000 gage measures 0.5 inches thick, while one marked as 36 gage measures just 0.004 inches in thickness.

USS vs. SAE

• While general-purpose and industrial washers in the U.S. follow USS standards, flat washers for cars, commercial vehicles and aerospace applications follow size standards established by the Society of Automotive Engineers. For a specific bolt size, SAE and USS washer dimensions generally are similar, but the USS washer generally is thicker, with a larger inner and outer diameter. For example, a USS washer for a 3/8-inch bolt has an inner diameter of 7/16 inch, compared to 13/32 for an equivalent SAE washer. USS washers give a bit more flexibility than SAE washers, and often work with more than one bolt size. SAE washers provide tighter tolerances, and their smaller outer diameter allows them to fit in tighter spaces than USS washers.