Flat washers, also sometimes called plain washers, are used under bolt heads or nuts to increase the bearing area or offer greater hardness, so that the nut or bolt can be rotated without distortion of the bearing surface. Flat washers are also used to give tightness to a joint, to prevent leakage, and especially to distribute pressure under the head of a nut or bolt.
There are two common flat washer styles, based on the evolution of this part: USS (United States Standard) flat washers and SAE flat washers (originally approved by Society of Automotive Engineers). USS flat washers have a larger outer diameter than SAE flat washers. Differences between USS and SAE flat washers are reflected in each item's actual physical measurements; unless the buyer has a specific USS or SAE requirement, the buyer should consider the measurements, based on need, and not whether the flat washer is USS or SAE.
It is important to note that the nominal size of a washer does not match the actual size of its internal diameter. The internal diameter of a washer is larger than its nominal size so that when one matches a, for example, ¾-20 bolt with a ¾ washer, the ID of the ¾ will be large enough to accommodate the diameter of the bolt’s shaft. A ¾ flat washer is .812”, not .750”, as one would likely assume. This makes it easy to match bolts or screws with the appropriate size washer. When ordering a washer for a use other than matching it to a screw or a bolt, however, if the ID is important to the use to which the washer is to be put, one should determine the actual inner diameter of the washer before placing an order.
In addition, Fender flat washers have a large outside diameter in comparison with the inside diameter (hollow section). Malleable flat washers are made from cast iron or galvanized iron. The Beveled flat washer offers a square outer diameter, and is generally thicker than USS or SAE flat washers.