Flat Washers

Flat washers are used to increase the bearing surface of a nut or fastener's head thus spreading the clamping force over a larger area. They can be useful when working with soft materials and oversized or irregular shaped holes.

Washer size refers to its nominal hole size and is based on screw size. Its outside diameter (OD) is always larger. Size and OD are usually specified in fractional inches, although decimal inches may be used instead. Thickness is typically listed in decimal inches although we often convert it to fractional inches for convenience.

Grade 2 flat washers should only be used with Grade 2 hex cap screws (hex bolts)—use hardened flat washers with Grade 5 and 8 cap screws. Because Grade 2 flat washers are made of soft, low carbon steel, they will "yield" (compress, cup, bend, etc.) under the higher torque values normally associated with Grade 5 and 8 cap screws. As a result, there will be a reduction in clamping force as the washer yields. Also, use only SAE pattern flat washers with Grade 5 and 8 cap screws because, as it is explained, the oversized hole in USS washers is too large to properly support the bearing surface of the cap screw's head and one washer size smaller will be too tight and may cause the head to fail.

Although not required by any standard, some experts believe that a fastener's head should mate with the "rounded" side of a hardened flat washer because the "sharp" washer side can damage the fastener's underhead fillet (the concave junction where the head and shank meet), which can lead to fastener failure.

Flat washers are commonly available in a variety of materials including aluminum, brass, nylon, silicon bronze, stainless steel and steel. Unplated or uncoated steel, referred to as "plain finish," has not been surface treated to prevent rust other than a light coating of oil for temporary protection. Consequently, common finishes for steel are zinc plating and hot dip galvanizing.

Hot dip galvanized steel, stainless steel and silicon bronze are usually recommended if the flat washers will be used with pressure preservative treated wood such as "ACQ" (Alkaline Copper Quaternary)—check local building codes and contact your lumber supplier for recommendations. In marine environments, stainless steel may show signs of pitting corrosion when submerged in salt water that lacks free oxygen, which makes silicon bronze a preferred material because stainless steel needs oxygen to create its self-healing, corrosion-resisting chromium oxide film.

There are different types of flat washers that are intended for different applications. See the list below for a description of each.

**Table 1. Flat Washers.** Common flat washer styles and types.
Flat Washers
Fender Washers
Because of their large outside diameter (OD), fender washers offer an extra-wide bearing surface that is useful when working with oversized or irregularly shaped holes and when fastening soft materials that could benefit from increased support.
Machine Screw Flat Washers
Machine screw flat washers are the smallest type of flat washer because their outside diameter (OD) is usually just a little more than twice their inside diameter (ID).
Mega Thick Flat Washers
Mega thick flat washers are extra-thick and range in thickness from 0.100" to 0.313" making them useful in spacing applications. Available in SAE and USS patterns.
SAE Flat Washers
SAE flat washers offer a tighter screw fit, are thinner and have a smaller outside diameter (OD) than comparably sized USS washers. Use hardened SAE flat washers with hex cap screw Grades 5 and 8.
Square Plate Washers
Square plate washers are large, thick and flat and are typically used in structural bolting applications.
Structural Flat Washers
Structural flat washers are used in exterior building applications and in construction with A325 bolts, studs and nuts.
Thick Flat Washers
Thick flat washers are thicker than standard washers and range in thickness from 0.072" to 0.146", which makes them usable as spacers. Available in SAE and USS patterns.
T-Slot Washers
T-Slot washers are flat washers that have one beveled edge and about the same outside diameter (OD) of a comparably sized SAE washer, but are roughly double the thickness of their SAE washer counterpart. Normally used with T-Slot Bolts but also as spacers because of their thickness, T-Slot Washers are heat treated (when indicated) for added strength.
USS Flat Washers
USS flat washers offer a looser screw fit, are thicker and have a larger outside diameter (OD) than comparably sized SAE washers. The oversized hole in USS washers is too large to properly support the bearing surface of a Grade 5 and 8 hex cap screw head and one washer size smaller will be too tight and may cause the head to fail. Consequently, use hardened SAE flat washers with hex cap screw Grades 5 and 8.

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