Flat Washer are used to increase the bearing surface of […]
Flat Washer 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 outside diameter are usually specified in fractional inches, although decimal inches may also be used instead. Thickness is typically listed in decimal inches although we often convert it to fractional inches for convenience.
Generally, Grade 2 flat washers should only be used with Grade 2 hex 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. As a result, there will be a reduction in clamping force as the washer yields.
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.
As a rule of thumb, hot dip galvanized steel, stainless steel and silicon bronze are usually recommended if the flat washers will be used with pressure preservative treated wood, just 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.