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.
Inch 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.
Metric Flat Washers are used to increase the bearing surface area of a bolt/screw head or nut possibly because the hole is oversized or the mating material is soft. They are thin disks with a concentric hole matched to the size of the fastener with which they are mated. Small Pattern Flat Washers are used with screws that have a small diameter head like Cheese Head and Socket Head Cap Screws. Fender Washers have an outside diameter that is about 3 times their screw (nominal) size. Use Class 100 HV steel washers with fasteners Class 6/Class 6.8 and below; 140 HV below Class 8/Class 8.8; 200 HV is used with Class 8/Class 8.8 and below; and 300 HV steel is suitable for Class 10/Class 10.9 and below. Steel and stainless steel are available along with plain and zinc plated finishes.