Flat Washers Basics
Flat washers are used under bolt heads or nuts to increase the bearing surface area, or to offer protection so that the nut or bolt can be rotated without distortion of the bearing surface.
Flat Washer Types
ASTM F844 USS (United States Standard) Flat Washers
USS flat washers were developed to satisfy engineering requirements in the majority of industrial applications in all fields of manufacturing, maintenance and repair. The Outer Diameter (OD), Inner Diameter (ID) and Thickness (Th) specifications were developed to meet the broadest possible number of applications.
SAE (Society of Automotive Engineers) Flat Washers
SAE flat washers were developed and specified by the Society of Automotive Engineers to meet the requirements of automotive manufacturer applications as broadly as possible. They feature smaller outside diameter when compared with USS washers.
Fender washers are flat washers with a larger outer diameter in proportion to its center hole. They are used where an extra wide bearing surface is required. Fender washers were originally designed for auto body repair work and they are also used in sheet metal, plumbing, and electrical work. Can be used in various applications, such as to attach signs to posts and to install drywall and wood paneling.
ASTM F436 Flat Washers
F436 flat washers are thru hardened structural flat washers. They are designed for general purpose and structural use with bolts, nuts, studs, and other internally and externally threaded fasteners. Dimensionally comparable to the low carbon SAE washer, but manufactured from a higher grade of steel and heat treated. F436 washers are suitable for use with many fastener grades including B7, B16, L7, and A325 and A490 structural bolts.
Thru Hardened SAE (Society of Automotive Engineers) Flat Washers
Thru hardened SAE flat washers are dimensionally equivalent to their low carbon SAE flat washer counterparts, but with tighter tolerances on thickness, manufactured from a higher grade of steel and heat treated.
Extra Thick Thru Hardened SAE (Society of Automotive Engineers) Flat Washers
Extra thick thru hardened SAE flat washers are generally used in extreme heavy duty applications with high strength ASTM A325, A490, and Grade 8 bolts and nuts. They can be as much as 50% thicker than standard thru hardened washers.
Thru Hardened USS (United States Standard) Flat Washers
Thru hardened USS flat washers are dimensionally similar to their USS flat washer counterparts, but with tighter tolerances on thickness, manufactured from a higher grade of steel and heat treated.
18-8 Stainless Steel Flat Washers
18-8 stainless steel flat washers are used most often in applications that require general atmospheric corrosion resistance, such as chemical and food processing equipment. Stainless steel has a higher corrosion resistance than carbon steel.
Brass Flat Washers
Brass fasteners are often used by electrical and communications industries, builders hardware, and some marine applications. These brass flat washers provide corrosion resistance in wet environments, offer thermal and electrical conductivity, and are nonmagnetic.
Round Plate Washers
Round plate washers, often called dock washers, are similar to a fender washer, but larger and thicker. With up to a 4" outside diameter, this thick flat washer is ideal for heavy duty load bearing applications.
Common Finishes of Flat Washers
Flat washers are available in a variety of finishes and coatings, which play a role in the suitability of the fasteners in different applications, such as enhancing resistance to corrosion from environmental elements or chemicals. Coatings on flat washers can also effect consistency of torque values for installations.
Black or Plain Finish Flat Washers
Flat washers are unfinished or will have a minimally protective finish. Black flat washers offer near zero corrosion resistance, though black oxide coatings or oiled coatings do offer increased corrosion resistance over unfinished flat washers.
- Black flat washers are commonly referred to as plain finish, black, black oxide, black phosphate, bare or light oil coated.
Galvanized Flat Washers
Flat washers are coated with a sacrificial zinc coating that acts as an anode to prevent the fastener underneath from corroding.
- Hot Dipped Galvanized: Fasteners are dipped in a bath of molten zinc.
Plated Flat Washers
Flat washers are coated with a thin layer of zinc, typically via electroplating.
- Plated SAE flat washers will not corrode as quickly when covered with this protective coating, even when a scratch or cut exposes the steel to air or moisture. Zinc or cadmium plating acts as a sacrificial anode, and will always tarnish and corrode first thereby protecting the steel underneath.
Teflon or PTFE Coated Flat Washers
Flat washers have a PTFE coating (commonly referred to as the Dupont tradename Teflon Flat Washers) and are formulated with superior characteristics for maximum fastener life.
- Technical components include a tensile strength of up to 4,000 psi and an operating pressure up to 100,000 psi. The absence of thread locking oxides enable easy and safe fastener removal. PTFE coatings have extreme chemical resistance and are ideal for applications including insulating piping, boiler rooms, water filled piping, water lines and more.