Washers are disks of metal or non-metallic material placed beneath a nut, an axle bearing, or a joint, to relieve friction, prevent leakage, isolate, prevent loosening, or distribute pressure. 

Image Credit: Alloy Fasteners 

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Types of Washers

There are many different types of washers for a variety of applications.  Lock washers are used to secure fasteners. Spring washers are a load bearing device that provides a preload between two surfaces. Other common types include flat washers, C-washers, D-shaped washers, countersunk or finishing washers, fender washers, SAE washers, torque washers, and shoulder washers.

Spring washers possess a spring rate, which is a characteristic that describes the deflection range and load capacity of the washer. Deflection and load ratings can be manipulated by stacking spring washers in series or parallel configurations. More detailed information is available in Spring Washer Selection Guide.

Hooke's Law

F = -kx

Where:

F = Load

x = Deflection "Displacement"

k = Spring Rate "Spring Constant"

Sizes

Washers are measured and characterized by either imperial or metric design units. The dimension of washers can be specified by standard nominal values, gaged values, or may be customized for particular applications. Nominal ANSI sizes refer to the size of the fastener that the washer is designed to mate with. Nominal SAE, USS, and JIS sizes not only specify the inner diameter of the washer but also dictate the outer diameter and thickness of the washer.

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Nominal Values

Nominal ANSI values used to describe the size of the washer most closely represent the inner diameter of the washer. The graph below illustrates the tolerance of these dimensions and contains a defining alpha character. N is used to denote Type A plain washer narrow series or SAE washers. W refers to Type A plain washer wide series, or USS washers.

Image Credit: Piping Handbook, 7th Edition

Thickness / Working Height

Washer thickness or working height is an important design criterion for all types of washers. This value may be used to describe the unloaded working height of a spring washer or the axial cross section of almost all other types of washers. 

Gage Thickness for Plain Washers

Flat washers that are manufactured or stamped out of tube or strip steel may reference a gaged thickness value. The gaged value is a standardized dimension used for tube and strip steel products. The following table can be used to determine the imperial measurement. 

Materials of Construction

Materials of construction dictate the physical properties of the washer. In some application where the washer goes through cyclic loading, as is the case for spring washers, yield strength and fatigue resistance are important design criteria and require the use of high-alloyed steels or spring steel. Softer materials are generally used when washers are subjected to lighter loads or mate with soft surfaces that could be damaged by a metallic washer. The following table illustrates how several grades of steel offer varied material properties: 

Mechanical Guidelines for Fasteners - Screws, Nuts, Bolts, Washers, Pins

Table Credit: Instock Fasteners 

Metallic materials of construction available for washers include aluminum, brass, copper base alloy or bronze, copper, nickel base alloy, spring steel, steel, hardened steel, stainless steel, and titanium.

Non-metallic materials of construction include ABS, acetal, asbestos, felt, leather, nylon, polyester, polycarbonate, polyethylene, PVC, polypropylene, PTFE, and rubber.

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Corrosion Resistance

Non-steel washer springs are typically produced to prevent corrosion. For steel washers, an abridged variety of corrosion prevention methods are listed below. 

• Phosphatingincorporates a zinc-phosphate layer and corrosion protective oil. This is sufficient for most applications but can be abrasive.
• Browningproduces an oxidized surface which is coated with oil. While not as good as phosphating, this is used when phosphating's abrasive finish is unacceptable.
• Metallic coatingslike zinc, cadmium, and nickel are all common. Zinc acts as a sacrificial anode that is attacked by corrosive materials before the washer's material. Cadmiumprovides high-quality protection, but is environmentallyand biologically toxic.Nickel can be used as a coating and is resistant to many corrosives butsince itacts as a cathode,the base metal can be attacked. For this reason, anickelfinish must always be a dense, non-porous coating.
• Electroplatingcan use almost any metal as a surface coating, buthydrogen embrittlementis a major drawback of this method, which leads toa shortened fatigue life.
• Peen platinguses a peening body along with a metallic powder that will be embedded into the washer inthe shot peen process.
• Metal sprayis reserved for large spring washers where regular plating methods are challenging. This results in a thick coating, but is not as adhesive to the washer as other metallic coatings.
• Chemical platinguses a nickel-phosphor alloy precipitated onto the surface with a chemical process. This has outstanding corrosion and abrasion resistance.
• Dacromet coatinginvolves zinc and aluminum flakes in a chromatic compound which is baked onto the washer.

Standards

ASME B18.22M - This Standard covers general specification and dimensions for flat, round hole washers, both soft (as fabricated) and hardened, intended for use in general purpose applications.

ADS AGS970 - Washers (aluminum alloy)

Resources

eHow - What is the Purpose of a Lock Washer?

BOKER'S Inc. - Flat Washers

Instock Fasteners - Guide to Fastener Sizes