Description

 

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. 

 

Selecting types of washers

Image Credit: Alloy Fasteners 

 

 

Types

 

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
Spring washers, sometimes called disc springs, lend their mechanical capabilities to the unique profile of the material: the irregularities of the washer compress with a proportionate resistance to return to their predeflected shape. Spring washers are employed in applications where assemblies need a part to take up play, maintain assembly tension, compensate for expansion or contraction in materials, or to absorb intermittent shock loads and provide a controlled reaction under dynamic loads.
Belleville washers can support high loads with small deflections. The load and deflection capability is dependent on height/thickness ratio. These are common in thermal expansion applications. 

 Washers Selection Guide

Image Credit: SEASTROM

Crescent washer is meant for lighter loads and produces a small deflection. There is a uniform spring rate throughout the washer's deflection. This is used in flexible, load-cycling applications.

 Washers Selection Guide

Image Credit: SEASTROM

Dome washers have a very high load capacity with a small overall deflection. While similar in appearance to a crescent washer, a dome washer has ground curves to create a flatter load-bearing surface.

 Washers Selection Guide

Image Credit: SEASTROM

Finger washers are notable for their split construction with protruding flanges. They combine the flexibility of a crescent washer with the distributed load points of a wave washer. They are common for noise and vibration dampening instances.

 Washers Selection Guide

Image Credit:Raymond

Wave washers offer moderate load capacity and deflection, and are typically used as cushions or spacers. These have multiple waves within the washer. More information about this type of washer, as well as a specification filter, is available in GlobalSpec's Wave Washer Selection Guide.

 Washers Selection Guide

Image Credit: SEASTROM

Single wave washers are similar to crescent washers but have flattened load points on the washer. This is meant to minimize abrasion against sensitive surfaces.

 Washers Selection Guide

Image Credit: SEASTROM

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 GlobalSpec's Spring Washer Selection Guide.

 

Hooke's Law

F = -kx

 

Where:

F = Load

x = Deflection "Displacement"

k = Spring Rate "Spring Constant"

 

 

Lock Washers
Lock washers are designed to secure fasteners that have a tendency to rotate or lose friction. Although though there are several different types of lock washers and is each designed for use with a particular fastener or application, they all work under the same principle. The washer is designed to exert a load, partially deform, and lock a fastener in place. A split coil or teeth of the lock washer bite into the head of the fastener and against another flat surface. In some applications they are deployed along with a flat washer in order to distribute the load evenly without deforming the assembly that the fastener is secured to.
Helical lock washers, also known as split lock washers, have a single coil of non-continuous flat wire filament. Each end of the coil is bent outwards towards a mating surface. When secured, the lock washer flattens while each end of the coil bites into the mating surfaces.

 Washers Selection Guide

Image Credit: J.W.Winco, Inc.

External tooth lock washers have a cylindrical inner diameter with several teeth along the outside diameter that are aligned at an obscure angle to the face of the washer. They are designed for use with wide headed fasteners. When secured, the teeth bite into a mating surface while they resist the compressional force.

 Washers Selection Guide

Image Credit: Brass Parts Components

Internal tooth lock washers have a cylindrical outer diameter with several teeth along the inside diameter that are aligned at an obscure angle to the face of the washer. They are designed for use with shallow headed fasteners. When secured, the teeth bite into a mating surface while they resist the compressional force.

 Washers Selection Guide

Image Credit: Brass Parts Components

 

 

Other Common Types

Flat washers, also referred to as Type A plain washers, are thin, flat, and circular general-purpose washers with a centrally located hole. Standardized flat washers may be designed by the use of imperial or metric dimensions. Standard imperial washers include SAE washers, for use with fine threaded nuts and bolts, and USS flat washers, for use with coarse threaded bolts and nuts. Standard metric washers are available in several gages as defined by JIS standards.

 Washers Selection Guide

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C-washers have a slot cut from the center to the perimeter.  Typically the slot is the same width as the "center hole," allowing the washer to be removed, replaced, or inserted without completely removing the fastener.

 Washers Selection Guide

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D-shape washers have a flat edge cut away on the parameter. They are mounted with the flat edge of the washer facing a flat surface or recessed hole that prevents the washer from turning.

Washer Selection Guide

Image Credit: SEASTROM

Countersunk washers, sometimes referred to as finishing washers, have a countersink that captures the head of the fastener. When secured they provide a flush surface and are available in several shapes including: 90 degree countersunk, angle countersunk, flanged, un-flanged, and rolled flange among others. 

 Washers Selection Guide

Image Credit: American Fastener

Fender washers have a large outside diameter in comparison to the inside diameter. The name is derived from their use in the automotive industry where they are used to mount fenders. They distributed a load evenly across a large surface area. Flat washers whose outer diameters measure more than three times the inner diameter are commonly referred to as fender washers.

 Washers Selection Guide

 

 

Image Credit: Grainger

SAE washers, also known as Type A plain washers, narrow series, are approved by the Society of Automotive Engineers. SAE washers are smaller and lighter than other flat washers and are for use with fine thread bolts and nuts. They are designed using imperial units and while smaller SAE washers are defined by a gaged number, larger SAE washers are defined by imperial units.

 Washers Selection Guide

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Torque washers have sharp prongs that sink into the mating surface and prevent rotation of the washer.  They are commonly used with carriage bolts in woodworking applications.

Washers Selection Guide

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Shoulder washers have an integral cylindrical sleeve. The sleeve is designed to mate with a cutout and segregates the fastener from the material it is secured to.

 Washers Selection Guide

Image Credit: SEASTROM

 

 

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.

 

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.

 

Washers Selection Guide

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. 

 

   
Thickness
Gage Number
 
Gage Measurement (in inches)
00000
 
0.500
0000
 
0.454
000
 
0.425
00
 
0.380
0
 
0.340
1
 
0.300
2
 
0.284
3
 
0.259
4
 
0.238
5
 
0.220
6
 
0.203
7
 
0.180
8
 
0.165
9
 
0.148
10
 
0.134
11
 
0.120
12
 
0.109
13
 
0.095
14
 
0.083
15
 
0.072
16
 
0.065
17
 
0.058
18
 
0.049
19
 
0.042
20
 
0.035
21
 
0.032
22
 
0.028
23
 
0.025
24
 
0.022
25
 
0.020
26
 
0.018
27
 
0.016
28
 
0.014
29
 
0.013
30
 
0.012
31
 
0.010
32
 
0.009
33
 
0.008
34
 
0.007
35
 
0.005
36
 
0.004

 

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

 

Steel Grade
Material & Treatment
Proof Load Stress
Yield Strength
Tensile Strength
Core Hardness, Rockwell
Standards Authority
   
ksi (1000 lbs/sq inch) Min.
ksi (1000 lbs/sq inch) Min.
ksi (1000 lbs/sq inch) Min.
   
Low-carbon grade 2
Low/medium carbon steel, cold-forged
55
57
74
B70-B100
Society of Automotive Engineers
Grade A
Low/medium carbon steel
90-100
95-105
60
B69-B100
American Society for Testing & Materials
Grade B
Low/medium carbon steel
70-133
76-139
60-100
B69-B95
American Society for Testing & Materials
Grade 5
Medium carbon steel, quenched & tempered
74-85
81-82
105-120
C19-C34
Society of Automotive Engineers
Grade 8
Medium carbon alloy steel, quenched & tempered
120
130
150
C33-C39
Society of Automotive Engineers
Grade C
Medium carbon steel
144
154
N/A
B78-C38
American Society for Testing & Materials
2-H
Medium carbon steel
144
154
N/A
B78-C38
Society of Automotive Engineers
Strength guidelines may vary by part thickness, number of threads/inch, finish, and other design or production factors.

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.

 

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

 

 

Read user Insights about Washers

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  • Preload Springs, Spacers, and Washers

    Preload springs, spacers, and washers are meant to maintain tension in an assembly where some slack may be present. Their capabilities can eliminate rattle, compensate for expansion or contraction of the assembly materials, or absorb intermittent shock loads. These products are made of elastic deformable materials, most commonly convoluted ductile, high-strength metal alloys which come in machined, welded, and open-ring varieties.

  • Spring Washers

    Spring washers, sometimes called disc springs, lend their mechanical capabilities to the unique profile of the material: the irregularities of the washer compress with a proportionate resistance to return to their predeflected shape. Spring washers are employed in applications where assemblies need a part to take up play, maintain assembly tension, compensate for expansion or contraction in materials, or to absorb intermittent shock loads and provide a controlled reaction under dynamic loads.

  • Wave Washers

    Wave washers or wave springs are wavy metal washers designed to provide a compensating spring force and sustain a load or absorb shock.