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The complete guide to washers
Source: | Author:Joanne | Published time: 2023-01-16 | 585 Views | Share:

Are washers necessary?

A washer is a disk placed underneath a nut, an axle bearing or joint. When to use a washer and when not to really depends on the materials you’re using. But first, let’s address the purpose of washers.

    What are washers for?

    Their purpose is to help threaded fasteners do a more reliable job while protecting the surface from damage during tightening. A perfect example of this is when the material you’re fastening to is weaker than the fastener material itself. Without a washer, the fastener can distort the material it’s being installed into. To prevent this, you need a large bearing surface, which the washer provides.

    If the fastener and the material it’s being driven into are the same – for example, metal to metal – then a washer isn’t necessary. That said, certain types of washers can still be advantageous, such as a lock washer, to prevent the fastener and nut from loosening. The bottom line is, it’s smart to always use washers. 

    At a glance: purpose of a washer

    • Distributes the load of the threaded fastener to prevent damage to the material being fastened

    • Prevents fastener from moving or corroding

    • Absorbs vibrations

    • Can sometimes act as a spacer

    • Enhances the lifespan of application

    Different types of washers

    Not just any washer will do for all applications. Lots of questions surround the application of washers, such as what washer should I use? What goes first, flat washer or lock washer? What are the different washer uses?

    For every type of washer, there are subcategories, but here are the three main types you should know about.

    1. Plain washers

    Their purpose is to distribute the fastener’s load while reducing heat and friction during the tightening process. They can also be used as spacers, as is common in industrial and domestic applications. Different types of plain washers include:

    Flat washers

    Flat washers

    Also called a flat nut washer. Offers full insulation, abrasion and spacing.

    Use: manufacturing, maintenance and repair

    Finishing washers

    Finishing washers

    Also known as flush countersunk washers. Used with flat or oval head screws. Provides a reliable boring surface.

    Use: cabinetry and furniture

    Shoulder washers

    Shoulder washers

    Used as a bushing to insulate fasteners or shafts. For this reason, they’re made of non-conductive materials, such as nylon.

    Use: electronic equipment

    2. Lock washers

    The purpose of a lock washer is to prevent the fastener from rotating or losing friction due to vibration or torque. There are many lock-washer types, such as washers with teeth, but they all do this by holding the nut and bolt in place. Some bite into the bolt and the nut with their ends. Lock washers are a favorite in transportation industries such as automotive and aerospace. You’ll also find them in household appliances such as washing machines.

    Tooth lock washers – external

    Tooth lock washers – external

    Tooth-like serrations prevent screws, nuts and bolted joints from loosening, through the use of friction.

    Use: where high-mechanical strength is needed, such as automotive & white goods

    Retaining washers

    Retaining washers

    Internal teeth grip screw shank. Nylon provides insulation against electricity and moisture while absorbing vibration.

    Use: electronic equipment

    Sealing washers

    Sealing washers

    Also known as tap washers. Creates a close-fitting seal between a screw head and the surface its fastened to. Prevents movement and dust and liquid ingress.

    Use: Plumbing applications or to maintain an ingress-protection rating between cord grips and equipment.

    3. Spring washers

    What are spring washers for? Some people specify these as a separate category of washers, but they’re actually a type of lock washer. These provide axial load to fasteners to limit movement in case of vibration or thermal expansion. Spring washers are perfect for applications that require a degree of flexibility. All of these reasons are why actuators on airplanes, including the flight controls and landing gear, are examples of spring washer uses.

    Spring washers – single wave

    Spring washers – single wave

    Deflects in application, creating more friction with the assembly, reducing the risk of the bolt or nut coming loose. Absorbs shock.

    Use: pre-load shafts or bearings

    Cup washers

    Cup washers

    Provides insulation by protecting the head of metal screws from electrical contact. Also provides an aesthetic finish and an anti-tampering measure.

    Use: Isolate screws used as stand-offs on PCBs and bolts that secure electrical components

    Using washers as spacers

    Washer or spacer? In effect, washers are a type of spacer. They position different components, for example, by keeping a bolt head from the surface material when tightened. Washer and spacer differences lie in the thickness and height of each – though sometimes they can be similar. A spacer can look like a thick washer and a washer can look like a short spacer.

    Which one you use isn’t really a case of spacer vs. washer. When a washer is used as a spacer, it’s usually done within a machine as a temporary measure until it can be replaced with a spacer, as it can stop friction that causes loss of movement.

    Does the washer go against the nut or the bolt?

    Spring washers only go on the nut side. There’s no one answer for lock-washer applications.

    What order do lock washers go on?

    Scenario

    What to do

    Using a lock washer to keep nut or bolt from loosening

    Place washer on side that will turn and drive into the surface. This is typically the nut side.

    If bolt is screwing into threads in any part of assembly

    Place washer on the bolt side.

    Protect surface of application

    Place washer on side that will drive into surface during tightening.

    Thin assembly material

    Use larger washer to improve load and pressure distribution. Place washer against the material.

     

    Lock washer, flat washer order

    If there’s a chance your washer will mushroom during tightening, use a flat washer with a smaller diameter above the larger washer. This means you will put the flat washer on first.

    Washer on PCB board

    Washer materials

    Washers are available in metal, rubber and plastic. When high strength is needed, go with a metal washer. Rubber is ideal when flexibility and a tight seal are needed. Plastic is perfect for smaller applications. An example is electronics, which need washers that resist vibrations.

    Nylon vs. rubber washers

    When you hear of rubber washers, the material is typically nitrile, which is a synthetic rubber. This material and nylon share some of the same properties, especially when it comes to sealing washers. Rubber is softer and more flexible, which enables these washers to provide effective seals to irregular surface areas. A drawback to rubber versus nylon is that rubber distorts quickly under high compression forces. Consequently, it can shorten the life of the washer.

    Metal washers

    Types of metal washers vary, as do the metals. Stainless steels vary as well, depending on their grade. For instance, some stainless steels are not very corrosion resistant, while others are. 

    Softer metals, such as copper, can also resist vibration in machines, helping to keep connections secure. Of course, coppers can also be hard if treated. Steel is similar. Low-carbon steel washers are common, as is stronger steel washers.

    The chart below is not intended to be the final word on metal characteristics, as so much depends on how the metal has been treated. Still, it’s a good starting point to decide what characteristics you’re after if you’re using metal.

    Characteristics

    Stainless steel

    Aluminum

    Copper

    Brass

    Galvanized steel

    Resists compression



    Resists shearing, bending & cracking



    Resists corrosion

    Ductility/malleable




    Conducts electricity

    Good sealing capability




    Plastic washer materials

    Plastics are more cost effective than metals. If your application doesn’t require high strength, consider using a plastic washer. They’re especially ideal for electronics, when electrical isolation and resistance to vibration is needed. Some are available in colors, but you can also get clear plastic washers. Your choices are vast, from plastic sealing washers to high-density polyethylene washers. Compare the materials below:


    Nylon 6/6

    HDPE

    LDPE

    PC

    POM

    PVC

    PEEK®

    PPS

    PVDF

    PP

    Tensile strength – pull apart (psi)

    12,400

    4,000

    1,400

    9,500

    9,800

    7,500

    14,000

    12,500

    7,800

    5,400

    Dielectric strength – insulation (v/mil)

    300 – 400

    450 – 500

    460-700

    380

    500

    544

    480

    450

    280

    711

    Bending stiffness (psi)

    410,000

    200,000

    30,000

    345,000

    370,000

    481,000

    590,000

    600,000

    310.000

    225,000

    Resistance to chemicals


    Dilute acids

    1

    5

    5

    3

    2

    (variable)

    4

    4

    4

    5

    4

    Dilute alkalis

    2

    5

    5

    1

    3

    4

    4

    4

    4

    4

    Oils & greases

    5

    2 (variable)

    2

    (variable)

    2

    3

    3

    (variable)

    4

    4

    5

    2

    (variable)

    Aromatic hydrocarbons

    5

    1

    1

    1

    5

    1

    4

    4

    5

    1

    Halogenated hydrocarbons

    3

    (variable)

    1

    1

    1

    2(variable)

    2(variable)

    4

    4

    5

    1

    Alcohols

    1

    5

    5

    3

    2(variable)

    3(variable)

    4

    4

    3 (variable)

    4

     

    Flat washer sizes: SAE and USS

    Flat washers are the most commonly used washers. Standards are set by the SAE, Society of Automotive Engineers, and USS (United States Standards). SAE washers are thinner and smaller than USS. Half-inch washers, washers with 2-inch holes and more are easily available.

    SAE flat washers

    Size: numbers & in.

    Inside diameter (in.)

    Outside diameter (in.)

    Thickness (in.)

    #6

    5/32

    3/8

    3/64

    #8

    3/26

    7/16

    3/64

    #10

    7/32

    1/2

    3/64

    ¼

    9/32

    5/8

    1/16

    5/16

    11/32

    11/16

    1/16

    3/8

    13/32

    13/16

    1/16

    7/16

    15/32

    59/64

    1/16

    1/2

    17/32

    1-1/16

    3/32

    9/16

    19/32

    1-3/16

    3/32

    5/8

    21/32

    1-5/16

    3/32

    3/4

    13/16

    1-1/2

    9/64

    7/8

    15/16

    1-3/4

    9/64

    1

    1-1/16

    2

    9/64

    1-1/8

    1-3/16

    2-1/4

    9/64

    1-1/4

    1-5/16

    1-1/2

    5/32

    1-1/2

    1-7/16

    3

    3/16

     

    USS flat washers

    Size (in.)

    Inside diameter (in.)

    Outside diameter (in.)

    Thickness (in.)

    3/16

    ¼

    9/16

    3/64

    ¼

    5/16

    ¾

    1/16

    5/16

    3/8

    7/8

    5/64

    3/8

    7/16

    1

    5/64

    7/16

    ½

    1-1/4

    5/64

    1/2

    9/16

    1-3/8

    7/64

    9/16

    5/8

    1-1/2

    7/64

    5/8

    11/16

    1-3/4

    9/64

    3/4

    13/16

    2

    5/32

    7/8

    15/16

    2-1/4

    11/64

    1

    1-1/16

    2-1/2

    11/64

    1-1/8

    1-1/4

    1-3/4

    11/64

    1-1/4

    1-3/8

    3

    11/64

    1-3/8

    1-1/2

    3-1/4

    3/16

    1-1/2

    1-5/8

    3-1/2

    3/16

    1-5/8

    1-3/4

    3-3/4

    3/16

    1-3/4

    1-7/8

    4

    3/16

    1-7/8

    2

    4-1/4

    3/16

    2

    2-1/8

    4-1/2

    3/16

    2-1/2

    2-5/8

    5

    15/64

    3

    3-1/8

    5-1/2

    9/32


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