The complete guide to washers
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.
Our guide looks at the purpose of washers and other information you need to know, such as:
What are washers for?
The purpose of washers 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. You can learn more in our guide, How to keep bolts from loosening.
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:
Also called a flat nut washer. Offers full insulation, abrasion and spacing.
Use: manufacturing, maintenance and repair
Also known as flush countersunk washers. Used with flat or oval head screws. Provides a reliable boring surface.
Use: cabinetry and furniture
Learn more in our Guide to finishing 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
Read more in our guide, Shoulder washers explained.
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-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
Internal teeth grip screw shank. Nylon provides insulation against electricity and moisture while absorbing vibration.
Use: electronic equipment
Find out more in our Guide to retaining 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
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
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 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. You can learn about grades in our guide Understanding stainless-steel grades.
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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