Nuts and washers come in a large variety of styles, each of which is suited for a specific type of job. Washers are sometimes needed when nuts are used to make a fitting more secure or to make up for space discrepancies on a project (for example, one side may not be perfectly level with another, and, by adding a washer to one side, the nuts will be at the same level).
Hex nuts are used for many general-purpose projects. Because hex nuts work for many applications, they are one of the most common styles used. They have six sides, which is where the name "hex" comes from.
Locknuts provide resistance against vibration, which might cause a nut to come loose over time. There are several styles of locknuts, but they all prevent loosening from vibration.
Wing nuts have two "wings," which make adjusting these nuts by hand easy. Wing nuts are convenient for projects that require frequent tightening or loosening.
Slotted nuts, also known as castle nuts (due to their resemblance to a castle tower), are useful for projects where vibration is a concern. A pin fits through the slots on the nut and through a hole in the bolt to prevent movement.
Square nuts have a larger surface than hex nuts, which makes square nuts easier to grip with wrenches. Square nuts are sometimes welded to machinery.
Spring lock washers provide resistance against vibration. There are three parameters to look at when choosing the correct size for your project: inside diameter, outside diameter and minimum thickness range.
Tooth lock washers come with a variety of teeth types: internal, external, internal-external and countersunk external. Internal teeth are hidden by the bolt or screw, so they provide a cleaner appearance. The external teeth provide the best locking capabilities. Internal-external teeth provide more surface area and are commonly used for electrical grounding to ensure a solid ground contact is made. Countersunk teeth work well with round screwheads.
Spherical washers have a male and female part. The sections fit together and are good for compensating for misalignments.
Round hole washers are a basic washer style. They are flat metal discs with a round hole in the middle. They are used for shoring up different spaces between screws and the material being connected. They also provide a dampening effect for vibration, which protects the surface material they are connected to, as well as keeping the fastener in place.
