Axle nuts are pieces of hardware that are used to help secure a wheel to an axle. The axle is a rotating shaft that helps secure the wheels to the rest of the vehicle and facilitate movement, and the axle nuts will be secured to lugs that are attached to either end of the axle. The configuration of the lugs will vary depending on what type of vehicle the wheel is being secured to; some vehicles will only feature one lug, while others may feature six, eight, or more.
On a bicycle, for example, the front wheel may be secured to the fork using two axle nuts. These nuts will thread directly onto the wheel's axle, which runs through the hub of that wheel. The rear wheel may work the same way, which means the entire bike will feature four axle nuts. Many modern bicycles will not feature any nuts at all; instead, a quick-release system is used. This system features a hollow axle through which a quick release skewer is run. On one side of the skewer, a threaded cap secures the axle. On the other side, a lever can be pivoted to either tighten or loosen the skewer, thereby securing the wheel or loosening it for removal.
On automobiles, each wheel is likely to feature several axle nuts. Most cars have at least four nuts per wheel, while larger vehicles may feature eight or more per wheel. These nuts are sometimes known as lug nuts instead, since they thread onto the lugs that keep the wheel in place. Another type of axle nut exists, which is usually much larger than the lug nuts. This nut will sit in the center of the axle to help secure components on either end of the axle shaft in place. When this axle nut is removed, the components on the ends of the axle can be removed for repair, maintenance, or replacement.
The materials used to make axle nuts can vary, though the most common material is steel. High grades of steel are usually used for durability and heat resistance. Steel tends to be exceptionally strong, and when it is galvanized, it will be resistant to rust and corrosion from exposure to moisture or other corrosive materials. Steel also has some give to it, meaning when it is impacted or torqued, it will bend slightly, preventing the axle nuts or other steel components from snapping or otherwise failing.