A truck axle is a component in the drive system of any truck; it is essentially a link between wheels, and it can either be fixed to the wheels and rotate with them, or it can be fixed to the frame or other components of the vehicle, allowing the wheels to rotate around the axle. A truck is likely to have more than one axle, depending on the size and purpose of the vehicle, and the term "truck axle" can refer to just the axle itself or all of the components that make up the axle, such as the axle casing.
The purpose of a truck axle is multiple: it acts to keep two parallel wheels in line with each other, it helps support the weight of the vehicle as well as any cargo, and it may even act as part of the steering, suspension, or drive system. In some trucks, the truck axle will be responsible for transferring power from the drive shaft to the wheels. The use of several other components in conjunction with the truck axle will be necessary to make this system possible.
The suspension system of a truck may rely on either one or many truck axles to support the weight of the vehicle. A solid axle may be mounted to leaf springs, for example, which will endure much of the suspension duties, but the leaf springs are designed to work in conjunction with the axle to provide stability during rough conditions as well as turning. In other cases, the axle may be used as a steering component, allowing the driver to navigate the vehicle by manipulating the components within or surrounding the axle.
Larger trucks may feature several axles to endure the weight of cargo being hauled by the vehicle. An 18-wheeler, for example, is a type of truck that will feature 18 wheels mounted on four, five, or six axles. This helps distribute the load more evenly, helping stabilize the truck and prevent damage to other components. Axles can crack under extreme load or after enduring an impact, however, so axles can be replaced if necessary.
Many axles are built to accommodate axle bearings that allow the wheels and other moving parts to move smoothly. The location and purpose of such bearings can vary according to the type of axle being used on the truck, though most axles are likely to work in conjunction with wheel bearings that allow the wheels to rotate freely.