central shaft for a rotating wheel or gear

An axle is a central shaft for a rotating wheel or gear. In some cases the axle may be fixed in position with a bearing or bushing sitting inside the hole in the wheel or gear to allow the wheel or gear to rotate around the axle. In other cases the wheel or gear may be fixed to the axle, with bearings or bushings provided at the mounting points where the axle is supported. Sometimes, especially on bicycles, the latter type is referred to as a spindle.

Train wheels are affixed to a straight axle, such that both wheels rotate in unison.

Vehicle axles

This dump truck has an airlift pusher axle, shown in the raised position, a so-called dead axle, present to maintain equal power supply.

Axles are an important structural component of a wheeled vehicle. The axles maintain the position of the wheels relative to each other and to the vehicle body. Since for most vehicles the wheels are the only part touching the ground, the axles must bear the weight of the vehicle plus any cargo.