The axle is installed through a hub during a vehicle's construction to keep the axle perpendicular to the wheel, which is installed onto the hub assembly. The hub aligns the axle and wheel so that the wheels turn with the axle. The bearing connects an axle to the hub shell. This provides a tighter alignment between the axle and hub while allowing the wheel to rotate freely. Bearings are often lubricated to reduce wear on the hub during use. For larger vehicles, including all types of automobiles, the hub and bearing often come in the same assembly.
Torque is applied to a drive shaft on a vehicle from a powered gear system. Systems generating this sort of power include gasoline-powered engines or foot-powered bicycle chain drives. The drive shaft uses this torque to spin the bearing, which in turn spins the wheel. Hubs will also rotate as part of a hub bearing assembly; on simpler vehicles, the bearing spins within the hub.
A steering system connects to the axles on a vehicle causing the axle to rotate with the steering mechanism controlled by the driver. The bearing keeps the axle tight to the hub so that the wheels remain perfectly aligned with the axle during a turn.
During use, bearings reduce the amount of wear from friction on the hub. The type of bearing depends on the amount of torque that will be transmitted from the drive shaft to the wheel. Most hub and bearing systems are greased to increase lubrication.
