An axle shaft is a solid steel shaft that runs from the differential and gear set of an axle housing to the wheel. Used in two distinct configurations, the axle shaft can be a straight shaft with splines machined into each end to engage both the differential on one end and a drive flange on the other. It also can be a straight shaft with splines machined into the differential end and a flange on the other to mount a wheel to the axle. The first design is primarily used with a full floating axle design, while the latter is commonly used on passenger cars and light pickup trucks.
As a rule, this shaft is the thickest piece of steel on any given vehicle chassis. Designed to withstand the twisting force of the drive train as well as to support a portion of the vehicle's weight, an axle shaft is hardened to further enhance the natural strength of the steel used to manufacture the axle. Each specific shaft is hardened in a particular manner and design to best withstand the purpose for which it is designed to function. This hardening is the defining factor in creating an axle that will not break under stress from intended use.
On the typical passenger vehicle, the axle shaft has the wheel flange machined into the axle itself and comes as a single component. The wheel bearing is commonly pressed on to the outside end of the axle nearest to the wheel flange, and the wheel studs are installed through the flange. This design utilizes the wheel bearing to support the weight of the vehicle by placing the wheel bearing at the outer edge of the axle housing. In this configuration, the axle flange delivers power to propel the vehicle to the tire and wheel assembly.
In a full-floating axle, the axle shaft is a straight axle with machined splines on each end. This axle, common in heavy trucks as well as high-performance and racing applications, does not support any of the vehicle's weight. The wheel assembly is attached to the axle housing by way of an inner and outer wheel bearing and rolls freely upon them while supporting all of the vehicle's weight. The shaft is slid down the axle tube and into the splined differential while a drive flange is placed on the wheel and over the outer spline of the axle shaft. Once the flange is tightened, the axle will drive the wheel; if the axle should happen to break, the wheel will continue to roll freely.