Axle

On cars and trucks, several senses of the word axle occur in casual usage, referring to the shaft itself, its housing, or simply any transverse pair of wheels.

In a solid, "live-axle" suspension system, the rotating inner axle cores (or half-shafts) serve to transmit driving torque to the wheels at each end, while the rigid outer tube maintains the position of the wheels at fixed angles relative to the axle, and controls the angle of the axle and wheels assembly to the vehicle body.

In other types of suspension systems, the axles serve only to transmit driving torque to the wheels: the position and angle of the wheel hubs is made independent from the axles by the function of the suspension system.

This is typical of the independent suspensions found on most newer cars, and even SUVs, and on the front of many light trucks.

Independent drive-trains still need differentials (or diffs), but without fixed axle-housing tubes attached.

The diff may be attached to the vehicle frame or body, and/or be integrated with the transmission (or gearbox) in a combined transaxle unit.

The axle (half-)shafts then transmit driving torque to the wheels, usually via constant-velocity joints.

Even when the suspension is not independent, split axles permit the use of a differential, allowing the left and right drive wheels to be driven at different speeds as the automobile turns, improving traction and extending tire life.

Truck designs use such a configuration to provide a greater weight capacity than a single axle.

Modern front-wheel drive cars typically combine the transmission (gearbox and differential) and front axle into a single unit called a transaxle.

Some simple vehicle designs, such as leisure go-karts, may have a single driven wheel where the drive axle is a split axle with only one of the two shafts driven by the engine, or else have both wheels connected to one shaft without a differential (kart racing).

The liftable tandem drive axle was invented in 1957 by the Finnish truck manufacturer Vanajan Autotehdas, a company sharing history with Sisu Auto.

Needle bearings and separate lip seals are used in semi-floating axles with axle retained in the housing at their inner ends typically with circlips which are 3¾-round hardened washers that slide into grooves machined at the inner end of the shafts and retained in/by recesses in the differential carrier side gears which are themselves retained by the differential pinion gear (or "spider gear") shaft.

Axles that are pressed into ball or tapered roller bearings, which are in turn retained in the axle housings with flanges, bolts, and nuts do not "float" and place axial loads on the bearings, housings, and only a short section of the shaft itself, that also carries all radial loads.

This setup allows the axle shaft to be the means of propulsion, and also support the weight of the vehicle.

The semi-floating design is found under most ½-ton and lighter trucks, as well as in SUVs and rear-wheel-drive passenger cars, usually being smaller or less expensive models.